Electron beams are essential in such common technological applications as microwave ovens, radio transmitters, medical x-ray scanners, industrial lamination machines, and ultrahigh-resolution lithography. But they are equally essential in the more exotic world of nuclear and high-energy physics. Because it interacts with nucleons through well-understood quantum electrodynamics, an energetic electron beam may serve as a pointlike probe of the nuclear quark structure (see box 1). Alternatively, the beam can produce highly specialized forms of electromagnetic radiation in what are known as light sources, diverse accelerator facilities used to study the structure and dynamics of substances ranging from biological materials to nanocomposites. To generate its highly tunable light, for example, a typical free-electron laser relies on a relativistic stream of electrons delivered in a train of picosecond pulses (see box 2).
For accelerators, electrons are typically generated through either photoemission or thermionic emission. Field emission, in which electrons tunnel...
