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.
Spontaneous transition of the plasma confinement mode was observed in the helicalaxis heliotron device "Heliotron J" for three different plasma heating schemes, i.e. ECH-only, NBIonly and the combination of ECH and NBI. The transition seems to occur above a certain critical density. In addition to the confinement transition, a spontaneous shift of the hitting position of the divertor plasma flux on the wall was observed. This shift could be related with the change of the edge field topology caused by non-inductive toroidal currents.
Recent 70-GHz, 0.4-MW ECH experiments in Heliotron J have revealed the existence of the spontaneous confinement transition, like that of the H-mode, at rather low threshold line-averaged densities of 1.2-1.6 × 10 19 m -3 . The transition was discovered in two edge-iota windows: one is 0.54 < ι (a)/ 2π < 0.56 at separatrix discharge plasmas and the other is 0.62 < ι (a)/2π < 0.63 at partial walllimiter discharge plasmas. The energy confinement time for the separatrix discharge plasmas was found to be enhanced beyond the normal ISS95 scaling in the transient H-mode phase, being 50% longer than that in the "before transition" phase. Keywords:heliotron, stellarator, H-mode, transition phenomena, confinement improvementThe research program regarding Heliotron J (a low shear helical-axis heliotron having four periods, ι /2π = 0.557, R = 1.2 m, and a < 0.2 m) is focused on the establishment, characterization, and improvement of plasma confinement in the next-generation heliotron line [1,2]. The characterization of 70-GHz, 0.4-MW ECH plasma confinement in Heliotron J was studied with special regard to its magnetic configuration effects [3]. The experiments revealed that the energy confinement characteristics in the normal confinement mode (T e < 1.5 keV, T i CX < 0.2 keV, -n e = (0.2 -3.0) × 10 19 m -3 , W p diam < 3 kJ, and B 0 < 1.5 T) indicate the existence of "good" confinement plasmas whose energy confinement time becomes 1.5-2 times longer than the ISS95 scaling [4]. At present, it is not clear whether these good confinement plasmas are essentially different from those of the traditional ISS95 scaling. Beside these good confinement plasmas, the spontaneous confinement improvement mode, like that of the H-mode, was author's e-mail: sano@iae.kyoto-u.ac.jp recently discovered during the ECH with strong gas puffing in two edge-iota windows. Figure 1 shows the time evolution of the observed H-mode behavior in which the sudden drops in both H α and SOL probe signals and the subsequent strong rises in both line-integrated density and diamagnetic energy content were observed at the H-mode transition. In the case of the vacuum edge iota ι (a)/2π = 0.542, the peak increment of the plasma energy content was about 70% while that in the case of ι(a)/2π = 0.623 was about 30%. These confinement improvements remained transient on an energy confinement timescale and the post-transition steady-state phase was not yet attained. ECE measurements revealed that, after the transition, the core electron temperature inside r/a < 2/3 slightly increased or was well maintained despite the strong increase in density. On the other hand, in the SOL region, Langmuir probe measurements revealed that, after the transition, the SOL density fluctuation also dropped in the fre-
Studies of global energy confinement and toroidal plasma current behaviour for the second harmonic 70 GHz ECH at B = 1-1.5 T are described with emphasis on the magnetic configuration effects in the helical-axis heliotron 'Heliotron J'. At low densities of ne < 0.4×10 19 m −3 , the electron temperature reached T e ≈ 1 keV in the core region, indicating the production of collision-less plasmas of electron collisionality ν * 0.1, where ν * = ν/(v e /π R 0 q). For medium densities of 0.5 × 10 19 m −3 < ne < 2 × 10 19 m −3 , the preferable energy confinement time, 1.5-2 times larger than that of the ISS95 scaling, was obtained under the condition of localized central heating at B ≈ 1.25 T for the standard configuration of Heliotron J. The measurements of the toroidal current under perpendicular microwave injection revealed the change of the current flow direction as a function of the poloidal magnetic field. The measured current behaviour was found to be qualitatively consistent with that of the bootstrap current predicted from neoclassical theory. The observed flow reversal showed that a proper selection of the field configuration could control the bootstrap current in the helical-axis heliotron. In addition, the current control through the electron cyclotron current drive scenario with oblique injection of microwaves was experimentally examined.
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