H-mode power threshold studies in mixed ion species plasmas at ASDEX Upgrade

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.