Psfc/ja-01-36 Electron Bernstein Waves in Spherical Tokamaks

In the electron cyclotron range of frequencies, the high-β NSTX plasmas are overdense to the traditional extraordinary X mode and/or the ordinary O mode. For low harmonics of the electron cyclotron frequency, the X and O modes are cutoff at the edge and for high harmonics the plasma is essentially optically thin to these modes. Thus, heating of such plasmas by electron cyclotron waves or diagnosing of such plasmas by electron cyclotron wave emission becomes problematic. However, such plasmas are optically thick to electron Bernstein waves (EBW) which are strongly absorbed or emitted by thermal electrons in the Doppler shifted vicinity of the fundamental or harmonics of the electron cyclotron resonance [1]. In order to propagate out into the vacuum region, EBWs have to mode convert to the X and O modes at the upper hybrid resonance. In this paper we discuss the approximate kinetic model that has been developed to study this mode conversion process. From this model we show that the emission coefficients for the X and O modes from EBWs are directly related to the excitation coefficients of EBWs from X and O modes. INTRODUCTION We have previously shown that the high-β NSTX-type plasmas can be heated by waves in the electron cyclotron range of frequencies by mode converting the X mode or the O mode to electron Bernstein waves (EBW) at the upper hybrid resonance (UHR) [1]. From ray tracing analysis, we have also shown that EBWs are locally and strongly absorbed at the Doppler shifted electron cyclotron resonance or its harmonics. For spherical tokamak plasmas, the location of the EBW energy deposition is significantly different for excitations on or away from the equatorial plane [1,2]. The strong and localized absorption implies that thermal emission of EBWs can occur for frequencies corresponding to the local electron cyclotron frequency. This emission then converts at the UHR to the X and O modes which are then observed in the vacuum region. The excitation and emission of EBWs has been studied on the Wendelstein 7-AS [3]. More recently, experiments on CDX-U and NSTX [4] and MST [5] have observed emission of EBWs via the mode conversion process and studied its dependence on the edge properties of the plasma. In this paper we formulate an approximate kinetic description of the mode conversion process that includes the propagating EBWs. From numerical solutions of this model we find that, for the same plasma parameters, the fraction of the EBW power that is mode converted to the X mode (emission coefficient) is the same as the fraction of the X mode power that is converted to the EBWs (excitation coefficient) when the X mode is launched into the plasma. A fraction of the emitted EBW power also converts to the O mode. This fraction is the same as the fraction of the O mode power that is converted to the EBWs if the O mode was to be launched from the outside. Thus, the X and O