Second harmonic EC plasma ramp-up through production of energetic electrons on the QUEST spherical tokamak

Plasma current is started up solely by electron cyclotron wave (ECW) in QUEST spherical tokamak. The beam of 28 GHz second harmonic ECW is injected obliquely from low field side, and hence parallel refractive index at the resonant layer is N_|| = 0.78. A steering antenna, consisting of two quasi-optical mirrors, can turn direction of the focused Gaussian beam. Polarization of the ECW is controlled as X-mode is superior to O-mode. In the experiment, achieved peak plasma current has been high record, as non-inductive RF start-up, reaching I_p > 75 kA. The count number and the averaged energy of Hard X-ray attributed to energetic electrons increase with I_p ramp-up. Analysis through TASK-WR simulation demonstrates that power of 28 GHz ECW is absorbed dominantly to energetic electrons via single-pass cyclotron damping, and that X-mode absorption into particle is adequately higher than that of O-mode.