@article{oai:repo.qst.go.jp:00082470,
 author = {Kobayashi, Shinji and Nagasaki, Kazunobu and Stange, T. and Mizuuchi, Tohru and Okada, Horoyuki and Minami, Takashi and Kado, Shinichiro and Satoshi, Yamamoto and Ohshima, Shinsuke and Hada, K. and Weir, G.M. and Konoshima, Shigeru and Nakamura, Yuji and Kemmochi, Naoki and Yoshiaki, Ohtani and Lu, X.X. and Panith, A. and Toi, Kazuo and SUZUKI, Yasuhiro and Nagasaki, Kazunobu and Mizuuchi, Tohru and Minami, Takashi and Kado, Shinichiro and Satoshi, Yamamoto and Yoshiaki, Ohtani},
 issue = {6},
 journal = {Plasma Physics and Controlled Fusion},
 month = {May},
 note = {Here, we report on a study of seed plasmas produced by non-resonant 2.45 GHz microwave heating in Heliotron J, which is useful for plasma start-up by neutral beam injection (NBI) under the conditions of low beam power (<1 MW), low accerelation voltage (<30 kV) and small device size (~1 m). The pre-ionization technique using the non-resonant heating produced the electron density in the order of 1017–1018 m−3 with the energy exceeds the CII ionization potential (30 eV). The existence of the relativistic electrons, which was essential to produce the seed plasmas, was confirmed experimentally during the pre-ionization phase by the synchrotron radiation and the hard x-ray emission measurements. The production mechanism of the relativistic electrons was considered to be stochastic interactions between the electrons and the microwave-frequency field. By optimizing the additional gas fueling during the microwave launch, a higher seed-plasma density more than 4 × 1018 m−3 was obtained, resulting that a rapid density evolution after NBI turn-on was successful even in the low NBI power (0.3 MW) condition. The seed plasmas produced by the pre-ionization method is useful to realize the plasma start-up under low absorption power condition in larger heliotron/stellarator devices such as perpendicular NBI in W7-X or deuterium NBI in LHD.},
 title = {Study of seed plasma generation for NBI plasma start-up using non-resonant microwave launch in Heliotron J},
 volume = {62},
 year = {2020}
}