Role of recovery pass beam phase error in RF system design for same cell energy recovery FELs

Abstract Recovery of residual energy in the electron beam leaving the FEL interaction region allows considerable improvement in two problem areas of particular concern in high average power designs: (1) the RF power required to generate a given average optical output power is reduced, and (2) the power and energy of the beam which must be dumped are reduced, with concomitant reductions in the amount of heat which must be removed and in the radiation shielding requirements. Recirculation of the beam for a second pass through the linac allows the residual beam power to be recovered in the same RF structure used for acceleration, minimizing the investment in structure and yielding a compact layout. If the energy recovered from the beam is adjusted so that the part which interacted with the FEL optical fields is reduced to the same energy as the part of the beam which did not (“differential” energy recovery), then a relationship between the RF power required, the power delivered to the FEL optical mode, the beam current, and the linac structure's external coupling coefficient is established. For a 100 kW optical output example using eight TESLA-type super conducting cavities, a minimum of 250 kW of RF power is required if Q e is adjusted to 2×10 6 . This would be less power than that required for beam loading in the injector linac.