Merits of a sub-harmonic approach to a single-pass, 1.5-Angstrom FEL

SLAC/SSRL and collaborators elsewhere are studying th physics of a single-pass, FEL amplifier operating in th 1 -- 2 Angstrom, wavelength region based on electron beams from the SLAC linac at ∼ 15 GeV energy. Hoping to reduce the total wiggler length needed to reach saturation when starting from shot noise, we have examined the benefits of making the first part of the wiggler resonant at a subharmonic wavelength (e.g. 4.5 Angstrom) at which the gain length can be significantly shorter. This leads to bunching of the electron beam at both the subharmonic and fundaments wavelengths, thus providing a strong coherent ''seed'' for exponential growth of radiation at the fundamental in the second part of the wiggler. Using both multi-harmonic and multi-frequency 2D FEL simulation codes, we have examined the predicted performance of such devices and the sensitivity to electron beam parameters such as current, emittance, and instantaneous energy spread