PELLET FUELING OF ELMY H-MODE DISCHARGES ON ASDEX UPGRADE

Efforts have been made to develop scenarios allowing more flexible pla sma density control by injecting cryogenic solid hydrogen pellets. Whi le the injection of pellets during ohmic discharges was found to be mo st efficient and also improves the plasma performance, an increase in the auxiliary heating power causes a deterioration of the pellet fuell ing efficiency. A further strong reduction of the pellet fuelling effi ciency by an additional process was observed for the more reactor rele vant conditions of shallow particle deposition during H mode phases. W ith injection during type I ELMy H mode phases, each pellet was found to trigger the release of an ELM and therefore cause particle losses m ainly from the edge region. In the type I ELMy H mode, only sufficient pellet penetration allowed noticeable, persistent particle deposition in the plasma by the pellets. Applying adequate pellet injection cond itions and favourable scenarios using combined pellet/gas puff refuell ing, significant density ramp-up to densities exceeding the empirical Greenwald limit by up to a factor of two was achieved even for strongl y heated H mode plasmas.