Prospects for Off-Axis Neutral Beam Current Drive in the DIII-D Tokamak

AbstractOff-axis neutral beam (NB) current drive (CD) (NBCD) has the potential to supply substantial off-axis CD for the demonstration steady-state, Advanced Tokamak scenarios. A modification of the two existing DIII-D NB beamlines is proposed to allow off-axis CD with NB injection (NBI) vertically steered to drive current as far off axis as half the plasma radius. The profile and magnitude of the driven current is calculated using the NUBEAM Monte Carlo module in the TRANSP and ONETWO transport codes. When the beam is injected in the same direction as the toroidal field, off-axis CD of [approximately equal to]45 kA/MW is calculated at normalized radius (square root of the toroidal flux), ρ = 0.5 with full-width at half-maximum of 0.45 in ρ. The dimensionless CD efficiency is comparable or somewhat better than that for electron cyclotron CD (ECCD) at the same location and plasma parameters. The efficiency stays nearly constant in going from on-axis to off-axis CD. The localization and magnitude of the off...