<title>High-efficiency FEL operation using microwave reacceleration in a combined accelerator/wiggler</title>

Abstract

A possible approach to high efficiency FEL operation is to combine a microwave linear accelerator and magnetic wiggler into a single structure. As the electrons lose energy to the radiation at the FEL oscillation wavelength (e.g. 10 micrometers ), energy is replaced by the microwave linac. The electron beam acts as a catalyst for the conversion of microwave power to infrared power. Several advantages to the accelerator/wiggler are: it is possible to obtain high conversion efficiency in a short length; small- signal gain reduction can be avoided; power extraction may be increased by increasing length; there is little detrapping; and electron beam energy out of the wiggler is relatively monochromatic, permitting efficient energy recovery. The expected performance of the high efficiency FEL will be calculated by computer simulation based on parameters measured on a full scale, six period model. The gain and efficiency will be estimated with and without the presence of the accelerating microwave field. Simulation results predict an efficiency greater than 15% in a 135 m long accelerator/wiggler. A fast (< 1 microsecond(s) time constant) variable attenuator will be used to modulate the microwave field in time so that small signal gain reduction is avoided. The optimum value of microwave field as a function of time will be calculated and the sensitivity of the gain to the microwave field will be discussed.