Drift-cyclotron loss-cone instability in 3D simulations of a sloshing-ion simple mirror

TL;DR

This study investigates the stability of collisionless sloshing beam-ion plasma in a 3D magnetic mirror, revealing how drift-cyclotron loss-cone modes grow and can be mitigated by external cool ions, with implications for plasma confinement.

Contribution

It introduces a combined simulation approach to analyze DCLC instability in sloshing-ion plasma and demonstrates the role of external cool ions in improving confinement.

Findings

DCLC modes grow and saturate within 1-10 μs.

Cool ions from external sources enhance beam-ion confinement.

Sloshing ions do not trap cool ions, affecting stability strategies.

Abstract

The kinetic stability of collisionless, sloshing beam-ion (45{\deg} pitch angle) plasma is studied in a 3D simple magnetic mirror, mimicking the Wisconsin High-temperature superconductor Axisymmetric Mirror (WHAM) experiment. The collisional Fokker-Planck code CQL3D-m provides a slowing-down beam-ion distribution to initialize the kinetic-ion/fluid-electron code Hybrid-VPIC, which then simulates free plasma decay without external heating or fueling. Over 1-10 $\mu$s, drift-cyclotron loss-cone (DCLC) modes grow and saturate in amplitude. DCLC scatters ions to a marginally-stable distribution with gas-dynamic rather than classical-mirror confinement. Sloshing ions can trap cool (low-energy) ions in an electrostatic potential well to stabilize DCLC, but DCLC itself does not scatter sloshing beam-ions into said well. Instead, cool ions must come from external sources such as charge-exchange collisions with a low-density neutral population. Manually adding cool ~1 keV ions improves beam-ion confinement several-fold in Hybrid-VPIC simulations, which qualitatively corroborates prior measurements from real mirror devices with sloshing ions.