Introduction

This chapter is meant to provide an overall introduction to the physics of free-electron lasers. It begins with a short historical perspective on the early history of free-electron lasers. This is followed by brief explanation of the principles of free-electron laser operation. This includes an explanation of the ponderomotive wave and the resonance condition as well as the pendulum equation governing wave trapping in the ponderomotive wave. Formulae for the gain in the low-gain regime and the exponentiation rate in high-gain configurations such as the Compton and Raman regimes are given, and the Pierce parameter is introduced. Basic expressions for the efficiency and bandwidth in these regimes are also introduced. Free-electron lasers have been built in a number of different configurations including both low-gain/high-Q oscillators and high-gain/low-Q oscillators (referred to as regenerative amplifiers or RAFELs for short), master oscillator power amplifiers (MOPAs), and systems where the optical field grows from shot noise on the electron beam to saturation in a single pass through the wiggler and which is referred to as self-amplified spontaneous emission (SASE). Optical klystrons consisting of two wigglers separated by a dispersive section that usually consists of a magnetic chicane composed of dipole magnets have also been demonstrated. In an optical klystron, the electron beam is injected into the first wiggler (called the modulator) in synchronism with a high-intensity laser pulse at the resonant wavelength which causes some initial bunching of the beam at the resonant wavelength. As the beam traverse the chicane, this bunching is enhanced which preconditions the beam for rapid amplification of the optical pulse in the second wiggler (called the radiator). When the radiator is tuned to a harmonic of the fundamental resonant wavelength in the modulator, this interaction is termed high-gain harmonic generation (HGHG). While all currently operating or designed free-electron lasers obey classical physics, quantum mechanical effects can be important when the spreading of the electron wave packets over the course of the wiggler approaches the ponderomotive wavelength. This condition is explained, and a quantitative measure of when quantum mechanical effects become important is derived. Finally, a description of the historical development and present status of free-electron laser experiments are given as well as an extensive bibliography of these experiments.