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The GaAsP/GaAs superlattice (SL) structure has been widely recognized as the most efficient spin polarized electron source with 90% maximum polarization and more than 1% quantum efficiency. The main spin depolarization mechanisms in these structures are: interband absorption smearing due to band-edge fluctuations, hole scattering between the heavy hole (HH) and light hole (LH) states that causes a broadening of the LH band, spin precession due to an effective magnetic field generated by the lack of crystal inversion symmetry and spin orbit coupling.

Spin-polarized electrons are commonly used in high energy physics. Future work will benefit from greater polarization. Polarizations approaching 90% have been achieved at the expense of yield. The primary paths to higher polarization are material design and electron transport. Our work addresses the latter. Photoexcited electrons may be preferentially emitted or suppressed by an electric field applied across the active region. We are tuning this forward bias for maximum polarization and yield, together with other parameters, e.g., doping profile. Preliminary measurements have been carried out on bulk and thin film GaAs. As expected, the yield change far from the bandgap is quite large for bulk material. The bias is applied to the bottom (non-activated) side of the cathode so that the accelerating potential as measured with respect to the ground potential chamber walls is unchanged for different front-to-back cathode bias values. The size of the bias to cause an appreciable effect is rather small reflecting the low drift kinetic energy in the zero bias case.

The SLC polarized electron source (PES) can meet the expected requirements of the International Linear Collider (ILC) for polarization, charge and lifetime. However, experience with newer and successful PES designs at JLAB, Mainz, Nagoya and elsewhere can be incorporated into a first-generation ILC source that will emphasize reliability and stability without compromising the photocathode performance. The long pulse train for the ILC may introduce new challenges for the PES, and in addition more reliable and stable operation of the PES may be achievable if appropriate R&D is carried out for higher voltage operation and for a simpler load-lock system. The outline of the R&D program currently taking shape at SLAC and elsewhere is discussed. The principal components of the proposed ILC PES, including the laser system necessary for operational tests, are described.

Electron spin polarization in excess of 70% has been observed in photoemission from a 0.1- mu m-thick epitaxial layer of In/sub x/Ga/sub 1-x/As, with x approximately=0.13 grown on a GaAs substrate. Under these conditions, the epitaxial layer is expected to be highly strained by the 0.9% lattice mismatch, as confirmed by X-ray diffractometer measurements of the lattice parameter. The electron polarization and the quantum efficiency have been measured as a function of the excitation photon energy from 1.25 to 2.0 eV. A significant enhancement of the electron polarization occurs in the vicinity of 1.33 eV, where the expected strain-induced level splitting permits optical excitation of a single band transition. Measurements made on a control sample of 1.14- mu m thickness, significantly larger than the critical thickness for pseudomorphic strain, show no polarization enhancement. >

The authors describe the design of the ultra-high vacuum system for the beam-line from the 160 kV polarized electron gun to the linac injector in the Stanford Linear Collider (SLC). The polarized electron source is a GaAS photocathode, requiring 10/sup -11/ Torr-range pressure for adequate quantum efficiency and longevity. The photocathode is illuminated by 3 ns-long laser pulses. Photocathode maintenance and improvements require occasional substitution of guns with rapid restoration of UHV conditions. Differential pumping is crucial since the pressure in the injector is more than 10 times greater than the photocathode can tolerate and since electron-stimulated gas desorption from beam loss in excess of 0.1% of the 20 nC pulses may poison the photocathode. The design for the transport line contains a differential pumping region isolated by a pair of valves. Exchange of guns requires venting only this isolated region, which can be restored to UHV rapidly by baking. The differential pumping is performed by nonevaporable getters and an ion pump. >