Free-Electron Laser Operation and Self-Amplified Spontaneous Emission Using a Step-Tapered Undulator

We present an experimental and theoretical evaluation of a new method of enhancing the efficiency and gain of the free-electron laser (FEL) and observations of self-amplified spontaneous emission at start-up of the step-tapered FEL. The stepped undulator is divided into two uniform sections of different deflection strengths, the upstream K-1 and the downstream K-2, and a step of Delta K = K-2 - K-1 approximate to 0.03 with K-1 < K-2 for mid-infrared operation.