The effects of linear-accelerator noise on the Los Alamos free-electron laser

The optical power produced by the Los Alamos free-electron laser has been flawed by a peristent instability. This instability appears as fluctuations in output power in a frequency band centered around 100 kHz. We have found that the fluctuations are dependent on the strength of the lasing and on variations in the electron micropulse's arrival time. When the lasing is strong, the fluctuations are small; when the lasing is weak, the fluctuations are large. The variations in the electron micropulse's arrival time primarily are due to fluctuations in the accelerator gradients. The primary noise sources in the accelerator are the electron gun and the klystron amplifier chain. In addition, this noise is uncontrolled because of a lack of bandwidth in the feedback controls for the RF system. Appropriate improvements are being made to eliminate these fluctuations in optical power.