Stable operation regimes in short-pulse FEL oscillators

We have investigated the stable operation regime of the short-pulse FEL oscillator system. Two different stable-focus regimes of stable operation have been identified. The one is a single-mode stable regime (SSR) and the other is a bump stable regime (BSR). The former is operated with a large cavity detuning and gives a single-mode stationary state without saturation. And the latter is operated near perfect cavity synchronism with a low current and gives a multi-mode stationary state after a transient bump of the macropulse power. Using properly defined collective phase variables, we describe the electron dynamics and the phase evolution of the optical field in the stable operation regimes. (author)