Excitation and emission of electron cyclotron waves in spherical tori

Abstract

The conventional ordinary and extraordinary modes in the electron cyclotron range of frequencies are not suitable for heating of and/or driving currents in spherical tori (ST) plasmas. However, electron Bernstein waves (EBWs) offer an attractive possibility for heating and current drive in this range of frequencies. In this paper, we summarize our theoretical and numerical results that describe the excitation of EBWs in ST plasmas when the extraordinary mode or the ordinary mode are coupled into the plasma from an external source. In our discussion on the conversion of the ordinary mode to EBWs (via the slow extraordinary mode) we illustrate very important physics, relevant to this conversion process, that has been ignored in previous studies. This particular physics has to do with the conversion of the slow extraordinary mode to the fast extraordinary mode that can then propagate out of the plasma and thus reduce the mode conversion to EBWs. This reduction in the mode conversion can occur even when the wave numbers are such that the ordinary mode cutoff and the slow extraordinary mode cutoff are coincident in space. Furthermore, we also consider the emission of EBWs from a thermal plasma. This emission mode converts to extraordinary and ordinary modes in the vicinity of the upper hybrid resonance. We describe the general relationship between the conversion coefficients when exciting EBWs using either the extraordinary mode or the ordinary mode, and the emission coefficients when thermally emitted EBWs convert to the extraordinary and ordinary modes.