Cold intense electron beams from LN2-cooled GaAs-photocathodes

To study electron-ion interactions at the Heidelberg heavy-ion storage ring, electron beams with low-energy spreads and dc-currents of milliamperes are desired. Measurements of the photoelectron energy distribution showed that electron beams with energy spreads of 5-8 meV can be obtained from GaAs photocathodes, cooled to about LN2-temperature. However, in order to get milliamperes beam currents, the laser illumination has to be increased up to 1 W, causing substantial cathode heating. The presented new electron gun design based on sapphire-substrate transmission-mode photocathodes, cooled by LN2, stabilizes the GaAs bulk temperature under 1 W laser illumination at about 95 K and thereby provides the prerequisites for an electron gun being operated at milliampere-currents with low-energy spreads