Polarized Electron Source Research Articles

Polarized electron sources

We review current methods for production of polarized electron beams. Major focus is devoted to photoemission from GaAs, which is the way that current facilities rely on. Some aspects of this kind of technology such as the physics behind generation of polarized beams and its advantages and limitations are considered. Finally, the perspectives for implementation of a GaAs photocathode in a RF gun are discussed.

歪んだＧａＡｓ薄膜を用いたスピン偏極電子線源

歪んだＧａＡｓ薄膜を用いたスピン偏極電子線源

Electron beam position stabilization with a piezo-electric optical correction system

A piezo-electrically controlled optical correction system was successfully used to reduce the helicity-correlated pulse-to-pulse position differences of a laser spot to better than ±100 nm at a pulse rate of 600 Hz. Using a simple feedback algorithm, average position differences of Δx =−3.5±4.2 nm and Δy =2.6±6.6 nm were obtained over a 6 h period. This optical correction system was successfully used in the polarized electron source at the Bates Linear Accelerator Center to stabilize the position of the electron beam during the recent SAMPLE experiment. Because this experiment measures a parity violating signal at the 10 −6 level, it is sensitive to systematic effects which are correlated with the helicity of the incident electrons. One potentially large systematic effect is the helicity-correlated motion of the incident electron beam. By using this optical correction system, electron beam position differences at the location of the experiment were routinely kept well below ±100 nm, with averages over the entire two month run of Δx =6.4±2.3 nm and Δy =−3.5±2.0 nm.

A simplified GaAs polarized electron source

We report operational and construction details of a simplified GaAs polarized electron source. It is contained in a modified 4.63 in. Conflat four-way cross, and uses a single 56 ℓ/s turbomolecular pump. The design incorporates multiple cesiators to extend source lifetime, a new spring-clamp GaAs crystal mounting design to provide uniform crystal heating, and a very simple tubular 90° electrostatic deflector. We also discuss matters related to preparing, heat cleaning, and activating the GaAs crystal.

Characterization of the SELPO-M polarized electron source on a 100 kV platform

Abstract A source of polarized electrons based on a helium flowing afterglow has been developed at Orsay. This source, SELPO-M, has been placed on a 100 kV platform and set up in such a way as to allow its connection to the MAMI accelerator. Its performances in terms of current, polarization and optical properties are given. Its figure of merit (IP 2 ≃75 μA) appears highly competitive as compared to strained GaAs sources.

Optically Pumped Electron Spin Filter

This paper reports the first experimental demonstration of an optically pumped electron spin filter. Unpolarized electrons produced in a cold-cathode discharge drift through a mixture of spin-polarized Rb and a nitrogen or helium buffer gas. Through spin-exchange collisions with the Rb, the drifting electrons become polarized along the optical pumping axis. We study the role of the buffer gas in both the optical pumping and the spin transfer to the free electrons. This spin filter produces electron beams with currents and polarizations comparable to first-generation GaAs polarized electron sources.

Beam optical system of the polarized electron source of the Amsterdam pulse stretcher AmPS

Abstract The beam optical system of a Polarized Electron Source (PES) is described. The PES is used for nuclear physics experiments with internal gas targets on the MEA-AmPS acceleration facility at NIKHEF. The PES beam optical system consists of a 100 keV photoelectron gun, a 100 keV beam line, and a 400 keV post-accelerator. Both the electrostatic and the beam trajectory calculations in the gun have been performed with the SAM code. To calculate the beam envelope in the PES beam line the MAD code was used. The input for MAD was obtained from beam trajectory calculations in the gun, extrapolated for a beam with non-zero initial emittance. Results of the beam line performance test are given.

Some «Exotic» Aspects of Physics With Microtips

ABSTRACTThis paper focuses on some experiments done using field emitter arrays (FEAs) produced at CEA-LETI, experiments which are outside of the display or microwave amplifier contexts. Among various studies done at CEA-LETI with FEAs we have selected:- the search towards a spin polarized electron source,- a study on the spatial coherence of the electron beam emitted by a single emitter,- the incorporation of an optical command in a FEA and finally,- the application of a FEA to electron injection into rare gas liquids.

Surface charge limit in NEA superlattice photocathodes of polarized electron source

The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.

Photocathode lifetime improvement by using a pulsed high voltage on the photocathode gun of the polarized electron source at NIKHEF

The first result on a dramatic improvement of the photocathode lifetime of the polarized electron source at NIKHEF is presented. The improvement was obtained after replacing the original DC power supply with a pulsed power supply for the photocathode gun high voltage. The pulsed high voltage power supply provides a negative Gauss-like pulse, with an amplitude of 100 kV, and a full-width of 600 μs, with repetition rates up to 10 Hz. Contrary to using DC high voltage, no deterioration of the vacuum in the acceleration chamber is observed. A photocathode lifetime of 180 h has been measured, using a strained layer InGaAsP photocathode. The lifetime is independent of whether or not the photocathode gun is operated at the pulsing rate of 1 Hz.

新しいスピン偏極電子線源とその応用

新しいスピン偏極電子線源とその応用

A bakeable in-line ultrahigh-vacuum valve

A bakeable ultrahigh-vacuum valve, with a coarse screw thread and conical springs, eliminates slip common to differential thread valves and reduces the bakeout-induced compression and release of the valve disk. The straight-through valve isolates a polarized electron source from gas scattering regions and maintains precision electron optical paths using a right-angled insertable electron lens.

Computer stabilized spin polarized electron source

A systematic optimization of the components of our polarized electron source has markedly improved its long term stability over earlier versions. The major factors shortening the lifetime of the source have been identified. The essential parts of the spin polarized electron source, such as the vacuum system, crystal holder, cleaving mechanism, caesium dispenser, oxygen admittance tube, and computer control activation features are discussed. The lifetime of the source now exceeds 500 h with a constant polarization of 28.5%.

Beam characterization of the Orsay He-afterflow polarized electron source

The measured optical properties of the Orsay polarized electron source, based on the Penning ionization reaction are presented. The upper limit on the beam energy spread is 0.25 eV, corresponding to our experimental resolution. The highest normalized emittance obtained at low current is . The behaviour of these beam characteristics as a function of different relevant parameters is discussed.

A flowing afterglow as a polarized electron source

A source of polarized electrons based on a helium flowing afterglow has been developed at Orsay. The different parts of the device are described. The results about the polarization of the metastable He atoms, the electron current and polarization and optical properties of the output electron beam are given. They reveal electron polarizations in the range 70–82% for currents up to 30 μA, about 60% for 100 μA, an average emittance of 0.5π mm mrad and an energy spread of 0.25 eV.

Emission of spin-polarized electrons from strained InGaP and InGaAsP photocathodes

Spectra of photoemission quantum yield and electron spin polarization are measured on the wide-bandgap epitaxial InGaP and InGaAsP layers, which are promising for use as high efficient and stable sources of polarized electrons. Pseudomorphic epitaxial layers with a thickness of 100–600 nm and band gaps of 1.4-1.9 eV are grown on GaAs substrates with various magnitudes of lattice mismatch and, thus, with various in-plane compressive strains. The magnitude of the strain-induced splitting of the valence band is measured by the photocurrent spectroscopy. The surface preparation technique includes removal of oxides in the solution of HCl in isopropanol and transfer to the Mott polarimeter under nitrogen atmosphere, followed by final heat cleaning in UHV. For thin ( d = 100–200 nm) highly strained pseudomorphic layers a polarization up to P = 0.66 is obtained. Due to strained-induced splitting of the valence band this value exceeds the theoretical limit P 0 = 0.5 for the unstrained material. Although the magnitudes of electron spin polarization are lower than those reported earlier for strained GaAs layers, high photoemission quantum yield Y in InGaAsP layers provide high values of the combination P 2 Y that is directly related to the figure of merit of polarized electron beam. Since March, 1995 the InGaP photocathodes are successfully explored in the polarized electron source at Mainz Microtron.

Material optimization for a polarized electron source from strained GaAs:Be grown on an InGaP pseudosubstrate

We report material optimization for a polarized electron source from strained GaAs:Be grown on an Inx0Ga1−x0P pseudosubstrate, i.e., a fully relaxed Inx0Ga1−x0P buffer layer on a GaP substrate, whereby the large lattice mismatch between the Inx0Ga1−x0P and the GaP was relieved by a linearly graded InxGa1−xP buffer layer. By changing the In composition x0 in the Inx0Ga1−x0P, strained GaAs:Be active layers with various lattice mismatch were obtained, and the effect of growth conditions on the residual strain in GaAs as well as the strain-induced enhancement of electron-spin polarization were studied. The results show that the residual strain in the GaAs layer is very sensitive to the growth temperatures, and that by choosing the right growth conditions, the strain in GaAs can be maintained even when the film exceeds its theoretical critical layer thickness. Enhanced electron-spin polarization has been observed from samples with larger strain.

Advances in Polarized Electron Sources

In this chapter, we review recent advances in the technology of polarized electron sources. Significant electron-spin polarization above 50% has been achieved with new photocathode structures based on the conventional GaAs photoemitter. Since 1991, significant enhancements of polarization utilizing strained heterostructures and GaAs-A1GaAs superlattices have been reported. We discuss the basic theoretical background and the techniques required to build such strained structures in detail. The operational performance of strained GaAs cathodes now in use at the Stanford Linear Accelerator Center (SLAC) for colliding-beam and fixed-target experiments is reviewed as well.

Spin Relaxation of Electrons in Strained-GaAs-Layer Photocathode of Polarized Electron Source

The luminescence polarization method using a mode-locked Ti:sapphire laser and a streak camera is applied to the measurement of the spin relaxation time and the lifetime of electrons in the strained-GaAs-layer photocathode of a polarized electron source. The spin relaxation time and the electron lifetime are 105 ps and 45 ps at room temperature, respectively. Electron-hole scattering is thought to be the main mechanism of the spin relaxation of our strained-GaAs photocathode.

The Stanford linear accelerator polarized electron source

The Stanford 3-km linear accelerator at SLAC has operated exclusively since early 1992 using a polarized electron beam for its high-energy physics programs. The polarized electron source now consists of a diode-type gun with a strained-lattice GaAs photocathode DC biased at high voltage and excited with circularly polarized photons generated by a pulsed, Ti:sapphire laser system. The electron polarization at the source is > 80%. To date the source has met all the beam requirements of the SLC and fixed target programs with < 5% downtime.