Optimization of Divertor

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Particle control in divertor region is of great importance for production and confinement of high performance plasma in magnetic confinement device. In order to develop a new type of divertor pumping system with a "superpermeation", which is the interaction between V-group metal membranes and hydrogen atoms, we carried out plasma-membrane experiments on basic surface physics and proof-of-principle experiments on hydrogen pumping in divertor region in fusion devices. In the divertor region, high-energy ion sputtering and impurity (metal and carbon) deposition on the membrane surface can be usually seen for vacuum components. The influence of such events on hydrogen permeation performance of membrane is investigated in the basic plasma-membrane experiments. The degradation of hydrogen permeation rate is observed and the method to improve the permeation rate is also found in the experiments. On the other hand, hydrogen particle pumping with a Nb membrane has been first time in the world d … More emonstrated in the divertor plasma in the JFT-2M tokamak. This suggests that the membrane pumping system is available in the divertor region. As a result, we have a possibility in prospect to realize a membrane pumping system or a panel pumping system as a divertor pumping method.We have studied properties of divertor plasma in the Heliotron type, large and small devices (LHD, Uragan-3M). In the divertor configuration, the heat and particle flows eventually on to the divertor plates. We find that the heat deposition patterns on the divertor plates are not up-down symmetric. Degree of the asymmetry is higher for the smaller device (Uragan-3M) with higher RF power. The asymmetry is reversed with reversal of the magnetic field. These observations are explained by the direct (non-diffusive) loss (the Bx∇B drift) of the ion from the confinement volume. The density and potential fluctuations in the edge beyond the last closed surface are measured by the probes. The particle flux estimated by the fluctuation measurement is comparable to that evaluated from the particle balance. Furthermore more than 50 % of the particle loss due to the fluctuations take place during the burst phase. Less