@article{oai:repo.qst.go.jp:00085237,
 author = {Shinichiro, Kojima and Hanada, Kazuaki and Idei, Hiroshi and Onchi, Takumi and Ikezoe, Ryuya and Nagashima, Yoshihiko and Hasegawa, Makoto and Kuroda, Kengoh and Nakamura, Kazuo and Higashijima, Aki and Nagata, Takahiro and Kawasaki, Shoji and Shimabukuro, Shun and Elserafy, Hatem and Masaharu, Fukuyama and Ejiri, Akira and Taiichi, Shikama and Yoneda, Nao and Yoneda, Ryota and Kariya, Tsuyoshi and Takase, Yuichi and Murakami , Sadayoshi and Bertelli, Nicola and Ono, Masayuki and Shinichiro, Kojima and Hanada, Kazuaki and Idei, Hiroshi and Ikezoe, Ryuya and Masaharu, Fukuyama and Ejiri, Akira and Taiichi, Shikama and Kariya, Tsuyoshi and Takase, Yuichi and Murakami , Sadayoshi},
 issue = {10},
 journal = {Plasma Physics and Controlled Fusion},
 month = {Oct},
 note = {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 Tebulk ∼ 140 eV and no hard x-ray (HXR) emission with a low Ip of ∼15 kA; it abruptly transitions to a regime that exhibits a low Tebulk of ∼10 eV and a strong HXR emission with a high Ip 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 Tetail, while the power transfer from the tail electrons to the bulk electrons decreases with 1/Tetail0.5. This causes a rapid transition to a state with high Tetail while reducing Tebulk 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.},
 title = {Observation of second harmonic electron cyclotron resonance heating and current-drive transition during non-inductive plasma start-up experiment in QUEST},
 volume = {63},
 year = {2021}
}