Optimization of the safety factor profile for high noninductive current fraction discharges in DIII-D

In order to assess the optimum q profile for discharges in DIII-D with 100% of the current driven noninductively ( f NI = 1), the self-consistent response of the plasma profiles to changes in the q profile was studied in high f NI , high β N discharges through a scan of q min and q 95 at two values of β N . As expected, both the bootstrap current fraction, f BS , and f NI increased with q 95 . The temperature and density profiles were found to broaden as either q min or β N is increased. A consequence is that f BS does not continue to increase at the highest values of q min . A scaling function that depends on q min , q 95 , and the peaking factor for the thermal pressure was found to represent well the f BS /β N inferred from the experimental profiles. The changes in the shapes of the density and temperature profiles as β N is increased modify the bootstrap current density ( J BS ) profile from peaked close to the axis to relatively flat in the region between the axis and the H-mode pedestal. Therefore, significant externally driven current density in the region inside the H-mode pedestal is required in addition to J BS in order to match the profiles of the noninductive current density ( J NI ) to the desired total current density ( J ). In this experiment, the additional current density was provided mostly by neutral beam current drive with the neutral-beam-driven current fraction 40–90% of f BS . The profiles of J NI and J were most similar at q min ≈ 1.35–1.65, q 95 ≈ 6.8, where f BS is also maximum, establishing this q profile as the optimal choice for f NI = 1 operation in DIII-D with the existing set of external current drive sources.