The Physics and Technology of Neutral Beam Current Drive: Ready for the Steady-State Tokamak Power Plant?

Enabling steady-state operation of a tokamak fusion reactor, i.e. with a pulse length that is not constrained by physics but only by maintenance needs, requires all plasma current to be driven non-inductively. Neutral beam injection (NBI) promises very high current drive efficiency. Recent quantitative analysis of the composition of the plasma current in highly non-inductive discharges in ASDEX Upgrade indicated that the various non-inductive contributions are predicted with good accuracy by the established models. This result reinforces confidence that the current drive predictions for DEMO and power plant scenarios that are based on the same physics models yield realistic results. On the other hand, designing an NBI beamline for a steady-state tokamak reactor presents technological challenges that go far beyond those of the ITER beamline. Energetic efficiency of the entire current drive system, which is no major concern for an experimental reactor, is of paramount importance for the economic viability of a power plant. Presently, the efficiency of NNBI beamlines is mainly limited by neutralisation efficiency, and new, experimentally not yet proven, neutralizer concepts are needed. The neutraliser is also the key component that largely determines the design parameters of most other beamline components. Conceptual system studies that explore a wide variety of technological options are therefore urgently needed, and IPP’s NBI group is getting actively involved within EUROfusion’s work package heating and current drive. The beam transport modelling tools developed to this end are also applied profitably to the study of poorly understood beam transport phenomena in the ASDEX Upgrade beamlines and the planning of future upgrades of the system that could increase NBI’s operational space.