Sakae Araki

Observation of pulsed x-ray trains produced by laser-electron Compton scatterings

X-ray generation based on laser-electron Compton scattering is one attractive method to achieve a compact laboratory-sized high-brightness x-ray source. We have designed, built, and tested such a source; it combines a 50 MeV multibunch electron linac with a mode-locked 1064 nm laser stored and amplified in a Fabry-Perot optical cavity. We directly observed trains of pulsed x rays using a microchannel plate detector; the resultant yield was found to be 1.2x10(5) Hz in good agreement with prediction. We believe that the result has demonstrated good feasibility of linac-based compact x-ray sources via laser-electron Compton scatterings.

Feasibility of optical diffraction radiation for a non-invasive low-emittance beam diagnostics

Abstract A “proof-of-principle” experiment on the optical diffraction radiation (ODR) as a single-pulse beam profile monitor is planned using an electron beam extracted from the KEK-ATF damping ring. The main goals of this experiment are the following: (i) To measure the yield and the angular distributions of the optical diffraction radiation from a large-size target at different wavelengths, impact parameters and beam characteristics for a comparison with analogous characteristics of optical transition radiation from a foil with identical optical parameters and for a verification of the model assumption (perfectly conducting semi-infinite target). (ii) To investigate the ODR angular distributions from a tilted target with a slit for observing the interference effects. (iii) To compare the results obtained by simulations based on classical approaches, taking into account the optical characteristics of the equipment and the beam parameters. (iv) To estimate the prospects of using ODR as a new non-invasive tool for ultrarelativistic beams. We estimated that the ODR photon yield in 10% bandwidth for 500 nm is about 106 photons/bunch with an impact parameter of 100 μm . This indicates that the ODR monitor is a promising candidate for single-pulse beam-profile measurements, and that it will be an extremely useful instrument for future linear colliders (JLC, NLC, TESLA and CLIC).

Production of gamma rays by pulsed laser beam Compton scattering off GeV-electrons using a non-planar four-mirror optical cavity

As part of the positron source R&D for future e+−e− colliders and Compton based compact light sources, a high finesse non-planar four-mirror Fabry-Perot cavity has recently been installed at the ATF (KEK, Tsukuba, Japan) [1]. The first measurements of the gamma ray flux produced with a such cavity using a pulsed laser is presented here. We demonstrate the production of a flux of 2.7 ± 0.2 gamma rays per bunch crossing ( ~ 3 × 106 gammas per second) during the commissioning.

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.

Photon generation by laser-Compton scattering at the KEK-ATF

We performed a photon generation experiment by laser-Compton scattering at the KEK-ATF,aiming to develop a Compton based polarized positron source for linear colliders. In the experiment, laser pulses with a 357 MHz repetition rate were accumulated and their power was enhanced by up to 250 times in the Fabry-Perot optical resonant cavity. We succeeded in synchronizing the laser pulses and colliding them with the 1.3 GeV electron beam in the ATF ring while maintaining the laser pulse accumulation in the cavity. As a result, we observed 26.0 +/- 0.1 photons per electron-laser pulse crossing, which corresponds to a yield of 10(8) photons in a second

Photon Generation by Laser-Compton Scattering Using an Optical Resonant Cavity at the KEK-ATF Electron Ring

We studied gamma-ray generation by the laser-Compton scattering using a Fabry-Perot optical resonant cavity at the KEK-ATF electron storage ring. The laser power was enhanced up to 388 W in the optical resonant cavity with an injection power of 7 W in the ATF operation environments. The yield of photons for a crossing of a laser pulse and an electron bunch was 3.3 +/- 0.6, which was consistent with a numerical estimate. In this paper, we report construction, installation and future prospect toward the polarized positron generation for the International Linear Collider.

Observation of the stimulated coherent diffraction radiation in an open resonator at LUCX facility

We present an initial test of a new type of a pre-bunched beam pumped free electron maser based on Stimulated Coherent Diffraction Radiation (SCDR) generated in an open resonator. A fast Schottky Barrier Diode (time response o1 ns) has enabled us to investigate the properties of the radiation stored in the cavity as well as the intrinsic properties of the cavity itself. We observed a turn-by-turn SCDR generated by a multibunch beam. When the cavity length was exactly a half of the bunch spacing a clear resonance was observed. Moreover, turn-by-turn measurements revealed the cavity quality factor of 72.88, which was rather high for an open resonator in the wavelength range of 3–5 mm. An exponential growth of the photon intensity as a function of the number of bunches was also demonstrated.

Performance Studies of a Laser Wire Beam Profile Monitor

We describe a new type of beam profile monitor developed for measurements of an electron beam as small as 10 µm in size. This monitor is based on the Compton scattering of electrons with a laser light target. A thin and intense laser beam (laser wire) is produced by injecting a CW laser beam into a Fabry–Perot optical cavity. We were able to obtain a stable laser wire having a beam waist of 14.52±0.55 µm and a power gain of 220±20. This monitor was installed in the ATF damping ring at KEK, and was used to measure its vertical emittance. We report in this paper the development and performance studies of this monitor in detail.

Development of a laser wire beam profile monitor (2)

We describe in this paper a new beam profile monitor which is suitable for a low-emittance circulating electron beam. The monitor, termed as a laser wire profile monitor, utilizes a CW laser and an optical cavity to minimize interference with an electron beam. A laser beam with a very thin waist is realized by employing the cavity of nearly concentric mirror configuration while the intensity is amplified by adjusting the cavity length to a Fabry–Perot resonance condition. We have built and tested a prototype cavity. It is found that a beam waist of 12 μm and an effective power of 3300 mW were achieved in the laser wire with good long-term stability. It will be installed in the ATF damping ring at KEK, in which a transverse electron beam size of about 10 μm is expected.

Status of optical diffraction radiation experiment at KEK-ATF extraction line

Abstract At KEK-ATF extraction line detailed investigation of backward optical diffraction radiation (ODR) and its implementation to transversal beam parameter diagnostics is planned. We designed and constructed a target chamber with a precise target movement mechanism, a precise laser alignment system and an optical system. The first step to achieve success in the experiment is the tuning of the optical system using backward optical transition radiation (OTR). We believe that, if we are able to measure OTR spectral-angular characteristics with a proper accuracy, we may obtain good results for ODR measurements as the OTR effect is well experimentally and theoretically studied. Therefore, the first stage of our experiment assumes the measurements of backward OTR to analyze all possible types of radiation coming from the target, which could be obstacles for the ODR measurements. In the first series of experiments we have found that the angular distribution of the detected optical radiation is strongly asymmetric. One of the possible reasons for the asymmetry is the synchrotron radiation contribution from bending or steering magnets.