Overview of coordinated spherical tokamak research in Japan

Abstract

Spherical tokamak (ST) research in Japan has produced many innovative results: (i) plasma start-up to I p > 70 kA was achieved by electron cyclotron wave (ECW) with N ∥ = 0.75, while electron heating to T e > 500 eV was achieved with N ∥ = 0.26 on QUEST. (ii) The radiofrequency (RF)-induced transport model was combined with the x-ray emission model, and extended magnetohydrodynamics equilibrium with kinetic electrons was developed to interpret fast-electron-dominated lower hybrid wave sustained plasmas on TST-2. (iii) Density as high as 30 times the cutoff density was achieved by electron Berstein wave current drive combined with electron beam injection on LATE. (iv) Multiple plasmoids formed by tearing instability in the elongated current sheet were observed, and flux closure and ion heating by plasmoid-mediated fast magnetic reconnection were observed on HIST. (v) Optimization of ECW-assisted inductive start-up with a vertical field with positive decay index was performed on TST-2. (vi) Stabilization of the vertical displacement event by a set of upper and lower helical field coils was demonstrated on TOKASTAR-2. (vii) A 6 h discharge was achieved by cool-down of the center stack cover on QUEST, where the plasma duration limit was consistent with the wall saturation time estimated by modeling. (viii) Extension of ion heating by plasma merging was achieved on TS-3U, TS-4U, UTST, MAST, and ST40.

NSTX-U theory, modeling and analysis results

“…New simulation and modeling capabilities have been developed to predict boundary turbulence, which influences particle and heat loads to divertor targets, and to predict operational limits of PFCs. The recent results in analysis, simulation and modeling of NSTX and NSTX-U, as well as future experiments on NSTX-U and other STs [154,[157][158][159], continue to advance the physics basis and technical solutions required for optimizing the configuration of next-step steady-state tokamak fusion devices.…”

Review of commercial nuclear fusion projects

“…HTS magnet technology also in this case provides the most promising route to compact, cost-effective fusion energy with this design. Experimental spherical tokamaks operating all over the World includes The National Spherical Torus eXperiment-Upgrade (NSTX-U) in the US (Menard et al, 2017), QUEST in Japan (Takase et al, 2022) and MAST-U and START in the United Kingdom (Sykes et al, 1999). Relevant projects are carried on by Tokamak Energy and UKAEA (United Kingdom Atomic Energy Agency).…”

Development of an arch antenna for LHCD in a spherical tokamak

“…Spherical tokamaks (STs), such as Globus-M and M2, NSTX-U, TST-2, etc, have grown rapidly worldwide since the 1990s to advance the goal of obtaining economic power from nuclear fusion [1][2][3][4][5]. These STs usually work at a low aspect ratio and look like a cored apple instead of a ring doughnut [6].…”