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

Hajima, Ryoichi; Nishimori, Nobuyuki; Nagai, Ryoji; Minehara, Eisuke

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 for various gain and loss parameters has clearly shown that the steep peak of detuning curve at the perfect synchronism appears only in high-gain and small-loss regime and the FEL extraction efficiency exceeds the scaling law of short-bunch FEL oscillators. A numerical analysis indicates that the lasing at the perfect synchronism is quasi-stationary superradiance with random fluctuation. It is also found that shot-noise in the electron bunch is inevitable to simulate the lasing at the perfect synchronism. Second-order autocorrelation measurements show that FEL pulses shorter than four optical cycles are successively generated for a number of round trips at the perfect synchronism, which is consistent with numerical results.