Experimental measurement of short pulse guiding effects in a free-electron laser

We present the results of computer simulations of the MKIII FEL at Stanford (1988) using the Los Alamos National Laboratories FELEX simulation code. The simulations predict a tapering of the spot size through the length of the optical pulse that results from the effect of slippage on optical guiding. We show that this effect can be measured using optical autocorrelation, and present a simple mathematical model that may be used to calculate autocorrelation functions measured after the beam passes through an aperture, and compare these autocorrelation functions to reveal the effect of the spatial taper. We then present an autocorrelation experiment on the MKIII free-electron laser at Stanford, the inconclusive results of which probably stem from remnant laser instability and drift. However, the maximum difference in normalized autocorrelation functions, under experimental conditions with 50% single pass gain, was 5%. This corresponds to a difference in spot size between the front and back of the pulses of approximately 25%.