Electron beam dynamics through a return-arc and a deceleration path of the JAERI energy-recovery linac

An energy-recovery linac for a high-power FEL has been developed at Japan Atomic Energy Research Institute, and a research program towards a 10 kW FEL is in progress. Since FEL interaction introduces large energy spread in the electron beam, special attention should be paid to both transverse and longitudinal phase space manipulation through the return-arc and the deceleration path. We present beam dynamics simulations coupled with a 1D FEL code, and discuss the limitation of FEL extraction efficiency determined by phase space acceptance of the return path.