First direct comparison of whole beam and single beamlet divergences in a negative ion source with simultaneous BES and CFC tile calorimetry measurements

Abstract

Neutral beam injection (NBI) systems are required for heating and current drive in the next generation fusion experiment ITER, and strict requirements are placed on the beamlet core divergence (<7 mrad) for transmission into the tokamak. The measurement of single beamlet divergence is challenging due to the multi-beamlet nature of the negative ion sources that are required for such systems; diagnostic systems compatible with large high power ion beams can only provide spatially averaged measurements, leading to mixing of beamlet signals within diagnostic results. To improve the understanding of this effect, a direct comparison has been made between the single beamlet and multi-beamlet divergence by a combination of both Beam Emission Spectroscopy (BES) and 1D carbon fiber composite tile calorimetry, in a joint campaign by IPP and Consorzio RFX. The measurements performed so far in this campaign at the BATMAN Upgrade Test Facility have led to two major results. First, an excellent agreement is found for single beamlet divergences from the two diagnostics, showing that the results from these diagnostic systems can be compared for single beamlets. Second, the contribution of beamlet deflection, caused by an alternating magnetic field at the extract grid, to the divergence as measured using BES has been quantified with up to a factor of 3 increase when compared with the single beamlet value. This demonstrates that further investigation is needed into how mixing of information from a beamlet affects diagnostic results with a combination of both simulation and experiment, which will be performed in a next step.