A simulation study on energy recovery from spent electron beams in an S-band linac

The feasibility of energy recovery with an S-band linac was studied based on two-dimensional particle-in-cell simulations. A 1-m standing-wave accelerating tube with 20 cells was chosen for the numerical model, with a 4.5-cell RF gun as the injector which is to be used for our IR FEL project. For ample reduction of beam energy to the dump with a typical beam energy from the gun of 5.49 MeV, the phase acceptance of the re-injection of the spent beam is found to be ±0.16π rad which corresponds to a feasible longitudinal deviation of ±27 ps. Furthermore, for a lower beam energy to the dump and a higher wall plug efficiency, simulations with a lower beam energy of 2.42 MeV from the gun were carried out. Counter re-injection of the spent beam is found to be effective for compensating longitudinal velocity variations of low-energy beams along the tube, while degradation in the transverse emittance due to enhanced space–charge effect was found.