Session JO04: Beams: Laser-Driven Lepton Acceleration

Live

One of the most challenging application of plasma accelerators is the development of a plasma-based collider for high-energy physics studies. Fast and accurate simulation tools are essential to study the physics toward configurations that enable the production and acceleration of very small beams with low energy spread and emittance preservation over long distances, as required for a collider. The Particle-In-Cell code WarpX is being developed by a team of the U.S. DOE Exascale Computing Project (with non-U.S. collaborators on part of the code) to enable the modeling of chains of tens of plasma accelerators on exascale supercomputers, for collider designs. We will present our latest application of the code to the modeling of up to 10 consecutive multi-GeV stages on the GPU-accelerated Summit supercomputer, together with the latest developments that made it possible.   

Live

In plasma-based accelerators, a crucial requirement for many applications is to keep the beam emittance from degrading. One source of emittance degradation is through Coulomb collision. This paper shows that the emittance growth due to Coulomb collision can be correctly captured in particle-in-cell simulations, with a proper Monte Carlo binary collision module implemented. In addition, the theory of the emittance growth due to Coulomb collision is extended from a monoenergetic matched beam to a mismatched beam with energy spread.   

Live

Plasma-based accelerators are promising candidates for particle acceleration in the next generation of light sources and lepton colliders due to their high-accelerating gradients. Plasmas accelerators are asymmetric for the accelerated bunch charge's sign as the background electrons and ions react at different timescales due to the disparity of their mass. Thus, progress for positron acceleration has been much slower than for electrons. A possible solution is the use of hollow plasma channels, but there beam breakup instabilities dominate the bunch dynamics. Using theory and three-dimensional particle-in-cell simulations, we show that the long-term plasma dynamics (ionic timescales) in the blowout's regime aftermath can generate thin and warm plasma channels. These channels are ready for high-gradient and high-quality positron acceleration, free from beam breakup instabilities. We explore regimes suitable for proof-of-principle experimental verification with today's technology.   

Live

Future, compact high-energy (TeV-class) colliders will require the production and acceleration of high-quality positron beams. We explored the possibility of using a 10 GeV-class electron beam from a laser-driven plasma accelerator, such as the ones under development at the BELLA Center of LBNL, to produce a positron beam. This can be achieved by capturing the particles generated during the pair decay of Bremsstrahlung radiation that is created when the relativistic electron beam travels through a high-Z solid target. We model the production of positron beams with the Monte Carlo based Geant4 library, and their capture and acceleration in plasmas using the WarpX particle-in-cell code. In this talk we will discuss scenarios relevant to future experiments at the BELLA Center.   

Live

Discretizing Maxwell's equations in Galilean (comoving) coordinates allows to derive a pseudo-spectral solver that mitigates the numerical Cherenkov instability, in boosted-frame particle-in-cell simulations of laser-plasma acceleration. In this talk, we present a generalization of previous work on Galilean spectral solvers, by incorporating spatial derivatives of arbitrary order. This increases the locality of the solver, and thereby enables efficient parallelization by domain decomposition on many distributed compute units. The method is applied to typical boosted-frame simulations of laser-wakefield acceleration.   

Live

Reliable modeling of laser-plasma accelerators (LPAs), where a short and intense laser pulse propagates in an underdense plasma over long distances, is a computationally challenging task. This is due to the great disparity among the scales involved in the modeling, ranging from the micron scale of the laser wavelength to, for instance, the meter scale of the laser-plasma interaction length for a 10 GeV-class LPA. To reduce such imbalance the time-averaged ponderomotive approximation (TAPA) may be used. Here, plasma particle dynamics is analytically averaged over the laser frequency, and only spatio-temporal scales associated with the laser envelope are retained in the calculations, resulting in significant computational savings. In this talk, we characterize the accuracy and robustness of the TAPA for a range of laser parameters of interest for present and future LPAs, and we show that the error introduced by the averaging process is small in all relevant cases.   

Live

Laser wakefield acceleration (LWFA) experiments have demonstrated acceleration gradients of over 40 GV/m, almost three orders of magnitude above conventional radio-frequency accelerators. This tremendous acceleration cannot be maintained indefinitely, however, primarily due to dephasing~and depletion.~Recent advances in optics~have shown that reflective elements such as axi-parabolas [Smartsev et al, Optics Letters, 2019] can produce a line-focus. When used in~conjunction with an echelon, it becomes possible to tune the speed at which the laser peak intensity travels along this line-focus. As a result, this scheme can prevent dephasing by driving a wakefield at the vacuum speed of light [Palastro et al. PRL 2020]. Here, we perform particle-in-cell simulations with Osiris of superluminal plasma~waves excited by superluminal perturbations. By comparing to equivalent subluminal cases, we found that at superluminal speeds, plasma waves can~support arbitrarily high electric fields, free from the wave-breaking limits typical of subluminal plasma waves. We discuss the implications of these results~in the development of more compact and efficient LWFAs.   

Live

Controlled electron acceleration via direct laser energy coupling in an ion channel is proposed based on a new insight into the involved particle-laser resonances. The latter is acquired by dropping the usual paraxial assumption and identifying the resonances for all allowed electron propagation angles. As a result, full resonances and corresponding electron injection are quantified using the angle explicitly. Applications to controlling electron trapping and optimizing beam divergence are examined. These findings are verified and augmented by test-particle simulations.   

Live

A planar laser pulse propagating in vacuum can exhibit an extremely large ponderomotive force. This force, however, cannot impart net energy to an axial electron: As the pulse overtakes the electron, the initial impulse from its rising edge is completely undone by an equal and opposite impulse from its trailing edge. Here we show that planar-like ``flying focus'' pulses can break this fundamental symmetry, imparting relativistic energies to electrons. The intensity peak of a flying focus---a moving focal point resulting from a chirped laser pulse focused by a chromatic lens---can travel at any subluminal velocity, forward or backward. As a result, an electron can gain enough momentum in the rising edge of the intensity peak to outrun and avoid the trailing edge. Accelerating the intensity peak can further boost the momentum gain. Theory and simulations demonstrate that these dynamic intensity peaks can backwards accelerate electrons to the MeV energies required for radiation and electron diffraction probes of high-energy-density materials.   

Live

We developed a laser-wakefield electron acceleration (LWFA) experimental capability by focusing one beamlet (1 ps, 250 J) of the Advanced Radiographic Capability (ARC, LLNL) onto a gas tube target filled with helium. When a picosecond laser pulse (intensity around 10$^{\mathrm{18\thinspace }}$W/cm$^{\mathrm{2}})$, is focused on a gas target with an electron density of about 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$, electrons are accelerated to multi-100 MeV energies by the interplay of direct laser acceleration and self-modulated LWFA. Such beams can be used for the development of x-ray sources using betatron, Compton scattering and bremsstrahlung mechanisms. Experiments were conducted at the National Ignition facility with the ARC short pulse focused at intensities around 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$ onto 3 mm plastic gas tubes filled with helium at atmospheric pressure. The tubes are closed with 1 \textmu m thick mylar windows that are blown off with long pulses 5-10 ns before the short pulse. The newly developed W-NEPPS (Wakefield NIF Electron Proton Positron Spectrometer), measured electron energies of 10-150 MeV, with charges approaching a microcoulomb. Performed under the auspices of the U.S. DOE by LLNL under Contract No. DE-AC52-07NA27344, supported by the DOE Office Science Early Career Research Program (FES) SCW 1575-1. LLNL-ABS-811953   

Live

Self-modulated laser wakefield accelerators (SMLWFA's) driven by picosecond-scale, kilojoule-class lasers enable particle beams and x-ray sources that could be coupled to experiments driven by large-scale, high-energy lasers such as the OMEGA laser at the Laboratory for Laser Energetics (LLE) or the National Ignition Facility at Lawrence Livermore National Laboratory. We report on the development of a SMLWFA platform for the OMEGA EP Laser System at LLE. This platform is the first laser-plasma accelerator driven by a short-pulse, kilojoule-class laser (OMEGA EP) connected to a high-energy-density-science (HEDS) driver (OMEGA). Initial experiments demonstrated electron beams with electron energies exceeding 200 MeV, divergences as low as 32 mrad, charge exceeding 700 nC, and laser-to-electron conversion efficiencies up to 11{\%}. The total charge in the electron beam is found to scale with both $a_{\mathrm{0}}$ and plasma density. These electron beams are, to our knowledge, the highest-charge electron beams produced from a laser-plasma accelerator and show promise as a path to MeV-class radiography sources and improved flux for broadband sources of interest at HEDS facilities.   

Live

Direct Laser Acceleration (DLA) is observed in an experiment on the Titan laser system at the Jupiter Laser Facility in a low density (3x10$^{\mathrm{17}}$ cm$^{\mathrm{-3}})$ regime of a self-modulated laser wakefield accelerator. A forking structure is show in the highest energies (\textgreater 50 MeV) of an accelerated electron beam (\textless 50 mrad) previously shown to indicate the effects of DLA during the acceleration process. The experimental results are confirmed in a quasi-3D simulation using OSIRIS under similar conditions to the experiment, i.e. 1 ps laser pulse, and moderate a$_{\mathrm{0}} \quad =$ 2.7. The simulation results show a stable SM-LWFA for the length of a 10 mm accelerator and confirm DLA being the dominant acceleration mechanism for the highest energy electrons.   

Live

There is interest in generating moderately relativistic electrons (10--100 MeV) to produce X-rays for the probing of hot, dense material. One way to produce such hot electrons involves using a high-intensity picosecond laser, which generates relativistic plasma waves and goes unstable due to self-modulational and Raman scattering instabilities. Hot electrons, produced by a combination of plasma wakefield acceleration and direct laser acceleration (DLA), can radiate X-rays via betatron emission or bremsstrahlung radiation. Recent work has determined the energy contribution to hot electrons from the laser fields (DLA) and plasma wakefields by calculating work done by fields perpendicular to and parallel to the laser propagation direction, respectively. However, a finite-width laser in reality has fields along the laser propagation direction. Using a quasi-3D decomposition into azimuthal modes in the particle-in-cell code OSIRIS, we extract the longitudinal laser fields from the longitudinal wakefields in an SM-LWFA simulation and find that the work done by the axial laser fields is significant and negative. In addition, we show evidence of DLA through particle tracking even without the formation of an ion channel. Examples for both short- and long-pulse lasers is presented.