Observation of second harmonic electron cyclotron resonance heating and current-drive transition during non-inductive plasma start-up experiment in QUEST

Noninductive plasma current start-up using 2nd harmonic electron cyclotron resonance heating (ECRH) with oblique radio frequency (RF) injection is demonstrated in a Q-shu University experiment with steady-state spherical tokamak. A strong transition was observed in the heating and plasma current ramp-up. The initial bulk electron heating regime exhibits T (ebulk) similar to 140 eV and no hard x-ray (HXR) emission with a low I (p) of similar to 15 kA; it abruptly transitions to a regime that exhibits a low T (ebulk) of similar to 10 eV and a strong HXR emission with a high I (p) of similar to 50 kA. This behavior is distinctly different from that observed in previous fundamental ECRH experiments. The mechanism of the heating and current drive transition are investigated considering wave power absorption and plasma power balance. The results indicate that the transition is caused by the favorable heating of tail electrons where the RF power absorption at the 2nd harmonic increases nearly linearly with T (etail), while the power transfer from the tail electrons to the bulk electrons decreases with 1/T (etail) (0.5). This causes a rapid transition to a state with high T (etail) while reducing T (ebulk) towards colder ion temperature. The understanding of the transition mechanism helps to consider plasma current start-up using 2nd harmonic ECRH for tokamak reactors such as JT-60 SA and ITER.