Measurements of ECH absorption on ATF using a polarization-controlled beam launcher

Electron cyclotron heating (ECH) is used on the Advanced Toroidal Facility (ATF) experiment at ORNL for plasma formation and heating. A 53.2 GHz gyrotron generates 200 kW cw which is transported to ATF in 6.35 cm evacuated waveguide. Power is launched into ATF using a recently completed polarization controlled beam launcher which can launch a linear polarized beam with a /minus/20 dB diameter of 12 cm at the plasma center. The launcher consists of a Vlasov mode converting antenna, a Teflon-copper laminate polarization rotating grating, and a spherical focusing mirror. The plane of polarization can be remotely adjusted by rotating the grating with a motorized vacuum feedthrough. First pass plasma absorption is monitored in two planes of polarization using a dual-polarized detector looking through a dome shaped scattering cut-off screen. During plasma operation, the detected signals indicate that absorption under ideal conditions is nearly complete. With low density or a shifted resonance zone, absorption is small and there are cases where there is mode coupling to the perpendicular polarization. This is presumably due to shear in the ATF magnetic field. 2 refs., 3 figs