Photon statistics of the SASE FEL

The photon statistics of the free-electron laser radiation are examined. The photon number moments are calculated for the multimode self-amplified spontaneous emission including the effects of the Gaussian free-electron laser spectral distribution. It is shown that, in the parameter regime where the pulse duration is of the order of the coherence time, the expectation value of the photon number moments deviate from those predicted by a negative-binomial probability distribution.