First lasing of the UCSB 30 μm free-electron laser

Third harmonic lasing has been achieved in a Free-Electron Laser specifically designed to operate in that mode. So far, 40 W at 41 μm wavelength and 5.16 MeV beam energy has been measured. One of the most difficult challenges has been suppression of lasing at the fundamental wavelength. This is currently achieved with an intracavity cesium iodide restrahlen filter, but eventually, the original grating design will be needed to provide full tunability. Operation of this FEL demands electron beam stability and control system accuracy at the limits of the present system. Further progress in the development of this FEL may require significant improvement in these areas.