We have developed and demonstrated a versatile, compact electron source that can produce a monoenergetic electron beam up to 50 mm in 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 cm 2 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• at 1 keV. The beam intensity can be controlled from 10 − 10 9 electrons cm −2 s −1 .
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