H-mode access by pellet fuelling in the MAST tokamak

Abstract

Access into H-mode is studied in the MAST tokamak when plasma is fuelled by cryogenic pellets. It is shown that pellet fuelling from the high-field side allows access to H-mode in plasmas heated by neutral beams. Simple and two-stage L-H transitions are identified. The results of comparison of the L-H transitions with pellet injection with transitions where plasmas are fuelled solely by gas puffing depend on the gas puff geometry: fuelling by high-field side gas leads to an L-H transition at the density comparable to the transition with pellet injection. In contrast low-field gas can completely prevent the L-H transition.