Nuclear Instruments and Methods in Physics Research A 475 (2001) 579–582 Optimization of power output and study of electron beam energy spread in a Free Electron Laser oscillator A. Abramovich a, *, Y. Pinhasi a , A. Yahalom a , D. Bar-Lev b , S. Efimov b , A. Gover b a Department of Electrical and Electronic Engineering, Faculty of engineering, The College of Judea and Samaria, P.O. Box 3, Ariel 44837, Israel b Department of Electrical Engineering, Physical Electronics, Ramat-Aviv, Tel-Aviv 69978, Israel Abstract Design of a multi-stage depressed collector for efficient operation of a Free Electron Laser (FEL) oscillator requires knowledge of the electron beam energy distribution. This knowledge is necessary to determine the voltages of the depressed collector electrodes that optimize the collection efficiency and overall energy conversion efficiency of the FEL. The energy spread in the electron beam is due to interaction in the wiggler region, as electrons enter the interaction region at different phases relative to the EM wave. This interaction can be simulated well by a three-dimensional simulation code such as FEL3D. The main adjustable parameters that determine the electron beam energy spread after interaction are the e-beam current, the initial beam energy, and the quality factor of the resonator out-coupling coefficient. Using FEL3D, we study the influence of these parameters on the available radiation power and on the electron beam energy distribution at the undulator exit. Simulations performed for I ¼ 1:5A, E ¼ 1:4MeV, L ¼ 20% (internal loss factor) showed that the highest radiated output power and smallest energy spread are attained for an output coupler transmission coefficient T m D30%. r 2001 Published by Elsevier Science B.V. PACS: 41.60.m; 41.60.Cr; 41.85.Qg Keywords: FEL oscillator; Electron beam diagnostic 1. Introduction Good electron beam transport along the beam- line of a Free Electron Laser (FEL) oscillator is essential in order to enable efficient energy exchange between the beam electrons and the electromagnetic wave inside the interaction region. Good transport is particularly important in electron beam energy recovery schemes such as a depressed collector in an Electrostatic Accelerator FEL (EA-FEL) [1–3]. At the entrance to the interaction region all the electrons have very nearly the same energy. In passing through the interaction region electrons may lose or gain a different amount of energy from the electromag- netic wave, depending on their entrance phase; consequently electrons have a large energy spread at the interaction region exit. Since the beam energy spread is generated by the nonlinear interaction process taking place in the resonator, it should be considered in the design of the FEL resonator. The resonator parameters (losses, output coupling coefficient) should be optimized for attaining both maximum output power emission and minimum energy spread [4,5]. *Corresponding author. Fax: +972-3-9066-238. 0168-9002/01/$-see front matter r 2001 Published by Elsevier Science B.V. PII:S0168-9002(01)01589-3