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Abstract
The problem of coupling to and driving current with the fast wave in the Lower Hybrid Range of Frequencies is addressed theoretically and experimentally. Previous experimental work on current drive with slow and fast waves in the LHRF is reviewed. A numerical algorithm for solving the problem of coupling electromagnetic energy in the LHRF from a phased array of identical rectangular waveguides to a plane-stratified, magnetized cold plasma is implemented, and a 4 x 3 array designed to launch fast waves is modeled. Surface modes of propagation are found to have a significant effect on coupling to fast waves.
The results of a medium power (P$\sb{\rm RF}$ $\le$ 200 kW) fast wave current drive experiment at 800 MHz on the Princeton Large Torus are presented. Two couplers were used: a phased array of six unshielded loop antennas, and a 4 x 3 array of dielectric-loaded waveguides. Plasma loading measurements with the loop antennas were not able to conclusively demonstrate fast wave coupling, but coupling measurements with the waveguide array showed dependence on phasing and $\overline n\sb{e}$ in agreement with theoretical expectations of fast wave loading. With either coupler, rf-induced effects on fast electrons, such as current drive and enhanced ECE, exhibit the same density limit $\overline n\sb{crit}$ $\simeq$ 1.0 $\times$ 10$\sp{13}$ cm$\sp{-3}$ as has been observed in previous 800 MHz slow wave experiments on PLT. The current drive efficiency observed below $\overline n\sb{crit}$ was comparable to the efficiency obtained with the 800 MHz slow wave system, and the formation of a fast ion tail accompanied by evidence of parametric decay at the plasma surface above $\overline n\sb{crit}$ was also similar to the slow wave launch case.
These results are explained by fast-to-slow mode conversion within the plasma, by either a ray-tracing effect or by mode conversion induced by scattering from ambient density fluctuations.
