Formation of the hot tail seeds for runaway electron generation during disruptions with lower hybrid waves in the HL-2A tokamak

The hot tail generation is expected to be the dominant mechanism for the runaway electron (RE) seed formation during disruptions, especially in large devices with high electron temperature such as international thermonuclear experimental reactor. This issue has been studied in the HL-2A tokamak by using the superthermal electrons produced by lower hybrid waves (LHWs), which can adjust the hot tail distribution. It was observed that RE generation was significantly enhanced during disruptions with LHWs. The measurements show that the multitudinous superthermal electrons with energy of 40-60 keV created by LHWs greatly transform the landscape of hot tail distribution. The tail electrons can be directly converted into REs under the acceleration of the high toroidal electric field during disruptions. Runaway current plateaus are more likely to be formed than in normal disruptions without LHWs. However, some abnormal phenomena have also been observed, that is, RE generation was not enhanced and no runaway current plateau was formed during some disruptions with LHWs. It is found that this is attributed to the complete loss of RE seeds caused by strong magnetic fluctuations, which prevents the generation of REs during disruptions. This may provide a way to avoid the generation of REs during disruptions by actively exciting magnetic fluctuations.