On the role of dissociative recombination on the effectiveness of a plasma neutralizer in DEMO fusion plant

In order to reduce the recirculating electrical power of DEMO the wall-plug efficiency of the additional heating systems plays a crucial role. The efficiency of the heating neutral beams (HNBs) of ITER is mostly limited by the gas neutralizer, whose efficiency is theoretically limited to about 55%. The use of a plasma neutralizer is a valuable alternative to increase the neutralization efficiency to about 80%, with a consequent improvement of the overall efficiency. The effectiveness of a plasma neutralizer increases with the ionization degree of the plasma, which is thus the key parameter of the system. The concept of a so-called “beam driven” plasma neutralizer (BDPN), in which the power to sustain the plasma is provided by the ion beam itself, is very appealing, and is explored in several works. Up to now though, a proof of principle for a plasma neutralizer was demonstrated only in experiments with an external plasma source and the scaling to an ITER-relevant plasma neutralizer suggests that a power in the order of 0.5 MW should be injected from outside to sustain a plasma with a 30% ionization degree. This work tries to bridge the gap between the models predicting the feasibility of a high efficiency BDPN and the experimental scaling. In particular, the role of the volume losses is highlighted and a much better agreement with experiment based scaling is obtained.