Impact of operational parameters on single beamlet deflection in a negative ion source for NBI applications

For current and future large scale tokamaks, neutral beams for heating and current drive are generated from the neutralisation of large negative ion beams with energies up to 1 MeV and current of up to 40 A. To improve efficiency and prevent high heat loads on beamline components, permanent magnets are used to deflect co-extracted electrons out of the beam at a low energy. This field also affects the negative ions as they are accelerated, causing beamlets to exit the grid system with a residual offset and deflection angle. This adversely affects the overall divergence of the beam, and compensation is foreseen in future devices. Measurements of the residual deflection of a single beamlet have been carried out at the BATMAN Upgrade test facility by calculating relative beamlet angles from beam emission spectroscopy (BES) spectra, and through the use of one-dimensional carbon fibre composite (1D-CFC) tile calorimetry to find beamlet positions. It is described how these measurements can be made, and that they are limited to relative measurements only, for a single beamlet and for a single line of sight. The amount of beamlet deflection is shown to change significantly, by up to 0.6°(10 mrad), depending on the operational parameters used. As is to be expected the beamlet deflection angle is observed to be affected by changes to the voltages of the acceleration system. However, the beamlet deflection angle is also observed to change with RF power and other source parameters, which, to a first approximation, should only affect beamlet divergence, and not the deflection. These changes to beamlet deflection through parameters other than the grid voltages used may have consequences for systems planning to use suppression systems for the zig-zag deflection. The effectiveness of the suppression system may be reduced due to changes in the source parameters, which could lead to beam losses and high heat loads on downstream components.