Download or read book Performance of the SLC Polarized Electron Source with High Polarization written by and published by . This book was released on 1993 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: For the 1992 operating cycle of the SLAC Linear Collider (SLC), the polarized electron source (PES) during its maiden run successfully met the pulse intensity and overall efficiency requirements of the SLC. However, the polarization of the bulk GaAs cathode was low ((approximately)27%) and the pulse-to-pulse stability was marginal. We have shown that adequate charge for the SLC can be extracted from a strained layer cathode having P{sub e}(approximately)80% even though the quantum efficiency (QE) is

Download or read book Polarized Electron Sources written by and published by . This book was released on 1995 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: Polarized electron sources for high energy accelerators took a significant step forward with the introduction of a new laser-driven photocathode source for the SLC in 1992. With an electron beam polarization of>80% and with (approximately)99% uptime during continuous operation, this source is a key factor in the success of the current SLC high-energy physics program. The SLC source performance is used to illustrate both the capabilities and the limitations of solid-state sources. The beam requirements for future colliders are similar to that of the SLC with the addition in most cases of multiple-bunch operation. A design for the next generation accelerator source that can improve the operational characteristics and at least minimize some of the inherent limitations of present sources is presented. Finally, the possibilities for producing highly polarized electron beams for high-duty-factor accelerators are discussed.

Download or read book Physics Results with Polarized Electrons at SLAC written by and published by . This book was released on 1996 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: Polarized electron beams can play an important role in the dynamics of interactions at high energies. Polarized electron beams at SLAC have been an important part of the physics program since 1970, when they were first proposed for use in testing the spin structure of the proton. Since 1992, the SLAC linear accelerator and the SLC have operated solely with polarized electrons, providing data for tests of QCD in studies of the spin structure of the nucleon and tests of the electroweak sector of the Standard Model. In the following sections, the performance of the source is summarized, and some of the recent results using the polarized beams are discussed.

Download or read book Advanced Strained Superlattice Photocathodes for Polarized Electron Sources written by and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Polarized electrons have been essential for high-energy parity-violating experiments and measurements of the nucleon spin structure. The availability of a polarized electron beam was crucial to the success of the Stanford Linear Collider (SLC) in achieving a precise measurement of the electroweak mixing angle, and polarized electron beams will be required for all future linear colliders. Polarized electrons are readily produced by GaAs photocathode sources. When a circularly polarized laser beam tuned to the bandgap minimum is directed to the negative-electron-affinity (NEA) surface of a GaAs crystal, longitudinally polarized electrons are emitted into vacuum. The electron polarization is easily reversed by reversing the laser polarization. The important properties of these photocathodes for accelerator applications are: degree of polarization of the extracted beam; ability to extract sufficient charge to meet accelerator pulse-structure requirements; efficiency and stability of operation; and absence of any asymmetries in the beam properties (charge, position, energy, etc.) upon polarization reversal. The performance of GaAs photocathodes has improved significantly since they were first introduced in 1978 [1]. The theoretical maximum polarization of 50% for natural GaAs was first exceeded in 1991 using the lattice mismatch of a thin InGaAs layer epitaxially grown over a GaAs substrate to generate a strain in the former that broke the natural degeneracy between the heavy- and light-hole valence bands [2]. Polarizations as high as 78% were produced for the SLC from photocathodes based on a thin GaAs epilayer grown on GaAsP [3,4]. After 10 years of experience with many cathode samples at several laboratories [5], the maximum polarization using the GaAs/GaAsP single strained-layer cathode remained limited to 80%, while the quantum efficiency (QE) for a 100-nm epilayer is only 0.3% or less. Two factors were known to limit the polarization of these cathodes: (1) the limited band splitting; and (2) a relaxation of the strain in the epilayer since the 10-nm critical thickness for maintaining perfect strain is exceeded for a 1 % lattice-mismatch [6]. Strained superlattice structures, consisting of very thin quantum well layers alternating with lattice-mismatched barrier layers are excellent candidates for higher polarization. Due to the difference in the effective mass of the heavy- and light-holes, a superlattice exhibits a natural splitting of the valence band, which adds to the strain-induced splitting. In addition, each of the SL layers is thinner than the critical thickness. Polarized photoemission from strained InGaAs/GaAs [7], InGaAdAlGaAs [8], and GaAs/GaAsP [9,10] superlattice structures have been reported in the literature. For this Phase II program, SVT Associates worked with the Stanford Linear Accelerator Center (SLAC) and University of Wisconsin at Madison to create photocathodes with improved polarization by employing GaAs/GaAsP superlattices. These superlattices consist of alternating thin layers of GaAs and GaAsP. The thicknesses and alloy compositions are designed to create a strained GaAs photoemission layer. Under strain, the heavy-hole and light-hole valence bands in GaAs split, removing degeneracy and allowing high polarization, theoretically 100%. This final report discusses the efforts and results achieved, comparing the device performance of newly created superlattice photocathodes grown by molecular beam epitaxy (MBE) with the devices created by other fabrication technologies, and efforts to optimize and improve the device operation.

Download or read book Polarization at SLAC written by and published by . This book was released on 1995 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: A highly polarized electron beam is a key feature. for the Current physics program at SLAC. An electron beam polarization of 80% can now be routinely achieved for typically 5000 hours of machine operation per year. Two main Physics programs utilize the polarized beam. Fixed target experiments in End Station A study the collision of polarized electrons with polarized nuclear targets to elucidate the spin structure of the nucleon and to provide an important test of QCD. Using the SLAC Linear Collider, collisions of polarized electrons with unpolarized positrons allow precise measurements of parity violation in the Z-fermion couplings and provide a very precise measurement of tile weak mixing angle. This paper discusses polarized beam operation at SLAC, and gives an overview of the polarized physics program.

Download or read book Superlattice Photocathodes for Accelerator Based Polarized Electron Source Applications written by and published by . This book was released on 1999 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: A major improvement in the performance of the SLC was achieved with the introduction of thin strained-layer semiconductor crystals. After some optimization, polarizations of 75-85% became standard with lifetimes that were equal to or better than that of thick unstrained crystals. Other accelerators of polarized electrons, generally operating with a much higher duty factor, have now successfully utilized similar photocathodes. For future colliders, the principal remaining problem is the limit on the total charge that can be extracted in a time scale of 10 to 100 ns. In addition, higher polarization is critical for exploring new physics, especially supersymmetry. However, it appears that strained-layer crystals have reached the limit of their optimization. Today strained superlattice crystals are the most promising candidates for better performance. The individual layers of the superlattice can be designed to be below the critical thickness for strain relaxation, thus in principle improving the polarization. Thin layers also promote high electron conduction to the surface. In addition the potential barriers at the surface for both emission of conduction-band electrons to vacuum and for tunneling of valence-band holes to the surface can be significantly less than for single strained-layer crystals, thus enhancing both the yield at any intensity and also decreasing the limitations on the total charge. The inviting properties of the recently developed AlInGaAs/GaAs strained superlattice with minimal barriers in the conduction band are discussed in detail.

Download or read book Highly Polarized Electron Source by Using in GaAs GaAs Strained Layer Superlattice written by Tsunehiko Omori and published by . This book was released on 1994 with total page 14 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Polarized Electron Source Developments written by and published by . This book was released on 1990 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Presently, only two methods of producing beams of polarized electrons for injection into linear accelerators are in use. Each of these methods uses optical pumping by circularly polarized light to produce electron polarization. In one case, electron polarization is established in metastable helium atoms, while in the other case, the polarized electrons are produced in the conduction band of appropriate semiconductors. The polarized electrons are liberated from the helium metastable by chemi-ionization, and from the semiconductors by lowering the work function at the surface of the material. Developments with each of these sources since the 1988 Spin Physics Conference are reviewed, and the prospects for further improvements discussed.

Download or read book Recent Advances in Polarized Electron Sources written by Charles K. Sinclair and published by . This book was released on 1999 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Measurement of Electron Beam Polarization at the SLC written by and published by . This book was released on 1987 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The polarimeters needed to monitor and measure electron beam polarization at the Stanford Linear Collider are discussed. Two types of polarimeters, are to be used. The first is based on the spin dependent elastic scattering of photons from high energy electrons. The second utilizes the spin dependence of elastic electron-electron scattering. The plans of the SLC polarization group to measure and monitor electron beam polarization are discussed. A brief discussion of the physics and the demands it imposes on beam polarization measurements is presented. The Compton polarimeter and the essential characteristics of two Moeller polarimeters are presented. (LEW).

Download or read book Polarization Developments written by and published by . This book was released on 1993 with total page 6 pages. Available in PDF, EPUB and Kindle. Book excerpt: Recent developments in laser-driven photoemission sources of polarized electrons have made prospects for highly polarized electron beams in a future linear collider very promising. This talk discusses the experiences with the SLC polarized electron source, the recent progress with research into gallium arsenide and strained gallium arsenide as a photocathode material, and the suitability of these cathode materials for a future linear collider based on the parameters of the several linear collider designs that exist.

Download or read book Improved Superlattices for Spin Polarized Electron Sources written by V. M. Ustinov and published by . This book was released on 2006 with total page 7 pages. Available in PDF, EPUB and Kindle. Book excerpt: Photoemission of polarized electrons from heterostructures based on InAlGaAs/GaAs superlattices with minimum conduction-band offsets is investigated. The comparison of the excitation energy dependence of the photoemission polarization degree with the calculated spectra makes it possible to determine the polarization losses at different stages of the photoemission. A maximum polarization of P = 91% and a quantum efficiency of QE = 0.5% are close to the best results obtained for photocathodes that are based on strained semiconductor superlattices.

Download or read book Photocathode Performance Measurements for the SLC Polarized Electron Gun written by and published by . This book was released on 1993 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: A low-voltage test system is used to qualify various III-V semiconductor materials as photocathodes for the SLC. The system features a load lock to introduce samples, high pumping speed, a sensitive residual gas analyzer, and an infrared temperature detector. Heat cleaning, cesiation, and oxidation procedures have been studied to optimize cathode activation for achieving an optimum NEA surface. VGF GaAs, MBE-grown AlGaAs, MBE GaAs layered on AlGaAs, and MOCVD GaAsP cathodes with different active layer thicknesses and doping concentrations have been tested for quantum efficiency and lifetime. New higher-polarization strained-layer GaAs on GaAsP photocathodes have also been tested. Results and operational experience are discussed.

Download or read book Polarization Study for NLC Positron Source Using EGS4 written by and published by . This book was released on 2000 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: SLAC is exploring a polarized positron source to study new physics for the NLC project. The positron source envisioned in this paper consists of a polarized electron source, a 50-MeV electron accelerator, a thin target less-than-or-equal-to 0.2 radiation length for positron production, and a capture system for high-energy, small angular divergence positrons. The EGS4 code was used to study the yield, energy spectra, emission-angle distribution, and the mean polarization of the positrons emanating from W-Re and Ti targets hit by longitudinally polarized electron and photon beams. To account for polarization within the EGS4 code a method devised by Flottmann was used, which takes into account polarization transfer for pair production, bremsstrahlung, and Compton interactions. A mean polarization of 0.85 for positrons with energies greater than 25 MeV was obtained. Most of the high-energy positrons were emitted within a forward angle of 20 degrees. The yield of positrons above 25 MeV per incident photon was 0.034, which was about 70 times higher than that obtained with an electron beam.

Download or read book Summary of Polarized E written by and published by . This book was released on 2007 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: The development of polarized electron sources in the 1970s capable of generating beams for injection into electron accelerators has been a major enabling factor for spin physics with electrons during the past quarter century. These sources continue to be refined for higher polarization and better operability. Recent developments were presented at this workshop in both plenary sessions and in 2 separate parallel sessions. The ILC plans to utilize not only a polarized electron source but also a polarized positron source. The current state of two types of positron sources were presented. This paper is a brief summary of all of these presentations.

Download or read book Highly Polarized Electron Emission from Strain Compensated Superlattices and Superlattices with High Valence Band Splitting written by and published by . This book was released on 2004 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: Results of experimental and theoretical studies of polarized electron emission from new short-period strain-compensated InAlGaAs-GaAsP superlattices and InAlGaAs-AlGaAs superlattices with a high-valence-band splitting are presented. The superlattice structures investigated are found to be very promising for spin-polarized electron sources.

Download or read book The Stanford Linear Collider written by and published by . This book was released on 1995 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Stanford Linear Collider (SLC) is the first and only high-energy ee− linear collider in the world. Its most remarkable features are high intensity, submicron sized, polarized (e−) beams at a single interaction point. The main challenges posed by these unique characteristics include machine-wide emittance preservation, consistent high intensity operation, polarized electron production and transport, and the achievement of a high degree of beam stability on all time scales. In addition to serving as an important machine for the study of Z° boson production and decay using polarized beams, the SLC is also an indispensable source of hands-on experience for future linear colliders. Each new year of operation has been highlighted with a marked improvement in performance. The most significant improvements for the 1994-95 run include new low impedance vacuum chambers for the damping rings, an upgrade to the optics and diagnostics of the final focus systems, and a higher degree of polarization from the electron source. As a result, the average luminosity has nearly doubled over the previous year with peaks approaching 103° cm−2s−1 and an 80% electron polarization at the interaction point. These developments as well as the remaining identifiable performance limitations will be discussed.