MHD simulation of tilt instability during the dynamic FRC magnetic compression process

TL;DR

This paper uses MHD simulations to study the tilt instability in FRCs during magnetic compression, revealing how growth rates depend on compression rates and how toroidal flow can delay instability onset.

Contribution

It provides new insights into the nonlinear evolution of tilt instability in FRCs under dynamic compression, highlighting the stabilizing role of toroidal flow.

Findings

Tilt mode causes confinement loss in nonlinear phase.

Growth rate increases with compression ramp rate.

Toroidal flow reduces tilt growth and delays instability.

Abstract

The nonlinear evolution of the tilt instability in a field reversed configuration (FRC) during the dynamic magnetic compression process has been investigated using magnetohydrodynamic (MHD) simulations with the NIMROD code [C. R. Sovinec \textit{et al.}, J. Comput. Phys. \textbf{195}, 355 (2004)]. The tilt mode induces significant deformations in the linear growth phase and results in complete confinement loss of the FRC in the nonlinear phase, with no evidence of dynamic nonlinear stabilization. The growth rate of the tilt mode increases with the compression field ramping rate and approaches an asymptotic value. Toroidal flow can reduce both the growth rate and the nonlinear saturation amplitude of the tilt mode. The stabilizing effect of the toroidal rotation is enhanced with higher compression field ramping rates due to the spontaneous toroidal field generation and increased flow shear during compression. Although the tilt mode remains unstable with a toroidal rotation Mach number close to 0.5, the onset of tilt distortion can be delayed, allowing a magnetic compression ratio up to 5.3 before the compressional heating terminates.