Feasibility Studies of Off-Axis Neutral Beam Current Drive in DIII-D

The objective of off-axis neutral beam (NB) current drive (CD) is to supplement the off-axis electron cyclotron current drive for development of steady state, advanced tokamak scenarios. A modification being considered is to tilt the present neutral beam lines (BL) by raising the source end of the BL by $\approx $~1.5~m. The driven current is calculated using the TRANSP and ONETWO/Nubeam Monte-Carlo codes taking into account finite orbit effects. When the beam is injected in the same direction as the toroidal field, a wide but localized off-axis CD ($\approx $~40~kA/MW at $\rho $~= 0.5 with FWHM of 0.45) is calculated. The normalized CD efficiency ($\zeta $~= 0.22) is comparable or somewhat better than electron cyclotron current drive. Sensitivities to fast ion diffusion and the use of the off-axis CD for scenarios with high steady-state performance in DIII-D will be discussed.