Extremely low emittance electron beams are required for next generation accelerators. GaAs semiconductor photocathodes with negative electron affinity (NEA) surfaces have an intrinsic advantage for generating such low emittance beams and the thermal emittance as low as is expected in ideal case. The thermal emittance of photoelectrons was measured for two different NEA photocathodes: a bulk-GaAs photocathode and a GaAs-GaAsP superlattice strained photocathode. The normalized root-mean-sqare emittances for the beam radius of 1.0 mm were as low as and , respectively. A comparison of these results shows that the superlattice photocathode minimizes the thermal emittance for photon excitation energies higher than the band gap energy.
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