• H.L. Andrews, C.A. Brau, J.D. Jarvis
  Vanderbilt University, Nashville, TN
• R. Durant, C.F. Guertin, T.H. Lowell, M.R. Mross, A. O'Donnell
  Vermont Photonics, Bellows Falls

We present further experimental observations of evanescent waves in a Smith-Purcell free-electron laser (FEL) with vertical conducting walls bounding the sides of the grating.  Evanescent waves are the basis of oscillator operation of the Smith-Purcell FEL. They have group veloctiy anti-parallel to the electron beam and for sufficiently high current, provide feedback to bunch the electron beam. A grating with side walls supports multiple transverse modes; we have observed emission from the two lowest such modes. The observed wavelengths and wavelength shift with changing operating voltage agree with theoretical predictions. Additionally, strong radiation from the upstream end of the grating confirms the negative group velocity.