Mechanism of runaway electron beam formation during plasma disruptions intokamaks

A new physical mechanism of formation of runaway electron beams during plasma disruptions intokamaks isproposed. The plasma disruption is caused by a strong stochastic magnetic field formed due tononlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjecturedthat the runaway electron beam is formed in the central plasma region confined insidethe intact magnetic surface located between q = 1 and the closestlow–order rational magnetic surfaces [q = 5/4 or q = 4/3,…]. Itresults in that runaway electron beam current has a helical nature with a predominantm/n = 1/1 component. The thermal quench and currentquench times are estimated using the collisional models for electron diffusion and ambipolarparticle transportin a stochastic magneticfield, respectively. Possible mechanisms for the decay of the runawayelectron current owing to an outward drift electron orbits and resonance interaction ofhigh–energy electrons with the m/n = 1/1 MHD mode are discussed.