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Author(s): Swanson, KK; Tsai, HE; Barber, SK; Lehe, R; Mao, HS; Steinke, S; Van Tilborg, J; Nakamura, K; Geddes, CGR; Schroeder, CB; Esarey, E; Leemans, WP | Abstract: Control of the properties of laser-plasma-accelerated electron beams that were injected along a shock-induced density downramp through precision tailoring of the density profile was demonstrated using a 1.8 J, 45 fs laser interacting with a mm-scale gas jet. The effects on the beam spatial profile, steering, and absolute energy spread of the density region before the shock and tilt of the shock were investigated experimentally and with particle-in-cell simulations. By adjusting these density parameters, the electron beam quality was controlled and improved while the energy (30-180 MeV) and energy spread (2-11 MeV) were independently tuned. Simple models that are in good agreement with the experimental results are proposed to explain these relationships, advancing the understanding of downramp injection. This technique allows for high-quality electron beams with percent-level energy spread to be tailored based on the application.

Laser plasma acceleration has been intensely investigated for its ability to produce energetic, ultrashort electron bunches in a compact distance. A high intensity laser pulse propagating through a plasma expels the electrons from the optical axis via the ponderomotive force, leaving behind a column of ions and driving a density wake. The accelerating electric fields present in the wake can reach several orders of magnitude greater than those found in radio-frequency cavities, allowing for compact systems much smaller than those using conventional accelerators. This compact source can provide electrons for various applications including stages for a high energy collider or for production of x-ray pulses from coherent undulator radiation. However, these applications require tunable, stable and high-quality electron beams. We report on a study of controlled injection along a shock-induced density downramp of laser-plasma- accelerated electrons through precision tailoring of the density profile produced from a mm-scale gas jet. Using BELLA Center’s TREX Ti:Sapphire laser, the effects of the plasma density profile and the tilt of the shock front on the beam spatial profile, steering, and energy were investigated experimentally. To explain these rela- tionships, we propose simple models which agree well with experimental results. Using this technique, electron beam quality was tailored, allowing for the production of high-quality electron beams with percent-level energy spreads over a range of energies.

Author(s): Van Tilborg, J; Barber, SK; Tsai, HE; Swanson, KK; Steinke, S; Geddes, CGR; Gonsalves, AJ; Schroeder, CB; Esarey, E; Bulanov, SS; Bobrova, NA; Sasorov, PV; Leemans, WP | Abstract: Active plasma lenses have attracted interest in novel accelerator applications due to their ability to provide large-field-gradient (short focal length), tunable, and radially symmetric focusing for charged particle beams. However, if the discharge current is not flowing uniformly as a function of radius, one can expect a radially varying field gradient as well as potential emittance degradation. We have investigated this experimentally for a 1-mm-diameter active plasma lens. The measured near-axis field gradient is approximately 35% larger than expected for a uniform current distribution, and at overfocusing currents ring-shaped electron beams are observed. These observations are explained by simulations.

A new one-dimensional analytical model of a thin double layer foil interaction with a laser pulse is presented. It is based on one-dimensional electrodynamics. This model can be used for the study of high intensity laser pulse interactions with overdense plasmas, leading to frequency upshifting, high order harmonic generation, and ion acceleration in different regimes.

We present results of an experiment where two independent Laser-Plasma-Accelerator (LPA) stages are coupled at a short distance by a plasma mirror. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed reconstruction of the temporal field structure and determination of the plasma density. Injection into the wakefield of the second stage was verified by a 100 MeV energy gain of the electron beam.

Laser-plasma accelerators (LPAs) have the potential to drive compact free-electron lasers (FELs). Even with LPA energy spreads typically at the percent level, the e-beam brightness can be excellent, due to the low normalized emittance (

Beam breakup due to the hose instability for electron transport through hollow plasma channels is examined numerically and compared with analytical theory. Wakefields are simulated for different geometries, including cylindrical hollow channels and the “bubble” regime, and the simulations are carried out with advanced Particle-In-Cell algorithms. Results are compared to previous estimates for beams accelerated in a plasma.

Electron beams of high energy and narrow, controllable energy spread using significantly reduced laser power are demonstrated by combining control of Laser-Plasma Accelerator (LPA) structure and injection. A high-energy accelerating structure was formed by controlling the phase front of the drive laser in order to obtain collimated propagation over the length of the plasma. This produced electron energies nearly double those previously achieved using comparable lasers. Injection into the accelerator was controlled by using the beat between “colliding” laser pulses to kick electrons at a controlled location into a plasma wave that was operated below the threshold for self injection. This resulted in the production of bunches with controllable energy. Stability of charge, pointing, and energy were demonstrated. With the injection location fixed by the colliding pulses, beam energy up to 200 MeV was obtained using 10 TW drive laser pulses, controlled by plasma density and by target location with respect to t...

In this paper we discuss how we used two laser pulses of different colors to generate femtosecond, ultralow emittance (∼10–2 mm mrad) electron beams in a laser-driven plasma-based accelerator. A long-wavelength pump pulse was used to excite a large wake without fully ionizing a high-Z gas. A short-wavelength injection pulse, with a small ponderomotive force and large peak electric field, ionizes a fraction of the remaining bound electrons into trapped wake orbits with a small quiver momentum. Two-dimensional (2D) Particle-In-Cell (PIC) simulations show that the transverse emittance of the injected beam can be an order of magnitude smaller than previously achieved in laser-plasma accelerators.

The laser group velocity in discharged capillary channels plays a critical role in laser-plasma accelerators. The laser travel time (and thus the group velocity) was measured through two-pulse frequency-domain interferometry and was found to depend on the on-axis plasma density and laser spot size. Experiments and multi-mode simulations were found to be in agreement.

The plasma properties of a plasma waveguide are critical to the performance of laser-plasma accelerators (LPAs). By measuring the group velocity in plasma channels through spectral interferometry, the density in the channel is retrieved. In this paper, experimental methods and results are presented for the plasma density in LPA-relevant plasma channels of various lengths.

This paper presents a novel diagnostic of laser spectral phase, based on spectral blueshifting from ionization of a known gas target, which allows in-situ measurements of high-peak-power laser pulses. A simulation was created to deconvolve the effects of high-order dispersion, optical compression, and gas density profile on blueshifted spectra. This simulation demonstrates sensitivity to third and fourth order spectral phase as well as neutral density and coarse spatial profile. Experimental results from the Berkeley Lab Laser Accelerator (BELLA), a laser source rigorously characterized by numerous time-profile measurements, are presented and compared with simulations in order to constrain the target density profile and illustrate the utility of this diagnostic technique.

We present initial experiments on staging of two separately driven laser plasma accelerators (LPAs) towards high energy physics and beam deceleration experiments. A study establishing long term stability of electron beams accelerated by an LPA in density downramp configuration [1] is presented, demonstrating the appropriateness of these beams as an injector for staged acceleration. Subsequently, these injector beams are used to longitudinally probe the fully characterized wakefield [2] excited in a discharge-capillary-based second stage accelerator.

A plasma decelerating stage is investigated as a compact alternative for the disposal of high-energy beams (beam dumps). This could benefit the design of laser-driven plasma accelerator (LPA) applications that require transportability and / or high-repetition-rate operation regimes. Passive and laser-driven (active) plasma-based beam dumps are studied analytically and with particle-in-cell (PIC) simulations in a 1D geometry. Analytical estimates for the beam energy loss are compared to and extended by the PIC simulations, showing that with the proposed schemes a beam can be efficiently decelerated in a centimeter-scale distance.

We study plasma wave excitation by an incoherent combination of a large number of low energy laser pulses (i.e., without constraining the pulse phases). We show that, in spite of the incoherent nature of electromagnetic fields within the volume occupied by the pulses, the excited wakefield behind the driver is regular and its amplitude is equal to that obtained using a single, coherent pulse with the same energy. Incoherent combination of multiple laser pulses may enable a technologically simpler path to high-repetition rate, high-average power laser-plasma accelerators, and associated applications.

A summary of progress at Lawrence Berkeley National Laboratory is given on: (1) experiments on down-ramp injection; (2) experiments on acceleration in capillary discharge plasma channels; and (3) simulations of a staged laser wakefield accelerator (LWFA). Control of trapping in a LWFA using plasma density down-ramps produced electron bunches with absolute longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV Ic FWHM, respectively) and with central momenta of 0.76 +- 0.02 MeV Ic, stable over a week of operation. Experiments were also carried out using a 40 TW laser interacting with a hydrogen-filled capillary discharge waveguide. For a 15 mm long, 200 mu m diameter capillary, quasi-monoenergetic bunches up to 300 MeV were observed. By detuning discharge delay from optimum guiding performance, self-trapping was found to be stabilized. For a 33 mm long, 300 mu m capillary, a parameter regime with high energy bunches, up to 1 Ge V, was found. In this regime, peak electron energy was correlated with the amount of trapped charge. Simulations show that bunches produced on a down-ramn and iniected into a channel-guided LWFA can produce stable beams with 0.2 MeV Ic-class momentum spread at high energies.

Acceleration of electrons using intense laser pulses that excite multi-gigavolt fields in plasmas will be described and the path forward to practical accelerators. Experiments with the

Experimental and theoretical investigations of laser guiding in plasma channels are reported. Intense (

Trapping of plasma electrons in the self-modulated laser wakefield accelerator (LWFA) via the coupling of Raman backscatter to the wake is examined analytically and with three-dimensional (3-D) test particle simulations. The trapping threshold for linear polarization is much less than for circular and occurs for wake amplitudes of δn/n∼25%, which is well below wave breaking. Self-channeling provides continuous focusing of the accelerated electrons which, along with relativistic pump laser effects, can enhance the energy gain by a factor ⩾2. The colliding pulse method for injecting electrons in the standard LWFA is examined. Simulations of test electrons in 3-D fields indicate the production of relativistic (⩾25 MeV) high-quality electron bunches with ultrashort durations (a few femtoseconds), small energy spreads (a few percent), and low normalized emittances (1 mm mrad).

The propagation of a short and intense laser pulse in a plasma channel is investigated through the nonlinear paraxial wave equation. An analytic condition describing quasi-matched laser propagation (i.e., when the second order normalized-intensity-weighted transverse moment of the laser-pulse remain constant) is obtained. For any arbitrary temporal laser pulse profile and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse, i.e., the slice-dependent laser waist, ensuring quasi-matched propagation. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes spot size oscillations. The analytical and semi-analytical results obtained for both cases are validated through comparison with numerical simulations.

Transverse stability of the drive beam is critical to plasma wakefield accelerators. A long, relativistic particle beam propagating in an overdense plasma is subject to beam envelope modulation and hosing (centroid displacement) instabilities. Coupled equations for the beam centroid and envelope are derived. The growth rate for beam hosing is examined including return current effects (where the beam radius is of order the plasma skin depth) in the long-beam, strongly-coupled, overdense regime.

Recent progress on a staged laser-plasma accelerator (LPA) experiment is reported. The experiment utilizes a 40 TW class laser which is split into two laser pulses. The first laser drives the first LPA module to produce an electron beam. The second laser drives the second LPA module in a dark-current free regime and accelerates the electron beam from the first LPA. Injection from the first stage, triggering of plasma mirror and reflection of the second laser, and laser guiding in the second stage with the laser pulses reflected off the plasma mirror has been successfully demonstrated. Optical spectral redshift of driving laser is analyzed to estimate the wakefield amplitude and electron energy gain in the second stage.

The operational plasma density and laser parameters for future colliders based on laser-plasma accelerators are discussed. Beamstrahlung limits the charge per bunch at low plasma densities. Reduced laser intensity is examined to improve accelerator efficiency in the beamstrahlung-limited regime.

The presence of low emittance beams in laser-plasma accelerators is indicated by single-shot spectroscopic measurements of betatron X-rays. By matching the X-ray betatron spectra to analytical and numerical models of betatron radiation, the electron bunch radius inside the plasma is estimated to be close to 0.1 micron. Correlations of the bunch radius with electron beam parameters are presented. Photon-counting spectra and statistical fitting are used to establish confidence ranges. Combined with simultaneous electron spectrum and divergence measurements, the normalized transverse emittance is estimated to be as low as 0.1 mm mrad consistent with three-dimensional particle-in-cell simulations.

Controlled injection and staging are critical elements to the improvement of the stability, quality, and tunability of laser plasma accelerators (LPAs) [1, 2]. This paper will discuss techniques such as colliding pulse [3], density tailoring [4, 5], and ionization injection [6, 7, 8], which have recently allowed for the generation of high-energy femtosecond electron beams with percent-level energy spread and stability, and state-of-the-art emittance. Progress on staged acceleration will also be presented, which promises to further increase beam quality, allow for mapping of the wakefield, and overcome the limit of pump depletion [9].

Laser plasma accelerators have produced femtosecond electron bunches with a relative energy spread ranging from 100% to a few percent. Simulations indicate that the measured energy spread can be dominated by a correlated spread, with the slice spread significantly lower. Measurements of coherent optical transition radiation are presented for broad-energy-spread beams with laser-induced density and momentum modulations. The long-range (meter-scale) observation of coherent optical transition radiation indicates that the slice energy spread is below the percent level to preserve the modulations.

Electron injection in a laser plasma wakefield accelerator by transverse colliding pulses is studied using two dimensional Particle-in-Cell simulations. It is demonstrated that beams with extremely small transverse emittance can be obtained, injected by the transverse standing wave created by the colliding pulses. Ionization of neutral gas is used to increase the final injection charge but keep the emittance small.

Use of counter-propagating pulses to control electron injection in laser-plasma accelerators promises to be an important ingredient in the development of stable devices. We discuss the colliding pulse scheme and associated diagnostics.

A design study has been carried out for a second-generation experiment on laser guiding and wakefield excitation in a channel. From simple scaling laws for the wakefield amplitude, dephasing length, the relativistic group velocity factor /spl gamma//sub g/, and energy gain with and without guiding, we find that the parameter regime for a compact single stage GeV accelerator favors laser systems producing short pulses (10 fs/spl les//spl tau//spl les/100 fs), each containing an energy on the order of 100 mJ to a few J's. Taking the dephasing length as the maximum acceleration distance, plasma channels with lengths of 1-10 cm and densities of 10/sup 17/-10/sup 19/ cm/sup -3/ need to be produced; whereas the design study has been primarily concerned with diffraction and channel guiding, dephasing and depletion limits, and linear wakefield theory, aspects of the effect of the plasma wave on the evolution of the laser pulse are discussed. We find that transverse and longitudinal pulse distortions could indeed affect the generated plasma wave phase velocity and amplitude, and hence may limit the achievable energy gains over the one-dimensional (1-D) linear estimates. Some issues for experiments on prototype small accelerators (100 MeV-1 GeV, cm scale) are also discussed.

Transverse stability of the drive beam is critical to plasma wakefield accelerators. A long, relativistic particle beam propagating in an overdense plasma is subject to beam envelope modulation and centroid displacement (hosing) instabilities. Coupled equations for the beam centroid and envelope are derived and solved. It is shown that the hosing growth rate is comparable to self-modulation, and coupling of the self-modulation enhances beam hosing and induces harmonic content. Large amounts of hosing significantly alters the structure of the plasma wakefields.

Reported are the results of a harmonic generation experiment in a simple gas (hydrogen) using 1-ps, 1- mu m laser pulses with a range of intensities extending from below to far above the laser ionization saturation threshold. The scaling with intensity above saturation of the third harmonic generated by a single laser-pulse in a filled gas cell is observed not to fit with a simple model that takes into consideration volume ionization effects alone. In another experiment, a pump-probe type, an upper limit on the conversion efficiency of third-harmonic generation in a preformed plasma is determined. It is found to be in agreement with the efficiency predicted by a relativistic harmonic generation theory. >

Plasma accelerators may be driven by the ponderomotive force of an intense laser or the space‐charge force of a charged particle beam. The implications for accelerator design and the different physical mechanisms of laser‐driven and beam‐driven plasma acceleration are discussed. Driver propagation is examined, as well as the effects of the excited plasma wave phase velocity. The driver coupling to subsequent plasma accelerator stages for high‐energy physics applications is addressed.

The wavefront of a short laser pulse after interaction in a laser‐plasma accelerator (LPA) was measured to diagnose laser‐guiding quality. Experiments were performed on a 100 TW class laser at the LOASIS facility of LBNL using a hydrogen‐filled capillary discharge waveguide. Laser‐guiding with a pre‐formed plasma channel allows the laser pulse to propagate over many Rayleigh lengths at high intensity and is crucial to accelerate electrons to the highest possible energy. Efficient coupling of laser energy into the plasma is realized when the laser and the channel satisfy a matched guiding condition, in which the wavefront remains flat within the channel. Using a wavefront sensor, the laser‐guiding quality was diagnosed based on the wavefront of the laser pulse exiting the plasma channel. This wavefront diagnostic will contribute to achieving controlled, matched guiding in future experiments.

Decoupling injection from acceleration is a key challenge to achieve compact, reliable, tunable laser‐plasma accelerators (LPA) [1, 2]. In colliding pulse injection the beat between multiple laser pulses can be used to control energy, energy spread, and emittance of the electron beam by injecting electrons in momentum and phase into the accelerating phase of the wake trailing the driver laser pulse [3, 4, 5, 6, 7]. At LBNL, using automated control of spatiotemporal overlap of laser pulses, two‐pulse experiments showed stable operation and reproducibility over hours of operation. Arrival time of the colliding beam was scanned, and the measured timing window and density of optimal operation agree with simulations [8]. The accelerator length was mapped by scanning the collision point.

Laser-plasma collider designs point to staging of multiple accelerator stages at the 10 GeV level, which are to be developed on the upcoming BELLA laser, while Thomson Gamma source designs use GeV stages, both requiring efficiency and low emittance. Design and scaling of stages operating in the quasi-linear regime to address these needs are presented using simulations in the VORPAL framework. In addition to allowing symmetric acceleration of electrons and positrons, which is important for colliders, this regime has the property that the plasma wakefield is proportional to the transverse gradient of the laser intensity profile. We demonstrate use of higher order laser modes to tailor the laser pulse and hence the transverse focusing forces in the plasma. In particular, we show that by using higher order laser modes, we can reduce the focusing fields and hence increase the matched electron beam radius, which is important to increased charge and efficiency, while keeping the low bunch emittance required for applications.

Electron injection and trapping in a laser wakefield accelerator by high‐order field ionization is studied theoretically and by particle‐in‐cell simulations. To obtain low energy spread beams we use a short region of gas mixture (H+N) near the start of the stage to trap electrons, while the remainder of the stage uses pure H and is injection‐free. Effects of gas mix parameters, such as concentration and length, on the final electron injection number and beam quality are studied. Laser polarization and shape effects on injection number and final electron emittance are also shown.

Plasma Channel Diagnostic Based on Laser Centroid Oscillations A. J. Gonsalves, K. Nakamura, C. Lin, J. Osterhoff, S. Shiraishi, C. B. Schroeder, C. G. R. Geddes, Cs. Toth, E. Esarey, and W. P. Leemans Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Abstract. A technique has been developed for measuring the properties of discharge-based plasma channels by monitoring the centroid location of a laser beam exiting the channel as a function of input alignment offset between the laser and the channel. The centroid position of low-intensity ( 10 18 Wcm −2 are used for laser plasma acceleration, the intensity was kept low in these experiments so that non-linear effects such as self-focusing [11] could be neglected and the plasma properties measured in a straight- forward way. The technique relies on the fact that a transverse offset of the laser pulse at the entrance of the plasma channel causes the laser beam to undergo transverse oscillations about the channel axis. These oscillations depend on the initial centroid offset and angle with respect to the channel axis, and on the channel properties. As will be shown, measuring the dependence of the exit laser centroid position on the input and on channel formation timing provides direct information on the channel matched spot size and profile. This technique reduces the measurement error compared to interferometry and provides increased precision in channel alignment to minimize pointing errors in LPA applications [12].

An overview is presented of the design of a 10 GeV laser plasma accelerator (LPA) that will be driven by a PW‐class laser system and of the BELLA Project, which has as its primary goal to build and install the required Ti:sapphire laser system for the acceleration experiments. The basic design of the 10 GeV stage aims at operation in the quasi‐linear regime, where the laser excited wakes are largely sinusoidal and offer the possibility of accelerating both electrons and positrons. Simulations show that a 10 GeV electron beam can be generated in a meter scale plasma channel guided LPA operating at a density of about 1017 cm−3 and powered by laser pulses containing 30–40 J of energy in a 50–200 fs duration pulse, focused to a spotsize of 50–100 micron. The lay‐out of the facility and laser system will be presented as well as the progress on building the facility.

The numerical modeling code INF&RNO (INtegrated Fluid & paRticle simulatioN cOde, pronounced “inferno”) is presented. INF&RNO is an efficient 2D cylindrical code to model the interaction of a short laser pulse with an underdense plasma. The code is based on an envelope model for the laser while either a PIC or a fluid description can be used for the plasma. The effect of the laser pulse on the plasma is modeled with the time‐averaged poderomotive force. These and other features allow for a speedup of 2–4 orders of magnitude compared to standard full PIC simulations while still retaining physical fidelity. The code has been benchmarked against analytical solutions and 3D PIC simulations and here a set of validation tests together with a discussion of the performances are presented.

Plasma density gradient control of wake phase velocity and trapping threshold in a laser wakefield accelerator produced electron bunches with absolute longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76±0.02 MeV/c, stable over a week of operation. Simulations validated against diagnostics show that use of such bunches as a wakefield accelerator injector can produce stable beams with 0.2 MeV/c‐class momentum spread at high energies. Preservation of bunch momentum spread requires high simulation momentum accuracy, and related self‐trapped simulations showed that high order particle weight effectively suppresses simulation momentum errors allowing design of low emittance stages.Plasma density gradient control of wake phase velocity and trapping threshold in a laser wakefield accelerator produced electron bunches with absolute longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76±0.02 MeV/c, stable over a week of operation. Simulations validated against diagnostics show that use of such bunches as a wakefield accelerator injector can produce stable beams with 0.2 MeV/c‐class momentum spread at high energies. Preservation of bunch momentum spread requires high simulation momentum accuracy, and related self‐trapped simulations showed that high order particle weight effectively suppresses simulation momentum errors allowing design of low emittance stages.

A design of a compact free‐electron laser (FEL), generating ultra‐fast, high‐peak flux, XUV pulses is presented. The FEL is driven by a high‐current, 0.5 GeV electron beam from the Lawrence Berkeley National Laboratory (LBNL) laser‐plasma accelerator, whose active acceleration length is only a few centimeters. The proposed ultra‐fast source (∼10 fs) would be intrinsically temporally synchronized to the drive laser pulse, enabling pump‐probe studies in ultra‐fast science. Owing to the high current (≳10 kA) of the laser‐plasma‐accelerated electron beams, saturated output fluxes are potentially greater than 1013 photons/pulse. Devices based both on self‐amplified spontaneous emission and high‐harmonic generated input seeds, to reduce undulator length and fluctuations, are considered.

Laser wakefield acceleration experiments were carried out by using a hydrogen‐filled capillary discharge waveguide. For a 15 mm long, 200 μm diameter capillary, quasi‐monoenergetic e‐beams up to 300 MeV were observed. By de‐tuning discharge delay from optimum guiding performance, self‐trapping was found to be stabilized. For a 33 mm long, 300 μm capillary, a parameter regime with high energy electron beams, up to 1 GeV, was found. In this regime, the electron beam peak energy was correlated with the amount of trapped electrons.

Staging laser plasma accelerators is an efficient way of mitigating laser pump depletion in laser driven accelerators and necessary for reaching high energies with compact laser systems. The concept of staging includes coupling of additional laser energy and transporting the electron beam from one accelerating module to another. Due to laser damage threshold constraints, in‐coupling laser energy with conventional optics requires distances between the accelerating modules of the order of 10 m, resulting in decreased average accelerating gradient and complicated e‐beam transport. In this paper we use basic scaling laws to show that the total length of future laser plasma accelerators will be determined by staging technology. We also propose using a liquid jet plasma mirror for in‐coupling the laser beam and show that it has the potential to reduce distance between stages to the cm‐scale.

Design considerations for a next‐generation electron‐positron linear collider based on laser‐plasma‐accelerators are discussed. Several of the advantages and challenges of laser‐plasma‐based accelerator technology are addressed. An example of the parameters for a 1 TeV laser‐plasma‐based collider is presented.

Laser plasma accelerators are able to produce high quality electron beams from 1 MeV to 1 GeV. The next generation of plasma accelerator experiments will likely use a multi‐stage approach where a high quality electron bunch is first produced and then injected into an accelerating structure. In this paper we present scaled particle‐in‐cell simulations of a 10 GeV stage in the quasi‐linear regime. We show that physical parameters can be scaled to be able to perform these simulations at reasonable computational cost. Beam loading properties and electron bunch energy gain are calculated. A range of parameter regimes are studied to optimize the quality of the electron bunch at the output of the stage.

Unphysical heating and macroparticle trapping that arise in the numerical modeling of laser wakefield accelerators using particle-in-cell codes are investigated. A dark current free laser wakefield accelerator stage, in which no trapping of background plasma electrons into the plasma wave should occur, and a highly nonlinear cavitated wake with self-trapping, are modeled. Numerical errors can lead to errors in the macroparticle orbits in both phase and momentum. These errors grow as a function of distance behind the drive laser and can be large enough to result in unphysical trapping in the plasma wake. The resulting numerical heating in intense short-pulse laser-plasma interactions grows much faster and to a higher level than the known numerical grid heating of an initially warm plasma in an undriven system. The amount of heating, at least in the region immediately behind the laser pulse, can, in general, be decreased by decreasing the grid size, increasing the number of particles per cell, or using smoother interpolation methods. The effect of numerical heating on macroparticle trapping is less severe in a highly nonlinear cavitated wake, since trapping occurs in the first plasma wave period immediately behind the laser pulse.

International audience; In laser wakefield accelerators, electron motion is driven by intense forces that depend on the plasma density. Transverse oscillations in the accelerated electron orbits produce betatron radiation. The electron motion and the resulting betatron radiation spectrum can therefore be controlled by shaping the plasma density along the orbit of the electrons. Here, a method based on the use of a plasma with a longitudinal density variation (density depression or step) is proposed to increase the transverse oscillation amplitude and the energy of the electrons accelerated in a wakefield cavity. For fixed laser parameters, by appropriately tailoring the plasma profile, the betatron radiation emitted by these electrons is significantly increased in both flux and energy.