Wiggler-free FEL with an intense helical beam

Abstract

We describe a new theoretical and numerical framework for magnetohydrodynamic (MHD) simulations with an incorporated anisotropic pressure tensor, which can play an important role in collisionless plasmas. The classical approach to handle the anisotropy is based on application of the double adiabatic approximation, assuming that the pressure tensor is well described only by those components that are oriented parallel and perpendicular to the local magnetic field. This gyrotropic assumption, however, fails around magnetically neutral regions, where the cyclotron period may become comparable to or even longer than the system's dynamical time, which causes a singularity in the mathematical expression. In this paper, we demonstrate that this singularity can be completely removed by direct use of the 2nd-moment of the Vlasov equation, combined with an ingenious gyrotropization model. Numerical tests are used to verify that our model properly reduces to the standard MHD results or the double adiabatic formulation in an asymptotic manner under the limit of fast isotropization and fast gyrotropization, respectively.