First results on energy recovery in the Jefferson Lab IRFEL

A recirculating, energy-recovering linac is used as driver accelerator for Jefferson Lab's high average power FEL. CW beam of 5 mA design current is transported from the superconducting RF (SRF) linac to the wiggler for lasing, and then recirculated back to the linac for deceleration and energy recovery. About 75% of the beam power is extracted before the beam is transported to the beam dump. Energy recovery reduces power consumption, RF equipment capital costs, and beam dump shielding requirements. It is arguably essential as FEL technology is scaled to higher average power levels. To date, 4 mA of CW beam has been energy recovered successfully. There is no evidence of RF instabilities due to the energy aperture of the transport system, momentum compaction or the phase of the decelerating beam. HOM power from the beam has interfered with the operation of the IR interlock detectors, designed to protect the warm waveguide window from thermal runaway. Installation of copper screens appears to have solved the problem. More detailed studies of the HOM spectra and their correlation to the beam properties are planned.