The influence of injector design on neutral beam optimisation for DEMO

The requirements for neutral beam injection (NBI) on DEMO are assessed and the consequences for the design of the injectors discussed. Optimization of current drive requires NBI within a 2 m × 2 m envelope in the plasma poloidal cross section at large tangency radii. This is compatible with beamlines of 20 m length and moderate high voltage stand-off distances between injectors. However, q-profile control will necessitate at least three beamlines of different injector types and may not be compatible with shinethrough power limits. Material irradiation studies show that there is no significant design issue for radial distances greater than 3 m from the tokamak wall.