Top Launch ECCD Experiments in High-qmin AT Scenario Plasmas on the DIII-D Tokamak

Studies of many tokamak reactors have shown that non-inductive off-axis current drive is a requirement for a steady-state device in the AT regime. Following the successful demonstration of doubling off-axis ECCD via top launch ECCD compared to low field side (LFS) launch in L-mode and low-b H-mode plasmas on DIII-D using a top launcher installed at toroidal 300 deg, a second top launch system was installed at toroidal 90 deg. Current drive experiments in moderate betan (β_N~3) high qmin (>2) AT scenario plasmas using two top launch systems are conducted. Top launch ECCD deposition are measured by ECE system utilizing modulated ECH while top launch ECCD current drive are measured using MSE system utilizing constant EC waveform. Measurements are compared with prediction using ray tracing code TORAY and quasi-linear Fokker-Planck code CQL3D. Comparisons with current drive aimed at the same location via LFS launch in these plasmas are also been made. Both top launchers on DIII-D have fixed aiming and are identical. Numerical studies are carried out to investigate how to improve the absorption, current drive efficiency and narrow top launch ECCD deposition profile in these plasmas.