Observation of nonthermal electrons further acceleration and long-lasting associated with magnetic reconnection and turbulence bursting in tokamak plasma

Abstract

Vital to magnetized plasma performance, the acceleration of nonthermal electrons significantly influences the current drive of radio frequency (RF) waves in plasma. In EAST, we observed nonthermal electrons initially <215 keV in energy, being locally accelerated to 600 keV within 100 milliseconds, which corresponds to the total growth time of the magnetic island. Surprisingly, these fast electrons (FE) lasted for 1.4 seconds, several times longer than the estimated relaxation time of 0.26 seconds, and exceeded 1/10 of the discharge length. Turbulence generation at island’s X point is attributed to the unexpected confinement and repopulation of FEs. This phenomenon may have positive implications for the steady-state long pulse H mode operation in EAST, especially regarding non-inductive current sustainment. For future RF electron heating-dominant devices, the mechanisms elucidated in this study have immediate implications for optimizing RF current drive efficiency.