The back transition and hysteresis effects in DIII-D

The 'back' transition from H-mode to L-mode has been studied on DIII-D as a part of our investigation of the L-H transition power threshold scaling. Based on a density-dependent scaling for the H-mode power threshold, ITER will require substantial hysteresis in this parameter to remain in H-mode as ne rises. Defining the hysteresis in terms of the ratio of sustaining to threshold power, PHL/PLH may need to be as small as 50% for ITER. Operation of DIII-D at injection powers slightly above the H-mode threshold results in an oscillatory behaviour with multiple forward-backward transitions in the course of a discharge. These discharges represent a unique system for studying various control parameters that may influence the H ↔ L state transition. Careful analysis of the power flow through the edge gives values for the sustaining power well below the corresponding threshold powers (PHL/PLH=35%-70%), indicating that substantial hysteresis can be achieved in this parameter. Studies of other control parameter candidates such as the edge temperature during the back transitions are less clear: the amount of hysteresis seen in these parameters, if any, is primarily dependent on the nature (ELMing, ELM-free) of the parent H-state. (author)