Observation of two threshold fields for runaway-electron generation in tokamaks

Two different threshold electric fields, characterizing a lower field required for significant seed RE generation and sustainment and a higher field required for the RE avalanche onset, have been experimentally observed in the flattop of EAST discharges. These results demonstrate that the threshold electric field for the RE avalanche onset is 1.2-fold higher than the RE detection onset field required for the primary generation. An updated 0-D model, including the delay in the RE avalanche onset, suggests that the RE population should be less than the previous prediction. The delay in the RE avalanche onset also opens a possibility for RE suppression at relatively low electron density.