High-density H-mode operation achieved using efficient plasma refueling byinboard pellet launch

A systematic study was performed on high-density H-mode operation in the to kamaks ASDEX Upgrade and JET using inboard pellet launch refueling. The pel let particle flux was found to correlate with the achieved density enhancem ent. After injection of each pellet the decay of the density enhancement st arts on a fast time scale until about half of the pellet inventory is expel led, slowing down significantly as the base density is approached. Whereas the overall slow decay happens in the particle confinement time, its first phase results from a sequence of ELMs following each injection. Loss of par ticles in an ELM sequence is accompanied by a loss of energy, causing a red uction of the plasma energy. Full plasma energy recovery after an ELM seque nce occurs faster than the slow density decay, allowing transient operation at high densities maintaining full confinement. However, confinement degra dation by inappropriate discharge scenarios must be avoided. Pellet-induced ELM bursts result in a particle flux from the plasma recycling at the wall , adding up with gas fluxes from other sources. Insufficient pumping can th en lead to a neutral gas pressure increase causing confinement degradation. Also, excessive temperature reduction by pellets close to rational surface s can create conditions likely to catalyze the growth of neoclassical teari ng modes (NTMs) at high beta (N), which may then be triggered by a succeedi ng pellet. (C) 2001 Elsevier Science B.V. All rights reserved.