Stabilization of the Alfvén-ion cyclotron instability through short plasmas: Fully kinetic simulations in a high-beta regime

Abstract

The Alfvén-ion cyclotron (AIC) mode is an instability that can be driven in magnetized plasmas with anisotropic pressure. Its chief deleterious effect is the driving of enhanced pitch-angle scattering of ions. Although the AIC mode has been observed in several mirror devices, it has not yet been observed in FRC devices developed by TAE Technologies [H. Gota et al., Nucl. Fusion 61, 106039 (2021)]. Previous theoretical work [T. Tajima et al., Phys. Rev. Lett. 39, 201 (1977)] has suggested that sufficient axial inhomogeneity, quantified by a critical axial plasma length, can stabilize this mode. This stabilization mechanism is examined in fully kinetic particle-in-cell simulations with one spatial dimension modeling a simplified magnetic mirror geometry for a plasma with β∼1. A fast-ion population provides the driving anisotropy for the AIC mode, and the resulting effect on the fast-ion pitch angle distribution is examined. The severity of mode activity is recorded for a scan of plasma lengths for multiple fast-ion injection angles. This scan yields critical lengths that show good qualitative agreement with those from the past theoretical work.