Effect of plasma rotation on the beam-driven current

In a rotating plasma, with co-neutral beam injection (NBI), the Doppler shift of the NBI particles, as viewed in the frame of the plasma, can result in a significant reduction in the beam-driven (Ohkawa) current when the rotation is strong (i.e. with rotational Mach numbers, M ⩾ 0.5). The correction applies to the toroidal fast ion current calculated for a non-rotating plasma and is independent of the normal Zeff and electron trapping terms. A simple analytical model is presented to estimate the magnitude of the effect for plasmas with arbitrary toroidal rotation and the conditions where this is important have been identified. This model has been compared with the results from existing Monte Carlo neutral beam codes and found to reproduce their results. The important parameters in this problem are the ratio, , of the NBI injection particle velocity (in the laboratory frame) to the critical velocity of the plasma, and the ratio ρϕ = vϕ/vcrit which is related to the rotational Mach number. A phase plot in dimensionless (ρLab, ρϕ) space is presented which enables the fast ion current drive efficiencies to be compared for different tokamaks. For strongly rotating plasmas, the degradation in fast ion current efficiency is significant for ρLab ⩽ 1. However, when ρLab is larger than this, the degradation in fast ion current drive is less severe. Approaches to improve the fast ion current drive efficiency are briefly discussed.