Fast Compressional Alfven Waves in a Tokamak

Enhancement of cyclotron damping is theoretically evaluated based upon the wave E-field polarization which is modified by the two-ion hybrid resonance. The degree of enhancement and its asymmetry about the plasma center is consistent with experimental observations. The enhancement factor is very large at low plasma densities at which most of the past damping strength measurements have been reported, but approaches unity at high densities. This implies the predominance of pure cyclotron damping in future reactor-like deuterium plasmas in spite of the presence of minority protons. A simple antenna and a pair of limiters are sufficient to couple rf power with an overall power deposition efficiency of about 75 %. There is no evidence that parasitic loading accounts for a significant amount of the power. 1. IntroductiQn One promising method of additional heating for bringing a tokamak plasma to thermonuclear temperatures is irradiation of the 'plasma by electromagnetic fields in the ion cyclotron range .of frequencies ("ICRF" heating)*. The impressed radio frequency (rf) field near the ion cyclotron frequency generates in the plasma fast compressional Alfven waves whose energy is absorbed by the ions (and possibly by electrons) through various field-particle interactions. One possible mechanism for dissipating wave energy is ion cyclotron damping. In one-ion species plasmas immersed in the inhomogeneous B-field of a tokamak, fundamental and harmonic cyclotron resonances occur at different major radius locations for a given wave frequency. Damping of the fast wave is governed by the small left-handed component (E+) of the wave E-field in the resonance layer. In the fundamental resonance layer, where the wave frequency equals the fundamental cyclotron frequency of the constituent ions, ions experience acceleration proportional to the magnitude of the E+. However, their coherent motion tends to shield the E+-field nearly completely from the resonance layer. The fundamental cyclotron damping is consequently weak in one-ion species plasmas. In the second harmonic resonance layer ion acceleration is proportional to the ion Larmor radius (e*) and to the perpendicular gradient of the E+. The gradient can be approximated by kjE+Iin one-ion species plasmas, and the wave damping strength is proportional to 1/2e(kJE)Z. In' recent tokamak experiments using deuterium as the working .gas, small concentrations of protons were always found as impurities. Such a mixture represents a more complicated situation: the proton fundamental cyclotron and the deuteron second harmonic cyclotron resonances occur at the same location, and, furthermore, a new resonance between these ion …