Nonlinear wakefields and electron injection in cluster plasma

Abstract

Laser and beam driven wakefields promise orders of magnitude increases in electric field gradients for particle accelerators for future applications. Key areas to explore include the emittance properties of the generated beams and overcoming the dephasing limit in the plasma. In this paper, the first in-depth study of the self-injection mechanism into wakefield structures from nonhomogeneous cluster plasmas is provided using high-resolution two dimensional particle-in-cell simulations. The clusters which are typical structures caused by ejection of gases from a high-pressure gas jet have a diameter much smaller than the laser wavelength. Conclusive evidence is provided for the underlying mechanism that leads to particle trapping, comparing uniform and cluster plasma cases. The accelerated electron beam properties are found to be tunable by changing the cluster parameters. The mechanism explains enhanced beam charge paired with large transverse momentum and energy which has implications for the betatron x-ray flux. Finally, the impact of clusters on the high-power laser propagation behavior is discussed.

Spatial profile of accelerated electrons from ponderomotive scattering in hydrogen cluster targets

“…While the simulations already reproduce the experimentally observed energy spectra and emission angles quite well, the results could further be optimised by randomising the particles' positions and introducing clusters of various sizes into the interaction volume. As seen by Mayr et al, randomisation of cluster location leads to some changes in the results while leaving the main features of the acceleration scheme intact [15].…”

“…the discontinuity between the outer ring-structure and the central spot in the experimental angular spectrum arises only for such an intermediate target when a sufficient amount of electrons is focused to the central axis via LWFA and/or DLA. A discussion of the influence of clusters for LWFA is given by Mayr et al [15]. The presence of clusters introduces highly localised changes in potential that can give electrons the momentum kick needed to be injected into the wake.…”

“…Over the last years, several more advanced setups have been proposed that aim to improve on some aspects of LWFA like beam emittance [9] or increasing the injection of electrons into the wake [10,11]. As there is a further trend going to higher-density targets [12] due to the stronger fields that can be applied, cluster targets and targets doped with nano-particles have been considered in several publications [4,[13][14][15]. Specifically in the works by Cho et al [13] and Aniculaesei et al [14], the presence of nano-particles was shown to aid electron injection in LWFA similar to ionisation injection [10].…”

“…Specifically in the works by Cho et al [13] and Aniculaesei et al [14], the presence of nano-particles was shown to aid electron injection in LWFA similar to ionisation injection [10]. Mayr et al [15] recently have numerically considered a target consisting of argon clusters which has shown that the injection mechanism in the presence of clusters is modified: electrons are ripped from the clusters by the laser pulse and can be injected if they are close to the central axis. This allows for a continuous injection on electrons due to the random positions of clusters within the target volume.…”