Toshiyuki Okugi

Design of a polarized positron source for linear colliders

We propose a design of a polarized positron source for linear colliders. The design is based on electron–positron pair creation from polarized g-rays which are produced by Compton scattering of circularly polarized laser light off a highenergy electron beam. Polarized positrons are created from those g-rays incident on a thin conversion target. A future linear collider of the TeV-energy region requires an extraordinary large number of positrons (B1 � 10 10 positrons/ bunch) in a multi-bunch time structure. To meet these requirements, our design employs a high-current, low-emittance electron beam of 5:8 GeV; 10 CO2 lasers, and 200 laser–electron collision-points. At each collision point, a pair of specially designed parabolic mirrors is installed to achieve efficient head-on collisions. This system allows us to produce high-intensity polarized g-rays, which effectively generate high-intensity polarized positrons with the magnitude of polarization greater than 50%: r 2003 Elsevier Science B.V. All rights reserved.

Observation of High Intensity X-Rays in Inverse Compton Scattering Experiment

Abstract We report the first results of high-intensity X-ray generation using Inverse Laser Compton scattering. This experiment was carried out by a US–Japan collaboration at the Brookhaven National Laboratory (BNL) Accelerator Test Facility (ATF) in September 1999. The 3.5 ps X-ray pulse at 6.5 keV, containing 3×10 6 X-ray photons was generated by the interaction of 60 MeV, 0.5 nC electron bunches and CO 2 laser pulses of 600 MW peak power.

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).

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.

Polarized positron source for the linear collider, JLC

Abstract A comprehensive description of a polarized positron project is presented in terms of physics motivations for utilizing a polarized positron in electron–positron collider experiments, a proof-of-principle experiment and a conceptual design of a polarized positron source for the future linear collider JLC. In order to verify a proposed method of creating highly polarized positron beams via successive two fundamental processes, i.e. Compton scattering and pair creation, we have been performing basic experiments both at KEK and BNL. First observation of positrons was made at KEK using an electron beam of 1.26 GeV and a laser of 2.33 eV. High-intensity picosecond X-rays were also generated at BNL using a specially designed Compton chamber. In order to realize polarized positron beams of the JLC which have considerably high intensity, i.e. 0.7×10 10 e + / pulse and a complicated multi-bunch structure, we have achieved a possible scheme for the Compton scattering system and a positron capture section into an L-band linac.

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.

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.

SR monitor for the ATF damping ring

The ATF Damping Ring (ATF DR) aims to demonstrate the technical feasibility of a low-energy part of next-generation electron-positron linear colliders by producing an electron bunch train with extremely small emittance. To diagnose the beam profile and radiation damping there, a synchrotron radiation (SR) monitor which uses visible light has been designed and built. This paper presents the system configuration of this SR monitor and the preliminary results from the initial beam commissioning.