Magnetohydrodynamic Stability at a Separatrix: Part II

TL;DR

This paper develops an analytic method using conformal transformations to accurately compute the stability parameter Δ' at a plasma separatrix, avoiding numerical issues near the X-point, and discusses implications for Peeling mode stability.

Contribution

It generalizes conformal transformation techniques to systems with non-zero boundary conditions, enabling analytic calculation of Δ' at a separatrix without numerical divergences.

Findings

Analytic expressions for vacuum energy and Δ' near a separatrix.

Derived a relation for growth rate: ln(γ/γ_A) = -0.5 ln(q'/q).

Discussed implications for Peeling mode stability.

Abstract

In the first part to this paper\cite{part1} it was shown how a simple Magnetohydrodynamic model could be used to determine the stability of a Tokamak plasma's edge to a Peeling (External Kink) mode. Stability was found to be determined by the value of $\Delta'$, a normalised measure of the discontinuity in the radial derivative of the radial perturbation to the magnetic field at the plasma-vacuum interface. Here we calculate $\Delta'$, but in a way that avoids the numerical divergences that can arise near a separatrice's X-point. This is accomplished by showing how the method of conformal transformations may be generalised to allow their application to systems with a non-zero boundary condition, and using the technique to obtain analytic expressions for both the vacuum energy and $\Delta'$. A conformal transformation is used again to obtain an equilibrium vacuum field surrounding a plasma with a separatrix. This allows the subsequent evaluation of the vacuum energy and $\Delta'$. For a plasma-vacuum boundary that approximates a separatrix, the growth rate $\gamma$ normalised by the Aflven frequency $\gamma_A$ is then found to have $\ln(\gamma/\gamma_A)=-{1/2} \ln (q'/q)$. Consequences for Peeling mode stability are discussed.