High-gain, short wavelength FEL in the Raman regime

While FEL technology has reached the EUV and X-ray regime at existing machines such as LCLS and SACLA, the scale of these projects is often impractical for university research and industrial applications. Sub-millimeter period undulators can reduce the size of a high-gain EUV FEL, but will impose stringent conditions on the electron beam. In particular, a high-gain EUV FEL based on undulators with a sub-mm period [1] requires electron beam currents upwards of 1 kA at energies below 100 MeV. Coupled with the small gap of such undulators and their low undulator strengths, K<0.1, such beam parameters bring longitudinal space-charge effects to the foreground of the FEL process. When the characteristic length associated with electron beam plasma behaviour becomes comparable to the gain-length, the relevant theoretical FEL model transitions from the Compton to the Raman limit [2], where collective field effects dominate. In this work, we investigate the behaviour of the FEL's gain-length and efficiency in these two limits. The starting point for the analysis is the one-dimensional FEL theory including space-charge forces. The derived results is compared to numerical results of Genesis 1.3 simulations. This theoretical model predicts that in the Raman limit, the gain-length scales as the beam current to the one-fourth power while the efficiency grows as the square root of the beam current. This advantage in efficiency may prove critical in the development of very compact short wavelength FELs, such as the Keck Foundation-funded SAMURAI FEL at UCLA.