Mode media interactions in an FEL (Free Electron Laser)

It is well known that the e-beam gain and refractive properties in an FEL alters the optical mode behavior from that of a bare resonator. We report here on 3-D simulations with FELEX and experiments on the 10.6 micron wavelength Los Alamos FEL performed to assess the importance of this effect for a range of e-beam parameters. Two sets of measurements are made for a near concentric cavity with optical axis displaced from the electron beam axis. The first is a static measurement made during cavity mode saturation of the optical beam centroid referred to each resonator mirror. The optical detector scheme for this part of the experiment uses a 256 element integrating pyroelectric array. The second is a dynamic measurement made during cavity mode decay of the motion of the optical beam centroid and waist referred to each resonator mirror. The optical detection scheme for this part of the experiment consists of a three-part split mirror detector viewing two different optical paths. These measurements quantify the gain and refractive properties of the electron beam when compared to FELEX simulations. Implications for FEL alignment schemes and future high-power FELs will be discussed. 6 figs., 1 tab