High-Gain, Long-Pulse Free-Electron-Laser Oscillator

We report the first operation of a high-gain, long-pulse (> 1 μsec), free-electron-laser oscillator. The growth rate is measured by variation of the interaction length of the electron beam within the oscillator cavity. The observed power gain per pass of up to ${10}^4$ and the buildup of cavity power to multimegawatt levels are in agreement with high-gain oscillator theory. Growth rates have also been measured as a function of wiggler field strength.