GEM-PIC-RZ: a Galilean electromagnetic particle-in-cell code in quasi-cylindrical geometry with nonuniform grids

Abstract

Laser-plasma accelerators span scales from the micrometre wavelength of the driving laser to acceleration lengths of millimetres or metres. Full particle-in-cell (PIC) simulations resolve the laser carrier but are constrained by the Courant condition, whereas quasi-static models are computationally efficient but do not resolve it. We present GEM-PIC-RZ, a quasi-cylindrical implementation of the Galilean electromagnetic PIC scheme. The method retains the slow evolution of the forward field and therefore resolves the laser carrier while allowing a propagation step governed by the evolution of the laser and plasma rather than by the cell size. User-defined nonuniform grids and a charge-conserving regularization of the bubble closure support efficient simulations across these scales. Comparisons with the full PIC code FBPIC show agreement for the laser wake, the depletion and redshift of a few-cycle pulse, and ionization injection; the measured CPU time is about 2\,\% of that for FBPIC in the reported runs; the injected charge carries a model uncertainty of about 15\,\% from the regularization. An envelope model, in contrast, underestimates the depletion of the few-cycle pulse fivefold. Self-injection in a sharp density down-ramp is not yet reproduced and remains an open problem.