Ryunosuke Kuroda

Efficient Propagation of Polarization from Laser Photons to Positrons through Compton Scattering and Electron-Positron Pair Creation

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2 x 10(4) e+ /bunch. The magnitude of positron polarization has been determined to be 73 +/- 15(stat) +/- 19(syst)% by means of a newly designed positron polarimeter.

Compact soft x-ray source using Thomson scattering

A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2m2. An infrared laser beam (λ0=1047nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2mJ/pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.

INJECTOR STUDY FOR COMPACT HARD X-RAY SOURCE VIA LASER COMPTON SCATTERING

Compact hard X-ray source via laser Compton sattering has been developed in SHI and AIST. Our system has the injector and the linac and the high power laser system. The injector has a photo-cathode rf gun with a solenoid magnet. To enhance the X-ray yeild, we are planning to increase electron beam charge up to 5 nC/bunch and to make multi-bunch beam. The beam tracking simulation in the injector have been performed by changing laser spot size, laser pulse width, rf phase and solenoid field to optimize the distance between the injector and the linac for 5 nC/bunch high charge beam. In addition, high charge multi-bunch beam simulation in rf-gun cavity have been carried out to investigate the influence by the beam loading and the wake field.

In-situ characterization of free-volume holes in polymer thin films under controlled humidity conditions with an atmospheric positron probe microanalyzer

A pulsed, slow positron beam, with a diameter of 200 μm, was extracted into air through a thin SiN window of an atmospheric positron probe microanalyzer (PPMA), and used to measure the ortho-positronium lifetimes τ in polyvinyl alcohol and polycaprolactam sub-μm-thick films. By measuring the variation of τ as a function of relative humidity, the effect of water molecules on the hole sizes, deduced from τ, was examined for the films with consideration to the chain mobility. The results demonstrate the usefulness of the atmospheric PPMA to the in-situ characterization of nanoscopic holes in thin films under practical conditions.

Slow Positron Beam Apparatus for Surface and Subsurface Analysis of Samples in Air

A technique for investigating atomic-scale defects and/or nanometer (sub-nanometer)-order pores near the surface of samples mounted in air (without vacuum) by positron annihilation spectroscopy (PAS) has been developed. The method relies on the extraction of slow positron beams from the vacuum chamber to air through a thin SiN membrane vacuum window. Using a positron beam with an injection energy of 2.6 keV and a vacuum window with a thickness of 30 nm, samples mounted in air can be investigated by PAS to a depth of ~100 nm.

Bunch Length Monitor Using Two-Frequency Analysis for RF Gun System

An rms (root mean square) bunch length monitor for a laser-driven photocathode rf gun system based on a two-frequency analysis technique has been developed. Typically, the photoelectron beam generated from the rf gun system has an energy of 3–5 MeV and an rms bunch length smaller than 20 ps down to 3–4 ps. This monitor is suitable for such electron beam measurement. The rms bunch length as a function of rf phase was experimentally measured using both the rms bunch length monitor and streak camera technique using a 50 MeV electron beam at the KEK accelerator test facility (KEK-ATF) injector section which has an rf gun system and a 3-m-long accelerator structure. A numerical simulation study was also performed using the PARMELA code. The availability of this monitor was clearly verified by comparing the results. Consequently, this monitor was installed in the rf gun system at Waseda University and the rms bunch length measurement for a 3.5 MeV electron beam was precisely performed using the monitor.

Application of high-energy photon CT system with laser-compton scattering to nondestructive test

A CT system using high-energy monochromatic photons is one of the best choices for nondestructive test of industrial products. We have developed a CT system for nondestructive test of industrial products using the laser-Compton photons, which were generated with an electron accelerator and conventional commercial laser system. We measured the profile, and found that they were a 0.8 - 1.5 mm in diameter. We also applied the CT system to the nondestructive test of the concrete specimen, and successfully obtained the CT image.

X-ray pulse generation by laser Compton scattering using a high-charge, laser-accelerated, quasi-monoenergetic electron beam

X-ray pulse generation is demonstrated by laser Compton scattering using a laser-accelerated quasi-monoenergetic electron (QME) beam containing 70 pC electrons with an energy of 60 MeV in the monoenergetic peak. A well-collimated X-ray beam with a divergence angle of approximately 5 mrad is produced. The X-ray photon number is estimated to be 2 × 107 per pulse, which is 10 thousand times larger than that achieved in a previous study using a laser-accelerated QME beam. The peak energy of the X-rays is estimated to be 60 keV by a numerical simulation using the measured characteristics of the QME beam.

Design of Terahertz-Wave Spectrophotometry by Compton Backscattering Using Relativistic Electron Bunches and their Coherent Synchrotron Radiations

We propose a new terahertz-wave spectrophotometry by Compton backscattering using relativistic electron bunches and coherent radiations generated by them. The terahertz-wave spectrophotometry can be realized simultaneously with Compton backscattering, where the characteristics in the terahertz-wave region are converted to those in the visible and ultraviolet regions. The number of Compton backscattered photons is estimated to be more than 100 counts per second with a wavelength divergence of 5% in the visible and ultraviolet regions using the compact S-band linac at National Institute of Advanced Industrial Science and Technology. This spectrophotometry becomes significant in energy recovery linacs.

Measurement of intense coherent synchrotron radiation at frequencies around 0.1 THz using the compact S-band linac

We measured intense radiation from an electron bunch in a millimeter wave region using the compact S-band linac. The dependence of the radiation on the electron-bunch charge was measured with an rf detector system at frequencies around 0.1 THz and was confirmed to be a coherent synchrotron radiation (CSR). The total power of the horizontally and vertically polarized CSRs, which were extracted through the Z-cut quartz window within 1 ns, was calculated to be about 88 and 30 nJ/pulse, excluding the absorption by the window. The two-dimensional distribution of the vertically polarized CSR was measured at a distance of about 0.7 m from the radiation point. The CSR distribution was comparatively uniform in the horizontal plane. Intense CSR, which was reflected in the vacuum chamber, was extracted with a delay of about 6 ns. This suggests that measurement of temporal structure is needed for CSR applications.