Coupling to the electron Bernstein wave using a phased array of waveguides in MST reversed field pinch

Abstract

Coupling to the electron Bernstein wave (EBW) via a phased array of waveguides is experimentally investigated in the Madison symmetric torus reversed field pinch (RFP). EBWs promise to provide localized heating and current drive in overdense plasmas such as those in the RFP, provided a technique can be developed to efficiently couple power from an externally launched electromagnetic wave to the electrostatic EBW. The choice of antenna structure and launched wave polarization are important factors in coupling waves with frequencies in the electron cyclotron range to EBW, especially on the RFP where the mode conversion to the EBW can take place in the near-field of the antenna. In this paper, a recently developed theory of coupling to EBW from a phased array of waveguides is tested against experiment. The theory predicts that coupling efficiency will vary with launch angle and that coupling depends sensitively upon the edge density profile. Amplitudes and phases of reflected power in each of the waveguides are measured experimentally and compared with predictions based upon density profiles measured by a Langmuir probe in the edge of the plasma. The parametric dependence of reflection has been studied for different polarizations, different launch angles, time varying density profiles and several frequencies of the launched wave. An asymmetry in reflection versus the launch angle predicted by theory was found experimentally for the X-mode, while the O-mode was symmetric. The dependence on density scale length predicted by theory was confirmed in the experiment. The phase of the reflected signal is shown to contain reflectometry-based information about the edge density profile. For appropriate phasing, measurements show that the total power reflection coefficient can be lower than 15%.