A Comparison Between the Fundamental and Third Harmonic Interactions in X-Ray Free-Electron Lasers

(TU-3-21)

H.P. Freund  

Science Applications International Corp., McLean, VA 22102

Studies are in progress for a free-electron laser to serve as a fourth generation light source. The current design for the Linac Coherent Light Source (LCLS) at SLAC employs a 14.35 GeV/3400 A electron beam in conjunction with a 13.2 kG/3.0 cm planar wiggler to generate 1.5 $\dot{A}$ X-rays using the fundamental interaction. However, at this short wavelength the interaction is extremely sensitive to the beam quality. In particular, the saturation efficiency is severely degraded from the ideal performance for energy spreads as low as 0.02%. Since this is the nominal design target, it is useful to consider an alternate scheme. One such scheme, which is considered in this paper, is to generate 4.5 $\dot{A}$ X-rays using the fundamental interaction by increasing the wiggler period to 4.44 cm, and rely on nonlinear harmonic generation to reach 1.5 $\dot{A}$ on the third harmonic. At this longer wavelength, both the fundamental and third harmonic interactions are less sensitive to beam quality, and it is found that the nonlinear harmonic generation mechanism yields higher powers at 1.5 $\dot{A}$ than the fundamental interaction at 1.5 $\dot{A}$ for beam energy spreads just slightly in excess of 0.02%.