All polarized electron sources in use on accelerators today are based
on photoemission from GaAs and related semiconductors or semiconductor structures.
The development of a highly polarized electron source capable of supporting
a linac-ring style electron-hadron collider is a challenging task. The average
electron beam current required to obtain the desired luminosity is about three
orders of magnitude above the highest values demonstrated to date in a high
polarization source. Even a much easier source providing only moderate polarization
(~ 40%) would operate well beyond the performance of present day sources.
The primary issues are obtaining an adequate quantum efficiency and photocathode
operational lifetime, and developing a suitable laser to illuminate the photocathode.
We will briefly review the state-of-the-art in present day polarized electron
sources, identify the current limiting phenomena, and suggest avenues for
future R&D which might lead to polarized sources matched to needs of
linac-ring colliders.