Fundamental Concepts

In the early 1970s John M.J.Madey, then at Stanford University, proposed the free-electron concept (Madey, 1971). The central idea is to extract light energy from electrons passing through a magnetic field with spatially periodic variations in intensity and direction. His team demonstrated first a free-electron laser amplifier (Elias et al., 1976) and later a free-electron oscillator (Deacon et al., 1977).

High-energy electrons emit light (called synchrotron radiation) when a magnetic field bends their path or otherwise accelerates (or decelerates) them. This emission resembles spontaneous emission from atoms or molecules, but free electrons lack discrete energy levels, so synchrotron radiation is not limited to discrete transitions.

In a free-electron laser, energetic electrons pass through a regular array of magnets with alternating polarity, called a wiggler or undulator magnet, as shown in Fig. 28.1. The regularly spaced magnetic fields bend the electron beam back and forth (thus the name wiggler ) periodically. After the electrons pass through part of the magnet array, they form clumps separated by the period of the magnetic field. Thus each...