Electron Landau damping of lower hybrid waves

Abstract

The role of electron Landau damping (ELD) in the lower hybrid resonance region is reviewed. First, it is shown that good matching of waves with parallel index n∥ large enough for efficient ELD from a finite-length antenna is possible only if the antenna is facing a tenuous plasma such that . This situation appears to prevail in recent experiments using phased waveguide arrays as launching structures. Moreover, it is shown that the relevant portion of the spectrum will be depleted by linear wave transformation before suffering appreciable ELD, unless the frequency is at least 1.5 times higher than the resonant frequency in the core of the plasma. Thus, in the case of a phased waveguide array designed for efficient ion heating in the resonance region, hardly more than a few per cent of the power will be absorbed by the electrons through ELD. On the other hand, provided that the antenna design is appropriate, ELD itself can be regarded as a simple and efficient method of heating large plasmas up to thermonuclear temperatures.