A. Brachmann

A Very High Charge, High Polarization Gradient-Doped Strained GaAs Photocathode

Abstract A high-gradient-doping technique is applied to strained polarized photocathodes. A 5.0– 7.5 nm p-type surface layer doped to 5×10 19 cm −3 is found sufficient to overcome the surface charge limit while maintaining high beam polarization. This technique can be employed to meet the charge requirements of the Next Linear Collider with a polarization approaching 80%.

Systematic study of polarized electron emission from strained GaAs/GaAsP superlattice photocathodes

Spin-polarized electron photoemission has been studied for GaAs∕GaAs1−xPx strained superlattice cathodes grown by gas-source molecular beam epitaxy. The superlattice structural parameters are systematically varied to optimize the photoemission characteristics. The heavy-hole and light-hole transitions are reproducibly observed in quantum efficiency spectra, enabling direct measurement of the band energies and the energy splitting. Electron-spin polarization as high as 86% with over 1% quantum efficiency has been observed.

SLAC's polarized electron source laser system and minimization of electron beam helicity correlations for the E-158 parity violation experiment

SLAC E-158 is an experiment designed to make the first measurement of parity violation in Moller scattering. E-158 will measure the right-left cross-section asymmetry, ALRMoller, in the elastic scattering of a 45-GeV polarized electron beam from unpolarized electrons in a liquid hydrogen target. E-158 plans to measure the expected Standard Model asymmetry of ∼10−7 to an accuracy of better than 10−8. To make this measurement, the photoemission-based polarized electron source requires an intense circularly polarized laser beam and the ability to quickly switch between right- and left-helicity polarization states with minimal right-left helicity-correlated asymmetries in the resulting beam parameters (intensity, position, angle, spot size, and energy), beamALRs. This laser beam is produced by a unique SLAC-designed flashlamp-pumped Ti:Sapphire laser and is directed through a carefully designed set of polarization optics. We analyze the transport of nearly circularly polarized light through the optical system and identify several mechanisms that generate beamALRs. We show that the dominant effects depend linearly on particular polarization phase shifts in the optical system. We present the laser system design and a discussion of the suppression and control of beamALRs. We also present results on beam performance from engineering and physics runs for E-158.

A HIGH-INTENSITY HIGHLY-POLARIZED ELECTRON BEAM FOR HIGH-ENERGY PHYSICS*

A new high-energy parity violation (PV) experiment at SLAC as well as particle-physics experiments using future e + e - colliders (such as NLC) operating at energies above the scale of unification of the electromagnetic and weak interactions require a highly-polarized electron beam with intensities previously unachievable due to a surface charge limit (SCL) effect at the cathode of the polarized electron source. A newly developed photocathode which allows these high intensities is being used for the SLAC PV experiment E-158. The intensity stability required for stable machine operation is determined by the source laser stability, which has been reduced to 0.5%. Temporal pulse shaping is performed on the laser beam using an improved pulse shaper. Details of the beam generation, energy compensation, and linac performance recently achieved for the SLAC parity violation experiment E-158 are discussed.

SLAC's polarized electron source laser system for the E-158 parity violation experiment

SLAC E158 is an experiment to make the first measurement of parity violation in Moller scattering. The left-right cross-section asymmetry in the elastic scattering of a 45-GeV polarized electron beam off unpolarized electrons in a liquid hydrogen target will be measured to an accuracy of better than 10-8, with the expected Standard Model asymmetry being approximately 10-7. An intense circularly polarized laser beam for the polarized electron source is required with the ability to quickly switch between left and right polarization states with minimal left-right asymmetries in the parameters of the electron beam. This laser beam is produced by a unique SLAC-designed, flash-lamp pumped, Ti:Sapphire laser. We present this laser system design and initial results from recent commissioning runs.

Helicity-correlated systematics for SLAC Experiment E158

Experiment E158 at the Stanford Linear Accelerator Center (SLAC) will make the first measurement of parity violation in Moller scattering. The left-right cross-section asymmetry in the elastic scattering of a 45-GeV polarized electron beam with unpolarized electrons in a liquid hydrogen target will be measured to an accuracy of better than 10/sup -8/, with the expected Standard Model asymmetry being approximately 10/sup -7/. Because helicity-correlated (left-right) charge and position asymmetries in the electron beam can give rise to systematic errors in the measurement, great care must be given to beam monitoring and control. We have developed beam current monitors that measure the charge per pulse at the 3 /spl times/ 10/sup -5/ level and RF cavity beam position monitors that measure the position per pulse to 1 /spl mu/m, which should allow precisions of 1 ppb and 1 nm for the final integrated charge and position asymmetries, respectively. In addition, since most helicity-correlated systematics in the electron beam can be traced back to the laser that drives the photoemission from the GaAs source cathode, we first use careful control of laser beam polarization, point-to-point imaging, and other techniques to minimize systematics. We also provide the capability of modulating in a helicity-correlated way the laser beams intensity and position as it strikes the photocathode, allowing the implementation of active feedbacks to ensure that the average charge and position asymmetries integrate close to zero over the course of the experiment. We present this system of precision beam monitoring and control and report on its performance during a recent commissioning run, T-437 at SLAC, which demonstrated charge and position asymmetry precisions of 12 ppb and 2 nm, respectively.

The ILC Polarized Electron Source

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.

Recent Polarized Photocathode R& D at SLAC

The SLAC high-gradient-doped MOCVD-grown GaAs cathode presently in use consists of a strained GaAs low-doped layer (with a small admixture of P) capped by a few nanometers of highly Zn-doped GaAs, which is heat-cleaned at relatively high temperature and then activated by Cs/NF{sub 3} co-deposition. The high-gradient-doped structure solves the problem of the surface charge limit that the previously-used SLAC cathodes had, and this preparation procedure has produced satisfactory results. However, the preparation procedure has a few weaknesses that prevent cathodes from achieving the ultimate desired performance. The peak polarization is limited to 80% due to strain relaxation in the relatively thick strained layers. Also dopant loss causes the surface charge limit effect to reappear after multiple high-temperature heat-cleanings. In this paper, we will discuss recent progress made at SLAC that addresses these limitations, including using the MBE growth technique with Be doping and using the superlattice structure. In addition, to reduce the heat-cleaning temperature, an atomic hydrogen cleaning technique is explored.

Transport Mechanisms in Polarized Semiconductor Photocathodes

We investigated the effect of an accelerating field on the spin polarization of photo‐generated electrons in a 100nm thick GaAs based photocathode active region. By decreasing the transport time of the electrons and the number of scattering events that cause depolarization, we expected to increase the polarization as was indicated by Monte Carlo simulations of the scattering and transport time statistics of the electrons.A tungsten (W) grid was deposited on the cathode surface to provide a uniform voltage distribution across the cathode surface. The metal grid formed a Schottky contact with the semiconductor surface. The bias voltage was primarily dropped at the metal semiconductor interface region, which is the cathode active region. For positive surface bias, the accelerating voltage not only increased the polarization, but it also enhanced the quantum efficiency of the photocathode. Preliminary results verify the bias effect on both quantum efficiency and polarization by a factor of 1.8 and 1% respecti...

The Polarized Electron Source for the International Collider (ILC) Project

The ILC project will be the next large high energy physics tool that will use polarized electrons (and positrons). For this machine spin physics will play an important role. The polarized electron source design is based on electron injectors built for the Stanford Linear Collider (polarized) and Tesla Test Facility (un‐polarized). The ILC polarized electron source will provide a 5GeV spin polarized electron beam for injection into the ILC damping ring. Although most ILC machine parameters have been achieved by the SLC or TTF source, features of both must be integrated into one design. The bunch train structure presents unique challenges to the source laser drive system. A suitable laser system has not yet been demonstrated and is part of the ongoing R&D program for ILC at SLAC. Furthermore, ILC injector R&D incorporates photocathode development, increasing available polarization, and improving operational properties in gun vacuum systems. Another important area of research and development is advancing the...