A visible free electron laser in France

The principles and evolution of free electron lasers (FEL) are reviewed, and the results from the recent LURE experiment, which produced a beam in the visible, are described. An FEL beam is produced when an electron stream passes through a magnetic field that forces it into a sinusoidal trajectory, causing it to emit light. The wavelength emitted is determined by the electron beam energy, the spacing of the magnets, and the strength of the magnetic field. An electron storage ring was used in the LURE trials, accompanied by a recirculating beam and an optical klystron. Lasing was demonstrated in 1976, as were the higher energies achievable with an electron beam. Intensity increases were attained by keeping the laser beam and electron beam in phase and bunching was enhanced by the FEL process. Further work is indicated in using positrons, reinstalling a magnetic undulator, and employing UV-resistant mirrors.