Runaway electrons in magnetic turbulence and runaway current termination in tokamak discharges*

The behaviour of runaway electrons in three types of magnetic turbulence in tokamak discharges is reviewed: (a) micromagnetic turbulence, (b) low-m/n magnetic islands in a sea of stochasticity, (c) macroscale magnetic turbulence. The confinement of runaway electrons is much better than that of bulk thermal electrons in (a) and (b), but is greatly degraded in (c). Spontaneous and intrinsic termination of runaway current, which will be favourable for tokamak fusion reactors in order to reduce the heat flux on the first wall, was first found in JT-60U by decreasing the safety factor at the plasma surface qs to around 2 or 3 by three different methods: (i) controlled inward plasma shift, (ii) a vertical displacement event, (iii) plasma current rampup.