http://arxiv.org/abs/1910.04591
Kinetic plasma simulations are nowadays commonly used to study a wealth of non-linear behaviours and properties in laboratory and space plasmas. In particular, in high-energy physics and astrophysics, the plasma usually evolves in ultra-strong electromagnetic fields produced by intense laser beams for the former or by rotating compact objects such as neutron stars and black holes for the latter. In ultra-strong electromagnetic fields, the gyro-period is several orders of magnitude smaller than the timescale on which we desire to investigate the plasma evolution. Some approximations are required like for instance artificially decreasing the electromagnetic field strength which is certainly not satisfactory. The main flaw of this downscaling is that it cannot reproduce single particle acceleration to ultra-relativistic speeds with Lorentz factor above $\gamma \approx 10^3-10^4$. In this paper, we design a new algorithm able to catch particle motion and acceleration to Lorentz factor up to $10^{15}$ or even higher by using Lorentz boosts to special frames where electric and magnetic fields are parallel. Assuming that these fields are locally uniform, we solve analytically the equation of motion in a tiny region smaller than the length scale of the gradient of the field. This analytical integration of the orbit severely reduces the constrain on the time step, allowing us to use very large time steps, avoiding to resolved the ultra high frequency gyromotion. We performed simulations in ultra-strong spatially and time dependent electromagnetic fields, showing that our particle pusher is able to follow accurately the exact analytical solution for very long times. This property is crucial to properly capture lepton electrodynamics in electromagnetic waves produced by fast rotating neutron stars.
J. Pétri
Fri, 11 Oct 19
23/76
Comments: Submitted to Journal of Computational Physics