Calculation of the non-inductive driven current profile due to helicon waves in DIII-D

Helicon current drive is an attractive solution for driving off-axis current to sustain steady state tokamak scenarios. Dedicated DIII-D experiments have been conducted with a MW-level helicon system and demonstrated helicon current drive for the first time in any device. Determination of the current drive efficiency requires isolating the non-inductive helicon current from all other current sources. We calculate the difference in the non-inductive current in plasmas with 370 kW of coupled helicon power versus those 1) without helicon and 2) with the helicon replaced by a comparable amount of ECH. These comparisons are essential to distinguish changes to the current profile that result indirectly from heating versus direct current drive. The Ohmic current profile is calculated from MSE-constrained EFITs under the assumption of neoclassical current diffusion. Other non-inductive current sources are calculated by models in TRANSP. Subtraction of the modeled current sources from the parallel current profile yields the remaining non-inductive current profile that could be due to the helicon waves. To mitigate systematic uncertainties, the same calculation is also performed in the shots without helicon and then subtracted from the result in the helicon shots. The inferred helicon current profile is peaked near the axis, as predicted by the GENRAY ray tracing code and consistent with the observed power deposition in this L mode experiment.