3D magnetic field measurements and improvements at the negative ion source BATMAN Upgrade

Abstract

BATMAN Upgrade is the testbed of the prototype radiofrequency-driven negative ion source, for the Heating Neutral Beams of ITER. Its extraction system corresponds roughly to one beamlet group of ITER (1/16th of ITER NBI extraction surface), from where negative ions are extracted together with undesired co extracted electrons. The latter are deflected out of the beam by a set of permanent magnets (Co-extracted Electron Suppression Magnets, CESMs) embedded in the second grid electrode. Two different configurations of CESMs are currently under investigation in the top and bottom halves of the grid system. The amount of co-extracted electrons can be reduced in the ion source by, among other means, applying a transversal magnetic filter field (MFF) to the plasma, created by either permanent magnets or a multi-kA current flowing vertically through the first grid electrode. A special device with five 3D Hall sensors has been developed and calibrated to measure the magnetic fields along each aperture axis in the grid system. The results have been compared with FEM simulations revealing very good agreement within the geometrical and material tolerances for the CESMs, but for the MFF a large discrepancy has been observed. The problem has been identified with a poor electric contact between pieces of the first grid electrode. The contact has been improved by setting annealed Cu washers as interlayer and the MFF matches the expected one. The poor electrical contact limited the maximum strength of the MFF but the results for ion source performance and beam optics are equivalent to those of the improved electrical contact. The effects of the MFF strength, its polarity and the MFF types on the ion source performance and single beamlet optics, regarding the vertical and horizontal beamlet deflection as well as the beamlet core divergence are discussed.