Effects of dimensionality and laser polarization on kinetic simulations of laser-ion acceleration in the transparency regime

A particle-in-cell study of laser-ion acceleration mechanisms in the transparency regime illustrates how two-dimensional (2D) S and P simulations (laser polarization out of and in the simulation plane, respectively) capture different physics characterizing these systems, visible in their entirety in often cost-prohibitive three-dimensional (3D) simulations [1]. The electron momentum distribution is virtually two-dimensional in 2D-P, unlike the more isotropic distributions in 2D-S and 3D, leading to greater heating in the simulation plane and a different timescale for the onset of relativistic transparency. The artificial longitudinal electron heating in 2D-P exaggerates the effectiveness of target-normal sheath acceleration (TNSA) so it is the dominant acceleration mechanism throughout the laser-plasma interaction, whereas 2D-S and 3D both have sizable populations accelerated through a combination of mechanisms. We perform a target length scan to optimize the peak ion energies in both 2D-S and 3D, and tracer analysis allows us to isolate the acceleration into stages of TNSA, hole boring (HB), and break-out afterburner (BOA) acceleration [2]. Supplemented by FFT analysis, we match the post-transparency BOA acceleration with a wave-particle resonance with a high-amplitude low-frequency electrostatic wave of increasing phase velocity, and we discuss the potential role of the Buneman instability in driving this mode [3].

[1] Stark, Yin, Albright, and Guo. Phys. Plasmas 24, 053103 (2017).

[2] Stark, Yin, Albright, Nystrom and Bird. Phys. Plasmas 25, 043114 (2018).

[3] Stark, Yin, and Albright. Phys. Plasmas 25, 062107 (2018).