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

完全同期FEL発振器における高効率・超短パルス発生

羽島 良一; 西森 信行; 永井 良治; 峰原 英介

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

ゲインの十分大きなFEL発振器では、電子バンチと光パルスの完全同期状態で高効率・超短パルスの発生が可能であることが原研FELで実証された。原研FELで行なった実験では、従来の理論で示されている短バンチFEL発振器の効率に関するスケーリング則を超えた高い効率が得られ、また、この時の光パルスは4サイクル以下の超短パルスであった。これらの実験結果は数値解析の結果とよく一致しており、「持続的なスーパーラディアンス発振」とよぶべき新しい発振形態であると考えられる。

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.