Low- and high-frequency nature of oblique filamentation modes. II. Vlasov–Maxwell simulations of collisionless heating process

Abstract

The nonlinear regime of electromagnetic oblique instabilities is investigated by means of a “noiseless” semi-Lagrangian Vlasov–Maxwell solver. Starting from an initial equilibrium configuration with two counterstreaming electron beams, qualitatively different nonlinear regimes are shown to exist depending on the nature of the solutions of the linear dispersion relation, whose properties have been discussed in the companion paper I [Ghizzo et al., Phys. Plasmas 27, 072103 (2020)]. This behavior is in contrast with existing theories of the oblique instability, which are based on the excitation of a single eigenmode at a time: nonlinear transitions toward regimes dominated by low-frequency modes are generally shown to be possible. The emphasis here is on gaining a better understanding of the multiplicity of electromagnetic oblique unstable modes and on modeling their back-reaction on plasma wave-particle interactions and energy conversion mechanisms. The latter are shown to depend on the saturation scenario of the different regimes of the oblique instability. A new regime is discussed, in which a stochastic heating occurs at the expenses of the magnetic energy first amplified by the oblique modes and in which a (reversible) violation of entropy conservation is made possible by large amplitude phase-space fluctuations of the distribution function.