M. Yoshioka

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.

Highly polarized electrons from GaAs-GaAsP and InGaAs-AlGaAs strained-layer superlattice photocathodes

GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes are presented as emission sources for highly polarized electron beams. The GaAs–GaAsP cathode achieved a maximum polarization of 92(±6)% with a quantum efficiency of 0.5%, while the InGaAs–AlGaAs cathode provides a higher quantum efficiency (0.7%) but a lower polarization [77(±5)%]. Criteria for achieving high polarization using superlattice photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra.

Highly polarized electron source using InGaAs-GaAs strained-layer superlattice

We have studied the polarization of photoemission from an In0.15Ga0.85As–GaAs strained-layer superlattice. The polarization of 82.7±0.3(stat.)±6.1(syst.)% was observed at laser wavelengths from 911 to 916 nm at room temperature. The quantum efficiency at the wavelength of 911 nm was ~0.015% in the vacuum of ~6×10-10 Torr with high cathode voltage of -4 kV.

Proposed Method to Produce a Highly Polarized e+ Beam for Future Linear Colliders

We propose a method to produce a spin-polarized e+ beam using e+e- pair-creation by circularly polarized photons. Assuming Compton scattering of an unpolarized e- beam and circularly polarized laser light, scattered γ-rays at the high end of the energy spectrum are also circularly polarized. If those γ-rays are utilized to create e± pairs on a thin target, the spin-polarization is preserved for e+s at the high end of their energy spectrum. By using the injector linac of Accelerator Test Facility at KEK and a commercially available Nd:YAG pulse laser, we can expect about 105 polarized e+s per second with a degree of polarization of 80% and a kinetic energy of 35–80 MeV. The apparatus for creation and measurement of polarized e+s is being constructed. We present new idea for possible application of our method to future linear colliders by utilizing a high-power CO2 laser.

KEKB accelerator control system

The KEKB accelerator control system including a control computer system, a timing distribution system, and a safety control system are described. KEKB accelerators were installed in the same tunnel where the TRISTAN accelerator was. There were some constraints due to the reused equipment. The control system is based on Experimental Physics and Industrial Control System (EPICS). In order to reduce the cost and labor for constructing the KEKB control system, as many CAMAC modules as possible are used again. The guiding principles of the KEKB control computer system are as follows: use EPICS as the controls environment, provide a two-language system for developing application programs, use VMEbus as frontend computers as a consequence of EPICS, use standard buses, such as CAMAC, GPIB, VXIbus, ARCNET, RS-232 as field buses and use ergonomic equipment for operators and scientists. On the software side, interpretive Python and SAD languages are used for coding application programs. The purpose of the radiation safety system is to protect personnel from radiation hazards. It consists of an access control system and a beam interlock system. The access control system protects people from strong radiation inside the accelerator tunnel due to an intense beam, by controlling access to the beamline area. On the other hand, the beam interlock system prevents people from radiation exposure by interlocking the beam operation. For the convenience of accelerator operation and access control, the region covered by the safety system is divided into three major access control areas: the KEKB area, the PF-AR area, and the beam-transport (BT) area. The KEKB control system required a new timing system to match a low longitudinal acceptance due to a low-alpha machine. This timing system is based on a frequency divider/multiply technique and a digital delay technique. The RF frequency of the KEKB rings and that of the injector Linac are locked with a common divisor frequency. The common divisor frequency determines the injection timing. The RF bucket selection system is also described. r 2002 Elsevier Science B.V. All rights reserved.

The interaction region of KEKB

Abstract An asymmetric-energy collider of 8 GeV electrons and 3.5 GeV positrons, KEKB, has been operating at KEK since 1998. On July 9, 2001 it achieved the highest record of luminosity, 4.5×10 33 cm −2 s −1 . This paper describes the interaction region (IR) extending about 8 m from the interaction point (IP) along each beam direction. The main machine components in the IR are focusing quadrupole magnets, compensation solenoids, and vacuum chambers, which are positioned on movable stages. They are designed to focus both beams into collisions with a finite angle crossing.

Beam-transport system of KEKB

The transport lines of the KEKB for positrons and electrons convey the beams separately from the injector linac to the KEKB rings. The length is about 500 m for each line. In order to make the maximum use of the existing tunnels and also to avoid interference with the AR, the beam lines took a serpentine course, resulting in a rather large curvature in the arcs. The consequences were a large number of bends in the arcs with high fields and also a large dispersion function and, thus, a large R56 component. The latter issue is crucial for the KEKB rings, since it results in a longer bunch length at injection. We adopted a special optics that reduces the R56 coefficient sufficiently. We have developed novel water-cooled ceramic chambers for kickers, eddy-current type septum magnets for injection, and a beam-abort system for the rings. The present paper describes the design and current status of the beam lines, the injection system, and the beam-abort system.

A High Polarization and High Quantum Efficiency Photocathode Using a GaAs?AlGaAs Superlattice

A charge of 2.3×1011 electrons in 2.5 ns at a laser wavelength of 757 nm with a corresponding quantum efficiency (QE) of 2.0% measured at 752 nm was extracted from a -120 kV biased, 20 mm diameter, GaAs–AlGaAs superlattice photocathode. The maximum electron polarization measured with material from the same wafer, but in a different system, was 71% at 757 nm for a QE of 1.0% measured at 752 nm. The quantity and temporal distribution of the extracted charge is consistent with a space charge limitation, rather than a cathode charge limit. The performance of this type of cathode makes it a possible candidate for future linear colliders.

Depolarization of photoemission at the surface and interior of an AlGaAsGaAs superlattice

Abstract We have studied the polarization of photoemission from an AlGaAsGaAs superlattice as a function of the total thickness of the superlattice. In three samples with thicknesses of 0.4, 0.1, and 0.05 μm the measured polarizations were 51.0, 69.9, and 73.9% respectively. We obtain 75.5% when we extrapolate these results to the case of a sample so thin that depolarization occurs only at the surface. We tested a fourth sample for which the thickness was 0.1 μm and the Be dopant concentration was a factor of ten below the high concentration of the first three samples. The fourth sample produced a polarization of 74.8%. This result confirms the expectation that reduced doping lowers the spin relaxation in the interior of the superlattice. We conclude that depolarization at the surface dominates interior depolarization when the sample is thin or the dopant concentration is low.

Polarized electron source for a linear collider in Japan

Abstract The research to develop the polarized electron source required by future linear colliders has been conducted by our collaboration. Recent advances in settling three difficult problems to realize the high polarization and high quantum efficiency, the long cathode lifetime and the multi-bunch beam generation are briefly described in this article.