Content area
Abstract
Lower-hybrid wave injection into tokamaks has been proposed for use in both bulk plasma heating and current drive schemes. However experimental investigations show turbulent regions near the outside of tokamak plasmas which will scatter an initially collimated, monochromatic wave in both angle and frequency. It is important to achieve a theoretical understanding of these processes to explain present results and predict the success of future experiments. In addition, the problem is of interest because of its relevance to other fields and basic physics.
In one regime the problem reduces to the solution of the equation
(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)
where F is the wave energy density and z and (OMEGA) are a dimensionless distance and frequency. We show how this equation can be solved in general and produce approximate solutions for both heavy and light angular scattering. The energy transmission coefficient drops off only inversely with the thickness of the layer, z(,0), and our approximate solution is in good agreement with numerical results. The root-mean-square frequency width of the transmitted wave will be proportional to(' )SQRT.(z(,0) for thin layers and to z(,0) for thick layers, while that of the back-reflected wave is proportional to(' )SQRT.(z(,0). These results are supported by random walk arguments.
These results are related to typical tokamak plasma parameters showing that for most experiments we expect the majority of the incident energy to reach the plasma center, unless the tokamak plasma is unusually small and dense, so long as the incident wave is not highly peaked in N(,(PARLL)). However the initial angular collimation may well be lost, reducing the probability of well defined resonance cones.
Although the frequency broadening of the lower-hybrid waves will not significantly affect their propagation it can be a few to many times the typical frequency of the turbulent density fluctuations--sufficient to account for measured spectral broadening in the Alcator tokamak.
