Optimization studies have been done for the helical axis heliotron configuration. One purpose is to find a configuration suitable for experimental studies of the basic properties of a helical axis heliotron. In the present study, the role of the bumpy field component (toroidal mirror ratio) in MHD stability and neoclassical confinement for this type of configuration is examined. The physical mechanism of the improvement of the neoclassical transport through control of the bumpy field component is clarified. The physics design and current status of the new helical axis heliotron device, Heliotron J, are also described.

Results obtained in the initial experimental phase of Heliotron J are reported. Electron beam mapping of the magnetic surfaces at a reduced DC magnetic field has revealed that the observed surfaces are in basic agreement with the ones calculated on the basis of the measured ambient field around the device. For 53.2 GHz second harmonic ECH hydrogen plasmas, a fairly wide resonance range for breakdown and heating by the TE02 mode has been observed in Heliotron J as compared with that in Heliotron E. With ECH injection powers up to ≈ 400 kW, diamagnetic stored energies up to ≈ 0.7 kJ were obtained without optimized density control.

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with the goal of demonstrating its improved helical confinement property in the helical-axis heliotron line. The design feature of Heliotron J is, as compared with that of Heliotron E, the reduced neoclassical transport (near the tokamak level according to the DKES code) and the enhanced average beta limit (4% for the Mercier criterion) with small bootstrap current, which carries the potential for developing the currentless 'quasiisodynamic' optimization. The device parameters are as follows: the major plasma radius of 1.2 m, the average plasma minor radius of 0.1-0.2 m, the magnetic field strength on the magnetic axis of 1.5 T, the vacuum rotational transform of 0.2-0.8 with a low magnetic shear, and the magnetic well depth of 1.5% at the plasma edge, with heating systems such as ECH(0.5 MW), NBI(1.5 MW), and ICRF(2.5 MW). The feasibility of the helical-axis heliotron concept and its ability to explore high-quality confinement surfaces with a divertor will be fully tested in Heliotron J. The first plasma was planned to be produced in the autumn of 1999.