Optimization of RF driven current for a large tokamak

Results are reported of a study of the current driven by radio-frequency waves at the electron cyclotron resonance frequency for a large tokamak with a plasma approaching ignition conditions. Using a ray tracing code in 3-D tokamak geometry the driven current is optimised with respect to injection geometry and plasma parameters. It is found that by suitable choice of the injected ksub(parallel to) spectrum a rather broad current profile can be achieved even with monochromatic radiation. For the optimum case of injection of X mode from the high field side of the resonance the variation of current with electron temperature is faster than the expected linear dependence. At high electron temperatures the 'effective' value of the normalised wave phase velocity is higher than previously supposed and the size of the current suggests this might be a practical scheme for a large high temperature device. It is estimated the current may be reduced by trapped electrons by as much as 60% but this estimate may be regarded as an upper limit. (author)