A conceptual system design study for an NBI beamline for the European DEMO

Neutral Beam Injection (NBI) as a robust, established heating and current drive (CD) method is considered for the European DEMO. Like on ITER, an NBI system for DEMO will have to use negative ions as the neutralisation efficiency of positive ions vanishes at the required >= 800 keV beam energy. The required specifications of an NBI on DEMO go significantly beyond those on ITER, particularly when NBI is chosen to provide bulk current drive for a steady-state tokamak DEMO. The NBI beamline's power efficiency, on ITER only about 27%, needs to be significantly increased to arrive at a tolerable recirculating power fraction. Envisaged solutions are the addition of energy recovery for the residual ions to a gas neutraliser or the replacement of the gas neutraliser by a photoneutraliser or a beam-driven plasma neutraliser. None of these concepts has been demonstrated on a relevant scale. In this article we outline our approach to a comprehensive system design study in order to explore a broad range of options for each beamline component and their mutual dependences, and we focus on the central role that the choice of the neutraliser concept - where needed in combination with energy recovery - plays for the layout of the whole beamline.