PROGRESS TOWARD SUSTAINED HIGH-PERFORMANCE ADVANCED TOKAMAK DISCHARGES IN DIII-D

Key elements of a sustained advanced tokamak discharge in DIII-D are a large fraction of the total current from bootstrap current (fBS) and parameters that optimize the capability to use electron cyclotron current drive (ECCD) at ρ ∼ 0.5 to maintain the desired current profile [1-4]. Increased fBS results from increasing both the normalized beta (βN) and the minimum value of the safety factor (qmin). Off-axis ECCD is, for the available gyrotron power, optimized at high βN, high electron temperature (Te) and low electron density (ne). As previously reported [2-4], these required elements have been separately demonstrated: density control at high βN with ne (le) 5 x 1019 m-3 using divertor-region pumping, stability at high β, and off-axis ECCD at the theoretically predicted efficiency. This report summarizes recent work on optimizing and integrating these results through evaluation of the dependence of the beta limit on qmin and q95, exploration of discharges with relatively high qmin, testing of feedback control of Te for control of the q profile evolution, and modification of the current profile time evolution when ECCD is applied