Excitation and emission of electron cyclotron waves in spherical tori

Abstract

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 offer an attractive possibility for heating and current drive in this range of frequencies. In this paper, we summarize our theoretical and numerical results which describe the excitation of electron Bernstein waves 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 electron Bernstein waves (via the slow extraordinary mode) we illustrate very important physics, relevant to this conversion process, that has been ignored in previous studies. The 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 electron Bernstein waves. 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 electron Bernstein waves 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 electron Bernstein waves using either the extraordinary mode or the ordinary mode, and the emission coefficients when thermally emitted electron Bernstein waves convert to the extraordinary and ordinary modes.

Recent progress on spherical torus research

“…352 The EBW mode conversion analysis predicts that the EBW mode conversion efficiency is reversible (i.e., the mode conversion efficiency is the same for EBW to X/O modes and X/O modes to EBW). 354 The first B-X EBW emission (EBWE) was measured on CDX-U by comparing the EBWE T e with Thomson scattering T e measurements, where near 100% conversion efficiency occurred with $50 eV electron temperature ST plasmas. 355 The first series of EBWE experiment on NSTX showed an efficient EBW coupling ($0.8 6 0.2) in L-mode plasmas, in good agreement with the numerical EBW modeling prediction of 0.65.…”

On electron Bernstein waves in spherical tori

“…However, electron Bernstein waves (EBWs) offer an attractive alternative in the EC frequency range as they have no density cutoffs, and damp strongly on electrons at the fundamental or any harmonic of the Doppler-shifted EC resonance [3]. Since EBWs cannot propagate in vacuum (like the X and O modes) they are excited, indirectly, by mode conversion of an externally launched O mode or X mode [3][4][5][6]. Successful experiments on mode conversion excitation of EBWs and their subsequent interaction with the plasma have been summarized by Laqua [7].…”

Efficient coupling of thermal electron Bernstein waves to the ordinary electromagnetic mode on the National Spherical Torus Experiment

“…Consequently, tools that are well established in low-β, high aspect ratio tokamaks and stellarators, such as electron temperature profile (T e (R)) measurements based on electron cyclotron emission (ECE), electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) cannot be employed in STs or other high-β magnetically-confined plasma devices. In recent years there has been a growing interest in utilizing electron Bernstein waves (EBWs) for these techniques, since they readily propagate in ST plasmas and are strongly absorbed at the electron cyclotron resonances [2][3][4][5][6][7]. In addition, recent Fokker-Planck numerical modeling predicts that electron Bernstein wave current drive (EBWCD )may be very efficient in high-β NSTX plasmas, even when current is driven far from the magnetic axis [8].…”