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We measured the quantum-efficiency (QE) dependence of the spin polarization of photoemission from a GaAs-AlGaAs superlattice over a wide range of QE values. In the QE range from 10-4 to 10-9, a change in the polarization from 76% to 48% was observed at a wavelength of 778.5 nm. The maximum polarization at 778.5 nm was observed at a QE one tenth of the maximum value. We also observed a similar dependence at other wavelengths. Such a behavior suggests that the vacuum level and surface potential barrier play important roles concerning the spin polarization of photoemission.

A charge of 2.3×1011 electrons in 2.5 ns at a laser wavelength of 757 nm with a corresponding quantum efficiency (QE) of 2.0% measured at 752 nm was extracted from a -120 kV biased, 20 mm diameter, GaAs–AlGaAs superlattice photocathode. The maximum electron polarization measured with material from the same wafer, but in a different system, was 71% at 757 nm for a QE of 1.0% measured at 752 nm. The quantity and temporal distribution of the extracted charge is consistent with a space charge limitation, rather than a cathode charge limit. The performance of this type of cathode makes it a possible candidate for future linear colliders.

We have studied the polarization of photoemission from an In0.15Ga0.85As–GaAs strained-layer superlattice. The polarization of 82.7±0.3(stat.)±6.1(syst.)% was observed at laser wavelengths from 911 to 916 nm at room temperature. The quantum efficiency at the wavelength of 911 nm was ∼0.015% in the vacuum of ∼6×10-10 Torr with high cathode voltage of -4 kV.

A new pulsed rf source, the “lasertron”, is being developed as a possible rf power source for future electron-positron linear colliders. In a series of systematic studies, a prototype lasertron was fabricated and tested. A peak power of 80 kW was attained at a 2.856 GHz rf frequency with a 1 µs time duration. This paper describes the results of experimentation on the lasertron, including the development of the photocathode and the laser system. Test results are compared with the analysis of beam dynamics in the lasertron.