Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas
Yuejiang Shi, Bing Liu, Shaodong Song, Yunyang Song, Xianming Song, Bowei Tong, Shikui Cheng, Wenjun Liu, Mingyuan Wang, Tiantian Sun, Dong Guo, Songjian Li, Yingying Li, Bin Chen, Xiang Gu, Jianqing Cai, Di Luo, D. Banerjee, Xin Zhao, Yuanming Yang, Wenwu Luo, Peihai Zhou, Yu Wang, Akio Ishida, Takashi Maekawa, Minsheng Liu, Baoshan Yuan, Yuankai Peng, the EXL-50 Team
Abstract
Abstract As a new spherical tokamak designed to simplify the engineering requirements of a possible future fusion power source, the EXL-50 experiment features a low aspect ratio vacuum vessel (VV), encircling a central post assembly containing the toroidal field coil conductors without a central solenoid. Multiple electron cyclotron resonance heating (ECRH) resonances are located within the VV to improve current drive effectiveness. Copious energetic electrons are produced and measured with hard x-ray detectors, carry the bulk of the plasma current ranging from 50–150 kA, which is maintained for more than 1 s duration. It is observed that over one ampere current can be sustained per watt of ECRH power issued from the 28 GHz gyrotrons. The plasma current reaches I p > 80 kA for high density (>5 × 10 18 m −2 ) discharge with 150 kW ECRH. An analysis was carried out combining reconstructed multi-fluid equilibrium, guiding-center orbits of energetic electrons, and resonant heating mechanisms. It is verified that in EXL-50 a broadly distributed current of energetic electrons creates a smaller closed magnetic-flux surface of low aspect ratio that in turn confines the thermal plasma electrons and ions and participate in maintaining the equilibrium force balance.