Overview of spherical tokamak research in Japan

Abstract

Nationally coordinated research on spherical tokamak is being conducted in Japan. Recent achievements include: (i) plasma current start-up and ramp-up without the use of the central solenoid by RF waves (in electron cyclotron and lower hybrid frequency ranges), (ii) plasma current start-up by AC Ohmic operation and by coaxial helicity injection, (iii) development of an advanced fuelling technique by compact toroid injection, (iv) ultra-long-pulse operation and particle control using a high temperature metal wall, (v) access to the ultra-high-β regime by high-power reconnection heating, and (vi) improvement of spherical tokamak plasma stability by externally applied helical field.

Investigation of hydrogen recycling in long-duration discharges and its modification with a hot wall in the spherical tokamak QUEST

“…(a) A cross-sectional view of the hot wall composed of heating-cooling panels and a radiation shield. (b) A photograph of the hot wall[43]. The holes in (b) are for plasma heating devices and diagnostics in the mid-plane area.…”

Overview of coordinated spherical tokamak research in Japan

“…The local helical coils of TOKASTAR-2 consist of two parallelogram-shaped coils, called helical field (HF) coils, located on the outboard side, and four fan-shape coils, called additional helical field (AHF) coils, located on the upper and lower sides. The effect of the helical field application on the plasma horizontal position was observed with magnetic field measurements [29,30] and with a high-speed camera [1,31] in TOKASTAR-2. The suppression of vertical instability was also studied in elongated plasmas but no clear effects of the helical field were observed [32].…”

“…The Subcommittee meets twice a year to discuss the overall strategy of Japanese ST research, exchange information on latest results, facilitate collaborations among different research groups, and coordinate participations in international collaborations. The roles of university ST research include: (1) unique research through creativity and innovation which might be considered too challenging for large ST devices, (2) establishment of the scientific basis for achieving ultra-high beta and ultra-long pulse (Fig. 1), (3) contribution to the scientific basis for practical and economically competitive fusion power, complementing the mainline tokamak research (JT-60SA, ITER, etc.…”

“…Spherical tokamak (ST) research in Japan [1] is being conducted as a nationally coordinated program of universityscale ST devices under the ST Research Coordination Subcommittee organized by the National Institute for Fusion Science (NIFS). Subcommittee members are leaders of ST research groups in Japan.…”

“…The Subcommittee meets twice a year to discuss the overall strategy of Japanese ST research, exchange information on the latest results, facilitate collaborations among different research groups, and coordinate participation in international collaborations. The roles of university ST research include: (1) unique research through creativity and innovation, which might be considered too challenging for large ST devices; (2) establishment of the scientific basis for achieving ultra-high beta and ultra-long pulse (figure 1); (3) contribution to the scientific basis for practical and economically competitive fusion power, complementing the mainline tokamak research (JT-60SA, ITER, etc); and (4) development and training of a future generation of world-leading tokamak scientists. Specific research topics include: (a) development of start-up, current drive, and control techniques without the use of the central solenoid (CS); (b) formation and sustainment of very high beta plasmas; and (c) demonstration of steadystate operation and the study of steady-state issues such as heat and particle control, divertor physics, and plasma-wall interaction.…”