Extended steady-state and high-beta regimes of net-current free heliotron plasmas in the large helical device

Motojima, Osamu*; Yamada, Hiroshi*; Komori, Akio*; Oyabu, Nobuyoshi*; Kaneko, Osamu*; Kawahata, Kazuo*; Mito, Toshiyuki*; Muto, Takashi*; Ida, Katsumi*; Imagawa, Shinsaku*; Nagayama, Yoshio*; Shimozuma, Takashi*; Watanabe, Kiyomasa*; Masuzaki, Suguru*; Miyazawa, Junichi*; Morisaki, Tomohiro*; Morita, Shigeru*; Odachi, Satoshi*; Ono, Noriyasu*; Saito, Kenji*; Sakakibara, Satoru*; Takeiri, Yasuhiko*; Tamura, Naoki*; Toi, Kazuo*; Tokitani, Masayuki*; Yokoyama, Masayuki*; Yoshinuma, Mikiro*; Ikeda, Katsunori*; Isayama, Akihiko; Ishii, Kameo*; Kubo, Shin*; Murakami, Sadayoshi*; Nagasaki, Kazunobu*; Seki, Tetsuo*; Takahata, Kazuya*; Takenaga, Hidenobu; LHD Experimental Group*

The performance of net-current free Heliotron plasmas has been developed by findings of innovative operational scenarios in conjunction with an upgrade of the heating power and the pumping/fueling capability in the Large Helical Device (LHD). Consequently, the operational regime has been extended, in particular, with regard to high density, long pulse length and high beta. Diversified studies in LHD have elucidated the advantages of net-current free heliotron plasmas. In particular, an Internal Diffusion Barrier (IDB) by combination of efficient pumping of the local island divertor function and core fueling by pellet injection has realized a super dense core as high as 510m, which stimulates an attractive super dense core reactor. Achievements of a volume averaged beta of 4.5 % and a discharge duration of 54-min. with a total input energy of 1.6 GJ (490 kW in average) are also highlighted. The progress of LHD experiments in these two years is overviewed with highlighting IDB, high and long pulse.