(OV/2-1) Extended Steady-State and High-Beta Regimes of Net-Current Free Heliotron Plasmas in the Large Helical Device

O. Motojima1), H. Yamada1), A. Komori1), O. Kaneko1), K. Kawahata1), T. Mito1), T. Mutoh1), N. Ohyabu1), K. Ida1), S. Imagawa1), Y. Nagayama1), T. Shimozuma1), K.Y. Watanabe1), S. Masuzaki1), J. Miyazawa1), S. Morita1), S. Ohdachi1), N. Ohno2), K. Saito1), S. Sakakibara1), Y. Takeiri1), N. Tamura1), K. Toi1), M. Tokitani3), M. Yokoyama1), M. Yoshinuma1), K. Ikeda1), A. Isayama4), K. Ishii5), S. Kubo1), S. Murakami6), K. Nagasaki7), T. Seki1), K. Takahata1), H. Takenaga4), LHD Experimental Group
1) National Institute for Fusion Science, Toki, Japan
2) EcoTopia Science Institute, Nagoya Univ., Nagoya, Aichi, Japan
3) Research Institute of Applied Machanics, Kyushu Univ., Kasuga, Fukuoka, Japan
4) Naka Fusion Institute, Japan Atomic Energy Agency, Naka, Ibaraki, Japan
5) Plasma Research Center, Tsukuba Univ., Tsukuba, Ibaraki, Japan
6) Department of Engineering, Kyoto Univ., Kyoto, Japan
7) Institute of Advanced Energy, Kyoto Univ., Uji, Kyoto, Japan

Abstract.  The performance of net-current free heliotron plasmas has been developed by an upgrade of the heating power and the pumping/fueling capability in the Large Helical Device (LHD) and understanding of the confinement physics of net-current free plasmas has been deepened. LHD is a superconducting magnetic confinement device employing a heliotron configuration. The heating capability is 15 MW of NBI, 2.9 MW of ICRF and 2.1 MW of ECH. The operational regime has been extended with regard to the pulse length and high beta. Achievements of a volume averaged beta of 4.5% and discharge duration of 54 min with a total input energy of 1.6 GJ are highlighted. These two major achievements emphasize the fundamental advantage of net-current free heliotron plasmas. Long pulse operation has been pursued by ICRF heating and a dynamic control of heat load on the divertor plate. A plasma with an ion temperature of 2 keV and a density of 8 × 1018 m-3 was created and maintained for more than 30 min. An essential reason for this success is that highly energetic trapped ions are well confined due to drift optimization. The high ion energy tail up to 1.6 MeV has been observed simultaneously. A current control of the helical coil which consists of 3 independent layers has enabled us to scan the plasma aspect ratio. Large aspect ratio reduces the Shafranov shift and is preferable to the MHD equilibrium beta limit as well as power deposition of the NBI heating while MHD stability is violated due to a suppression of the spontaneous magnetic well. The effect of MHD instabilities is still mitigated at larger aspect ratio compared to the regular operation. Consequently, the highest beta value of 4.5% has been achieved at aspect ratio of 6.6 and the magnetic field of 0.425 T. This high beta state is maintained for more than 10 times the energy confinement time. The beta value still increases with the heating power. An internal diffusion barrier has realized a super dense core as high as 5 × 1020 m-3. Complete detached plasma with the line-averaged density of 2 × 1020 m-3 has been maintained in quasi-steady state. New findings also enable us to extend the envelope of explorable physical parameter space. Diversified studies in LHD have elucidated the broad scope of steady-state high temperature plasmas.

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