K. Hahn et al., TRANSVERSE BEAM COMBINER FOR ILSE, Nuovo cimento della Società Italiana di Fisica. A. Nuclei, particles and fields, 106(11), 1993, pp. 1665-1670
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.