Invariant regimes of Spencer scaling law for magnetic compression of rotating FRC plasma

TL;DR

This study investigates the invariance of Spencer's scaling law for magnetic compression in rotating FRC plasmas through MHD simulations and theoretical analysis, revealing conditions under which the scaling law remains valid.

Contribution

It demonstrates that the pressure and radius scalings are invariant under magnetic compression up to a ratio of 6 even with initial flow, extending the applicability of Spencer's law.

Findings

Scaling invariance up to compression ratio 6 with initial flow

Toroidal flow affects compression through work and equilibrium reshaping

Conservation of angular momentum explains the invariant scaling

Abstract

The scaling laws for the magnetic compression of a toroidally rotating field reversed configuration (FRC) have been investigated in this work. The magnetohydrodynamics (MHD) simulations of the magnetic compression on rotating FRCs employing the NIMROD code [C. R. Sovinec \textit{et al.}, J. Comput. Phys. \textbf{195}, 355 (2004)], are compared with the Spencer's one-dimensional (1D) theory [R. L. Spencer \textit{et al.}, Phys. Fluids \textbf{26}, 1564 (1983)] for a wide range of initial flow speeds and profiles. The toroidal flow can influence the scalings directly through the alteration of the compressional work as also evidenced in the 1D adiabatic model, and indirectly by reshaping the initial equilibrium. However, in comparison to the static initial FRC equilibrium cases, the pressure and the radius scalings remain invariant for the magnetic compression ratio $B_{w2}/B_{w1}$ up to 6 in presence of the initial equilibrium flow, suggesting a broader applicable regime of the Spencer scaling law for FRC magnetic compression. The invariant scaling has been proven a natural consequence of the conservation of angular momentum of both fluid and magnetic field during the dynamic compression process.