Technology developments for a beam source of an NNBI system for DEMO

Abstract

Based on the requirements for heating and current drive of a DEMO (tokamak) device a neutral beam injection (NBI) system faces several challenges, from which the achievement of high wall-plug efficiency is the driving parameter for the beam line design. For boosting the wall-plug efficiency of the ITER NBI system from about 0.25 to target values around 0.4 (pulsed DEMO) or 0.5–0.6 (steady state DEMO), the standard gas neutralizer concept has to be modified or replaced by the innovative laser neutralizer concept for which the modular concept of the ion source allows for adaption. The technology developments of ITER’s high power RF-driven ion source are continued by optimization of the RF-driver geometry and the resulting overlap of the driver plasmas in a large expansion chamber. Insights in the caesium evaporation and distribution in the ion source could allow for a reduction of the caesium consumption. On the other hand, the caesium dynamics influences the temporal source performance and the beam uniformity such that investigations on alternatives to caesium became an R&D task. Finally, the status of proof-of-principle experiments on the laser neutralization technique and its challenges for up-scaling from a lab scale experiment to a beam line are discussed.