A. Enomoto

The KEKB injector linac

Abstract An 8-GeV electron/3.5-GeV positron injector for KEKB was completed in 1998 by upgrading the existing 2.5-GeV electron/positron linac. The main goals were to upgrade its accelerating energy from 2.5 to 8 GeV and to increase the positron intensity by about 20 times. This article describes not only the composition and features of the upgraded linac, but also how these goals were achieved, by focusing on an optics design and commissioning issues concerning especially high-intensity single-bunch acceleration to produce positron beams.

Plasma wake-field accelerator experiments at KEK☆

Abstract We report on a plasma wake-field accelerator experiment using a high-intensity 250 MeV electron beam of the linac at KEK (National Laboratory for High Energy Physics, Japan). The experiment provedthe plasma wake-fields excited by a train of several bunches accelerate a trailing bunch. We observed an approximately 4 MeV shift of beam energy in a 1 m long plasma with a density of 4 × 1011 cm−3. We describe the present status and the future plan of experimets based on a new scheme of a plasma wake-field accelerator driven by a train of multiple bunches.

Intense positron beam at KEK

Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Experimental study of positron production from a W single crystal by the KEK 8-GeV electron linac beam

Abstract We have measured the positron production efficiency from tungsten single-crystal targets using an 8 GeV electron beam. A single-bunch beam with a bunch width of 10 ps, a repetition rate of 2 Hz, and an intensity of 0.2 nC/bunch was incident on a target mounted on a precision goniometer. Positrons produced in the forward direction were detected by a magnetic spectrometer in the 10–20 MeV/c momentum range. Systematic data on the target-thickness dependence and the momentum distribution of the produced positrons were obtained for crystal targets. The results show that, when the crystal axis 〈1 1 1〉 is aligned to the electron beam direction, the positron yield increases compared to the amorphous case by factors of 6.5, 3.4 and 2.3 at 10 MeV/c for 2.2, 5.3 and 9.0 mm thick crystals, respectively. We observed that the positron yield from the 9.0 mm thick crystal is larger than the maximum yield attainable with 18–20 mm thick amorphous targets at 8 GeV.

Wakefield accelerator using twin linacs

Abstract A collinear wakefield test facility using two linacs with a common test section is described. Beams from one linac excite wakefields in a test medium s

Focusing system of the KEK positron generator

Abstract A distinctive feature of the KEK positron generator is the use of a compact positron focusing system and a high-current linac as a primary electron beam accelerator. The maximum positron peak currents obtained up to the present are 34 mA at 250 MeV and 17 mA at 2.5 GeV. This report describes the focusing system for the electron beam with a current of up to 10 A and for the positron beam with a large divergence. It reports an improvement of the electron/positron converter unit and its performance. In addition, experimental results are shown for interesting features of the positron focusing system: growth of the positron bunch length and electron mixing in the positron beam.

Control system for the photon factory 2.5 GeV electron linac

Abstract The 2.5 GeV electron linac of the KEK Photon Factory is controlled by a distributed processor network composed of eight minicomputers and many microprocessors. Each individual linac equipment has autonomous operation capability with a micro-processor-based device controller. The device controllers are interconnected to six subcontrol stations and a main operators console through a fiber optic communication network; it is effective to prevent control system malfunction by strong electromagnetic noises arising from pulse operation of high-power klystrons. The system configuration and performance are described.

An overview of the slow-positron beam facility at the photon factory, KEK

The KEK slow-positron source is in the final stage of construction. The beam line comprises a 31 m long vacuum duct within an axial magnetic field and a following electrostatic guided section. In order to vary the energy of a positron beam dedicated to depth-profile measurements, a high voltage station capable of applying 60 kV has been installed in the beam transport system. The target assembly (a water-cooled tantalum rod of 5 radiation lengths and a moderator with multiple tungsten vanes) and the following straight section (8 m; used for positron storage) are under high voltage. The beam duct located downstream is at ground potential. Positron beams passing through this region have a high kinetic energy. A focusing triplet quadrupole lens and a moderator on the retarding electrode are located at the end of the magnetic transport. This beam line has 9 right-angle-curved ducts, comprising a radius of curvature of 40 cm. Positrons with a maximum energy of 60 keV are guided by bending magnets attached to the beam-transport ducts. A transport system to switch from magnetically guided to electrostatically guided has been installed. The design of the brightness-enhancement stage of the positron beam for positron re-emission microscopy is in progress. In a preliminary experiments at 2.0 GeV with a 2 kW primary beam, 4×106e+/s of slow positrons were observed by detecting annihilation γ-rays at the end of the magnetic beam-transport line. Further improvements are expected by careful surface and thermal treatments of the moderator.

Multiple single-board-computer system for the KEK positron generator control

Abstract The KEK positron generator is controlled by means of a distributed microprocessor network. The control system is composed of three kinds of equipment: device controllers for the linac equipment, operation management stations and a communication network. Individual linac equipment has its own microprocessor-based controller. A multiple single board computer (SBC) system is used for communication control and for equipment surveillance; it has a database containing communication and linac equipment status information. The linac operation management that should be the most soft part in the control system, is separated from the multiple SBC system and is carried out by work-stations. The principle that every processor executes only one task is maintained throughout the control system. This made the software architecture very simple.

Improvement of the alignment system for the KEK 2.5-GeV electron linac

The alignment system for the KEK 2.5-GeV electron linac was improved regarding several points: replacement of the large gas laser (He-Ne) with a compact diode-pumped solid laser (Nd:YAG), the introduction of a single-mode, polarization-maintaining fiber between the laser and the light-injection point into the alignment vacuum duct, and the use of new optics for realizing a diffraction-limited beam along the entire linac length of 460 m. The new system works satisfactorily and serves as a light source of good quality. A preliminary measurement of the system resolution for the KEK 2.5-GeV electron linac is reported as well as the main points concerning the improvements. A new design of the linac alignment system for the KEKB injector linac is also proposed.