INTENSITY EFFECTS IN FINAL FOCUSING FOR HEAVY-ION INERTIAL FUSION

Focusing of heavy-ion beams on an indirectly driven inertial fusion ta rget can be achieved, in principle, under vacuum conditions in the rea ctor chamber. Due to performance limitations in the storage rings of t he driver accelerator, a relatively large number of small emittance be ams is required. We suggest that the effect of space charge can be des cribed in terms of three distinct mechanisms: an increase of the entra nce divergence angle; a current sensitivity of the focal spot; and the effect of a nonuniform distribution. The first two can be calculated by means of an envelope model. The resulting current sensitivity formu la is confirmed by a computer simulation taking into account the corre ct Coulomb forces between interacting beamlets. We find that for propa gation in vacuum the current sensitivity is considerable, in particula r for charge states Z > 1, since the spot size blow-up scales like Z4/ A2. The effect depends on the total current in a bundle-and not the nu mber of beamlets in it-and thus determines how many bundles have to be used. Possible cures are briefly discussed.