Initial beam emittance measurements for electron gun with NEA-GaAs type photocathodes

Extremely low emittance electron beams are necessary for new generation accelerators. The required emittances is as low as 0.1 π.mm.mrad and NEA-type photocathodes have an intrinsic advantage of generating such a low emittance. In this paper, emittance measurements of photoelectrons extracted from two kinds of NEA photocathodes are described. The measurement were carried out using a 200 kV polarized electron source at Nagoya University. The normalized vertical emittances of Bulk-GaAs and GaAs-GaAsP stained-layer superlattice photocathodes are as low as 0.12-0.18 ± 0.03 π.mm.mrad and 0.09 ± 0.01 π.mm.mrad near the bandgap energy in the case of 0.7 mm beam radius, respectively. These results showed the advantage of using NEA-GaAs type photocathodes for the generation of low emittance beams. (author)