An accelerator/wiggler for high efficiency FEL operation

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 μm), 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. A six period, full scale model of the accelerator/wiggler has been fabricated to check the computer simulations for both microwave and magnetic properties. Microwave measurements on this model indicate that the predicted Q is accurate, that there is an approximate 1.5% error in the predicted resonant frequency and that critical coupling to the structure in the presence of the electron beam can be achieved with a direct, rectangular waveguide feed.