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

A systematic study was performed on high-density H-mode operation in the tokamaks ASDEX Upgrade and JET using inboard pellet launch refueling. The pellet particle flux was found to correlate with the achieved density enhancement. After injection of each pellet the decay of the density enhancement starts on a fast time scale until about half of the pellet inventory is expelled, 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 particles in an ELM sequence is accompanied by a loss of energy, causing a reduction of the plasma energy. Full plasma energy recovery after an ELM sequence occurs faster than the slow density decay, allowing transient operation at high densities maintaining full confinement. However, confinement degradation 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 then lead to a neutral gas pressure increase causing confinement degradation. Also, excessive temperature reduction by pellets close to rational surfaces can create conditions likely to catalyze the growth of neoclassical tearing modes (NTMs) at high βN, which may then be triggered by a succeeding pellet.