Mode conversion coupling to electron Bernstein waves*

In NSTX, and generally in spherical tori, the emission of electron Bernstein waves (EBW) from the interior of the plasma can be observed in the vacuum region after mode conversion to the X mode and/or the O mode. The same mode conversion process comes into play when exciting EBWs through externally launched power. We consider the mode conversion process that couples the O mode to EBWs via the X mode. It is shown that density inhomogeneity can change the mode conversion efficiency significantly from that obtained analytically in [1]. Results obtained from a comprehensive model [2] are found to differ from the analytical result as the gradient scale length decreases. A detailed analysis comparing the analytical and computational results will be presented. \newline [1] E.\ Mj{\o}lhus, {\it J.\ Plasma Phys} {\bf 31}, 7 (1984). \newline [2] A.\ K.\ Ram and S.\ D.\ Schultz, Phys.\ Plasmas {\bf 7}, 4084 (2000).