Physics of Off-Axis Electron Cyclotron Current Drive

Electron cyclotron current drive is a key option for driving current off-axis in a tokamak, as needed for example for current profile control or for suppression of neoclassical tearing modes. Experiments in DIII-D at low beta have shown that the partial cancellation of the Fisch-Boozer co-current by the Ohkawa counter-current can cause strong deterioration of the current drive efficiency at larger minor radius. However, more recent experiments at higher power have shown that the loss in efficiency can be mostly recovered if the target plasma has higher electron beta, {beta}{sub e}. The improvement in efficiency with beta can be understood from a theoretical viewpoint by applying the Fokker-Planck code CQL3D, which shows excellent agreement with experiment over a wide range of parameters, thereby validating the code as an effective means of predicting the ECCD.