Observation of internal kink instability purely driven by suprathermal electrons in the HL-1M tokamak

Strong m = 1 MHD activities are observed in the HL-1M tokamak during off-axis electron cyclotron resonance heating (ECRH) when the cyclotron resonance location is placed just outside the q = 1 surface at the high-magnetic-field side of the magnetic surface. Addition of lower-hybrid waves to ECRH significantly enhances the MHD excitation, but lower-hybrid waves alone cannot excite or sustain the mode. This result is a clear demonstration of the suprathermal trapped electron effect on the instability because of the absence of energetic ions in the plasma.