Beam divergence of RF negative hydrogen ion sources for fusion

Abstract

Neutral beam injectors (NBI) for fusion facilities have strict requirements on the beam divergence (7 mrad for the ITER NBI at 1 MeV). Measurements of the single beamlet divergence of RF negative ion sources (at lower beam energy < 100 keV) show significantly higher values (9–15 mrad), also larger than filament arc sources at similar beam energies. This opened up questions whether the higher divergence is caused by different measurement or evaluation techniques, whether it is a direct cause of the RF source, e.g. due to a higher temperature of negative ions or an oscillating extraction meniscus, and whether it is a problem at all after full acceleration. In a joint effort between the labs modeling and diagnostic capabilities at the NNBI test facilities have been strongly extended and evaluation methods benchmarked. Particularly challenging is the strong increase in beamlet divergence at a lower filling pressure, seen both in filament arc and RF sources.Beside the source and beam investigations carried out in SPIDER (with selected, isolated apertures rather than the total of 1280 apertures) at Consorzio RFX, the IPP test facilities ELISE (640 apertures) and BATMAN Upgrade (70 apertures) contribute to the physics understanding of the beam optics in RF sources. The determination of the beam divergence is not straight-forward because effects originating from measuring the divergence of multiple beamlets (Beam Emission Spectroscopy) and/or constraints from the individual diagnostic (lateral heat conductance in CFC tiles) lead to difficulties. Still, the divergence requirement is not met at the limited total beam energy available at ELISE and BATMAN Upgrade (< 60kV). However, variation of the beam energy show a decrease of the divergence for higher energies and beam simulation for the ITER NBI accelerator predict that the divergence requirement will be met after full acceleration of the negative ion beam.