Application of High Harmonic Fast Waves to Off-Axis Current Drive in DIII-D

High harmonic fast waves, also called ``whistlers'' or ``helicons,'' may be an effective means of driving current off-axis in high performance discharges in \mbox{DIII-D}. Modeling using the GENRAY ray tracing code APP shows that fast waves launched with frequency 500~MHz tend to spiral around the magnetic axis. If the electron beta is above 1.7\%, the waves are damped around $\rho=0.5$ for a broad range of conditions. The fast wave current drive in the test discharge is 2 to 4 times larger per MW than that from the electron cyclotron heating or neutral beam injection systems on DIII-D. Interestingly, the current drive location and magnitude are nearly independent of the launched $n_{||}$ over the range 2 to 4. Use of a moderately large value, $n_{||}=3$, reduces the possibility of mode conversion to the slow wave. A traveling wave antenna is expected to be effective at launching the wave with a narrow spectrum of $n_{||}$, which also helps avoid mode conversion. A test of the physics of high harmonic fast wave current drive is planned for DIII-D.