Energy approach to stability analysis of the locked and rotating resistive wall modes in tokamaks

Abstract

A method is proposed for stability analysis of the locked and rotating resistive wall modes (RWMs) in tokamaks. The method is based on the relations describing the balance of energy permeating through the vessel wall. This is a natural extension of the traditional energy approach to the plasma stability tasks which allows incorporation of the energy outflow (absent in the classical energy principle) and its dissipation in the wall. The proposed method covers the locked and rotating modes with a complex growth rate. Its efficiency is proved by derivation of a general dispersion relation for such modes with further reduction to particular consequences for slow and fast RWMs. It is shown that in the latter case, when the skin depth becomes smaller than the wall thickness, the mode rotation essentially amplifies its damping, weakening and even suppressing the instability. This effect was earlier found in the frame of the slab model [V. D. Pustovitov, Phys. Plasmas 19, 062503 (2012)]. Here, it is confirmed with equations valid for toroidal geometry, which are obtained as a supplement to the standard energy principle. The presented results predict strong rotational stabilization of the fast RWMs, which occurs at the mode rotation frequency above a critical level. The estimates are given to allow comparison of these predictions with experimental results.