Fast disruptions by ballooning mode ridges and fingers in high temperature, low resistivity toroidal plasmas

Abstract

The nonlinear evolution of ballooning modes in high temperature, low resistivity tokamaks is investigated. Convection cells driven by unstable ballooning modes rapidly convect the hot central plasma to the wall in ridges whose two-dimensional poloidal projection resembles fingers. As the resistivity η is reduced in magnitude, the number of fingers increases, with the width of each individual finger becoming more narrow. Importantly, the rate at which energy is transported to the wall is unchanged as η is reduced. Because of the increasingly fine scale fingers generated as η is decreased, the time scale for diffusion across the fingers is independent of the magnitude of the resistivity. As a consequence, the magnetic field lines are decoupled from the plasma (not “frozen-in”) even as η approaches zero. The limit of resistive magnetohydrodynamics (MHD) as η→0 is not the same as ideal MHD with η≡0.