Neutral Beam Injection for a tokamak-based Volumetric Neutron Source

Abstract

Neutral Beam Injection (NBI) is an essential system of the Volumetric Neutron Source (VNS) currently studied by EUROfusion. The VNS is a medium-sized tokamak with a tritium plasma. Approximately 42 MW deuterium NBI provides heating, non-inductive current drive and fast ions for beam–target dominated fusion. The mission of the VNS is to test and qualify components such as breeding blankets in reactor-relevant conditions and exposure to high fluences of 14 MeV neutrons in continuous operation. Consistent with the size of the tokamak, the beam energy is around 120 keV, which can be achieved with the well-established technology of positive-ion-based NBI. However, no previously built NBI has been designed for continuous operation and in an environment with reactor-like neutron radiation, leading to numerous design challenges. The evolving conceptual design is based on ASDEX Upgrade’s well tested and reliable NBI injectors. Each injector beamline will have four beams from four individual sources that together provide $\sim 14\mathrm{MW}$ NBI. Three injectors are foreseen to operate simultaneously while the cryo pumps in a fourth injector regenerate. All beamlets of each of the four beams intersect between the toroidal field magnets at the midplane, maximising transmission and limiting the duct cross section. The remote handling concept foresees that an entire injector box is removed from the torus on rails and replaced by a spare. Repair and maintenance is then carried out in an active maintenance facility (AMF) to keep VNS down-times short. Ion source high-voltage conditioning uses a calorimeter at the AMF, eliminating the need for a calorimeter in each injector box. The paper reviews the current state of the NBI concept with an emphasis on solutions to the challenges arising from continuous operation; space limitations, beamline length and reionisation; operation in a nuclear environment and remote handling.