Dependence of EC toroidal injection angle on effective EC assisted startup

The effect of electron cyclotron (EC) wave toroidal injected angle on the quality of pre-plasma for the EC assisted startup is studied experimentally at DIII-D in support of ITER. The Deuterium alpha filterscope signal shows for the first time that the post-reflection, second pass absorption efficiency depends heavily on the toroidal injection angle and results in the change in pre-plasma density and temperature that is necessary for a successful startup. A 110 GHz, extraordinary-mode polarized EC wave is injected into the DIII-D vacuum vessel aimed for the second harmonic resonant absorption at a poloidally oblique angle from the upper outboard launcher. The toroidal injection angle is systematically scanned from radial to 20 degrees off-radial angle. It has been reported previously that the EC beam is absorbed at least twice at the resonance radius due to the reflection near the inboard midplane. The DYON and RT-4 simulations indicate that the power absorption efficiency changes with the toroidal injection angle and explains the experimental observation qualitatively. This result provides valuable data to validate models and optimize ITER's startup strategy, where the EC assist is necessary and EC is designed to be injected at a toroidally oblique angle.