A novel design for ion beam funneling by the use of a two-beam RFQ and a multigap-deflector

Heavy ion inertial fusion (HIIF) injector linacs start with a set of low frequency radio-frequency quadrupoles (RFQs), because of the small values of the current limits of linear accelerators in the low energy part. For a higher ion energy, the frequency is increased to reach a better accelerator efficiency. The accumulation of ion beam current in such a driver linac is done by multiple stages of funneling: in each stage the accelerator frequency is doubled and two beams with 180 degrees phase shift are combined to fill all the rf-buckets of the high frequency accelerator stage. In the ideal case, there is no change of the emittance and the beam current and brightness are doubled. For the first funneling stage a two-beam RFQ, where two beams are bunched and accelerated in a single r.f. cavity and a novel scheme for an rf funneling deflector operating at low voltages has been developed. With the use of convergent beams, a short funneling structure placed around the beam crossing position seems to be possible. (author)