Residual Ion Energy Recovery for the DEMO NBI - a Conceptual Design Study

The requirements for a neutral beam injection system for DEMO go significantly beyond those on ITER, particularly regarding availability and energy efficiency. The latter is mostly limited by the neutralisation efficiency, which at energies around 1 MeV is only about 55 % with a gas neutraliser like on ITER. Besides the technically challenging alternative by photoneutralisation - with a theoretically possible neutralisation efficiency of up to 100 %, but undemonstrated at any relevant scale - residual ion (RI) energy recovery (ER) has been proposed as a means of improving energy efficiency with a less efficient neutraliser. For this purpose the negative and positive RIs are first deflected out of the neutral beam in opposite directions, and subsequently decelerated by an electrically biased collector. The possible gain in energy efficiency depends not only on the fraction of the kinetic energy to which the RIs can be decelerated while still being effectively collected, but also on the additional neutral beam losses by reionisation due to the additional beamline length. We present a CAD model of the conceptual design of an energy recovery system integrated into a DEMO beamline with ITER-like parameters and beam shape. We use detailed 3D ion optics simulations to study charged particle trajectories, taking the effects of finite beamlet divergence and space charge into account. Heat loads on beamline components and transmission losses are an output of these simulations as is the beamline's energy efficiency gain. The dependence of the efficiency gain on a variety of design parameters, such as the neutralisation efficiency, which could e.g. be mildly increased with a plasma neutraliser, are studied using a simpler 0D model, in order to show under which conditions the integration of ER is economically attractive.