First observation of spiking behavior in the time domain in a free-electron laser

The first observation of spiking behavior, or sideband instability, of a free-electron laser (FEL) is reported in the time domain using an autocorrelator. The autocorrelation measurements on the MKIII FEL indicate the existence of three spikes within otherwise square-shaped micropulses, separated by 240 microns, where the minimum theoretical separation is 150 microns. The experimental results agree very well with computer simulations performed using similar operating parameters, which predicted three spikes separated by 200 microns. The simulation predicted a sideband with approximately 10 percent of the power of the fundamental wavelength, which also matched the observed spectrum. (S.A.V.)