Vacuum pumping concept for quasi-continuous NBI operation at a steady state fusion machine

A concept for a test facility based on a Volumetric Neutron Source (VNS), mainly for the qualification of tritium breeding technology, is currently being explored in EUROfusion. The VNS uses neutral beam injection (NBI) to produce beam-driven fusion. The required NBI injectors are well established, but experience with continuous operation is currently limited. An interesting issue in this context is the pumping of the NBI boxes. A continuous gas stream must be fed into the NBI box to supply the ion sources. At the same time, the pressure inside the NBI box must be low to avoid losses due to re-ionisation. This requires a very high pumping speed, which can only be achieved with gas-binding pumps inside the NBI box. The operation time of an NBI is limited due to the inventory of flammable gas inside, which is calculated at 3 h. Periodic regeneration is required to resume operation for the next cycle. With 4 NBIs installed, 3 in operation to provide beam power, only 1 can be in regeneration mode. Consequently, the available regeneration time is limited to 1/3 of the average NBI injector operating time, i.e. 1 h. With a suitable rough pumping system, it is expected that this time will be sufficient to bring the NBI injector back into operation. The feasibility study shows that with 4 NBI injectors installed on VNS (3 in operation, 1 in regeneration mode) and a suitable rough pumping system for a fast regeneration process, VNS can be continuously supplied with a beam power of 42 MW.