Transverse combining of four beams in MBE-4

Transverse beam combining is a cost-saving option employed in many designs for induction linac heavy ion fusion drivers. But resultant transverse emittance increase, due predominantly to anharmonic space charge forces, must be kept minimal so as not to sacrifice focusability at the target. A prototype combining experiment has been built, using the MBE-4 experiment. Four sources produce four 4 mA Cs⁺ beams at 200 keV. The ion sources are angled toward each other, so that the beams converge. Focusing upstream of the merge consists of 4 quadrupoles and a final combined-function element (quadrupole and dipole). All lattice elements are electrostatic. Due to the small distance between beams at the last element (~2 mm), the electrodes here are a cage of small wires, each at different voltage. The beams emerge into the 30 period transport lattice of MBE-4 where emittance growth due to merging, as well as the subsequent evolution of the distribution function, can be diagnosed. The combiner design, simulation predictions, and preliminary results from the experiment are presented