High efficiency off-axis current drive by high frequency fast waves

Modeling work shows that current drive can be done off-axis with high efficiency, as required for FNSF and DEMO, by using very high harmonic fast waves ("helicons" or "whistlers"). The modeling indicates that plasmas with high electron beta are needed in order for the current drive to take place off-axis, making DIII-D a highly suitable test vehicle for this process. The calculations show that the driven current is not very sensitive to the launched value of n_∥, a result that can be understood from examination of the evolution of n_∥ as the waves propagate in the plasma. Because of this insensitivity, relatively large values (∼3) of n_∥ can be launched, thereby avoiding some of the problems with mode conversion in the boundary found in some previous experiments. Use of a traveling wave antenna provides a very narrow n_∥ spectrum, which also helps avoid mode conversion.