INTENSITY EFFECTS IN FINAL FOCUSING FOR HEAVY-ION INERTIAL FUSION

Focusing of heavy-ion beams on an indirectly driven inertial fusion target can be achieved, in principle, under vacuum conditions in the reactor chamber. Due to performance limitations in the storage rings of the driver accelerator, a relatively large number of small emittance beams is required. We suggest that the effect of space charge can be described in terms of three distinct mechanisms: an increase of the entrance 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 formula is confirmed by a computer simulation taking into account the correct Coulomb forces between interacting beamlets. We find that for propagation in vacuum the current sensitivity is considerable, in particular 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 number of beamlets in it-and thus determines how many bundles have to be used. Possible cures are briefly discussed.