Non-inductive current start-up assisted by energetic electrons in Q-shu University experiment with steady-state spherical tokamak

Abstract

After intensive discharge cleaning of the chamber wall, non-inductive current start-up experiments have been successfully performed in QUEST in moderate vertical fields of about 1.0–1.5 mT with positive n-index. Simultaneously, with increasing plasma current, an asymmetric toroidal flow of energetic electrons was observed and direct measurements of current driven by this asymmetric flow were taken with a newly developed Langmuir probe technique. A numerical study of the energetic electron orbits indicates that the total current is enough to play a dominant role in the formation of a closed flux surface in QUEST.

Particle balance in long duration RF driven plasmas on QUEST

“…The experiments were conducted on QUEST [11][12][13][14][15], which is a medium-sized spherical tokamak (R = 0. height and diameter of the vacuum vessel (VV) is 2.8 m. Four pairs of poloidal field coils and three separated center solenoid coils are available and their combination produces various magnetic configurations [11]. Of late, the maximum pulse duration is reaching nearly 10 min.…”

Comparison between Non-Inductive Plasma Current Start-Up Using ECRH with and without Fundamental Resonance on QUEST

“…Because space is limited in the high-field side of a ST, non-inductive plasma current start-up is an indispensable technique. Experiments of non-inductive plasma current start-up using electron cyclotron resonance heating (ECRH) have been actively performed in ST such as LATE [2], MAST [3], TST-2 [4] and QUEST [5,6]. In the experiments, the combined effect of fundamental (1st) and second harmonic (2nd) ECRH provides a significant plasma heating [2,5], and it is also reported the energetic electrons confined in an open magnetic surface plays an essential role in plasma current start-up [5,6].…”

“…Experiments of non-inductive plasma current start-up using electron cyclotron resonance heating (ECRH) have been actively performed in ST such as LATE [2], MAST [3], TST-2 [4] and QUEST [5,6]. In the experiments, the combined effect of fundamental (1st) and second harmonic (2nd) ECRH provides a significant plasma heating [2,5], and it is also reported the energetic electrons confined in an open magnetic surface plays an essential role in plasma current start-up [5,6]. In conventional tokamaks, plasma production with 2nd ECRH has been reported [7,8] and a author's e-mail: miura@triam.kyushu-u.ac.jp * ) This article is based on the presentation at the 24th International Toki Conference (ITC24).…”

Recent progress on spherical torus research

“…125 After closed flux surfaces are formed, pressure-driven "bootstrap" currents can be generated within the flux surfaces to maintain the plasma current. 126 The ECH/EBW start-up experiments have been performed in a number of ST devices in recent years: LATE, 26,127 TST-2, 128,129 CPD, 130 QUEST, 131 and MAST. 132 An illustrative ECH/EBW start-up and ramp-up experiment was performed on the LATE device, which is shown in Fig.…”