Negative ion beam halo mitigation at the 1 MV testbed at IRFM

Beam experiments and calculations aimed at characterising and suppressing the negative deuterium ion beam halo have been performed at IRFM, CEA-Cadarache. Using a “classic” plasma grid, we established that no halo (<2%) is present during volume operation (no caesium present in the ion source) at low accelerator pressure (<0.01 Pa). Around 8% beam halo is measured in caesiated operation, at perveance match.We hypothesized that significant halo originates from negative ions being formed on the metal parts of the plasma grid (as could happen when neutral atoms from the source hit the plasma grid covered with caesium). Based on this hypothesis we tested a new design for the plasma grid, aimed at substantially reducing the halo. The test was successful and the halo was reduced from 8% to 3%.The single beamlet profiles show an asymmetry that is caused by the presence of the electron suppression magnets in the accelerator.