Synchronously injected amplifiers, a novel approach to high-average-power FEL

Two new FEL ideas based on synchronously injected amplifiers are described. Both of these rely on the synchronous injection of the optical signal into a high-gain, high-efficiency tapered wiggler. The first concept, called Regenerative Amplifier FEL (RAFEL), uses an optical feedback loop to provide a coherent signal at the wiggler entrance so that the optical power can reach saturation rapidly. The second idea requires the use of a uniform wiggler in the feedback loop to generate light that can be synchronously injected back into the first wiggler. The compact advanced FEL is being modified to implement the RAFEL concept. We describe future operation of the advanced FEL at high average current and discuss the possibility of generating 1 kW average power.