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Spherical tokamak (ST) research in Japan has produced many innovative results: (i) plasma start-up to Ip > 70 kA was achieved by electron cyclotron wave (ECW) with N∥ = 0.75, while electron heating to Te > 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., source:Y. Takase et al 2022 Nucl. Fusion 62 042011, source:https://doi.org/10.1088/1741-4326/ac29cf, identifier:0000-0002-4846-5267

Distributions of edgewise bending strain in helical coils with the geodesic winding based on virial theorem are analyzed theoretically and numerically. A force-balanced coil (FBC) is a multipole helical coil combining toroidal field (TF) coils and a solenoid helically wound on a torus. The combination reduces the net electromagnetic force in the direction of the major radius by canceling out the centering force due to the TF coil current and the hoop force due to the solenoid current. The FBC concept was extended using the virial theorem, which shows the theoretical lower limit of stress in the coils and their supporting structure. High-field coils should accordingly have the same averaged principal stresses in all directions, which is named the virial-limit condition. Since FBC winding is modulated to reduce the tilting force, the winding is slightly similar to but different from the shortest geodesic trajectory and has no tensile load. To apply FBC to high-temperature superconducting tapes, the degradation of superconducting properties originating from edgewise bending strain is an important problem. Since the geodesic trajectory is a kind of a straight line on a curved surface and curves only to the normal direction of the surface, it is expected that the tape with geodesic trajectories has a small residual stress. In this paper, we analyze the effect of the winding modulations including the geodesic modulation for the optimization of residual stress in helical windings., source:H. Tsutsui et al., "Edgewise Bending Strain in Helical Coils With Geodesic Windings Based on Virial Theorem," in IEEE Transactions on Applied Superconductivity, vol. 26, no. 4, pp. 1-4, June 2016, Art no. 4901704, doi: 10.1109/TASC.2016.2535779., source:https://doi.org/10.1109/TASC.2016.2535779, identifier:0000-0003-1134-4389

Nonlinear interplay of the electron temperature gradient (ETG) modes and the trapped electron modes (TEMs) was investigated by means of gyrokientic simulation. Focusing on the situation where both TEMs and ETG modes are linearly unstable, the effects of TEM-driven zonal flows on ETG turbulence were examined by means of entropy transfer analysis. In a statistically steady turbulence where the TEM driven zonal flows are dominant, it turned out that the zonal flows meditate the entropy transfer of the ETG modes from the low to high radial wavenumber regions. The successive entropy transfer broadens the potential fluctuation spectrum in the radial wavenumber direction. In contrast, in the situation where ETG modes are unstable but TEMs are stable, the pure ETG turbulence does not produce strong zonal flows, leading to a rather narrow spectrum in the radial wavenumber space and a higher transport level., source:https://doi.org/10.1585/pfr.10.1403047

0000-0002-4846-5267, Spherical tokamak (ST) research in Japan has produced many innovative results: (i) plasma start-up to Ip > 70 kA was achieved by electron cyclotron wave (ECW) with N∥ = 0.75, while electron heating to Te > 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.