  We have developed and demonstrated a versatile, compact electron source that
can produce a monoenergetic electron beam up to 50 mm diameter from 0.1 to 30
keV with an energy spread of <10 eV. By illuminating a metal cathode plate with
a single near ultraviolet (UV) light emitting diode (LED), a spatially uniform
electron beam with 15% variation over 1 cm2 can be generated. A uniform
electric field in front of the cathode surface accelerates the electrons into a
beam with an angular divergence of <1 degree at 1 keV. The beam intensity can
be controlled from 10^9 electrons/cm2s.
