TRANSVERSE BEAM COMBINER FOR ILSE

Previous heavy-ion fusion driver system studies suggest that transvers e beam combining significantly reduces driver cost. In a combiner, sev eral beams are brought together to a common transport channel which ac commodates the increased line charge density. Combining intense beams increases the transverse emittance mainly due to the heating of the be am by space charge forces as the non-uniform original beam configurati on becomes more uniform. The combiner itself introduces additional abe rrations, which are small for the present design. Those aberrations ar e due to the reduced available space for the focusing electrodes and r educed clearance from the beamlets to the surrounding electrodes, ther eby generating field aberrations and larger image forces. These aberra tions can also lead to particle loss. We have studied a particular des ign of the proposed induction linac system experiment (ILSE) combiner which is a first-order achromat that tolerates a rather large fraction al head-to-tail momentum tilt of +/- 10%. Using a 2D particle-in-cell code we have found that similar to 7% of particles are lost in the com biner. The emittance growth after the combiner is large enough so that the emittance growth due to combiner aberrations is unimportant. The scaled projection to a driver shows the growth is small enough to be t olerated. At present, methods of improving combiner design to reduce p article loss and to minimize emittance growth are being studied.