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Transport Mechanisms in Polarized Semiconductor Photocathodes

Description: We investigated the effect of an accelerating field on the spin polarization of photogenerated 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% respectively.
Date: December 18, 2006
Creator: Ioakeimidi, K.; Brachmann, A.; Clendenin, J.E.; Garwin, E.L.; Kirby, R.E.; Maruyama, T. et al.
Partner: UNT Libraries Government Documents Department

Low Emittance Guns for the ILC Polarized Electron Beam

Description: Polarized electron beams generated by DC guns are routinely available at several accelerators including JLAB, Mainz and SLAC. These guns operate with a cathode bias on the order of -100 kV. To minimize space charge effects, relatively long bunches are generated at the gun and then compressed longitudinally external to the gun just before and during initial acceleration. For linear colliders, this compression is accomplished using a combination of rf bunchers. For the basic design of the International Linear Collider (ILC), a 120 kV DC photocathode gun is used to produce a series of nanosecond bunches that are each compressed by two sub-harmonic bunchers (SHBs) followed by an L-band buncher and capture section. The longitudinal bunching process results in a significantly higher emittance than produced by the gun alone. While high-energy experiments using polarized beams are not generally sensitive to the source emittance, there are several benefits to a lower source emittance including a simpler more efficient injector system and a lower radiation load during transport especially at bends as at the damping ring. For the ILC, the SHBs could be eliminated if the voltage of the gun is raised sufficiently. Simulations using the General Particle Tracer (GPT) package indicate that a cathode bias voltage of {ge}200 kV should allow both SHBs to be operated at 433 or even 650 MHz, while {ge}500 kV would be required to eliminate the SHBs altogether. Simulations can be used to determine the minimum emittance possible if the injector is designed for a given increased voltage. A possible alternative to the DC gun is an rf gun. Emittance compensation, routinely used with rf guns, is discussed for higher-voltage DC guns.
Date: December 1, 2006
Creator: Clendenin, J.E.; Brachmann, A.; Ioakeimidi, K.; Kirby, R.E.; Maruyama, T.; Miller, R.H. et al.
Partner: UNT Libraries Government Documents Department

Polarization Possibilities of Small Spin-Orbit Interaction in Strained-Superlattice Photocathodes

Description: Strained-superlattice photocathodes based on InGaP/GaAs were investigated. The photocathode performance is found highly dependent on the superlattice parameters. The electron confinement energy in superlattice appears important.
Date: December 12, 2006
Creator: Maruyama, T.; Brachmann, A.; Clendenin, J.E.; Garwin, E.L.; Ioakeimidi, K.; Kirby, R.E. et al.
Partner: UNT Libraries Government Documents Department

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

Description: 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 design of DC and RF electron gun technology for polarized sources. This presentation presents the current status of the design and outlines aspects of the relevant R&D program carried out within the ILC community.
Date: December 1, 2006
Creator: Brachmann, A.; Clendenin, J.E.; Garwin, E.L.; Ioakeimidi, K.; Kirby, R.e.; Maruyama, T. et al.
Partner: UNT Libraries Government Documents Department

RF Guns for Generation of Polarized Electron Beams

Description: Several accelerators, including the SLC, JLAB, Mainz, Bates/MIT, and Bonn have successfully operated for medium and high energy physics experiments using polarized electron beams generated by dc-biased guns employing GaAs photocathodes. Since these guns have all used a bias on the order of 100 kV, the longitudinal emittance of the extracted bunch is rather poor. Downstream rf bunching systems increase the transverse emittance. An rf gun with a GaAs photocathode would eliminate the need for separate rf bunchers, resulting in a simpler injection system. In addition, the thermal emittance of GaAs-type cathodes is significantly lower than for other photocathode materials. The environmental requirements for operating activated GaAs photocathodes cannot be met by rf guns as currently designed and operated. These requirements, including limits on vacuum and electron back bombardment, are discussed in some detail. Modifications to actual and proposed rf gun designs that would allow these requirements to be met are presented.
Date: November 9, 2005
Creator: Clendenin, J.E.; Brachmann, A.; Dowell, D.H.; Garwin, E.L.; Ioakeimidi, K.; Kirby, R.E. et al.
Partner: UNT Libraries Government Documents Department

Comparison of ALINGAAS/GAAS Superlattice Photocathodes Having Low Conduction Band Offset

Description: The main advantage of superlattice (SL) structures as spin polarized electron emitters is the ability to provide a large splitting between the heavy hole (HH) and light hole (LH) valence bands (VB) over a large active thickness compared to single strained layers. Two important depolarization mechanisms in these structures are the scattering effects during the transit of the electrons in the active region and the depolarization that takes place in the band bending region (BBR) near the surface. In this paper, we systematically study the effects of the electron mobility and transit time by using an InAlGaAs/GaAs SL with a flat conduction band (CB). Initial results by the SPTU-SLAC collaboration using such structures grown by the Ioffe Institute showed polarization and quantum yield (QE) of 92% and 0.2% respectively. We report measurements using similar structures grown by SVT Associates. The results (polarization up to 90%) are also compared with simulations.
Date: March 31, 2006
Creator: Ioakeimidi, K,; Maruyama, T.; Clendenin, J.E.; Brachmann, A.; Garwin, E.L.; Kirby, R.E. et al.
Partner: UNT Libraries Government Documents Department