H-mode power threshold studies in mixed ion species plasmas at ASDEX Upgrade
U. Plank, T. Pütterich, C. Angioni, M. Cavedon, G. D. Conway, R. Fischer, T. Happel, A. Kappatou, R. M. McDermott, P. A. Schneider, G. Tardini, M. Weiland, ASDEX Upgrade Team
Abstract
Understanding the dependence of the H-mode power threshold, PLH on the main ion composition is crucial for H-mode operation in ITER. In this work, H-mode power threshold studies conducted at ASDEX Upgrade are presented. These were performed in hydrogen plasmas with helium doping and in mixed hydrogen-deuterium plasmas. PLH stays constant on the level of pure hydrogen in hydrogen plasmas with helium doping up to cHe = 20%. Power balance calculations with the transport code ASTRA show that the edge ion heat flux at the L-H transition is independent of helium concentration and applied auxiliary heating, supporting that a critical edge ion heat flux is needed to enter H-mode. A non-linear dependence of PLH on the relative hydrogen content is found. PLH and the edge ion heat flux are constant at the level of pure deuterium for between 0 and 0.5 and constant on the level of pure hydrogen for between 0.8 and 1.