Resistive Hose Instabilities in Fluid Runaway Electron Modeling.

A now-common approach for modeling runaway electron (RE) effects in macroscopic dynamics introduces a reduced fluid description for a separate beam-like electron species traveling parallel to magnetic field lines in a resistive MHD background plasma [Bandaru, et al., PRE 99, 063317(2019)]. The RE beam provides a source of resistance-free current density whose direction depends on the time-evolving magnetic field. Linear computations of a RE beam with the MHD+RE system of equations have revealed that the model exhibits instabilities akin to resistive hose instabilities [Rosenbluth, Phys. Fluids 3, 932(1960)]. The radial structure of the beam mode is localized near the origin but away from any rational surface. The beam mode has a dominant m=1 poloidal mode structure that does not require the existence of a q=1 rational surface. RE density sources and drift velocity effects can be neglected in the linear analysis, but the effect of RE drift orbits on the beam mode is discussed. Scaling of the growth rate of the instability with the resistivity of the bulk plasma beam shows the transition from tearing and kink instability to the beam instability. In the low resistivity limit, tearing modes modified by the presence of REs are dominant. A simplified analytic model is presented to complement numerical results from initial-value NIMROD calculations and a numerical eigenvalue solution. Initial nonlinear simulations of the beam instability are presented for conditions relevant to post-thermal quench tokamak plasmas.