Commissioning and Characterizing Top Launch ECCD Systems on the DIII-D Tokamak

Following the successful demonstration of doubling off-axis ECCD efficiency via top launch ECCD compared to low field side (LFS) launch on DIII-D in 2019 using a top launcher (TL) installed at toroidal 300 deg, a second TL system is installed at toroidal 90 deg. Commissioning and characterizing the new top launch system and the re-installed improved 300 deg top launch system has been carried out. Both systems are equipped with a waveguide switch enabling flexible switching between TL and LFS launch, so dedicated gyrotrons are not needed. The aiming of both TLs is fixed at the same design value which is predicted to be optimal for off-axis ECCD in a typical DIII-D AT plasma in reversed BT at 1.65 T. Injection at full power of the connected gyrotron with modulated or constant waveform for >4 second has been demonstrated for both TL systems. Experiments were performed to comprehensively characterize the polarization and aiming of the two TLs. The toroidal magnetic field and the inclination and ellipticity of the injected EC waves were systematically scanned in ohmic and H-mode plasmas. Comparisons were made between: (1) X-mode and O-mode launch, (2) TL and LFS launch, and (3) slow and fast modulation.