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

摘要

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_B 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,q95≈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.