INTENSITY EFFECTS IN FINAL FOCUSING FOR HEAVY-ION INERTIAL FUSION WITH SHORT BUNCHES

Focusing of heavy-ion beams on an indirectly driven inertial-fusion ta rget can be achieved under vacuum conditions in the reactor chamber wi th a relatively large number of small emittance beams. Effects of spac e charge can be described in terms of four distinct mechanisms: an inc rease of the entrance divergence angle; a current sensitivity of the f ocal spot; the effect of a non-uniform distribution; and effects from finite bunch lengths. The first two can be calculated by means of an e nvelope model. The resulting current sensitivity formula is confirmed by a computer simulation taking into account the Coulomb forces betwee n interacting bunched beamlets. We find that the current sensitivity i s considerable, in particular for charge states Z > 1, since the spot size blow-up scales like Z(4)/A(2). The effect depends on the total cu rrent in a bundle and not on the number of beamlets in it, and thus de termines how many bundles have to be used. Effects from finite bunch l engths are largest for large numbers of beamlets and result in intensi ty losses at the beam spot at the ends of the bunches.