Evaluation of different colling fluids for high-voltage Neutral Beam Injector grids

The Neutral Beam Injectors (NBIs) of the ITER experimental fusion reactor are designed to accelerate Deuterium negative ions with energy up to 1 MeV and current up to 40 A. The accelerator grids must be designed to operate at high voltages and to withstand high power densities (in the order of some tens of MW m^-2). They must maintain a proper alignment in all the foreseen operating scenarios, in order to obtain good beam optics, so the thermo-mechanical deformations must be maintained at very low values. Further requirements come from the need of keeping under control the maximum surface temperature in copper. With these requirements, the cooling of the grids represents a significantly critical aspect of the NBI design. Coolant properties have to satisfy high resistivity requirements and to be appropriate for the removal of high heat loads. The cooling circuits must match with the beam optic geometry and the space constrains severely affect the coolant distribution. This paper presents some studies of the grid cooling circuits design carried out with Computational Fluid Dynamics (CFD) numerical simulations and analytical methods. Cooling performances for different cooling fluids (water and dielectric coolants) have been investigated.