Effects of optical guiding on sideband instabilities in a free-electron laser

The effects of optical guiding on sideband instabilities in a Raman free-electron laser (FEL) are studied numerically and experimentally. An axisymmetric two-dimensional (2D) computer code that includes the effects of space charge and diffraction in an overmoded waveguide is developed to simulate sideband growth in the Columbia University FEL, which generates radiation of millimeter wavelength. It is found in both the simulation and the experiment that the effect of refractive optical guiding, which slows down the radiation group velocity, shifts the sidebands away from the signal carrier. We also find numerically that refractive optical guiding enhances the filling factor of the electron beam and perturbs the electron distribution, and thereby increases the sideband growth rate. We show that the sideband growth rate can be depressed by tuning the FEL so that the real part of the effective index of refraction associated with the electron beam decreases. The effect of wiggler tapering on the sideband growth is also studied with the 2D code. A significant reduction in the sideband growth rate in an efficiency-enhanced wiggler has been demonstrated and is qualitatively consistent with experimental measurements.