Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

A new physical mechanism of formation of runaway electron (RE) beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured 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 magnetic surfaces [\(q=5/4\) or \(q=4/3\), \dots]. It results in that runaway electron beam current has a helical nature with a predominant \(m/n=1/1\) component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high--energy electrons with the \(m/n=1/1\) MHD mode are discussed.