Analysis of the intensity fluctuations of SASE using an autoregressive model

We have, by the use of an autoregressive model, analyzed the intensity fluctuation of SASE produced with the high-intensity single-bunch beam from the L-band linac and the wiggler for the FEL system at the Institute of Scientific and Industrial Research, Osaka University. The power spectrum of the SASE intensity shows a flat and constant structure over the whole frequency range up to 0.7 Hz, which indicates that the intensity fluctuation of SASE is random, as expected, while a contribution originating from the fluctuation of the electron beam intensity shows up in the frequency region below 0.1 Hz and reaches approximately 20% of the total intensity in the frequency region lower than 0.01 Hz. The power spectrum of the electron beam intensity shows a peak correspondingly in the same frequency region below 0.1 Hz. Thus, a contribution of the intensity fluctuation of the electron beam to the total fluctuation of the SASE intensity is evaluated quantitatively. (author)