Influence of the shape of a conducting chamber on the stability of rigid ballooning modes in a mirror trap

TL;DR

This study investigates how the shape of a conducting chamber influences the stability of rigid ballooning modes in a mirror trap, highlighting the importance of chamber shape, plasma beta, and boundary conditions for stabilization.

Contribution

It provides a detailed analysis of the effects of chamber shape and boundary conditions on plasma stability, including the role of conducting end plates and various plasma pressure profiles.

Findings

Conducting shells can stabilize rigid modes if plasma beta exceeds a critical value.

Combining conducting end plates with lateral walls creates two stability zones, potentially merging into full stability.

Stability margins depend on plasma anisotropy, mirror ratio, and vacuum gap width.

Abstract

The MHD stabilization of ``rigid'' flute and ballooning modes with azimuthal number $m = 1$ in an axisymmetric mirror trap by means of a perfectly conducting lateral wall is studied both in the presence and in the absence of the end MHD anchors. Numerical calculations were carried out for an anisotropic plasma created by injection of a beam of neutral atoms into the minimum of the magnetic field at the right angle to the trap axis. The stabilizing effect of the conducting shell in the form of a straight cylinder is compared with a proportional chamber, which, on an enlarged scale, repeats the shape of the plasma column. It is confirmed that for effective wall stabilization of the rigid modes, the plasma beta ($\beta$, the ratio of the plasma pressure to the magnetic field pressure) must exceed some critical value $\beta_{\text{crit}}$. When conducting lateral wall is combined with conducting end plates imitating MHD end anchors, there are two critical beta values and, respectively, two stability zones $\beta<\beta_{\text{ crit}1}$ and $\beta > \beta_{\text {crit}2}$ that can merge, making the entire range of allowable beta values $0<\beta<1$ stable. The dependence of the critical betas on the plasma anisotropy, mirror ratio, and the width of the vacuum gap between the plasma and the lateral wall is examined. In contrast to the earlier works of other authors focused on to the stepwise plasma model, the stability margins are calculated for a number of diffuse radial pressure profiles with different peakedness and several axial magnetic field profiles.