Reducing the Energy Spread of Recirculating Linac by Non-isochronous Beam Dynamics

The Superconducting Linear Accelerator S-DALINAC at the University of Darmstadt (Germany) is a recirculating linac with two recirculations. Currently, acceleration in the linac section is done on crest of the accelerating field. The recirculation path is operated achromatic and isochronous. In this recirculation scheme the energy spread of the resulting beam in the ideal case is determined by the electron bunch length. Taking into account the stability of the RF system the energy spread increases drastically. In this work we will present a new non-isochronous recirculation scheme which helps cancelling out these errors from the rf-control. This scheme uses longitudinal dispersion in the recirculation paths and an acceleration off-crest with a certain phase with respect to the maximum. We will present beam dynamic calculations which show the usability of this system even in a Linac with only two recirculations and first experimental results.