(EX/P4-11) Particle Control under Wall Saturation in Long-pulse High-density H-mode Plasmas of JT-60U

H. Kubo1), T. Nakano, N. Asakura, H. Takenaga, K. Tsuzuki, N. Oyama, H. Kawashima, K. Shimizu, H. Urano, K. Fujimoto, JT-60 Team
Japan Atomic Energy Agency, Naka, Ibaraki-ken, 311-0193 Japan

Abstract.  Particle control is one of the key issues for steady-state operation. In short discharges, the first wall absorbs hydrogen particles and it works as a pump (wall pumping). The wall pumping is effective to control the plasma density. However, in future tokamak devices, wall retention increases in a long discharge. Then, it is expected that the wall retention is saturated and the wall pumping does not work. Therefore, for steady-state operation, particle control by active divertor pumping without the wall pumping should be established. In JT-60U, long-pulse operation up to 65 s with NBI of 12 MW for 30 s has become available since 2003. Global saturation of the wall retention has been observed in a latter phase of an ELMy H-mode plasma after several long-pulse discharges. In this paper, the particle control has been studied under the wall saturation in long-pulse high-density H-mode plasmas of JT-60U. The electron density has been successfully controlled by active divertor pumping in long-pulse high-density ELMy H-mode plasmas where wall pumping does not work and even outgas appears. The energy confinement and ELM activity was sustained, while the outgas rate increased and the gas puff rate decreased. Particle balance has been investigated to clarify mechanisms of the wall saturation. We have observed immediate outgas with appearance of divertor plasma detachment. The observation shows that dynamic equilibrium between particle injection and desorption is established under high flux in the attached divertor plasma and the wall retention decreases with particle flux reduction by the detachment. The wall saturation can be attributed to three mechanisms: increase in the wall retention by repeating the long-pulse high-density discharges, outgas from the divertor tiles due to a rise in the divertor tile temperature, and dynamic equilibrium between particle injection and desorption.

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