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Emittance preservation of an electron beam in a loaded quasi-linear plasma wakefield

Veronica K. Berglyd Olsen, Erik Adli, Patric Muggli

V. K. Berglyd Olsen et al., Phys. Rev. Accel. Beams 21, 011301 (2017), arXiv:1710.04858.

Abstract: We investigate beam loading and emittance preservation for a high-charge electron beam being accelerated in quasi-linear plasma wakefields driven by a short proton beam. The structure of the studied wakefields are similar to those of a long, modulated proton beam, such as the AWAKE proton driver. We show that by properly choosing the electron beam parameters and exploiting two well known effects, beam loading of the wakefield and full blow out of plasma electrons by the accelerated beam, the electron beam can gain large amounts of energy with a narrow final energy spread (%-level) and without significant emittance growth.

Schlieren Imaging for the Determination of the Radius of an Excitated Rubidium Column

A.-M. Bachmann, M. Martyanov, J. Moody, A. Pukhov, P. Muggli

A.-M. Bachmann et al., Nucl. Inst. and Meth. in Phys. Res. A (2017).

Abstract: AWAKE develops a new plasma wakefield accelerator using the CERN SPS proton bunch as a driver. The proton bunch propagates through a 10 m long rubidium plasma, induced by an ionizing laser pulse. The co-propagation of the laser pulse with the proton bunch seeds the self modulation instability of the proton bunch that transforms the bunch to a train with hundreds of bunchlets which drive the wakefields. Therefore the plasma radius must exceed the proton bunch radius. Schlieren imaging is proposed to determine the plasma radius on both ends of the vapor source. We use Schlieren imaging to estimate the radius of a column of excited rubidium atoms. A tunable, narrow bandwidth laser is split into a beam for the excitation of the rubidium vapor and for the visualization using Schlieren imaging. With a laser wavelength very close to the D2 transition line of rubidium (780 nm), it is possible to excite a column of rubidium atoms in a small vapor source, to record a Schlieren signal of the excitation column and to estimate its radius. We describe the method and show the results of the measurement.

A method to determine the maximum radius of defocused protons after self-modulation in AWAKE

M. Turner, E. Gschwendtner, P. Muggli

M. Turner et al., Nucl. Inst. and Meth. in Phys. Res. A (2017).

Abstract: The AWAKE experiment at CERN aims to drive GV/m plasma wakefields with a self-modulated proton drive bunch, and to use them for electron acceleration. During the self-modulation process, protons are defocused by the transverse plasma wakefields and form a halo around the focused bunch core. The two-screen setup integrated in AWAKE measures the transverse, time-integrated proton bunch distribution downstream the 10 m long plasma to detect defocused protons. By measuring the maximum radius of the defocused protons we attempt calculate properties of the self-modulation. In this article, we develop a routine to identify the maximum radius of the defocused protons, based on a standard contour method. We compare the maximum radius obtained from the contour to the logarithmic lineouts of the image to show that the determined radius identifies the edge of the distribution.

Interferometer-based high-accuracy white light measurement of neutral rubidium density and gradient at AWAKE

F. Batsch, M. Martyanov, E. Oz, J. Moody, E. Gschwendtner, A. Caldwell , P. Muggli

F. Batsch et al., Nucl. Inst. and Meth. in Phys. Res. A (2017), arXiv:1802.04604.

Abstract: The AWAKE experiment requires an automated online rubidium (Rb) plasma density and gradient diagnostic for densities between 1 and 10x1014 cm−3. A linear density gradient along the plasma source at the percent level may be useful to improve the electron acceleration process. Because of full laser ionization of Rb vapor to Rb+ within a radius of 1 mm, the plasma density equals the vapor density. We measure the Rb vapor densities at both ends of the source, with high precision using, white light interferometry. At either source end, broadband laser light passes a remotely controlled Mach-Zehnder interferometer built out of single mode fibers. The resulting interference signal, influenced by dispersion in the vicinity of the Rb D1 and D2 transitions, is dispersed in wavelength by a spectrograph. Fully automated Fourier-based signal conditioning and a fit algorithm yield the density with an uncertainty between the measurements at both ends of 0.11 to 0.46 % over the entire density range. These densities used to operate the plasma source are displayed live in the control room.

Simulation study of an LWFA-based electron injector for AWAKE Run 2

B. Williamson, G. Xia, S. Döbert, S. Karsch, P. Muggli

B. Williamson et al., Nucl. Inst. and Meth. in Phys. Res. A (2017).

Abstract: The AWAKE experiment aims to demonstrate preservation of injected electron beam quality during acceleration in proton-driven plasma waves. The short bunch duration required to correctly load the wakefield is challenging to meet with the current electron injector system, given the space available to the beamline. An LWFA readily provides short-duration electron beams with sufficient charge from a compact design, and provides a scalable option for future electron acceleration experiments at AWAKE. Simulations of a shock-front injected LWFA demonstrate a 43 TW laser system would be sufficient to produce the required charge over a range of energies beyond 100 MeV. LWFA beams typically have high peak current and large divergence on exiting their native plasmas, and optimisation of bunch parameters before injection into the proton-driven wakefields is required. Compact beam transport solutions are discussed.

Summary of Working Group 8: Advanced and Novel Accelerators for High Energy Physics

B. Cros, P. Muggli, C. B. Schroeder, C. Tang

B. Cros et al., Nucl. Inst. and Meth. in Phys. Res. A (2017), arXiv:1712.08343.

Abstract: We briefly summarize the work and discussions that occurred during the Working Group 8 sessions of the EAAC 2017, dedicated to advanced and novel accelerators for high energy physics applications.

Summary of Working Group 8: Advanced and Novel Accelerators for High Energy Physics

F. Braunmueller, M. Martyanov, S. Alberti, P. Muggli

F. Braunmueller et al., accepeted for publication in Nucl. Inst. and Meth. in Phys. Res. A (2017), arXiv:1801.02415.

Abstract: We present the set-up and test-measurements of a waveguide-integrated heterodyne diagnostic for coherent transition radiation (CTR) in the AWAKE experiment. The goal of the proof-of-principle experiment AWAKE is to accelerate a witness electron bunch in the plasma wakefield of a long proton bunch that is transformed by Seeded Self-Modulation (SSM) into a train of proton micro-bunches. The CTR pulse of the self-modulated proton bunch is expected to have a frequency in the range of 90-300 GHz and a duration of 300-700 ps. The diagnostic set-up, which is designed to precisely measure the frequency and shape of this CTR-pulse, consists of two waveguide-integrated receivers that are able to measure simultaneously. They cover a significant fraction of the available plasma frequencies: the bandwidth 90-140 GHz as well as the bandwidth 255-270 GHz or 170-260 GHz in an earlier or a latter version of the set-up, respectively. The two mixers convert the CTR into a signal in the range of 5-20 GHz that is measured on a fast oscilloscope, with a high spectral resolution of 1-3 GHz dominated by the pulse length. In this contribution, we will describe the measurement principle, the experimental set-up and a benchmarking of the diagnostic in AWAKE.

Signatures of the self-modulation instability of relativistic proton bunches in the AWAKE experiment

M. Moreira, J. Vieira, P. Muggli

M. Moreira et al., accepeted for publication in Nucl. Inst. and Meth. in Phys. Res. A (2017), arXiv:1803.01558.

Abstract: We investigate numerically the detection of the self-modulation instability in a virtual detector located downstream from the plasma in the context of AWAKE. We show that the density structures, appearing in the temporally resolving virtual detector, map the transverse beam phase space distribution at the plasma exit. As a result, the proton bunch radius that appears to grow along the bunch in the detector results from the divergence increase along the bunch, related with the spatial growth of the self-modulated wakefields. In addition, asymmetric bunch structures in the detector are a result of asymmetries of the bunch divergence, and do not necessarily reflect asymmetric beam density distributions in the plasma.

A Rubidium Vapor Source for a Plasma Source for AWAKE

G. Plyushchev, R. Kersevan, A. Petrenko, P. Muggli

G. Plyushchev et al., Journal of Physics D: Applied Physics, 51(2), 025203 (2017).

Abstract: We present the scheme for a rubidium vapor source that is used as a plasma source in the AWAKE plasma wakefield acceleration experiment. The plasma wakefield acceleration process requires a number of stringent parameters for the plasma: electron density adjustable in the (1-10)x1014cm-3 range, 0.25% relative density uniformity, sharp (<10cm) density ramps at each end, density gradient adjustable from -3 to +10% over 10m, and %-level density step near the beginning the plasma column. We show with analytical and direct Simulation Monte Carlo results that the rubidium density in the proposed source should meet these requirements. Laser ionization then transfers the above neutral vapor parameters to the plasma.

AWAKE readiness for the study of the seeded self-modulation of a 400GeV proton bunch

P. Muggli, for the AWAKE Collaboration

P. Muggli et al., Plasma Physics and Controlled Fusion, 60(1) 014046 (2017).

Abstract: AWAKE is a proton-driven plasma wakefield acceleration experiment. We show that the experimental setup briefly described here is ready for systematic study of the seeded self-modulation of the 400GeV proton bunch in the 10\,m-long rubidium plasma with density adjustable from 1 to 10×1014cm−3. We show that the short laser pulse used for ionization of the rubidium vapor propagates all the way along the column, suggesting full ionization of the vapor. % We show that ionization occurs along the proton bunch, at the laser time and that the plasma that follows affects the proton bunch.

GHz Modulation detection using a streak camera: suitability of streak cameras in the AWAKE experiment

K. Rieger, A. Caldwell, O. Reimann, and P. Muggli

K. Rieger et al., Review of Scientific Instruments 88, 025110 (2017).

Abstract: Using frequency mixing a modulated light pulse of ns duration is created. We show that, with a streak camera, we can detect via an FFT detection approach up to 450 GHz modulation in a pulse in a single measurement, which is an unusual application of a streak camera. This work is performed in the context of the AWAKE plasma wake eld experiment where modulation frequencies in the range of 80-280 GHz are expected.

AWAKE: A Proton-Driven Plasma Wakefield Acceleration Experiment at CERN

C.Bracco and P. Muggli et al.

C.Bracco et al., Nuclear and Particle Physics Proceedings 273-275, 175 (2016).

Abstract:The AWAKE Collaboration has been formed in order to demonstrate proton-driven plasma wakefield acceleration for the first time. This acceleration technique could lead to future colliders of high energy but of a much reduced length when compared to proposed linear accelerators. The CERN SPS proton beam in the CNGS facility will be injected into a 10 m plasma cell where the long proton bunches will be modulated into significantly shorter micro-bunches. These micro-bunches will then initiate a strong wakefield in the plasma with peak fields above 1 GV/m that will be harnessed to accelerate a bunch of electrons from about 20 MeV to the GeV scale within a few meters. The experimental program is based on detailed numerical simulations of beam and plasma interactions. The main accelerator components, the experimental area and infrastructure required as well as the plasma cell and the diagnostic equipment are discussed in detail. First protons to the experiment are expected at the end of 2016 and this will be followed by an initial three-four years experimental program. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders.

Proton-Beam-Driven Plasma Acceleration

E. Adli and P. Muggli

E. Adli and P. Muggli, Rev. Accl. Sci. Tech. 09, 85 (2016).

Abstract: We describe the main ideas, promises and challenges related to proton-driven plasma wakefield acceleration. Existing high-energy proton beams have the potential to accelerate electron beams to the TeV scale in a single plasma stage. In order to drive a wake effectively the available beams must be either highly compressed or microbunched. The self-modulation instability has been suggested as a way to microbunch the proton beams. The AWAKE project at CERN is currently the only planned proton-driven plasma acceleration experiment. A self-modulated CERN SPS beam will be used to drive a plasma wake. We describe the design choices and experimental setup for AWAKE, and discuss briefly the short-term objectives as well as longer-term ideas for the project.

Staging optics considerations for a plasma wakefield acceleration linear collider

C.A. Lindstrøm, E. Adli, J.M. Allen, J.P. Delahaye, M.J. Hogan, C. Joshi, P. Muggli, T.O. Raubenheimer, V. Yakimenko

C.A. Lindstrøm et al., Nucl. Instr. and Meth. in Phys. Res. A 829, 224 (2016).

Abstract: Plasma wakefield acceleration offers acceleration gradients of several GeV/m, ideal for a next-generation linear collider. The beam optics requirements between plasma cells include injection and extraction of drive beams, matching the main beam beta functions into the next cell, canceling dispersion as well as constraining bunch lengthening and chromaticity. To maintain a high effective acceleration gradient, this must be accomplished in the shortest distance possible. A working example is presented, using novel methods to correct chromaticity, as well as scaling laws for a high energy regime.

An accurate Rb density measurement method for a plasma wakefield accelerator experiment using a novel Rb reservoir

E. Öz, F. Batsch, P. Muggli

E. Öz et al., Nucl. Instr. and Meth. in Phys. Res. A 829, 321 (2016).

Abstract: A method to accurately measure the density of Rb vapor is described. We plan on using this method for the Advanced Wakefield (AWAKE) (Assmann et al., 2014 [1]) project at CERN , which will be the world׳s first proton driven plasma wakefield experiment. The method is similar to the hook (Marlow, 1967 [2]) method and has been described in great detail in the work by Hill et al. (1986) [3]. In this method a cosine fit is applied to the interferogram to obtain a relative accuracy on the order of 1% for the vapor density–length product. A single-mode, fiber-based, Mach–Zenhder interferometer will be built and used near the ends of the 10 meter-long AWAKE plasma source to be able to make accurate relative density measurement between these two locations. This can then be used to infer the vapor density gradient along the AWAKE plasma source and also change it to the value desired for the plasma wakefield experiment. Here we describe the plan in detail and show preliminary results obtained using a prototype 8 cm long novel Rb vapor cell.

Laser pulse propagation in a meter scale rubidium vapor/plasma cell in AWAKE experiment

A. Joulaei, J. Moody, N. Berti, J. Kasparian, S. Mirzanejhad, P. Muggli

A. Joulaei et al., Nucl. Instr. and Meth. in Phys. Res. A 829, 339 (2016).

Abstract: We present the results of numerical studies of laser pulse propagating in a 3.5 cm Rb vapor cell in the linear dispersion regime by using a 1D model and a 2D code that has been modified for our special case. The 2D simulation finally aimed at finding laser beam parameters suitable to make the Rb vapor fully ionized to obtain a uniform, 10 m-long, at least 1 mm in radius plasma in the next step for the AWAKE experiment.

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

E. Gschwendtner et al.

E. Gschwendtner et al., Nucl. Instr. and Meth. in Phys. Res. A 829, 76 (2016).

Abstract: The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world׳s first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected into the sample wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

Electron trapping and acceleration by the plasma wakefield of a self-modulating proton beam

K. V. Lotov, A. P. Sosedkin, A. V. Petrenko, L. D. Amorim, J. Vieira, R. A. Fonseca, L. O. Silva, E. Gschwendtner and P. Muggli

K. V. Lotov et al., Phys. Plasmas 21, 123116 (2014).

Abstract: It is shown that co-linear injection of electrons or positrons into the wakefield of the self-modulating particle beam is possible and ensures high energy gain. The witness beam must co-propagate with the tail part of the driver, since the plasma wave phase velocity there can exceed the light velocity, which is necessary for efficient acceleration. If the witness beam is many wakefield periods long, then the trapped charge is limited by beam loading effects. The initial trapping is better for positrons, but at the acceleration stage a considerable fraction of positrons is lost from the wave. For efficient trapping of electrons, the plasma boundary must be sharp, with the density transition region shorter than several centimeters. Positrons are not susceptible to the initial plasma density gradient.

High-efficiency acceleration of an electron beam in a plasma wakefield accelerator

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M. Litos, E. Adli, W. An, C. I. Clarke, C. E. Clayton, S. Corde, J. P. Delahaye, R. J. England, A. S. Fisher, J. Frederico, S. Gessner, S. Z. Green, M. J. Hogan, C. Joshi, W. Lu, K. A. Marsh, W. B. Mori, P. Muggli, N. Vafaei-Najafabadi, D. Walz, G. White, Z. Wu, V. Yakimenko, G. Yocky

M. Litos et al., Nature 515, 92 (2014).

Abstract: High-efficiency acceleration of charged particle beams at high gradients of energy gain per unit length is necessary to achieve an affordable and compact high-energy collider. The plasma wakefield accelerator is one concept being developed for this purpose. In plasma wakefield acceleration, a charge-density wake with high accelerating fields is driven by the passage of an ultra-relativistic bunch of charged particles (the drive bunch) through a plasma. If a second bunch of relativistic electrons (the trailing bunch) with sufficient charge follows in the wake of the drive bunch at an appropriate distance, it can be efficiently accelerated to high energy. Previous experiments using just a single 42-gigaelectronvolt drive bunch have accelerated electrons with a continuous energy spectrum and a maximum energy of up to 85 gigaelectronvolts from the tail of the same bunch in less than a metre of plasma. However, the total charge of these accelerated electrons was insufficient to extract a substantial amount of energy from the wake. Here we report high-efficiency acceleration of a discrete trailing bunch of electrons that contains sufficient charge to extract a substantial amount of energy from the high-gradient, nonlinear plasma wakefield accelerator. Specifically, we show the acceleration of about 74 picocoulombs of charge contained in the core of the trailing bunch in an accelerating gradient of about 4.4 gigavolts per metre. These core particles gain about 1.6 gigaelectronvolts of energy per particle, with a final energy spread as low as 0.7 per cent (2.0 per cent on average), and an energy-transfer efficiency from the wake to the bunch that can exceed 30 per cent (17.7 per cent on average). This acceleration of a distinct bunch of electrons containing a substantial charge and having a small energy spread with both a high accelerating gradient and a high energy-transfer efficiency represents a milestone in the development of plasma wakefield acceleration into a compact and affordable accelerator technology.

Laser Ionized Preformed Plasma at FACET

S Z Green, E. Adli, C.I. Clarke, S. Corde, S.A. Edstrom, A.S. Fisher, J. Frederico, J.C. Frisch, S. Gessner, S. Gilevich, P. Hering, M.J. Hogan, R.K. Jobe, M. Litos, J.E. May, D.R. Walz, V. Yakimenko, C.E. Clayton, C. Joshi, K.A. Marsh, N. Vafaei-Najafabadi, P. Muggli

S.Z. Green et al., Plasma Phys. Control. Fusion 56, 084011 (2014).

Abstract: The Facility for Advanced Accelerator and Experimental Tests (FACET) at SLAC installed a 10-TW Ti:Sapphire laser system for pre-ionized plasma wakefield acceleration experiments. High energy (500 mJ), short (50 fs) pulses of 800-nm laser light at 1 Hz are used at the FACET experimental area to produce a plasma column. The laser pulses are stretched to 250 fs before injection into a vapor cell, where the laser is focused by an axicon lens to form a plasma column that can be sustained over the desired radius and length. A 20-GeV electron bunch interacts with this preformed plasma to generate a non-linear wakefield, thus accelerating a trailing witness bunch with gradients on the order of several GV/m. The experimental setup and methods for producing the pre-ionized plasma for plasma wakefield acceleration experiments performed at FACET are described.

Self-modulation instability of ultra-relativistic particle bunches with finite rise times

J. Vieira, L.D. Amorim, Y. Fang, W.B.Mori, P. Muggli, L.O. Silva

J. Vieira et al., Plasma Phys. Control. Fusion 56, 084014 (2014).

Abstract: We study the evolution of the self-modulation instability using bunches with rise times longer than the plasma wavelength. Using particle-in-cell simulations we show that unlike long bunches with sharp rise times, there are large variations of the wake amplitudes and wake phase velocity before self-modulation instability saturation. These results show that use of bunches with sharp rise times is important to seed the self-modulation instability to ensure stable acceleration regimes.

Proton-driven plasma wakefield acceleration: a path to the future of high-energy particle physics

AWAKE Collaboration: R. Assmann, R. Bingham, T. Bohl, C. Bracco, B. Buttenschön, A. Butterworth, A. Caldwell, S. Chattopadhyay, S. Cipiccia, E. Feldbaumer, R.A. Fonseca, B. Goddard, M. Gross, O. Grülke, E. Gschwendtner, J. Holloway, C. Huang, D. Jaroszynski, S. Jolly, P. Kempkes, N. Lopes, K. Lotov, J. Machacek, S.R. Mandry, M. Meddahi, N. Moschuering, P. Muggli, Z. Najmudin, P. A. Norreys, E. Oz, A. Pardons, A. Petrenko, A. Pukhov, K. Rieger, O. Reimann, H. Ruhl, E. Shaposhnikova, L.O. Silva, A. Sosedkin, R. Tarkeshian, R.M.G.N. Trines, T. Tückmantel, J. Vieira, H. Vincke, M. Wing, G. Xia

AWAKE Collaboration, Plasma Phys. Control. Fusion 56 084013 (2014), available at arXiv:1401.4823.

Abstract: New acceleration technology is mandatory for the future elucidation of fundamental particles and their interactions. A promising approach is to exploit the properties of plasmas. Past research has focused on creating large-amplitude plasma waves by injecting an intense laser pulse or an electron bunch into the plasma. However, the maximum energy gain of electrons accelerated in a single plasma stage is limited by the energy of the driver. Proton bunches are the most promising drivers of wakefields to accelerate electrons to the TeV energy scale in a single stage. An experimental program at CERN - the AWAKE experiment - has been launched to study in detail the important physical processes and to demonstrate the power of proton-driven plasma wakefield acceleration. Here we review the physical principles and some experimental considerations for a future proton-driven plasma wake field accelerator.

Hosing Instability Suppression in Self-modulated Plasma Wakefields

J. Vieira, W.B. Mori and P. Muggli

J. Vieira et al., Phys. Rev. Lett. 112, 205001 (2014).

Abstract: We show that hosing of a long particle beam is stabilized for a beam that is 100% self-modulated if the associated plasma density perturbation is linear. These results indicate that when a train of particle bunches or laser pulses excite a linear wake, the train is stable against hosing. We derive scalings for maximum bunch tilts and SMI seeds needed to ensure stable propagation beyond saturation of the self-modulation instability. Numerical solutions of the reduced hosing equations and three-dimensional particle-in-cell simulations confirm our analytical findings.

Proceedings of the first European Advanced Accelerator Concepts Workshop 2013: Summary of working group 1: Electron beams from plasmas

P. Muggli and Z. Najmudin

P. Muggli et al., Nucl. Instr. Meth. Phys. Res. A 740(11), 39 (2014).

Abstract: We briefly summarize the contribution presented during the working group 1 (WG1) sessions.

A novel Rb vapor plasma source for plasma wakefield accelerators

E. Oz and P. Muggli

E. Oz et al., Nucl. Instr. Meth. Phys. Res. A 740(11), 197 (2014).

Abstract: We describe a novel plasma source developed at the Max Planck Institute for Physics that will be used for a proton driven plasma wakefield accelerator experiment at CERN. Rubidium vapor is confined in a 10 meter-long, 4 cm diameter, oil-heated stainless steel pipe. A laser pulse tunnel ionizes the vapor forming a 10-meter long, radius plasma with a range of densities around . Access to the source is provided using custom manufactured fast valves. The source is designed to produce a plasma with a density uniformity of at least 0.2% during the beam–plasma interaction.

The effect of plasma radius and profile on the development of self-modulation instability of electron bunches

Y. Fang, L.D. Amorim, J. Vieira, W.B. Mori and P. Muggli

Y. Fang et al., Phys. Plasmas 21, 056703 (2014).

Abstract: Plasmas available for PWFA experiments may have longitudinal and transverse density profiles that could affect the outcome of an experiment. This paper investigates the effect of plasmas with finite radius and inhomogeneous transverse density profiles on the wakefield excitation and the self-modulation instability (SMI) development in overdense plasmas. We focus here on the case of an electron bunch. Simulation results show that such plasmas generate larger focusing force for the propagating electron beam, and therefore higher growth rate for the SMI. Although the initial Ez amplitude is lower in such plasmas, the increased focusing force can dominate the development trend of the SMI, i.e., larger saturated Ez amplitude can be reached over similar plasma lengths.

Seeding of the Self-modulation Instability of a long Electron Bunch in a Plasma

Y. Fang, V. E. Yakimenko, M. Babzien, M. Fedurin, K. P. Kusche, R. Malone, J. Vieira, W.B. Mori and P. Muggli

Y. Fang et al., Phys. Rev. Lett. 112, 045001 (2014).

Abstract: We demonstrate experimentally that a relativistic electron bunch shaped with a sharp rising edge drives plasma wakefields with one to seven periods along the bunch as the plasma density is increased. The plasma density is varied in the 1015-1017 cm-3 range. The wakefields generation is observed after the plasma as periodic modulation of the correlated energy spectrum of the incoming bunch. We choose a low bunch charge of 50 pC for optimum visibility of the modulation at all plasma densities. The longitudinal wakefields creating the modulation are in the MV/m range and are indirect evidence of the generation of transverse wakefields that can seed the self-modulation instability, although the instability does not grow significantly over the short plasma length (2 cm). We show that the seeding provides a phase reference for the wakefields, a necessary condition for the deterministic external injection of a witness bunch in an accelerator. This electron work supports the concept of similar experiments in the future, e.g. SMI experiments using long bunches of relativistic protons.

Beam loading by distributed injection of electrons in a plasma wakefield accelerator

N. Vafaei-Najafabadi, K. A. Marsh, C. E. Clayton, W. An, W. B. Mori, C. Joshi, W. Lu, E. Adli, S. Corde, M. Litos, S. Li, S. Gessner, J. Frederico, A. S. Fisher, Z. Wu, D. Walz, R. J. England, J. P. Delahaye, C. I. Clarke, M. J. Hogan, and P. Muggli

N. Vafaei-Najafabadi et al., Phys. Rev. Lett. 112, 025001 (2014).

Abstract: We show through experiments and supporting simulations that propagation of a highly relativistic and dense electron bunch through a plasma can lead to distributed injection of electrons, which depletes the accelerating field; i.e. beam loads the wake. The source of the injected electrons is ionization of the second electron of rubidium (Rb II) within the wake. This injection of excess charge is large enough to severely beam load the wake, and thereby reduce the transformer ratio T. The reduction of the average T with increasing beam loading is quantified for the first time by measuring the ratio of peak energy gain and loss of electrons while changing the beam emittance. Simulations show that beam loading by RbII electrons contributes to the reduction of the peak accelerating field from its weakly loaded value of 43 GV/m to strongly loaded value of 26 GV/m.

Interaction of Ultra Relativistic e- e+ Fireball Beam with Plasma

P. Muggli, S.F. Martins, J. Vieira, L. O. Silva

P. Muggli et al., arXiv:1306.4380 (2013).

Abstract: Ab initio simulations of the propagation in a plasma of a soon to be available relativistic electron-positron beam or fireball beam provide an effective mean for the study of microphysics relevant to astrophysical scenarios. We show that the current filamentation instability associated with some of these scenarios reaches saturation after only 10 cm of propagation in a typical laboratory plasma with a density 1017/cc. The different regimes of the instability, from the purely transverse to the mixed mode filamentation, can be accessed by varying the background plasma density. The instability generates large local plasma gradients, intense transverse magnetic fields, and enhanced emission of radiation. We suggest that these effects may be observed experimentally for the first time.

Strategies for mitigating the ionization-induced beam head erosion problem in an electron-beam-driven plasma wakefield accelerator

W. An, M. Zhou, N. Vafaei-Najafabadi, K. A. Marsh, C. E. Clayton, C. Joshi, W. B. Mori, W. Lu, E. Adli, S. Corde, M. Litos, S. Li, S. Gessner, J. Frederico, M. J. Hogan, D. Walz, J. England, J. P. Delahaye, and P. Muggli

W. An et al., Phys. Rev. ST Accel. Beams 16, 101301 (2013).

Abstract: Strategies for mitigating ionization-induced beam-head erosion in an electron-beam-driven plasma wake field accelerator (PWFA) are explored when the plasma and the wake are both formed by the transverse electric field of the beam itself. Beam head erosion can occur in a preformed plasmas because of a lack of focusing force from the wake at the rising edge (head) of the beam due to the finite inertia of the electrons. When the plasma is produced by field ionization from the space charge field of the beam, the head erosion is significantly exacerbated due to the gradual recession (in the beam frame) of the 100% ionization contour. Beam particles in front of the ionization front cannot be focused (guided) causing them to expand as in vacuum. When they expand, the location of the ionization front recedes such that even more beam particles are completely unguided. Eventually this process terminates the wake formation prematurely, i.e., well before the beam is depleted of its energy. Ionization-induced head erosion can be mitigated by controlling the beam parameters (emittance, charge and energy) and/or the plasma conditions. In this paper we explore how the latter can be optimized so as to extend the beam propagation distance and thereby increase the energy gain. In particular we show that, by using a combination of the alkali atoms of the lowest practical ionization potential (Cs) for plasma formation and a precursor laser pulse to generate a narrow plasma filament in front of the beam, the head erosion rate can be dramatically reduced. Simulation results show that in the upcoming ``two-bunch PWFA experiments" on the FACET facility at SLAC national accelerator laboratory the energy gain of the trailing beam can be up to 10 times larger for the given parameters when employing these techniques. Comparison of the effect of beam head erosion in preformed and ionization produced plasmas is also presented.

Experimental Study of Current Filamentation Instability

B. Allen, V. Yakimenko, M. Babzien, M. Fedurin, K. Kusche, P. Muggli

B. Allen et al., Phys. Rev. Lett. 109, 185007 (2012).

Abstract: Current filamentation instability is observed and studied in a laboratory environment with a 60MeV electron beam and a plasma capillary discharge. Multiple filaments are observed and imaged transversely at the plasma exit with optical transition radiation. By varying the plasma density the transition between single and multiple filaments is found to be kpσr~2.2. Scaling of the transverse filament size with the plasma skin depth is predicted in theory and observed over a range of plasma densities. Lowering the bunch charge, and thus bunch density, suppresses the instability.

Experimental observation of suppressing CSR-induced beam energy spread with shielding plates

V. Yakimenko, M. Fedurin, V. Litvinenko, A. Fedotov, D. Kayran, P. Muggli

V. Yakimenko et al., Phys. Rev. Lett. 109, 164802 (2012).

Abstract: We describe the first direct observation of the significant suppression of the energy spread induced by coherent synchrotron radiation (CSR) by a pair of conductive plates placed inside a dipole magnet. In addition to various feedback loops improving the energy stability of the beam parameters, our key innovation for this experiment is the observation of the time-resolved energy-variation within the electron bunch, instead of the traditionally measured RMS energy spread. We present the results of the experiments and compare them with a rigorous analytical theory.

Dielectric wakefield acceleration of a relativistic electron beam in a slab-symmetric dielectric lined waveguide

G. Andonian, D. Stratakis, O. Williams, B. O’Shea, X. Wei, E. Hemsing, P. Muggli, M. Babzien, K. Kusche, M. Fedurin, V. Yakimenko, J.B. Rosenzweig

G. Andonian et al., Phys. Rev. Lett. 108, 244801 (2012).

Abstract: We report first evidence of wakefield acceleration of a relativistic electron beam in a dielectric-lined slab-symmetric structure. The high energy tail of a 60 MeV electron beam was accelerated by 150 keV in a 2cm long, slab-symmetric SiO2 waveguide, with the acceleration/deceleration clearly visible due to use of a beam with a bifurcated longitudinal distribution that serves to approximate well a driver-witness beam pair. This split-bunch distribution is verified by longitudinal reconstruc- tion analysis of the emitted coherent transition radiation. The dielectric waveguide structure is further characterized by spectral analysis of the emitted coherent Cerenkov radiation at THz frequencies, from a single electron bunch, and from a a relativistic bunch train with spacing selectively tuned to the second longitudinal mode (TM02). Start-to-end simulation results reproduce aspects of the electron beam bifurcation dynamics, emitted THz radiation properties, and the observation of acceleration in the dielectric lined, slab-symmetric waveguide.

A proposed demonstration of an experiment of proton-driven plasma wakefield acceleration based on CERN SPS

G. Xia, R. Assmann, R. A. Fonseca, C. Huang, W. Mori, L. O. Silva, J. Vieira, F. Zimmermann and P. Muggli

G. Xia et al., J. Plasma Phys., 1-7 (2012).

Abstract: The proton bunch-driven plasma wakefield acceleration (PWFA) has been proposed as an approach to accelerate an electron beam to the TeV energy regime in a single plasma section. An experimental program has been recently proposed to demonstrate the capability of proton-driven PWFA by using existing proton beams from the European Organization for Nuclear Research (CERN) accelerator complex. At present, a spare Super Proton Synchrotron (SPS) tunnel, having a length of 600 m, could be used for this purpose. The layout of the experiment is introduced. Particle-in-cell simulation results based on realistic SPS beam parameters are presented. Simulations show that working in a self-modulation regime, the wakefield driven by an SPS beam can accelerate an externally injected approx. 10 MeV electrons to approx. 2 GeV in a 10-m plasma, with a plasma density of 7x1014 cm-3.

Transverse self-modulation of ultra-relativistic lepton beams in the plasma wakefield accelerator

J. Vieira, Y. Fang, W.B. Mori, L. O. Silva and P. Muggli

J. Vieira et al., Phys. Plasmas 19, 063105 (2012).

Abstract: The transverse self-modulation of ultra-relativistic, long lepton bunches in high-density plasmas is explored through full-scale particle-in-cell simulations. We demonstrate that long SLAC-type electron and positron bunches can become strongly self-modulated over centimeter distances, leading to wake excitation in the blowout regime with accelerating fields in excess of 20 GV/m. We show that particles energy variations exceeding 10 GeV can occur in meter-long plasmas. We find that the self-modulation of positively and negatively charged bunches differ when the blowout is reached. Seeding the self-modulation instability mitigates the effect of the competing hosing instability. This work reveals that a proof-of-principle experiment to test the physics of bunch self-modulation can be performed with available lepton bunches and with existing experimental apparatus and diagnostics.

Teravolt-per-meter beam and plasma fields from low-charge femtosecond electron beams

J.B. Rosenzweig, G. Andonian, P. Bucksbaum, M. Ferrario, S. Full, A. Fukusawa, E. Hemsing, B. Hidding, M. Hogan, P. Krejcik, P. Muggli, G. Marcus, A. Marinelli, P. Musumeci, B. O'Shea, C. Pellegrini, D. Schiller, G. Travish

J.B. Rosenzweig et al., Nucl. Inst. Meth. Phys. Res. A 663(1), 98 (2011).

Abstract: Recent initiatives in ultra-short, GeV electron beam generation have been aimed at achieving sub-femtosecond (fs) pulses capable of driving X-ray free-electron lasers (FELs) in single-spike mode. This scheme foresees the use of very low charge beams, which may allow existing FEL injectors to produce few-100 as pulses, with very high brightness. Towards this end, recent experiments at SLAC have produced approximately 2 fs rms, low transverse emittance, 20 pC electron pulses. Here we examine the use of such pulses to excite plasma wakefields exceeding 1 TV/m, permitting a table-top TeV accelerator. We present a scheme for focusing the beam to very small dimensions, where the surface Coulomb fields are also at the TV/m level. These conditions access a new regime for high field for atomic physics, allowing frontier atomic physics experiments such as barrier suppression regime ionization. They also, critically, permit well-sub-fs plasma formation for subsequent wake excitation. We examine the use of such ultra-short beams for creating coherent sub-cycle IR radiation at unprecedented high power levels.

Phase Velocity and Particle Injection in a Self-Modulated Proton-Driven Plasma Wakefield Accelerator

A. Pukhov, N. Kumar, T. Tuckmantel, A. Upadhyay, K. Lotov, P. Muggli, V. Khudik, C. Siemon, and G. Shvets

A. Pukhov et al., Phys. Rev. Lett. 107, 145003 (2011).

Abstract: It is demonstrated that the performance of the self-modulated proton driver plasma wakefield accelerator is strongly affected by the reduced phase velocity of the plasma wave. Using analytical theory and particle-in-cell simulations, we show that the reduction is largest during the linear stage of self-modulation. As the instability nonlinearly saturates, the phase velocity approaches that of the driver. The deleterious effects of the wake’s dynamics on the maximum energy gain of accelerated electrons can be avoided using side-injections of electrons, or by controlling the wake’s phase velocity by smooth plasma density gradients.

Resonant Excitation of Coherent Cerenkov Radiation in Dielectric Lined Waveguides

G. Andonian, O. Williams,1 X. Wei, P. Niknejadi, E. Hemsing, J.B. Rosenzweig, P. Muggli, M. Babzien, M. Fedurin, K. Kusche, R. Malone, and V. Yakimenko

G. Andonian et al., Appl. Phys. Lett. 98, 202901 (2011).

Abstract: We report the observation of coherent Cerenkov radiation in the terahertz regime emitted by a relativistic electron pulse train passing through a dielectric lined cylindrical waveguide. We describe the beam manipulations and measurements involved in repetitive pulse train creation including comb collimation and nonlinear optics corrections. With this technique, modes beyond the fundamental are selectively excited by use of the appropriate frequency train. The spectral characterization of the structure shows preferential excitation of the fundamental and of a higher longitudinal mode.

Hollow plasma channel for positron plasma wakefield acceleration

W. D. Kimura, H. M. Milchberg P. Muggli, X. Li, W. B. Mori

W. D. Kimura et al., Phys. Rev. ST Accel. Beams 14, 041301 (2011).

Abstract: Plasma wakefield acceleration (PWFA) has demonstrated the ability to produce very high gradients to accelerate electrons and positrons. In PWFA, a drive bunch of charged particles passes through a uniform plasma, thereby generating a wakefield that accelerates a witness bunch traveling behind the drive bunch. This process works well for electrons, but much less so for positrons due to the positive charge attracting rather than repealing the plasma electrons, which leads to reduced acceleration gradient, halo formation, and emittance growth. This problem can be alleviated by having the positron beam travel through a hollow plasma channel. Presented are modeling results for producing 10-100 cm long hollow plasma channels suitable for positron PWFA. These channels are created utilizing laser-induced gas breakdown in hydrogen gas. The results show that hollow channels with plasma densities of order 1016cm-3 and inner channel radii of order 20μm are possible using currently available terawatt-level lasers. At these densities and radii, preliminary positron PWFA modeling indicates that longitudinal electric fields on axis can exceed 3GV/m.

Effect of temperature on ion motion in future plasma wakefield accelerators

R. Gholizadeh, T. Katsouleas, C. Huang, W. B. Mori and P. Muggli

R. Gholizadeh et al., Phys. Rev. ST Accel. Beams 14, 021303 (2011).

Abstract: We study the effect of plasma temperature on ion motion in a plasma wakefield accelerator with parameters typical of a future high-energy accelerator. We show that the collapse of the plasma ions caused by the extremely high fields of ultra-dense electron bunches can be prevented only by a very high plasma ion temperature.

Scaling of the longitudinal electric field and transformer ratio in a nonlinear plasma wakefield accelerator

I. Blumenfeld, C. E. Clayton, F. J. Decker, M. J. Hogan, C. Huang, R. Ischebeck, R. H. Iverson, C. Joshi, T. Katsouleas, N. Kirby, W. Lu, K. A. Marsh, W. B. Mori, P. Muggli, E. Oz, R. H. Siemann, D. R. Walz, and M. Zhou

I. Blumenfeld et al., Phys. Rev. ST Accel. Beams 13, 111301 (2010).

Abstract: The scaling of the two important figures of merit, the transformer ratio, T, and the longitudinal electric field, Ez, with the peak drive-bunch current, IP, in a nonlinear Plasma Wakefield Accelerator is presented for the first time. The longitudinal field scales as IP0.623+/-0.007, in good agreement with nonlinear wakefield theory (IP0.5), while the unloaded transformer ratio is shown to be greater than unity and scales weakly with the bunch current. The effect of bunch head erosion on both parameters is also discussed.

A Simple Method for Generating Adjustable Trains of Picosecond Electron Bunches

P. Muggli, B. Allen, V. E. Yakimenko, J. Park, M. Babzien, K. P. Kusche, W.D. Kimura

P. Muggli et al., Phys. Rev. ST Accel. Beams 13, 052803 (2010).

Abstract: A simple, passive method for producing an adjustable train of picosecond electron bunches is demonstrated. The key component of this method is an electron beam mask consisting of an array of parallel wires that selectively spoils the beam emittance. This mask is positioned in a high magnetic dispersion, low beta-function region of the beam line. The incoming electron beam striking the mask has a time/energy correlation that corresponds to a time/position correlation at the mask location. The mask pattern is transformed into a time pattern or train of bunches when the dispersion is brought back to zero downstream of the mask. Results are presented of a proof-of-principle experiment demonstrating this novel technique that was performed at the Brookhaven National Laboratory Accelerator Test Facility. This technique allows for easy tailoring of the bunch train for a particular application, including varying the bunch width and spacing, and enabling the generation of a trailing witness bunch.

Plasma Wakefield Acceleration Experiments at FACET

M.J. Hogan, T. O. Raubenheimer, A. Seryi, P. Muggli, T. Katsouleas, C. Huang, W. Lu, W. An, K.A. Marsh, W.B. Mori, C. E. Clayton, C. Joshi

M.J. Hogan et al., New J. Phys. 12, 055030 (2010).

Abstract: FACET Facilities for Accelerator science and Experimental Test beams at SLAC will provide high energy density electron and positron beams with peak currents of roughly 20 kA that will be focused down to a 10μm x 10μm transverse spot size at an energy of ~23 GeV. With FACET, the SLAC linac will support a unique program concentrating on second-generation research in plasma wakefield acceleration. Topics include high- gradient electron acceleration with a narrow energy spread and preserved emittance, beam loading, and high-gradient positron acceleration. This paper describes the FACET facility, summarizes the state of the art for plasma wakefield accelerators and discusses the plasma wakefield accelerator program to be conducted at FACET over the next five years.

Energy Gain Scaling with Plasma Length and Density in the Plasma Wakefield Accelerator

P. Muggli, I. Blumenfeld, C. E. Clayton, F. J. Decker, M. J. Hogan, C. Huang, R. Ischebeck, R. H. Iverson, C. Joshi, T. Katsouleas, N. Kirby, W. Lu, K. A. Marsh, W. B. Mori, E. Oz, R. H. Siemann, D. R. Walz, M. Zhou

P. Muggli et al., New J. Phys. 12, 045022 (2010).

Abstract: We present plasma wakefield acceleration experimental results showing that the energy gain by 28.5~GeV electrons scales with plasma length, and reaches 14 GeV over a plasma with a density of 2.6x1017cm-3 and a length of 31 cm. At this plasma density the average accelerating gradient is 36 GV/m. These results are in good agreements with the numbers obtained from particle in cell simulations describing the experiment. The linear scaling is also observed both at lower and higher plasma densities, at which smaller energy gains and accelerating gradients are measured. The systematic measurements of energy gain show the reproducibility and control of the acceleration process.

Preservation of beam emittance in the presence of ion motion in future plasma wakefield-based colliders

R. Gholizadeh, T. Katsouleas, P. Muggli, C. Huang, W. Mori

R. Gholizadeh et al., Phys. Rev. Lett. 104, 155001 (2010).

Abstract: The preservation of beam quality in a plasma wakefield accelerator driven by ultra high intensity and ultra low emittance beams, characteristic of future particle colliders, is a challenge. The electric field of these beams leads to plasma ions motion, resulting in a nonlinear focusing force and emittance growth of the beam. We propose to use an adiabatic matching section consisting of a short plasma section with a decreasing ion mass to allow for the beam to remain matched to the focusing force. We use analytical models and numerical simulations to show that the emittance growth can be significantly reduced

Optimization of Positron Trapping and Acceleration in an Electron-beam-driven Plasma Wakefield Accelerator

X. Wang, P. Muggli, T. Katsouleas, C. Joshi, W. B. Mori, R. Ischebeck, M. J. Hogan

X. Wang et al., Phys. Rev. ST Accel. Beams 12, 051303 (2009).

Abstract: Positron trapping and acceleration in a plasma wake using a four-bunch scheme [X. Wang et al., Phys. Rev. Lett. 101, 124801 (2008)] is numerically investigated through 2D particle-in-cell simulations. This scheme that integrates positron generation, trapping and acceleration into a single stage is a promising approach for positron acceleration in a future plasma-based linear collider. It consists of a plasma with an embedded thin foil target into which two closely spaced electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Some of the outstanding issues related to the quality of the accelerated positron beam load are discussed as a function of the beam and plasma parameters. Simulations show that a large number of positrons (107 ~ 108) can be trapped when the plasma wake is modestly nonlinear, and the positron-generating foil target is immersed in the plasma. Beam loading can reduce the energy spread of the positron beam load. The quality of the positron beam load is not very sensitive to the exact bunch spacing between the drive electron bunch and the positron beam load.

Enhancing parallel quasi-static particle-in-cell simulations with a pipelining algorithm

B. Feng, C. Huang, V. Decyk, W. B. Mori, P. Muggli, T. Katsouleas

B. Feng et al., Journal of Computational Physics, 228(15), 5340 (2009).

Abstract: A pipelining algorithm is described to overcome the limitation on scaling quasi-static particle-in-cell models of relativistic beams in plasmas to a very large number of processors. The pipelining algorithm uses multiple groups of processors and optimizes the job allocation on the processors in parallel computing. The algorithm is implemented on the quasi-static code QuickPIC and is shown to scale to over 103 processors and increased the scale and speed by two orders of magnitude over the non-pipelined model. The new approach opens the door to performing full scale 3-D simulations of future plasma wakefield accelerators or full lifetime models of beam interaction with electron clouds in circular accelerators such as the Large Hadron Collider (LHC) at CERN.

Review of High-energy Plasma Wakefield Experiments

Patric Muggli and Mark J. Hogan

P. Muggli and M.J. Hogan, Comptes Rendus Physique, 10(2-3), 116 (2009).

Abstract: Plasma wakefield accelerator (PWFA) experiments have made considerable progress in the past decade by using high-energy particle beams to drive large amplitude waves or wakes in a plasma. Electron beam driven experiments have measured the integrated and dynamic aspects of plasma focusing, the bright flux high-energy of betatron radiation photons, particle beam refraction at the plasma/neutral gas interface, and the structure and amplitude of the accelerating wakefield. Gradients spanning kT/m to MT/m for focusing and 100 MeV/m to 50 GeV/m for acceleration have been excited in plasmas with densities of 1014 to 1017 cm-3, respectively. The large accelerating gradient led to the energy doubling of 42 GeV electrons in only 85 cm of plasma. Positron beam driven experiments have evidenced the comparatively more complex dynamic and integrated plasma focusing, the subsequent halo formation and emittance growth in the positron beam and demonstrated accelerating gradients of nearly 100 MeV/m. This paper summarizes these experimental progress, illustrates the key enabling technologies that made the work possible, concludes with a brief discussion of proposed future directions, and suggests that the PWFA could one day revolutionize e-/e+ linear colliders.

Transverse Emittance and Current of Multi-GeV Trapped Electrons in a Plasma Wakefield Accelerator

N. Kirby, I. Blumenfeld, C.E. Clayton, F.J. Decker, M.J. Hogan, C. Huang, R. Ischebeck, R.H. Iverson, C. Joshi, T. Katsouleas, W. Lu, K.A. Marsh, S.F. Martins, W.B. Mori, P. Muggli, E. Oz, R.H. Siemann, D.R. Walz, and M. Zhou

N. Kirby et al., Phys. Rev. ST Accel. Beams 12, 051302 (2009).

Abstract: Multi-GeV trapped electron bunches in a plasma wakefield accelerator (PWFA) are observed with normalized transverse emittance divided by peak current, εN,x /It, below the level of 0.2 μm/kA. A theoretical model of the trapped electron emittance, developed here, indicates that emittance scales inversely with the square root of the plasma density in the nonlinear “bubble” regime of the PWFA. This model and simulations indicate that the observed values of εN,x /It result from multi-GeV trapped electron bunches with emittances of a few μm and multi-kA peak currents.

A High Density Hydrogen-Based Capillary Plasma Source for Particle-Beam-Driven Wakefield Accelerator Applications

Hao Chen, Efthymios Kallos, Patric Muggli, Thomas C. Katsouleas and Martin A. Gundersen

H. Chen et al., IEEE Trans. Plasma Sci. 37(3), 456 (2009).

Abstract: We report the generation of variable plasma densities up to 1019 cm-3 in hydrogen-filled, hollow cathode capillary discharges, and consider their applications as a practical plasma source for particle-beam-driven plasma wakefield accelerators. The capillary consists of a transparent, cylindrical borosilicate glass tube. The plasma density is determined as a function of time, using Stark broadening of the Hα line, with a resolution of 50 ns, and is found to decay exponentially with a typical time constant of several hundreds of nanoseconds. The time delay between the discharge and the drive electron beam can therefore be tuned to reach the density appropriate for the maximum acceleration gradient. The dependence of the plasma density on the capillary geometry and gas pressure is discussed, and the results of optical studies of the discharge channel formation process are presented. Implications of the results for beam-driven plasma accelerators are discussed.

Positron Injection and Acceleration on the Wake Driven by an Electron Beam in a Foil and Gas Plasma

X. Wang, R.Ischebeck, P. Muggli, T. Katsouleas, C. Joshi, W. B. Mori, M. J. Hogan

X. Wang et al., Phys. Rev. Lett. 101, 124801 (2008).

Abstract: A novel approach for generating and accelerating positron bunches in a plasma wake is proposed and modeled. The system consists of a plasma with an embedded thin foil into which two electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Monte Carlo and 3D PIC simulations show a large number of positrons (107~108) are trapped and accelerated to ~5 GeV over 1 meter with relatively narrow energy spread and low emittance.

Generation of Trains of Electron Microbunches with Adjustable Sub-picosecond Spacing

P. Muggli, V. Yakimenko, M. Babzien, E. Kallos, K. P. Kusche

P. Muggli et al., Phys. Rev. Lett. 101, 054801 (2008).

Abstract: We demonstrate that trains of subpicosecond electron microbunches, with subpicosecond spacing, can be produced by placing a mask in a region of the beam line where the beam transverse size is dominated by the correlated energy spread. We show that the number, length, and spacing of the microbunches can be controlled through the parameters of the beam and the mask. Such microbunch trains can be further compressed and accelerated, and have applications to free electron lasers (FELs) and plasma wakefield accelerators (PWFAs).

Halo Formation and Emittance Growth of Positron Beams in Plasmas

P. Muggli, B.E. Blue, C.E. Clayton, F.J. Decker, M.J. Hogan, C. Huang, C. Joshi, T.C. Katsouleas, W. Lu, W.B. Mori, C.L. O'Connell, R.H. Siemann, D. Walz, M. Zhou

P. Muggli et al., Phys. Rev. Lett. 101, 055001 (2008).

Abstract: An ultra-relativistic 28.5 GeV, 700 μm-long positron bunch is focused near the entrance of a 1.4m-long plasma with a density ne between ~1013 cm-3 and 5x1014 cm-3. Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of ~3 in the high emittance plane of the beam ~1 m downstream from the plasma exit. As ne increases the formation of a beam halo containing ~40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of ~3 and emittance ratio of ~5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.

Gigavolt per Meter Breakdown Limits on Wakefields Driven by Electron Beams in Dielectric Structures

M. C. Thompson, H. Badakov, A. M. Cook, J.B. Rosenzweig, R. Tikhoplav, G. Travish, I. Blumenfeld, M.J. Hogan, R. Ischebeck, N. Kirby, R. Siemann, D. Walz, P. Muggli, A. Scott, and R. Yoder

M. C. Thompson et al., Phys. Rev. Lett. 100, 214801 (2008).

Abstract: First measurements of the breakdown threshold in a dielectric subjected to GV/m wakefields produced by short (30–330 fs), 28.5 GeV electron bunches have been made. Fused silica tubes of 100 µm inner diameter were exposed to a range of bunch lengths, allowing surface dielectric fields up to 27 GV/m to be generated. The onset of breakdown, detected through light emission from the tube ends, is observed to occur when the peak electric field at the dielectric surface reaches 13.8±0.7 GV/m. The correlation of structure damage to beam-induced breakdown is established using an array of postexposure inspection techniques.

High-Gradient Plasma Wakefield Acceleration with Two Subpicosecond Electron Bunches

E. Kallos, T. Katsouleas, W.D. Kimura, K. Kusche, P. Muggli, I. Pavlishin, I. Pogorelsky, D. Stolyarov, V. Yakimenko

E. Kallos et al., Phys. Rev. Lett. 100, 074802 (2008).

Abstract: A plasma wakefield experiment is presented where two 60-MeV subpicosecond electron bunches are sent into a plasma produced by a capillary discharge. Both bunches are shorter than the plasma wavelength, and the phase of the second bunch relative to the plasma wave is adjusted by tuning the plasma density. It is shown that the second bunch experiences a 150 MeV/m loaded accelerating gradient in the wakefield driven by the first bunch. This is the first experiment to directly demonstrate high-gradient, controlled acceleration of a short-pulse trailing electron bunch in a high-density plasma.

Hosing Instability in the Blow-Out Regime for Plasma-Wakefield Acceleration

C. Huang, W. Lu, M. Zhou, C.E. Clayton, C. Joshi, W.B. Mori, P. Muggli, S. Deng, E. Oz, T. Katsouleas, M.J. Hogan, I. Blumenfeld, F.J. Decker, R. Ischebeck, R.H. Iverson, N. A. Kirby, and D. Walz

C. Huang et al., Phys. Rev. Lett. 99, 255001 (2007).

Abstract: The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al.

Ionization-induced electron trapping in ultrarelativistic plasma wakes

E. Oz, S. Deng, T. Katsouleas, P. Muggli, C. D. Barnes, I. Blumenfeld, F. J. Decker, P. Emma, M. J. Hogan, R. Ischebeck, R. H. Iverson, N. Kirby, P. Krejcik, C. O'Connell, R. H. Siemann, D. Walz, D. Auerbach, C. E. Clayton, C. Huang, D. K. Johnson, C. Joshi, W. Lu, K. A. Marsh, W. B. Mori, and M. Zhou

E. Oz et al., Phys. Rev. Lett. 98, 084801 (2007).

Abstract: The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.

Energy doubling of 42 GeV electrons in a metre-scale plasma wakefield accelerator

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Ian Blumenfeld , Christopher E. Clayton, Franz-Josef Decker, Mark J. Hogan, Chengkun Huang, Rasmus Ischebeck , Richard Iverson, Chandrashekhar Joshi, Thomas Katsouleas, Neil Kirby, Wei Lu, Kenneth A. Marsh , Warren B. Mori, Patric Muggli, Erdem Oz, Robert H. Siemann, Dieter Walz, Miaomiao Zhou

I. Blumenfeld et al., Nature 445, 741-744 (15 February 2007).

Abstract: The energy frontier of particle physics is several trillion electron volts, but colliders capable of reaching this regime (such as the Large Hadron Collider and the International Linear Collider) are costly and time-consuming to build; it is therefore important to explore new methods of accelerating particles to high energies. Plasma-based accelerators are particularly attractive because they are capable of producing accelerating fields that are orders of magnitude larger than those used in conventional colliders.In these accelerators, a drive beam (either laser or particle) produces a plasma wave (wakefield) that accelerates charged particles. The ultimate utility of plasma accelerators will depend on sustaining ultrahigh accelerating fields over a substantial length to achieve a significant energy gain. Here we show that an energy gain of more than 42 GeV is achieved in a plasma wakefield accelerator of 85 cm length, driven by a 42 GeV electron beam at the Stanford Linear Accelerator Center (SLAC). The results are in excellent agreement with the predictions of three-dimensional particle-in-cell simulations. Most of the beam electrons lose energy to the plasma wave, but some electrons in the back of the same beam pulse are accelerated with a field of 52 GVm. This effectively doubles their energy, producing the energy gain of the 3-km-long SLAC accelerator in less than a metre for a small fraction of the electrons in the injected bunch. This is an important step towards demonstrating the viability of plasma accelerators for high-energy physics applications.

Positron Production by X Rays Emitted by Betatron Motion in a Plasma Wiggler

D. K. Johnson, D. Auerbach, I. Blumenfeld, C. D. Barnes, C. E. Clayton, F. J. Decker, S. Deng, P. Emma, M. J. Hogan, C. Huang, R. Ischebeck, R. Iverson, C. Joshi, T. C. Katsouleas, N. Kirby, P. Krejcik, W. Lu, K. A. Marsh, W. B. Mori, P. Muggli, C. L. O’Connell, E. Oz, R. H. Siemann, D. Walz, M. Zhou

D. K. Johnson et al., Phys. Rev. Lett. 97, 175003 (2006).

Abstract: Positrons in the energy range of 3 to 30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1 to 50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.

Plasma Production via Field Ionization

C. L. O’Connell, C. D. Barnes, F.-J. Decker, M. J. Hogan, R. Iverson, P. Krejcik, R. Siemann, D. R. Walz, C. E. Clayton, C. Huang, D. K. Johnson, C. Joshi, W. Lu, K. A. Marsh, W. Mori, M. Zhou, S. Deng, T. Katsouleas, P. Muggli, E. Oz

C. L. O’Connell et al., Phys. Rev. ST Accel. Beams 9, 101301 (2006).

Abstract: Plasma production via field ionization occurs when an incoming particle beam is sufficiently dense that the electric field associated with the beam ionizes a neutral vapor or gas. Experiments conducted at the Stanford Linear Accelerator Center explore the threshold conditions necessary to induce field ionization by an electron beam in a neutral lithium vapor. By independently varying the transverse beam size, number of electrons per bunch, or bunch length, the radial component of the electric field is controlled to be above or below the threshold for field ionization. Additional experiments ionized neutral xenon and neutral nitric oxide by varying the incoming beam?s bunch length. A self-ionized plasma is an essential step for the viability of plasma-based accelerators for future high-energy experiments.

Hose Instability and Wake Generation by an Intense Electron Beam in a Self-Ionized Gas

S. Deng, C. D. Barnes, C. E. Clayton, C. O'Connell, F. J. Decker, R. A. Fonseca, C. Huang, M. J. Hogan, R. Iverson, D. K. Johnson, C. Joshi, T. Katsouleas, P. Krejcik, W. Lu, W. B. Mori, P. Muggli, E. Oz, F. Tsung, D. Walz, M. Zhou

S. Deng et al., Phys. Rev. Lett. 96, 045001 (2006).

Abstract: The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam's space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.

Multi-GeV Energy Gain in a Plasma-Wakefield Accelerator

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M. J. Hogan, C. D. Barnes, C. E. Clayton, F. J. Decker, S. Deng, P. Emma, C. Huang, R. H. Iverson, D. K. Johnson, C. Joshi, T. Katsouleas, P. Krejcik, W. Lu, K. A. Marsh, W. B. Mori, P. Muggli, C. L. O'Connell, E. Oz, R. H. Siemann, and D. Walz

M. J. Hogan et al., Phys. Rev. Lett. 95, 054802 (2005).

Abstract: A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8x1010 electrons is compressed to 20 µm longitudinally and focused to a transverse spot size of 10 µm at the entrance of a 10 cm long column of lithium vapor with density 2.8x1017 atoms/cm3. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

Possibility of a multibunch plasma afterburner for linear colliders

R. Maeda, T. Katsouleas, P. Muggli, C. Joshi, W. B. Mori, and W. Quillinan

R. Maeda et al., Phys. Rev. ST Accel. Beams 7, 111301 (2004).

Abstract: A concept for increasing the energy of a multibunch linear collider using plasma wakefields is examined. The realization of high beam quality and high efficiency (and high luminosity) requires more complexity than the original plasma afterburner concept proposed for doubling the energy of single bunch linear colliders. This paper discusses the possibilities of using alternate bunches in the train to drive the wake and accelerate upon it or alternately a few bunches to excite the wake and a single bunch to accelerate it. Simulation results indicate that an energy of collision/energy of linac ratio of 2.8 can be obtained with 4% energy spread and 0.29 relative luminosity by utilizing five drive bunches per accelerated bunch. The concept including transverse effects is modeled with 2D linear plasma wakefield theory.

Meter-Scale Plasma-Wakefield Accelerator Driven by a Matched Electron Beam

P. Muggli, B. E. Blue, C. E. Clayton, S. Deng, F.-J. Decker, M. J. Hogan, C. Huang, R. Iverson, C. Joshi, T. C. Katsouleas, S. Lee, W. Lu, K. A. Marsh, W. B. Mori, C. L. O'Connell, P. Raimondi, R. Siemann, and D. Walz

P. Muggli et al., Phys. Rev. Lett. 93, 014802 (2004).

Abstract: A high-gradient, meter-scale plasma-wakefield accelerator module operating in the electron blowout regime is demonstrated experimentally. The beam and plasma parameters are chosen such that the matched beam channels through the plasma over more than 12 beam beta functions without spreading or oscillating over a range of densities optimum for observing both deceleration and acceleration. The wakefield decelerates the bulk of the initially 28.5 GeV beam by up to 155 MeV; however, particles in the back of the same beam are accelerated by up to 280 MeV at a density of 1.9x1014 cm-3 as the wakefield changes sign.

Plasma Wakefield Acceleration in Self-ionized Gas or Plasmas

S. Deng, C. D. Barnes, C. E. Clayton, C. O'Connell, F. J. Decker, O. Erdem, R. A. Fonseca, C. Huang, M. J. Hogan, R. Iverson, D. K. Johnson, C. Joshi, T. Katsouleas, P. Krejcik, W. Lu, K. A. Marsh, W. B. Mori, P. Muggli, and F. Tsung

S. Deng et al., Phys. Rev. E68, 047401 (2003).

Abstract: Tunnel ionizing neutral gas with the self-field of a charged particle beam is explored as a possible way of creating plasma sources for a plasma wakefield accelerator [Bruhwiler et al., Phys. Plasmas (to be published)]. The optimal gas density for maximizing the plasma wakefield without preionized plasma is studied using the PIC simulation code OSIRIS [R. Hemker et al., in Proceeding of the Fifth IEEE Particle Accelerator Conference (IEEE, 1999), pp. 3672?3674]. To obtain wakefields comparable to the optimal preionized case, the gas density needs to be seven times higher than the plasma density in a typical preionized case. A physical explanation is given.

Plasma-Wakefield Acceleration of an Intense Positron Beam

B. E. Blue, C. E. Clayton, C. L. O'Connell, F.-J. Decker, M. J. Hogan, C. Huang, R. Iverson, C. Joshi, T. C. Katsouleas, W. Lu, K. A. Marsh, W. B. Mori, P. Muggli, R. Siemann, and D. Walz

B.E. Blue et al., Phys. Rev. Lett. 90, 214801 (2003).

Abstract: Plasma wakefields are both excited and probed by propagating an intense 28.5GeV positron beam through a 1.4m long lithium plasma. The main body of the beam loses energy in exciting this wakefield while positrons in the back of the same beam can be accelerated by the same wakefield as it changes sign. The scaling of energy loss with plasma density as well as the energy gain seen at the highest plasma density is in excellent agreement with simulations.

Ultrarelativistic Positron Beam Transport through Meter-scale Plasmas

M. J. Hogan, C. E. Clayton, C. Huang, P. Muggli, S. Wang, B. E. Blue, D. Walz, K. A. Marsh, C. L. O'Connell, S. Lee, R. Iverson, F.-J. Decker, P. Raimondi, W. B. Mori, T. C. Katsouleas, C. Joshi, and R. H. Siemann

M. J. Hogan et al., Phys. Rev. Lett. 90, 205002 (2003).

Abstract: We report on the first study of the dynamic transverse forces imparted to an ultrarelativistic positron beam by a long plasma in the underdense regime. Focusing of the 28.5 GeV beam is observed from time-resolved beam profiles after the 1.4 m plasma. The strength of the imparted force varies along the ~12 ps full length of the bunch as well as with plasma density. Computer simulations substantiate the longitudinal aberration seen in the data and reveal mechanisms for emittance degradation.A physical explanation is given.

Dynamic Focusing of an Electron Beam through a Long Plasma

C. O'Connell, F-J. Decker, M. J. Hogan, R. Iverson, P. Raimondi, R. H. Siemann, D. Walz, B. Blue, C. E. Clayton, C. Joshi, K. A. Marsh, W. B. Mori, S. Wang, T. Katsouleas, S. Lee, and P. Muggli

C. O'Connell et al., Phys. Rev. ST Accel. Beams 5, 121301 (2002).

Abstract: The focusing effects of a 1.4 m long, (0–2)x1014 cm-3 plasma on a single 28.5 GeV electron bunch are studied experimentally in the underdense or blowout regime, where the beam density is much greater than the plasma density. As the beam propagates through the plasma, the density of plasma electrons along the incoming bunch drops from the ambient density to zero leaving a pure ion channel for the bulk of the beam. Thus, from the head of the beam up to the point where all plasma electrons are blown out, each successive longitudinal slice of the bunch experiences a different focusing force due to the plasma ions. The time-changing focusing force results in a different number of betatron oscillations for each slice depending upon its location within the bunch. By using an electron beam that has a correlated energy spread, this time-dependent focusing of the electron bunch has been observed by measuring the beam spot size in the image plane of a magnetic energy spectrometer placed at the plasma exit.

High energy density plasma science with an ultrarelativistic electron beam

C. Joshi, B. Blue, C. E. Clayton, E. Dodd, C. Huang, K. A. Marsh, W. B. Mori, S. Wang, M. J. Hogan, C. O'Connell, R. Siemann, D. Watz, P. Muggli, T. Katsouleas, S. Lee

C. Joshi et al., Phys. Plasmas 9, 1845 (2002).

Abstract: An intense, high-energy electron or positron beam can have focused intensities rivaling those of today's most powerful laser beams. For example, the 5 ps (full-width, half-maximum), 50 GeV beam at the Stanford Linear Accelerator Center (SLAC) at 1 kA and focused to a 3 micron rms spot size gives intensities of 1020 W/cm2 at a repetition rate of .10 Hz. Unlike a ps or fs laser pulse which interacts with the surface of a solid target, the particle beam can readily tunnel through tens of cm of steel. However, the same particle beam can be manipulated quite effectively by a plasma that is a million times less dense than air! This is because of the incredibly strong collective fields induced in the plasma by the Coulomb force of the beam. The collective fields in turn react back onto the beam leading to many clearly observable phenomena. The beam paraticles can be: (1) Deflected leading to focusing, defocusing, or even steering of the beam; (2) undulated causing the emission of spontaneous betatron x-ray radiation and; (3) accelerated or decelerated by the plasma fields. Using the 28.5 GeV electron beam from the SLAC linac a series of experiments have been carried out that demonstrate clearly many of the above mentioned effects. The results can be compared with theoretical predictions and with two-dimensional and three-dimensional, one-to-one, particle-in-cell code simulations. These phenomena may have practical applications in future technologies including optical elements in particle beam lines, synchrotron light sources, and ultrahigh gradient accelerators.

Transverse Envelope Dynamics of a 28.5-GeV Electron Beam in a Long Plasma

C. E. Clayton, B. E. Blue, E. S. Dodd, C. Joshi, K. A. Marsh, W. B. Mori, S. Wang, P. Catravas, S. Chattopadhyay, E. Esarey, W. P. Leemans, R. Assmann, F. J. Decker, M. J. Hogan, R. Iverson, P. Raimondi, R. H. Siemann, D. Walz, T. Katsouleas, S. Lee, and P. Muggli

C. E Clayton et al., Phys. Rev. Lett. 88, 154801 (2002).

Abstract: The transverse dynamics of a 28.5-GeV electron beam propagating in a 1.4 m long, (0–2) x 1014 cm-3 plasma are studied experimentally in the underdense or blowout regime. The transverse component of the wake field excited by the short electron bunch focuses the bunch, which experiences multiple betatron oscillations as the plasma density is increased. The spot-size variations are observed using optical transition radiation and Cherenkov radiation. In this regime, the behavior of the spot size as a function of the plasma density is well described by a simple beam-envelope model. Dynamic changes of the beam envelope are observed by time resolving the Cherenkov light.

X-Ray Emission from Betatron Motion in a Plasma Wiggler

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S. Wang, C. E. Clayton, B. E. Blue, E. S. Dodd, K. A. Marsh, W. B. Mori, C. Joshi, S. Lee, P. Muggli, T. Katsouleas, F. J. Decker, M. J. Hogan, R. H. Iverson, P. Raimondi, D. Walz, R. Siemann, and R. Assmann

S. Wang et al., Phys. Rev. Lett. 88, 135004 (2002).

Abstract: The successful utilization of an ion channel in a plasma to wiggle a 28.5-GeV electron beam to obtain broadband x-ray radiation is reported. The ion channel is induced by the electron bunch as it propagates through an underdense 1.4-meter-long lithium plasma. The quadratic density dependence of the spontaneously emitted betatron x-ray radiation and the divergence angle of ~(1–3)x1014 radian of the forward-emitted x-rays as a consequence of betatron motion in the ion channel are in good agreement with theory. The absolute photon yield and the peak spectral brightness at 14.2-keV photon energy are estimated.

Energy Doubler for a Linear Collider

S. Lee, T. Katsouleas, P. Muggli, W. B. Mori, C. Joshi, R. Hemker, E. S. Dodd, C. E. Clayton, K. A. Marsh, B. Blue, S. Wang, R. Assmann, F. J. Decker, M. Hogan, R. Iverson, and D. Walz

S. Lee et al., Phys. Rev. ST Accel. Beams 5, 121301 (2002).

Abstract: The concept of using short plasma sections several meters in length to double the energy of a linear collider just before the collision point is proposed and modeled. In this scenario the beams from each side of a linear collider are split into pairs of microbunches with the first driving a plasma wake that accelerates the second. The luminosity of the doubled collider is maintained by employing plasma lenses to reduce the spot size before collision.

Measurements of Radiation near an Atomic Spectral Line from the Interaction of a 30 GeV Electron Beam and a Long Plasma

P. Catravas, S. Chattopadhyay, E. Esarey, W. P. Leemans, R. Assmann, F.-J. Decker, M. J. Hogan, R. Iverson, R. H. Siemann, D. Walz, D. Whittum, B. Blue, C. Clayton, C. Joshi, K. A. Marsh, W. B. Mori, S. Wang, T. Katsouleas, S. Lee, and P. Muggli

P. Catravas et al., Phys. Rev. E 64, 046502 (2001).

Abstract: Emissions produced or initiated by a 30-GeV electron beam propagating through a ~1-m long heat pipe oven containing neutral and partially ionized vapor have been measured near atomic spectral lines in a beam-plasma wakefield experiment. The Cerenkov spatial profile has been studied as a function of oven temperature and pressure, observation wavelength, and ionizing laser intensity and delay. The Cerenkov peak angle is affected by the creation of plasma, and estimates of neutral and plasma density have been extracted. Increases in visible background radiation, consistent with increased plasma recombination emissions due to dissipation of wakefields, were simultaneously measured.

Collective Refraction of a Beam of Electrons at a Plasma-gas Interface

P. Muggli, S. Lee, T. Katsouleas, R. Assmann, F. J. Decker, M. J. Hogan, R. Iverson, P. Raimondi, R. H. Siemann, D. Walz, B. Blue, C. E. Clayton, E. Dodd, R. A. Fonseca, R. Hemker, C. Joshi, K. A. Marsh, W. B. Mori, and S. Wang

P. Muggli et al., Phys. Rev. ST Accel. Beams 4, 091301 (2001).

Abstract: In a recent Brief Comment, the results of an experiment to measure the refraction of a particle beam were reported [P. Muggli et al., Nature 411, 43 (2001)]. The refraction takes place at a passive interface between a plasma and a gas. Here the full paper on which that Comment is based is presented. A theoretical model extends the results presented previously [T. Katsouleas et al., Nucl. Instrum. Methods Phys. Res., Sect. A 455, 161 (2000)]. The effective Snell's law is shown to be nonlinear, and the transients at the head of the beam are described. 3D particle-in-cell simulations are performed for parameters corresponding to the experiment. Additionally, the experiment to measure the refraction and internal reflection at the Stanford Linear Accelerator Center is described.

Boundary effects: Refraction of a particle beam

Patric Muggli, Seung Lee, Thomas Katsouleas, Ralph Assmann, Franz-Joseph Decker, Mark J. Hogan, Richard Iverson, Pantaleo Raimondi, Robert H. Siemann, Dieter Walz, Brent Blue, Christopher E. Clayton, Evan Dodd, Ricardo Fonseca, Roy Hemker, Chandrashekhar Joshi, Kenneth A. Marsh, Warren B. Mori, Shoquin Wang

P. Muggli et al., Nature 411, 43-43 (03 May 2001).

Simulations of Cerenkov wake radiation sources

N. Spence, T. Katsouleas, P. Muggli, W. B. Mori, and R. Hemker

N. Spence et al., Phys. Plasmas 8, 4995 (2001).

Abstract: The Cerenkov wakes stimulated by various drivers (an intense laser pulse, a train of laser pulses or beats and a relativistic particle bunch) propagating transverse to a dc magnetic field in a plasma are analyzed. In each case, the wake generated couples to the electromagnetic radiation of approximate frequency ωp at the plasma-vacuum boundary. The radiation amplitude is Ωcp times the amplitude of the wake excited in the plasma (for a sharp boundary). Two- and three-dimensional particle-in-cell simulations are used to verify the scaling laws. For the parameters of current plasma wake field accelerator experiments the results predict that generation of high-power ~GW coherent microwave to terahertz radiation is possible.

High power radiation from ionization fronts in a static electric field in a waveguide

J. R. Hoffman, P. Muggli, R. Liou, M. Gundersen, J. Yampolsky, T. Katsouleas, C. Joshi, and W. B. Mori

J. R. Hoffman et al., J. Appl. Phys. 90, 1115 (2001).

Abstract: The radiation produced when a relativistically moving plasma/gas boundary (i.e., an ionization front) passes between alternatively biased capacitor electrodes is studied. Results of an experiment based on a design which incorporates the capacitor electrodes into an X band waveguide are presented. The waveguided design effectively couples nearly three orders of magnitude more power into the output than the previously unguided designs. Linear theory is extended to include the depletion of the laser energy as it propagates through the ionizable gas (i.e., laser depletion), and the effect of finite output pulse duration.

E-157: A 1.4-m-long plasma wake field acceleration experiment using a 30 GeV electron beam from the Stanford Linear Accelerator Center Linac

M.J. Hogan, R. Assmann, F.-J. Decker, R. Iverson, P. Raimondi, S. Rokni, R.H. Siemann, D. Walz, D. Whittum, B. Blue, C.E. Clayton, E. Dodd, R. Hemker, C. Joshi, K.A. Marsh, W. B. Mori, S. Wang, T. Katsouleas, S. Lee, P. Muggli, P. Catravas, S. Chattopadhyay, E. Esarey, W.P. Leemans

M. J. Hogan et al., Phys. Plasmas 7, 2241 (2000).

Abstract: In the E-157 experiment now being conducted at the Stanford Linear Accelerator Center, a 30GeV electron beam of 2×1010 electrons in a 0.65-mm-long bunch is propagated through a 1.4-m-long lithium plasma of density up to 2×1014 e-/cm3. The initial beam density is greater than the plasma density, and the head of the bunch expels the plasma electrons leaving behind a uniform ion channel with transverse focusing fields of up to several thousand tesla per meter. The initial transverse beam size with σ=550?100 µm is larger than the matched size of 5 µm resulting in up to three beam envelope oscillations within the plasma. Time integrated optical transition radiation is used to study the transverse beam profile immediately before and after the plasma and to characterize the transverse beam dynamics as a function of plasma density. The head of the bunch deposits energy into plasma wakes, resulting in longitudinal accelerating fields which are witnessed by the tail of the same bunch. A time-resolved Cherenkov imaging system is located in an energy dispersive plane downstream of the plasma. It images the beam onto a streak camera allowing time-resolved measurements of the beam energy spectrum as a function of plasma density. Preliminary experimental data from the first three runs are compared to theory and computer simulations.

Guest Editorial, Special Issue on Laser and Plasma Accelerators, IEEE Transactions on Plasma Science

P. Muggli and T.M. Antonsen, Jr.

P. Muggli and T.M. Antonsen
, Jr.,IEEE Trans. On Plasma Sci., 28(4), p. 1054 (2000).

Investigation of a channeling high intensity laser-beam in underdense plasmas

Z. Najmudin, A.E. Dangor, A. Modena, C.E. Clayton, D. Gordon, C. Joshi, K.A. Marsh, P. Muggli, V. Malka, C. Danson, D. Neely, and F.N. Walsh

Z. Najmudin et al., IEEE Trans. On Plasma Sci., 28(4), p. 1057 (2000).

Abstract: The interaction of an intense short pulse laser (5×1018 W/cm2) with underdense plasma was extensively studied. The beam is found to be highly susceptible to the forward Raman scattering instability. At sufficiently high growth rates, this can lead to wavebreaking with the resultant production of a high flux of accelerated electrons (1011 for E=2 MeV). Some electrons are found to be accelerated well above the dephasing energy, up to 94 MeV. Self-scattered images intimate the presence of high-intensity channels that extend more than 3.5mm or 12 Rayleigh lengths. These filaments do not follow the axis of laser propagation, but are seen to be emitted within an 4 cone centered around this axis. Spectra of the self-scattered light show that the main contribution of the scattering is not from light captured within these filaments. But there is evidence for self-phase modulation from effects such as ionization and relativistic self-focusing. However, no clear correlation is observed between channel length and the number or energies of accelerated electrons. Evidence for high intensities within the channels is given by small-angle Thomson scattering of the plasma wave generated therein. With this method, the intensity is found to be of the order of 1018 W/cm2 greater than 12 Rayleigh lengths from focus.

Plasma Source Test and Simulation Results for the Underdense Plasma Lens Experiment at the UCLA Neptune Laboratory

H. Suk, C. E. Clayton, C. Joshi, T.C. Katsouleas, P. Muggli, R. Narang, C. Pellegrini, J.B. Rosenzweig

H. Suk et al., IEEE Transactions on Plasma Science, vol.28, no.1, pp.271-7 (2000).

Abstract: The planned plasma lens experiment at the UCLA Neptune Laboratory is described. In the experiment, electron beams with an energy of 16 MeV, a charge of 4 nC, and a pulse duration of 30 ps [full-width at half-maximum (FWHM)] are designed to be produced from the 1.625-cell photoinjector radio-frequency gun (=2.856 GHz) and PWT linac in the Neptune. The generated beams are passed through a thin underdense argon plasma with a density of low 1012 cm-3 range and a thickness of a few centimeters. For this experiment, a LaB6-based discharge plasma source was developed and tested. In this paper, the overview of the planned plasma lens experiment and the test results of the plasma source for various conditions are presented. In addition, computer simulations with a 2-1/2 dimensional particle-in-cell code (MAGIC) were performed and the simulation results are shown.

Nanocomposite of semiconducting ferroelectric antimony sulphoiodide dots-doped glasses

Y.H. Xu, F. Del Monte, J.D. Mackenzie, K. Namjoshi, P. Muggli, and C. Joshi

Y.H. Xu et al., Ferroelectrics, Vol. 230: (1-4), pp. 313-322 (1999).

Homogeneous meter-long plasma source for advanced accelerator applications

P. Muggli, K.A. Marsh, S. Wang, C.E. Clayton, S. Lee, T.C. Katsouleas, and C. Joshi

P. Muggli et al., IEEE Trans. on Plasma Science 27(3), pp. 791-799 (1999).

Abstract: A photo-ionized lithium source is developed for plasma acceleration applications. A homogeneous column of lithium neutral vapor with a density of 2×1015 cm-3 is confined by helium gas in a heat-pipe oven. A UV laser pulse ionizes the vapor. In this device, the length of the neutral vapor and plasma column is 25 cm. The plasma density was measured by laser interferometry in the visible on the lithium neutrals and by CO2 laser interferometry on the plasma electrons. The maximum measured plasma density was 2.9×1014 cm-3, limited by the available UV fluence (83 mJ/cm2), corresponding to a 15% ionization fraction. After ionization, the plasma density decreases by a factor of two in about 12 µs. These results show that such a plasma source is scaleable to lengths of the order of 1 m and should satisfy all the requirements for demonstrating the acceleration of electrons by 1 GeV in a 1 GeV/m amplitude plasma wake.

The Neptune photoinjector

J. Rosenzweig, S. Anderson, K. Bishofberger, X. Ding, A. Murokh, C. Pellegrini, H. Suk, A. Tremaine, C.E. Clayton, C. Joshi, K.A. Marsh, P. Muggli

J. Rosenzweig et al., Nucl. Instr. Meth. Phys. Res. A 410, pp. 437-451 (1998).

Abstract: The RF photoinjector in the Neptune advanced accelerator laboratory, along with associated beam diagnostics, transport and phase-space manipulation techniques are described. This versatile injector has been designed to produce short-pulse electron beams for a variety of uses: ultra-short bunches for injection into a next-generation plasma beatwave acceleration experiment, space-charge dominated beam physics studies, plasma wake-field acceleration driver, plasma lensing, and free-electron laser microbunching techniques. The component parts of the photoinjector, the RF gun, photocathode drive laser systems, booster linac, RF system, chicane compressor, beam diagnostic systems, and control system, are discussed. The present status of photoinjector commissioning at Neptune is reviewed, and proposed experiments are detailed.

Plasma Wave Generation in a Self-Focused Channel of a Relativistically Intense Laser Pulse

C. E. Clayton, K.-C. Tzeng, D. Gordon, P. Muggli, W. B. Mori, C. Joshi, V. Malka, Z. Najmudin, A. Modena, D. Neely, and A. E. Dangor

C. E. Clayton et al., Phys. Rev. Lett. 81, 100-103 (1998).

Abstract: Evidence for self-channeling of a relativistically intense laser pulse in an underdense plasma is presented through Schlieren and 90° Thomson sidescatter images. Using collective Thomson scattering of a probe beam, we observe that relativistically propagating plasma waves are excited over the entire length of the channel, up to 12 Rayleigh lengths (~4 mm). From the wave amplitude, the intensity inside the channel is estimated to be ~1018 W/cm2.

Generation of microwave pulses from the static electric field of a capacitor array by an underdense, relativistic ionization front

P. Muggli, R. Liou, C. H. Lai, J. Hoffman, T. C. Katsouleas, and C. Joshi

P. Muggli et al., Phys. Plasmas 5, 2112 (1998).

Abstract: The dc to ac radiation converter is a new device in which a relativistic ionization front directly converts the static electric field of an array of alternatively biased capacitors into a pulse of tunable radiation. In a proof-of-principle experiment frequencies between 6 and 21 GHz were generated with plasma densities in the 1012 cm-3 range and a capacitor period 2d=9.4 cm. In the present experiment, short pulses with frequencies between 39 and 84 GHz are generated in a structure with 2d=2 cm. The frequency spectra of these pulses are measured using a diffraction grating. The spectra are discrete, and their center frequency varies linearly with the gas pressure prior to ionization (or plasma density), as expected from theory. Their relative spectral width is around 18%, consistent with the expected number of cycles (six) contained in the pulses. An upper limit of 750 psec (bandwidth detection limited) is placed on the pulses length. The emitted frequency increases from 53 to 93 GHz when the capacitors are connected by pair to obtain a effective array period of 4 cm.

Observation of Electron Energies Beyond the Linear Dephasing Limit from a Laser-Excited Relativistic Plasma Wave

D. Gordon, K. C. Tzeng, C. E. Clayton, A. E. Dangor, V. Malka, K. A. Marsh, A. Modena, W. B. Mori, P. Muggli, Z. Najmudin, D. Neely, C. Danson, and C. Joshi

D. Gordon et al., Phys. Rev. Lett. 80, 2133-2136 (1998).

Abstract: The spatial extent of the plasma wave and the spectrum of the accelerated electrons are simultaneously measured when the relativistic plasma wave associated with Raman forward scattering of an intense laser beam reaches the wave breaking limit. The maximum observed energy of 94 MeV is greater than that expected from the phase slippage between the electrons and the accelerating electric field as given by the linear theory for preinjected electrons. The results are in good agreement with 2D particle-in-cell code simulations of the experiment.

Generation of ultrashort, discrete spectrum microwave pulses using the dc to ac radiation converter

P. Muggli, R. Liou, J. Hoffman, T. Katsouleas, and C. Joshi

P. Muggli et al., Appl. Phys. Lett. 72, 19 (1998).

Abstract: The output radiation of a dc to ac radiation converter is characterized. A relativistic ionization front passing through a capacitor array of period d=1cm produces short pulses of tunable radiation between 39 and 84 GHz with a gas pressure between 0 and 30 mT. The frequency spectra of the produced pulses are discrete and exhibit full widths at half maximum between 12% and 28%, consistent with the expected width for six cycles? pulses. An upper bound of 750 ps (detection bandwidth limited) is placed on the pulse widths. These are the shortest pulses produced by a source of coherent radiation in this frequency range.

Second harmonic generation in PVD waveguides

X.Y. Zhu, B. McKensy, P. Muggli, R. Brogle, and C. Joshi

X.Y. Zhu et al., Ferroelectrics 24(2), (1996).

Demonstration of Microwave Generation from a Static Field by a Relativistic Ionization Front in a Capacitor Array

C. H. Lai, R. Liou, T. C. Katsouleas, P. Muggli, R. Brogle, C. Joshi, and W. B. Mori

C. H. Lai et al., Phys. Rev. Lett. 77, 4764-4767 (1996).

Abstract: We present the results of a proof-of-principle experiment to demonstrate the generation of tunable radiation from a laser-ionized gas-filled capacitor array. This scheme directly converts a static electric field of wave number k0 into coherent radiation pulses of frequency ωp2/2k0c, where ωp is the plasma frequency. The radiation frequency can be tuned by varying gas pressure and/or capacitor spacing. In this experiment, well-polarized, short (less than 5 ns) microwave pulses have been generated over a frequency range of 6 to 21 GHz. The frequency of the detected signal, as measured with cut-off waveguides, scales linearly with the plasma density, and the relative power of the signal scales quadratically with the dc bias voltage in agreement with the theory.

Photoemission from diamond and fullerene films for advanced accelerator application

P. Muggli, R. Brogle, S. Jou, H.J. Doerr, R.F. Bunshah, and C. Joshi

P. Muggli et al., IEEE Trans. on Plasma Science 24(2), 428 (1996).

Abstract: The photoemission properties of thin diamond and fullerene films were investigated for advanced accelerator applications, using subpicosecond laser pulses at three different wavelengths (650, 325, and 217 nm). The quantum efficiency (QE) obtained at 217 nm with a boron-doped, p-type, (111) polycrystalline diamond film (2.6×l0-4) was only five times smaller than the QE obtained with a mirror polished copper sample (1.3×10-3) but more than nine times larger than the QE obtained with a pure diamond film or with natural diamond monocrystals. Similar results were obtained for the two-photon electron yields at 325 nm. The electron yields obtained with pure fullerene films were small and comparable to the ones observed with the pure diamond samples. With 650 nm pulses, the damage threshold of the (110) Type IIa natural diamond monocrystal (9.38×l04 µJ cm-2), defined here as the fluence leading to an onset of ion emission, was 25 times larger than the damage threshold for a copper sample (3.75×l03 µJ cm-2). The damage threshold of the boron-doped sample at the same wavelength was two times larger than that of copper. Damage thresholds with 325 nm pulses were lower, and with 217 nm pulses ion emission was observed at all fluences probably attributed to ablation of surface hydrocarbon contaminants. Results show that high-quality highboron concentration diamond films could be a good candidate for high-RF electron guns.

Nonlinear Optical Properties of Epitaxial KNbO3 Thin Films via Sol-Gel Technique

C.H. Cheng, Y. Xu, J.D. Mackenzie, P. Muggli, and C. Joshi

C.H. Cheng et al., Ceramic Transactions 55, 243-250 (1995).

Two-color photoemission produced by femtosecond laser pulses on copper

P. Muggli, R. Brogle, C. Joshi

P. Muggli et al., JOSA B, Vol. 12, Issue 4, pp. 553 (April 1995).

Abstract: Single-color illumination of a copper surface by a red or an ultraviolet femtosecond laser pulse yields a threephoton (red) or a two-photon (UV) photoemission process. A multicolor, multiphoton process is generated when the red and the UV pulses overlap both in space and in time on the photocathode. It is shown that this emission process results from the absorption by an electron of one red and one UV photon. It provides a means to correlate ultrashort laser pulses of different wavelengths.

High power gyrotrons at 8 GHz : 8 GHz gyrotron development at the Centre de Recherches en Physique des Plasmas, Lausanne

P. Muggli and M.Q. Tran, in "Gyrotron Oscillators, their Principles and Practice," C.J. Edgcombe editor, published by Taylor and Francis, pp. 295-303, (1993).

Parasitic Oscillation in and Suppression of a Gyrotron Backward Wave Mode in a Low-Q 8 GHz Gyrotron

P. Muggli, M. Q. Tran, and T. M. Tran


Abstract: The parasitic oscillation of the TEo02 gyrotron backward wave (gyro BW) mode is observed in a low-Q, 8 GHz TEo011, gyrotron. Although at low power (PBW < 5 kW), the oscillation of the gyro BW mode, simultaneously with the gyrotron mode, results in a maximum TEo012 mode efficiency of less than 0.25. The parasitic oscillation is suppressed by operating the gyrotron with a negative magnetic field gradient along the electron beam, which allows the maximum efficiency to reach 0.40 and the output power to be multiplied by a factor varying from 1.4 to 1.7. The optimum efficiency curve of the TEo011 mode indicates that the low-Q cavity behaves as a much higher Qdiff cavity. Excessive values of magnetic field gradient and alpha favor the TEo012 longitudinal mode, which oscillates in place of the TEo011 mode and limits its maximum output power. This competitive process is responsible for the high-Q-like behavior of the optimum efficiency curve.

Etude d'un gyrotron à cavité cylindrique: influence des réflexions de puissance et de l'oscillation d'un mode propageant.

P. Muggli, Thèse de doctorat, EPFL, #964 (Ph.D Dissertation)

Velocity ratio measurement using the frequency of the gyro-backward wave
P. Muggli, M.Q. Tran and T.M. Tran

P. Muggli et al., Physics of Fluids B-3(6), pp. 1315-1318, (1991).

Abstract: The operating diagram of a low quality factor, 8 GHz TEo01 gyrotron exhibits oscillations between 6.8 and 7.3 GHz. These oscillations are identified as the backward wave component of the TEo21 traveling mode. As the resonance condition of this mode depends on the average parallel velocity (vpar) of the beam electrons (ωBwc/γ-kparvpar), the measurement of ωBw for given Ωc, and γ is used as a diagnostic for the beam electrons velocity ratio α=vperp/vpar. The values of α, deduced from ωBw through the linear dispersion relation for the electron cyclotron instability in an infinite waveguide, are unrealistic. A nonlinear simulation code gives a values that are in very good agreement with the ones predicted by a particle trajectory code (+10% to +20%). It is found numerically that the particles? velocity dispersion in vpar and vperp increases ωBw. This effect explains part of the discrepancy between the values of α inferred from ωBw without velocity dispersion and the expected values.

Experimental measurement of competition between fundamental and second harmonic emission in a quasi-optical gyrotron

S. Alberti, M. Pedrozzi, M.Q. Tran, J. P. Hogge, T.M. Tran, P. Muggli, B. Jodicke and H. G. Mathews

S. Alberti et al., Phys. Fluids B2(7) (Letters), pp. 2544-2546, (1990).

Abstract: A quasi-optical gyrotron (QOG) designed for operation at the fundamental (fce=100GHz) exhibits simultaneous emission at fce and 2fce (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic. The experimental measurements show that the excitation of the second harmonic is only possible when the fundamental is present. The frequency of the second harmonic is locked by the frequency of the fundamental. Experimental evidence shows that when the second harmonic is not excited, total efficiency is enhanced.

Effect of power reflection on the operation of a low-Q 8 GHz gyrotron

P. Muggli, M.Q. Tran, T.M. Tran, H.-G. Mathews, G. Agosti, S. Alberti and A. Perrenoud

P. Muggli et al., IEEE Transactions on Microwave Theory and Techniques, MTT-38(9), pp.1345-1351, (1990).

Abstract: The operating characteristics of a low-Q (Qcav=225), 8 GHz gyrotron oscillator operating in the TEo011 mode and submitted to various mismatched loads are reported. Under matched conditions, output power np to 310 kW (η=35%) and maximum efficiency up to 43% have been measured. In general, power reflection from loads with different phases and amplitudes leads to an output power decrease. Excessive reflections cause mode switching (TEo011 to TEo012) or even arcing inside the tube. The effect of power reflection is seen to increase rapidly with current and output power. Nevertheless, we have observed that, as predicted by calculations, the maximum output power is not reached under matched conditions but with a specific nonzero value of the complex reflection coefficient.

Experimental measurements on a 100 GHz frequency tunable quasi-optical gyrotron

S. Alberti, M.Q. Tran, J. P. Hogge, T.M. Tran, A. Bondeson, P. Muggli, A. Perrenoud, B. Jodicke and H. G. Mathews

S. Alberti et al., Phys. Fluids B2(7), pp. 1654-1661 (1990).

Abstract: Experiments on a 100 GHz quasioptical (QO) gyrotron operating at the fundamental (ω=Ωce) are described. Powers larger than 90 kW at an efficiency of about 12% were achieved. Depending on the electron beam parameters, the frequency spectrum of the output can be either single moded or multimoded. One of the main advantages of the QO gyrotron over the conventional gyrotron is its continuous frequency tunability. Various techniques to tune the output frequency have been tested, such as changing the mirror separation, the beam voltage, or the main magnetic field. Within the limitations of the present setup, 5% tunability was achieved. The QO gyrotron designed for operation at the fundamental frequency exhibits simultaneous emission at 100 GHz (fundamental) and 200 GHz (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic.

Prospects for high power, quasi-optical gyrotrons operating in the millimeter wave range

T.M. Tran, M.Q. Tran, S. Alberti, J. P. Hogge, B. Isaak, P. Muggli and A. Perrenoud

T.M. Tran et al., IEEE Trans. Elect. Device 36, pp. 1983-1990 (1989).

Abstract: The prospects for high-power (several megawatts) quasi-optical gyrotrons operating in the high-frequency region (≥150 GHz) are considered. The analysis is mainly concerned with the physics of interaction between the annular electron beam and the electromagnetic field within the quasi-optical resonator, together with the constraints required by long-pulse or CW operations. It is shown that powers of several megawatts at frequencies exceeding 150 GHz are possible if one can maintain single-mode operation in the overmoded quasi-optical resonators. A thorough performance study of the magnetron injection gun for such high-power gyrotrons is carried out using the adiabatic theory as well as numerical simulations

Multimode simulation of the frequency spectrum of a quasi-optical gyrotron

M.Q. Tran, A. Bondeson, A. Perrenoud, B. Isaak and P. Muggli

M.Q. Tran et al., Int. J. Electron. 61(6), pp. 1029-1039 (1986).

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