Generation and termination of runaway electrons at major disruptions in JT-60U

The operation conditions to avoid runaway electron generation at the major disruption have been investigated in JT-60U tokamak plasmas. It has been found that runaway electrons are not observed for low Bt of ⩽ 2.2 T or low plasma current quench rates (Iγ ≡ -(dIp/dt)/Ip) of <50 s-1. Furthermore, they are not observed for low effective safety factors defined at the plasma edge (qeff) of ⩽ 2.5 even for high Iγ of 300-400 s-1, which is the case for uncontrolled disruptions accompanied by large plasma displacements (e.g., vertical displacement events (VDEs)). On the other hand, in controlled disruptions with small plasma shifts, qeff easily increases above 8, and runaway electrons are observed even for low current quench rates of 50-100 s-1. Furthermore, it has been found that in these position controlled disruptions the runaway current tail can rapidly decay even for zero or weakly positive plasma surface voltages. These observations of the avoidance and termination of runaway electrons suggest an anomalous loss mechanism for runaway electrons.