Study of seed plasma generation for NBI plasma start-up using non-resonant microwave launch in Heliotron J

• Published in: Plasma physics and controlled fusion Vol. 62; no. 6; pp. 65009 - 65020
• Main Authors: Kobayashi, S, Nagasaki, K, Stange, T, Mizuuchi, T, Okada, H, Minami, T, Kado, S, Yamamoto, S, Ohshima, S, Hada, K, Weir, G, Konoshima, S, Nakamura, Y, Kenmochi, N, Otani, Y, Lu, X X, Panith, A, Toi, K, Suzuki, Y
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.06.2020
• Subjects: Heliotron J; NBI plasma start-up; non-resonant microwave heating
• ISSN: 0741-3335; 1361-6587
• DOI: 10.1088/1361-6587/ab877e

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.