Measurements Of Gain Larger Than 10(5) At 12 Mu M In A Self-Amplified Spontaneous-Emission Free-Electron Laser

We report measurements of very large output intensities corresponding to a gain larger than 10(5) for a single pass free-electron laser operating in the self-amplified spontaneous emission (SASE) mode at 12 mu m. We also report the observation and analysis of intensity fluctuations of the SASE radiation intensity in the high-gain regime. The results are compared with theoretical predictions and simulations. [S0031-9007(98)07403-1].