High-efficiency ultrashort pulse generation in a high-gain FEL oscillator near the perfect synchronism

It has been verified experimentally in JAERI-FEL that a high-gain FEL oscillator has the maximum extraction efficiency at the perfect synchronism of optical-cavity length. The simultaneous measurement of FEL efficiency and absolute cavity length has clearly shown that a sharp peak of detuning curve at the perfect synchronism appears in high-gain and low-loss regime and the FEL extraction efficiency exceeds the scaling law of short-bunch FEL oscillators. A numerical analysis indicates that lasing at the perfect synchronism is quasi-stationary superradiance with random fluctuations, which is analogous to a SASE FEL. Second-order autocorrelation measurements show that FEL pulses shorter than four optical cycles are generated successively for a number of round trips at the perfect synchronism, which is consistent with numerical results.