EBW Experiments in the Madison Symmetric Torus

A phased pair of waveguides is being used to experimentally investigate coupling to the electron Bernstein wave (EBW) in the Madison Symmetric Torus (MST) reversed field pinch (RFP). Coupling experiments have been performed from 3.3 to 3.9 GHz at -1 Watt. These show that the coupling efficiency depends sensitively upon several factors, including the edge ne profile, the polarization (X or O mode), and N(perpendicular) (controlled by phase between the waveguides). The amplitude and phase of reflected power (in each arm of the antenna) are compared to simulations and show general agreement. The predicted (a)symmetry in reflection for (X)O-mode launch versus interguide phasing is observed. A boron nitride cover has been installed on the end of the antenna, acting as a limiter and preventing plasma from contacting the waveguide vacuum windows; this has a measurable effect on coupling both into vacuum and into the target plasma. A heating experiment at 3.6 GHz and 150 kW has been installed, and measures have been taken to prevent breakdown at the window. Tube performance has been improved over past tests at this power level, and the capability of a 10 millisecond pulse has been achieved.