Controlled high density operation on ASDEX upgrade by pellet refueling from the magnetic high-field side

A novel, highly efficient tokamak plasma refueling scenario with cryogenic deuterium (D) pellets injected from the magnetic high-field side was applied for the first time to demonstrate feedback-controlled operation beyond the empirical Greenwald density limit. In this scheme, a centrifuge injector was used to inject pellets via a funnel and a guiding tube at velocities of 240 m/s and repetition rates up to 30 Hz. The pellet particle fluxes achieved were capable to achieve stable operation at about 1.2 times the Greenwald limit with a supporting gas puff of about the same amount. In contrast to low-field side pellet injection, no significant pellet induced confinement degradation and density profile peaking were observed. The H-mode was maintained during high density operation with an energy confinement time normalized to the ELMy H-mode 0.85 * ITERH93P scaling of above 0.82. The density gradient at the plasma edge already achieved during the pre-pellet H-mode phase shows no significant increase during the pellet induced increase of plasma inventory. The particle flux through the edge is about 5 times higher during the pellet phase than before. This seems to indicate a strong increase of the particle diffusivity at the plasma edge enforced by the pellet particle source. (orig.)19 refs.SIGLEAvailable from TIB Hannover: RA 71(1/314) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman