Feasibility of high gain X-ray free electron lasers

We investigate the feasibility of a tunable VUV/soft X-ray FEL operating in a dedicated storage ring. Because of poor mirror reflectivity available at these wavelengths, the FEL must operate in the intermediate high gain regime (gain = 3-5). In order to obtain this high gain, a long interaction length (27 m), a high current (270 A) and strong magnetic fields (a_w > 1) are necessary. Consequently some velocity spread effects and possibly even some space charge effects must be considered. We use a one-dimensional, comprehensive small signal gain analytical model to conduct a parametric study of the FEL operating parameters and their scaling laws. The results of the analytic model are compared with results obtained with the 2-D FEL simulation code FRED, and the effect of optical guiding on the gain is estimated using an enhanced filling factor based on the observed guided mode size. We conclude that oscillation below 300 Å is feasible with existing mirror technology.