ABSTRACT
This chapter discusses the present reigning champion among the X-ray light sources: the free electron laser (FEL). It presents a historical introduction of FEL, and discusses the properties of radiation from a sequence of bends, wigglers and undulators. The chapter shows how their radiation spectra compare, and focuses on undulator resonance conditions and microbunching. It describes the precise physical meaning and exact definition of the undulator parameter K. The two major kinds of FELs are single pass and multi pass. While multi-pass FELs were built primarily in the earlier days of FEL technology, single-pass devices are dominating the arena today. The need to use single-pass systems is actually a necessity dictated by an absence of good mirrors in the X-ray spectral region. The single-pass system has to be a high-gain system, which puts extreme constraints on the quality of the electron beam, as well as on the accuracy of the undulator.
