Improving the longitudinal acceptance of low energy electron linacs

Abstract

Variation of the first neighbour coupling constant permits precisely controllable gradients in the accelerating fields in coupled-cavity linacs. A ramped accelerating gradient, combined with an increasing synchronous phase angle, as in radiofrequency quadrupoles, results in low energy electron linac designs with substantially larger longitudinal acceptances. The transverse focusing provided by the RFQ fields is not present, but moderate solenoidal fields are sufficient to compensate for RF and space charge defocusing forces. Designs are presented for both pulsed and high gradient cw linacs.