For a large part of the past century, the goal of higher energy drove the
development of particle accelerators. In recent times, however, the quest
for higher beam quality has become paramount, driven in large part by a
desire for brighter synchrotron light sources.  Existing,
third-generation, synchrotrons have produced countless discoveries in
fields such as structural biology and materials science.
Fourth-generation synchrotrons, otherwise known as free-electron lasers,
promise to substantially brighten some of the darker regions of the
spectrum, allowing some experiments that currently take hours to be
completed in less than a picosecond. The hard x-ray and teraHertz regions
are producing the most exciting developments. It turns out that the
brightness of the photon beam depends on the brightness of the electrons
beam, and therein lays the major challenge: How can I make a really
bright electron beam?