Lasing in Forbidden Regions: Second Harmonic FEL Oscillation

We have produced and measured for the first time second harmonic oscillation in the infrared region by the high average power Jefferson Lab Infrared Free Electron Laser. Although such lasing is ideally forbidden, since gain is zero on axis for a perfect electron beam in an infinite wiggler, the finite geometry allows sufficient gain in this situation for lasing to occur. We were able to lase at pulse rates up to 74.85 MHz and could produce over 4.5 watts average and 40 kW peak of IR power in a 40 nm FWHM bandwidth at 2925 nm. In agreement with predictions, the source preferentially lased in a TEM-01 mode. We present results of initial source performance measurements and comparisons of gain as calculated by different approaches