Stable operation regimes in short-pulse FEL oscillators

We have investigated the stable operation regime of the short-pulse FEL osc illator system. Two different stable-focus regimes of stable operation have been identified. The one is a single-mode stable regime (SSR) and the othe r 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 a nd gives a multi-mode stationary state after a transient bump of the macrop ulse power. Using properly defined collective phase variables, we describe the electron dynamics and the phase evolution of the optical field in the s table operation regimes.