Study of Spherical Tokamak of Burning Plasma with TF-CS Hybrid Force-Balanced Coils

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Stress distributions of a compact tokamak device Tbdoroki-II with TF-CS hybrid type forte-balanced coils were measured. The measured values were similar to analytically evaluated values, and the reduction of stress was experimentally confirmed.]A fully-inonduced current start up process of a spherical torus (ST) with a transport model of CDBM was numerically simulated. This results show the possibility of a current oscillation by interactions of profiles of internal transport barrier, pressure and boot strap current. A possibility of an over-drive of plasma current was also shown.A finite Larmor radius effect in ST is important because of the weakness of magnetic field compared with that of conventional tokamak. A computation code to calculate trajectory and diffusion coefficient without guiding center approximation was developed. The results were compared with those of conventional tokamak by guiding center approximation. Especially, we investigated an anomalous diffusion by a resonance of toroidal field ripple and banana particles, and evaluated an energy dependence of the diffusion coefficients. The result shows that Larmor motion affects the toroidal phase of banana tip, and softens the resonance. In order to investigate the physical mechanism of the resonance, we use the simple equilibrium configuration with a circular cross section of a conventional tokamak. We numerically found that the peaks of diffusion coefficients appears around the resonance energies of axisymmetric magnetic field. Moreover, distortion of trajectories by the resonance in a phase space makes island configurations, transverse motions by collisions enhance the diffusion.