Recent Experiments in Gamma 10

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Recent experimental results are presented on thermal barrier/plug potential formation and end plugging in an axisymmetrized tandem mirror GAMMA 10. GAMMA 10 device is a minimum-B anchored tandem mirror with outboard thermal barrier. Plug and barrier are produced in the axisymmetric mirror. Ramp coils set at both ends of the central cell throttle the fraction of passing particles through the min.-B anchors and into the plug/barrier cells. The min-B mirror is designed to be symmetric w. r. t. the midplane in order to cancell the geodesic curvature through each anchor/transition region as shown in Fig. 1. This results in zero average net drift from a flux surface for particles passing through the non-axisymmetric region and zero net parallel current flowing into the central cell, when azimuthal particle drfit caused by radial electric field is kept small in the anchor/transition region. Both-end plugging has been achieved by a combination of neutral beams and gyrotrons. The presence of Potential plug with thermal barrier at the axisymmetric mirror cells has been indicated by measured axial profiles of density, electron temperature and potential, as shown in Fig. 2. Radial distributions of the ion end-loss flux and of plug potentail are obtained with a movable end loss analyzer. Axial particle confinement time τ_<∥> ∿ 50 ms is estimated to be ten times as long as that of unplugged plasma. Measurements of axial potential profile show a 0.5 kV central-cell potential, 1.2 kV plug potential. The potential measurements were carried out by the use of heavy ion beam probe at the central cell and end loss analyzers at both ends. Central-cell density is higher than that in the plug. Strong end plugging is observed during the period when the plug density remains higher than the barrier density. Simultaneously we performed electron energy measurements in the plug and barrier regions from electron cyclotron emission, X-ray absorption and X-ray pin-hole camera image. These indicated that plug electrons have much higher average energy than central cell electrons. Measurements in the barrier region show that football-shaped hot electrons with beta value up to 5% has been stably produced in the axisymmetric mirror cell. Sloshing ions are produced by injecting neutral beams with a skew angle of 42°in the end cells. Plasma target in the anchor cell with line-density high enough to absorb high-energy neutral beams has been quickly produced with short-pulse-gun-injected plasma, central cell gas puffing and ICRF in the central cell. Stable central cell plasma (n≃2×10^<12>cm^<-3>, T_i≃200 eV) has been sustained only when ion-cyclotron-resonance surfaces exist both in the anchor and in the central cells. RF-trapping of the central cell stream into the anchorcells results in a favorable axial profile of the inital plasma for starting up the thermal barrier/plug potential. Radial profile of potential has been controlled with floating end plates which are divided radially into 5 and azimuthally into 4 segments. The floating potential of the end plates goes down negatively with a parabolic radial distribution. The decrease in the anchor/transition potentail lowers radial electric field strength which favors for realizing effectively axisymmetric confinement in GAMMA 10.
核融合科学研究所の論文

Recent Experiments in Gamma 10

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