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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 ∼ 140 eV and no hard x-ray (HXR) emission with a low I p of ∼15 kA; it abruptly transitions to a regime that exhibits a low T ebulk of ∼10 eV and a strong HXR emission with a high I p of ∼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.

An evaluation of high-field-side (HFS) X-mode injection for the electron-Bernstein-wave (EBW) scenario is performed using the GENRAY ray-tracing code. In the early stage of low-density plasma start-up, when the electron cyclotron resonance and upper hybrid resonance layers are close to each other, efficient and localized heating by the EBW is attainable. We show that, when the electron density rises, the HFS scenario spontaneously shifts to current drive with successful electron heating. This shift can be explained as a change in heating mechanism from collisional to electron cyclotron damping. Also, we discuss a possible OX -B scenario to continue the plasma current drive beyond the formation of an over-dense plasma.

High field side (HFS) injection of eXtraordinary X-mode for electron Bernstein wave (EBW) conversion was conducted in the QUEST tokamak. Radio frequency (RF; 8.2 GHz) power was delivered from the low field side (LFS) to the high field side HFS through waveguides, and from the HFS placed 20 cm above the midplane of the vacuum vessel. The aim was to compare the RF launches from the LFS and HFS. The plasma brightness, measured by a fast camera, as well as the H α signal captured along the mid-plane, was noticeably higher in the HFS launch than in the LFS launch. The HFS injection achieved a plasma current of approximately 130 A, versus 35 A in the LFS injection. The electron density n e predicted from the position of the upper hybrid resonance agreed with the line-averaged n e measured by an interferometer, confirming the effective conversion and subsequent damping of the EBW mode. The RF leakage of the HFS injection was less than one-sixth that of the LFS injection. These results indicate that HFS delivers better RF coupling and conversion efficiency to EBW than LFS injection. Such efficient plasma heating via EBW will significantly enhance the plasma production.

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