Y. Ogawa

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.

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.

Beam monitor utilizing transition radiation

Beam monitors utilizing electromagnetic waves, especially visible light radiated by charged particles, have several excellent features for beam diagnostics in accelerators: they are essentially free from environmental electromagnetic noise and are characterized by a high-speed time response. A beam monitor based on transition radiation is one of the most promising monitors concerning positions, sizes, emittance, energy and time structures of bunches for high-intensity, short-pulse beams. We started to develop beam monitors utilizing transition radiation for the beam diagnostics of high-intensity electron beams. Bunch-length measurements were performed at the KEK 2.5-GeV linac with this monitor and a streak-camera system.<<ETX>>

Present status and beam-stability issues of the KEKB injector linac

The KEKB injector linac was completely upgraded for the KEK B-Factory (KEKB) project in March, 1998. Many difficulties have been overcome during the elaborate commissioning of the upgraded linac since the end of 1997. The 3.5-GeV positron and 8-GeV electron beams have been injected to the KEKB rings with good performance. Much effort has also been continuing to stabilize the intensity and quality of the beams. Some experimental results on the beam stability issues am shown together with the recent operation status in this report. A beam test on a new scheme of a two-bunch injection was started in order to increase the positron intensity since March, 2001.

New pre-injector of the KEK 2.5-GeV linac and its performance

The injection system (pre-injector) of the KEK 2.5-GeV linac has been upgraded so that we can investigate intense beam acceleration for the KEK B-factory project. It requires intense beams to achieve a short injection time in practice. An outline of the new pre-injector and its performance is given.<<ETX>>

Reduction of outgassing rate of SUS316 and OFC class-1 by prebaking in a vacuum furnace: application to gravitational-wave experiments

Abstract A high performance uhv system is required for a large-scaled interferometer used for gravitational-wave experiments. Pressures of 10−6 Pa or less are necessary for its stable operation. Quick pumping is also required for frequent exposure to the atmosphere in the preliminary experiments. So materials used for very long vacuum ducts in the uhv system should have a low outgassing rate and a low gas-adsorption probability. Surface modification techniques such as high-temperature prebaking (550°C × 100 h) in the vacuum furnace and TiN coating were adopted for this purpose. Evacuation properties of the test ducts (1 m long × 0.15 m in diameter) made of SUS316 and OFC Class-1 prebaked in a vacuum furnace were investigated.

Recent Progress at KEKB

We summarize the machine operation of KEKB during past one year focusing on progress for this period.

Increase of positrons by a high-intensity two-bunch acceleration scheme at the KEKB linac

As the accumulation current of positrons increases in the KEKB ring, the injection time is becoming longer. It will thus be one of the most important issues affecting the accumulation of the integrated luminosity. As one of the steps, we introduced a high-intensity two-bunch acceleration scheme at the KEKB linac to intensify positrons by means of doubling the primary electrons. We recently obtained test results of 0.54 nC for the first bunch and 0.49 nC for the second bunch at the linac end. This scheme increased the positron intensity by nearly 65%. Since the linac frequency is not a harmonic number of the LER frequency, the best time interval between two bunches is 96.29 ns, corresponding to 49 LER-buckets. Even with this limitation, it is undoubtedly a very useful scheme for increasing the positron injection rate. The beam test results are described.

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.

Double-balanced bellows for vibration isolation between the inside and outside of a vacuum ; application to gravitational-wave experiments

Abstract In recent precise experiments, such as optical measurements with interferometry, physical quantities sometimes need to be observed in a ‘quiet’ vacuum, where all of the instruments in the vacuum are well isolated from the vibration of the vacuum tank including the vacuum-pumping system and, sometimes, from the seismic noise of the earth. We have developed a new technique for vibration isolation between the inside and outside of a vacuum (e.g. an optical bench and a tank), which involves a double-balanced bellows (DBB) to compensate for the atmospheric pressure. Both the detailed structure and the DBB mechanism as well as the results of measurements of the vibration characteristics are presented, showing the validity of using the DBB, especially in gravitational-wave experiments.