Prevention of sideband-induced detrapping in tapered-undulator free-electron lasers

The sideband instability of free-electron laser (FEL) oscillators has recently been observed1 in the Los Alamos experiments. This result underscores the expected need for sideband suppression in tapered-undulator oscillators. While the instability is predicted to actually enhance the power output of untapered systems, it is a possible threat to the potential high extraction of tapered systems. Simulations2 predict that the instability will lead to detrapping in FELs with relatively long, highly tapered undulators in which trapped electrons undergo multiple synchrotron oscillations while traversing the undulator. Sideband growth is associated with loss of extraction by one-half or more. Means for sideband suppression using multilayer dielectric mirror coatings are investigated. The objective is to identify filter functions which give adequate sideband suppression. It is found that the introduction of wavelength selectivity can suppress the instability, with recovery of full extraction.