Theory and observation of optical guiding in a free-electron laser

Optical guiding in a Raman free-electron laser (FEL) is studied theoretically and experimentally. Two complimentary theoretical approaches to the problem of optical guiding in a waveguide containing a filamentary electron beam are given and shown to be in good agreement with each other in the exponential gain regime. Evidence for optical guiding of 2-mm-wavelength radiation along the electron beam in the Columbia University FEL is obtained experimentally by analysis of spatial ring-down data of the optical wave profile and compared with numerical simulations. These data are presented for the exponential gain regime. A similar experiment at signal saturation conditions shows a much less well-defined ring-down. We give plausible experimental as well as theoretical arguments why the ring-down pattern is less well defined. Based on the observations presented in this paper, it is not possible to validate optical guiding at saturation.