Session GP1 - Poster Session IV.
POSTER session, Tuesday afternoon, October 30
Exhibit Hall B,
Mode-converted (MC) EBWs offer an attractive path for electron temperature measurement, heating, and current drive in overdense plasmas (ømega_pe\ggØmega_ce). A quad-ridged antenna was installed in CDX-U with a movable limiter, which shortens electron density scale length at the MC layer and hence optimizes MC efficiency. Electrostatic EBWs are expected to MC to X-mode electromagnetic waves. Measurements were made with both the X- and O-mode aligned antennas, and the X/O ratio was calculated. An X/O ratio >2 was observed with the antenna near the MC layer, in contrast to a ratio of 1.2 measured previously with an antenna outside the vessel. A ratio of \sim1 was seen with the antenna far from the MC layer, possibly due to reflections between the plasma and vessel wall causing polarization scrambling. Reduction of the X/O ratio was observed when the limiter was extended, likely due to polarization mixing caused by reflection or refraction at the limiter surface in front of the antenna.