An Introduction to Free-electron Lasers

Free-electron lasers (FEL) represent the pinnacle of current generation accelerator-based light sources. In addition to their unparalleled peak brightness, FELs offer highly tunable pulsed radiation with pulse durations as short as femtoseconds. In this paper, we will explore the fundamental operating principles behind these formidable instruments. We begin by introducing key concepts in conventional laser theory, thus forming a framework for subsequent discussion. Next, we examine the physics behind accelerating and maintaining a stable, relativistic electron beam. Then, we use classical EM theory to explain the radiation output from the FEL, as well as its different regimes of laser amplification. We conclude with a brief overview of future FEL research and development