Rotational stabilization of the resistive wall modes in tokamaks with a ferritic wall

Abstract

The dynamics of the rotating resistive wall modes (RWMs) is analyzed in the presence of a uniform ferromagnetic resistive wall with μ̂≡μ/μ0≤4 (μ is the wall magnetic permeability, and μ0 is the vacuum one). This mimics a possible arrangement in ITER with ferromagnetic steel in test blanket modules or in future experiments in JT-60SA tokamak [Y. Kamada, P. Barabaschi, S. Ishida, the JT-60SA Team, and JT-60SA Research Plan Contributors, Nucl. Fusion 53, 104010 (2013)]. The earlier studies predict that such a wall must provide a destabilizing influence on the plasma by reducing the beta limit and increasing the growth rates, compared to the reference case with μ̂=1. This is true for the locked modes, but the presented results show that the mode rotation changes the tendency to the opposite. At μ̂>1, the rotational stabilization related to the energy sink in the wall becomes even stronger than at μ̂=1, and this “external” effect develops at lower rotation frequency, estimated as several kHz at realistic conditions. The study is based on the cylindrical dispersion relation valid for arbitrary growth rates and frequencies. This relation is solved numerically, and the solutions are compared with analytical dependences obtained for slow (s/dw≫1) and fast (s/dw≪1) “ferromagnetic” rotating RWMs, where s is the skin depth and dw is the wall thickness. It is found that the standard thin-wall modeling becomes progressively less reliable at larger μ̂, and the wall should be treated as magnetically thick. The analysis is performed assuming only a linear plasma response to external perturbations without constraints on the plasma current and pressure profiles.