High Field Side Lower Hybrid Current Drive for Efficient, Off-axis Current Drive in DIII-D

For DIII-D, efficient off axis current at r/a~ 0.6-0.8 with peak current density ~0.4 MA/m$^2$ is sought for advanced tokamak plasmas. With high field side launch, lower hybrid waves can drive current peaked off-axis, r/a~0.6-0.8, and driven current up to 0.39 MA/MW couple with single pass absorption. Utilizing GENRAY+CQL3D, wave penetration was found to be strongly influenced by local q value. In AT discharges, the wave penetration is dominated by the poloidal upshift resulting in good wave penetration. Practically, a small inner gap and a metallic launcher at the plasma edge were thought to be potential issues. Experiments found that small inner gap had a negligible impact on plasma performance. Using a mock-up coupler, inner wall limited discharges with up to 10 MW injected power were investigated and the plasma mock-up coupler interaction was restricted to the carbon protection tiles. No significant Mo I source was observed in any discharge and the core Mo contamination was subtle to non-existent. These experimental and simulation results indicate that HFS LHCD is compatible DIII-D discharges and has potential to provide efficient off-axis current drive in DIII-D.