Modeling the effect of anisotropic pressure on tokamak plasmas normal modes and continuum using fluid approaches

TL;DR

This paper introduces MISHKA-A, a fluid code that models the effects of pressure anisotropy on tokamak plasma stability, revealing how anisotropy modifies the BAE gap and influences mode stability.

Contribution

Development of MISHKA-A, an extension of ideal MHD code, to analyze pressure anisotropy effects on plasma normal modes using three fluid closure models.

Findings

Anisotropy modifies the BAE gap and sound frequency in low beta, large aspect ratio plasmas.

The SA model maintains the ideal MHD BAE gap structure, while CGL introduces a low-frequency gap.

Mode stability depends on the relation between parallel and perpendicular pressures, consistent with the Bussac criterion.

Abstract

Extending the ideal MHD stability code MISHKA, a new code, MISHKA-A, is developed to study the impact of pressure anisotropy on plasma stability. Based on full anisotropic equilibrium and geometry, the code can provide normal mode analysis with three fluid closure models: the single adiabatic model (SA), the double adiabatic model (CGL) and the incompressible model. A study on the plasma continuous spectrum shows that in low beta, large aspect ratio plasma, the main impact of anisotropy lies in the modification of the BAE gap and the sound frequency, if the q profile is conserved. The SA model preserves the BAE gap structure as ideal MHD, while in CGL the lowest frequency branch does not touch zero frequency at the resonant flux surface where $m+nq=0$, inducing a gap at very low frequency. Also, the BAE gap frequency with bi-Maxwellian distribution in both model becomes higher if $p_\perp > p_\parallel$ with a q profile dependency. As a benchmark of the code, we study the m/n=1/1 internal kink mode. Numerical calculation of the marginal stability boundary with bi-Maxwellian distribution shows a good agreement with the generalized incompressible Bussac criterion [A. B. Mikhailovskii, Sov. J. Plasma Phys 9, 190 (1983)]: the mode is stabilized(destabilized) if $p_\parallel < p_\perp (p_\parallel > p_\perp)$.