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.[i] In this configuration, energetic electrons are generated due to a combination of plasma wakefield acceleration and direct laser acceleration (DLA) as a 100fs to 1ps long laser propagates in a tenuous plasma. The electrons then radiate X-rays due to their betatron motion. Recent work has shown that much—and even the majority—of the energy contribution to hot electrons can be from the DLA mechanism.[ii] However, properly determining the DLA contribution to the electron energy is challenging due to numerical issues including dispersion errors and the staggering of the velocity and magnetic field with respect to the electric field in most particle-in-cell codes. We present a customized finite-difference field solver designed to minimize errors in the dispersion relation of light waves in vacuum and to take into account the time-staggered electric and magnetic fields. Single-particle tests show that the new solver is much more accurate when compared to theory. We present preliminary results on the acceleration mechanisms of electrons for a variety of laser pulse durations, using both three-dimensional and quasi-3D simulation geometries, with and without the new solver.

*This work is supported by the DOE, NSF, and LLNL.

Authors: K. G. Miller, F. Li, X. Xu, N. Lemos,^† F. Albert,^† C. Joshi and W. B. Mori, University of California – Los Angeles

^†Lawrence Livermore National Laboratory

[i] F. Albert, et al., “Observation of Betatron X-Ray Radiation in a Self-Modulated Laser Wakefield Accelerator Driven with Picosecond Laser Pulses,” Phys. Rev. Lett. 118, 134801 (2017).

[ii] J. L. Shaw, et al., “Role of Direct Laser Acceleration of Electrons in a Laser Wakefield Accelerator with Ionization Injection,” Phys. Rev. Lett. 118, 64801 (2017).