H. Zushi, Y. Hirooka, R. Bhattacharyay,M. Sakamoto, Y. Nakashima, T. Yoshinaga, Y. Higashizono, K. Hanada, N. Nishino, N. Yoshida, K. Tokunaga, S. Kado, T. Shikama, Kawasaki, K. Okamoto, T. Miyazaki, H. Honma, K. N. Sato, K. Nakamura, H. Idei, M. Hasegawa, H. Nakashima, A. Higashijima


Active Particle Control Experiments and Critical Particle Flux Discriminating between the Wall Pumping and Fueling in the Compact Plasma Wall Interaction Device CPD Spherical Tokamak

Date of publication:

Oct. 2008

Key words:

21 IAEA Fusion Energy Conference, EX/P4-12


Two approaches associated with wall recycling have been performed in a small spherical tokamak device CPD (Compact Plasma wall interaction experimental Device), that is, 1) demonstration of active particle recycling control, namely, °»active wall pumping°… using a rotating poloidal limiter whose surface is continuously gettering by lithium, and 2) basic study of the key parameters which discriminates between °»wall pumping and fueling°…. For the former, active control of °»wall pumping°… has been demonstrated during 50 kW RF-current drive discharges whose pulse length is typically ~300 ms with a flat-top of ~250 ms. The rotating limiter is in the shape of cylinder with the diameter and axial length of 150 mm and 120 mm, respectively. Although the rotating limiter is located at the outer board, as soon as the rotating drum is gettered with lithium, hydrogen recycling measured with H spectroscopy decreases by about a factor of 3 not only near the limiter but also in the center stack region. A clear reduction in hydrogen recycling by factor of ~3 is found both near the limiter and center stack region. Also, the oxygen impurity level measured with O-II spectroscopy is reduced by about a factor of 3. As a consequence of the reduced recycling, rf driven current has nearly doubled at the same vertical magnetic field. The surface analysis has been conducted to investigate hydrogen and lithium distributions over the rotating drum after plasma exposure, showing that both H and LiH are uniformly distributed. For the latter, global plasma wall interaction with plasma facing components in the vessel is studied in a simple torus produced by electron cyclotron waves. A static gas balance (pressure measurement) without external pumping systems has been performed to investigate the role of particle flux on a transition of °»wall fueling°… to °»wall pumping°…. It is found that a critical particle flux exists to discriminate between them. Beyond the critical value, a large fraction (~80%) of pressure drop (°»wall pumping°…) is found, suggesting that almost all injected particles are retained in the wall. Below it, a significant pressure rise (°»wall fueling°…) is found, which indicates that particles are fuelled from the wall during/just after the discharge.

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