Sensitivity of transport and stability to the current profile in steady-state scenario plasmas in DIII-D

Abstract

Recent experiments on DIII-D have provided the first systematic data on the impact of the current profile on the transport and stability properties of high-performance, steady-state scenario plasmas. In a future tokamak, to achieve 100% noninductive conditions and produce net power, the current profile J must be sustained by a large fraction of bootstrap current JBS, which is nonlinearly coupled with the kinetic profiles. Systematic scans of qmin and q95 were performed to determine empirically the best alignment of the noninductive currents with J and the variation of the transport properties with q. Transport analysis indicates that χe and χi are sensitive to the details of J in a way that makes the pressure profile peaking and JBS scale nonlinearly with both q and β in the experiment. Drift wave stability analysis yields linear growth rates that do not reproduce experimental trends in χ with qmin and q95. At high beta, necessary to maximize fBS, the plasma duration is often limited by n=1 tearing modes, whose stability also depends on the J profile. Broadly deposited electron cyclotron (EC) current at mid-radius was found to supply part of the required noninductive current and to positively affect the tearing stability. The modes appear when JEC is turned off for stable cases and always appear when the EC deposition is shifted outwards. The variation in the EC scan results is consistent with PEST3 calculations, showing that the tearing stability becomes extremely sensitive to small perturbations of the equilibrium in wall-stabilized plasmas run close to the ideal MHD limit. These modeling results are being used to design new experiments with higher ideal and tearing limits. A new capability for off-axis neutral beam injection system will be used to explore higher qmin scenarios and different current alignments.