Mechanisms of plasma disruption and runaway electron losses in tokamaks

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak mechanisms of the formation of runaway electron beams and their losses are proposed. The plasma disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number MHD modes. It is hypothesized that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between q = 1 and the closest low–order rational [q = 4/3 or q = 3/2] magnetic surfaces. The thermal quench time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current decay stage is due to the ambipolar particle transport in a stochastic magnetic field. The runaway electron beam in the confined plasma region is formed due to their acceleration the inductive toroidal electric field. The runaway electron beam current is modeled as a sum of toroidally symmetric part and a small amplitude helical current with a predominant m/n = 1/1 component. The runaway electrons are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch wall and (ii)by the formation of stochastic layer of runaway electrons at the beam edge. Such a stochastic layer for high–energy runaway electrons is formed in the presence of the m/n = 1/1 MHD mode. It has a mixed topological structure with a stochastic region open to wall. The effect of external resonant magnetic perturbations on runaway electron loss is discussed. A possible cause of the sudden MHD signals accompanied by runaway electron bursts is explained by the redistribution of runaway current during the resonant interaction of high–energetic electron orbits with the m/n = 1/1 MHD mode.