Vanderbilt University FEL center for biomedical and materials research☆

Most early free electron laser (FEL) designs used simple optical cavities consisting of two concave dielectric mirrors. This type of optical cavity, though appropriate for proof of principle experiments, does not take advantage of the unique features of the FEL gain medium such as broadband tunability, high peak and average power and access to previously inaccessible wavelength ranges. This article will discuss various approaches which have been demonstrated or are under development to take advantage of these aspects of the FEL gain medium. The prospects and problems for each approach will be discussed. Broadband cavities have been demonstrated using holes in the mirrors, Brewster plate output coupling and a suspended intracavity mirror. High power designs using ring resonators and grazing incidence mirrors are now being developed. Access to the soft X-ray region may be accomplished using a whispering gallery ring resonator.

The gain medium of the FEL does not store energy. Because of this fact, the peak power can only be increased by cavity dumping. Several techniques will be discussed which accomplish this. Interferometric and dispersive cavities are being studied to tailor the spectral or temporal behavior of the laser to a given specification. Finally, several new schemes will be presented which hold promise as future optical cavities for FELs.