The negative-electron-affinity (NEA) photocathodes which produce polarized electrons are a vital component of electron accelerators such as that at DoE Jefferson Lab and the Stanford Linear Accelerator Center (SLAC). Future systems, such as the International Linear Collider (ILC), will require a polarized electron beam intensity at least 20 times greater than produced by strained GaAs, which is used in the current generation of photocathodes. Additionally, the degree of electron polarization needs to be increased beyond the 80% currently attainable and intrinsic material properties related to improving the surface charge limit must also be addressed, and the photocathodes should be more robust in an RF gun environment. The end result of the combined Phase I Phase II effort will be a new generation of robust photocathodes capable of yielding intense, highly polarized electron beams for use in advanced electron colliders. We have previously achieved & gt; 85% polarization using a strained superlattice formed from alternating layers of GaAs and GaAsP approximately ten monolayers thick. For this program we will apply a novel strain- compensated superlattice concept utilizing antimony-, arsenic-, and phosphorus-based material which should overcome material limitations of the GaAs/GaAsP alloys. In the Phase I we designed and fabricated an Sb- and P-based strain-compensated superlattice structure grown by molecular beam epitaxy. The Phase I program optimized the growth conditions to achieve the desired alloy composition and interface quality. Photocathode structures were grown and characterized. Novel Sb-based SL photocathodes studied in Phase I will be further optimized by investigating parameters that can affect the polarization and quantum efficiency of these photocathodes for high current electron guns. Further improvement on QE The performance of the optimized cathodes will be evaluated in realistic gun environment by Jefferson Lab. Commercial Applications and Other
Benefits: A successful project will produce a highly efficient polarized electron source for use in experimental research at DoE Jefferson Lab, SLAC, and other electron collider facilities. These devices have applications in other areas which include magnetic imaging research, surface analysis, Quantum computing and cryptography.
