Two-fluid model of rf current condensation in magnetic islands

TL;DR

This paper models rf current condensation in magnetic islands using a two-fluid approach, revealing how heat loss mechanisms influence stabilization efficiency and suggesting passive stabilization methods for NTMs.

Contribution

It extends previous models by incorporating two-fluid effects, showing how heat loss impacts the condensation effect and stabilization potential.

Findings

Avoiding collisional heat loss enhances the condensation effect.

Heat loss impact depends on diffusion and equilibration time ratio.

Large deposition profiles improve heating efficiency for stabilization.

Abstract

The stabilization of tearing modes with rf waves is subject to a nonlinear effect, termed rf current condensation, that has the potential to greatly enhance and localize current driven within magnetic islands. Here we extend previous investigations of this effect with a two fluid model that captures the balance of diffusive and thermal equilibration processes within the island. We show that the effective power, and resulting strength of the condensation effect, can be greatly enhanced by avoiding collisional heat loss to the ions. The relative impact of collisions on the overall power balance within the island depends on the ratio of the characteristic diffusion time scale and the electron-ion equilibration time, rather than the latter alone. Although relative heat loss to ions increases with island size, the heating efficiency does as well. In particular, we show that the latter safely dominates for large deposition profiles, as is typically the case for LHCD. This supports the possibility of passive stabilization of NTMs, without the precise aiming of the rf waves required for ECCD stabilization.