Ryukou Kato

1 ms Pulse Beam Generation and Acceleration by Photocathode Radio Frequency Gun and Superconducting Accelerator

We report the successful generation and acceleration of a 1-ms-long pulse and multibunch electron beam by a normal conducting photocathode RF gun and a super conducting accelerator at the KEK Superconducting Test Facility (STF). A 1.3 GHz normal conducting RF gun generates a 1 ms and 10 mA macropulse that fully satisfies the designed parameters. This is the longest macropulse generated by a normal conducting RF gun with a frequency of more than 1 GHz. A beam acceleration of up to 40 MeV was demonstrated with more than 60% of the designed average current. The accelerated beam properties were evaluated: the intensity and energy fluctuations were 3.8% and less than 0.08%, respectively. These beam properties are sufficient for the requirement of the STF operation including that in the quantum beam project, which is high-brightness-X-ray generation by inverse laser Compton scattering. The intensity fluctuation should be improved toward the International Linear Collider (ILC) for uniform acceleration.

Proposal for the edge-focusing wiggler for SASE

Abstract We are developing a new type of wiggler with a strong focusing force, named the edge-focusing wiggler. It is a Halbach-type wiggler made with permanent magnet blocks with an edge angle φ. An application is given for SASE in the shorter wavelength region. The characteristics of the wiggler are studied in detail using a three-dimensional magnetic field calculation program with the magnetic charge method.

Photonuclear reaction based high-energy x-ray spectrometer to cover from 2 MeV to 20 MeV.

A photonuclear-reaction-based hard x-ray spectrometer is developed to measure the number and energy spectrum of fast electrons generated by interactions between plasma and intense laser light. In this spectrometer, x-rays are converted to neutrons through photonuclear reactions, and the neutrons are counted with a bubble detector that is insensitive to x-rays. The spectrometer consists of a bundle of hard x-ray detectors that respond to different photon-energy ranges. Proof-of-principle experiment was performed on a linear accelerator facility. A quasi-monoenergetic electron bunch (Ne = 1.0 × 10(-6) C, Ee = 16 ± 0.32 MeV) was injected into a 5-mm-thick lead plate. Bremsstrahlung x-rays, which emanate from the lead plate, were measured with the spectrometer. The measured spectral shape and intensity agree fairly well with those computed with a Monte Carlo simulation code. The result shows that high-energy x-rays can be measured absolutely with a photon-counting accuracy of 50%-70% in the energy range from 2 MeV to 20 MeV with a spectral resolution (Δhν/hν) of about 15%. Quantum efficiency of this spectrometer was designed to be 10(-7), 10(-4), 10(-5), respectively, for 2-10, 11-15, and 15-25 MeV of photon energy ranges.

High Power Operation of the THz FEL at ISIR, Osaka University

The free‐electron laser based on the L‐band electron linac is under development to generate high power radiation in the THz region. Recent results of the development are presented, including the wavelength spectra and the output power vs. wavelength from 25 to 110 μm.

MEASUREMENT OF CHARACTERISTICS OF THE INFRARED FREE ELECTRON LASER WITH THE L-BAND LINEAR ACCELERATOR AT OSAKA UNIVERSITY

Abstract Characteristics of the free electron laser based on the L-band electron linear accelerator at Osaka University have been studied by measuring the output light with a fast infrared detector. The time resolution of the detector has been measured to be 170 ns with the self-amplified spontaneous emission from the single-bunch electron beam. The time spectrum of the output light from the free electron laser operated at a wavelength of 40 μm and the output power of 800 μJ/macropulse has been measured with the detector, and the gain of the free electron laser and the loss of the optical cavity in the round trip are derived to be 58 and 6%, respectively. The calculated gain based on a two-dimensional model agrees fairly well with the measured value.

Performances of THz cameras with enhanced sensitivity in sub-terahertz region

Uncooled microbolometer-type 640x480 and 320x240 Terahertz (THz) focal plane arrays (FPAs) with enhanced sensitivity in sub-THz region are developed, and incorporated into 640x480 and 320x240 cameras, respectively. The pixel in the THz-FPA has such a structure that an area sensitive to electromagnetic wave is suspended above read-out integrated circuit (ROIC). A thin metallic layer is formed on the top of the sensitive area, while a thick metallic layer is formed on the surface of ROIC. The structure composed of the thin metallic layer and the thick metallic layer behaves as an optical cavity. The THz-FPAs reported in this paper have a modified pixel structure which has several times longer optical-cavity length than NEC’s previous pixel does, by forming a thick SiN layer on the ROIC. The extended optical-cavity structure is favorable for detecting electromagnetic wave with lower frequency. Consequently, the Minimum Detectable Power per pixel (MDP) is improved ten times in sub-THz region, especially 0.5-0.6 THz. This paper presents spectral frequency dependences of MDP values for THz-FPA with the modified pixel structure and THz-FPA with the previous pixel structure, using THz free electron laser (FEL) developed by Osaka University. The modification of pixel structure extends high sensitivity region to lower frequency region, such as sub-THz region, and the wider spectral coverage of THz camera surely expands its applicability

Image reconstruction method for non-synchronous THz signals

Image reconstruction method for non-synchronous THz signals was developed for a combination of THz Free Electron Laser (THz-FEL) developed by Osaka University with THz imager. The method employs a slight time-difference between repetition period of THz macro-pulse from THz-FEL and a plurality of frames for THz imager, so that image can be reconstructed out of a predetermined number of time-sequential frames. This method was applied to THz-FEL and other pulsed THz source, and found very effective. Thermal time constants of pixels in 320x240 microbolometer array were also evaluated with this method, using quantum cascade laser as a THz source.

Characteristic measurements of higher harmonics generated in the SASE-FEL process

The nonlinear harmonic generation of self-amplified spontaneous emission (SASE) has been studied in the far-infrared region. Angular distributions of the second harmonic and the third harmonic as well as the fundamental of SASE have been measured. The third harmonic radiation is emitted on the electron beam axis, while the second harmonic radiation is emitted slightly off-axis in the horizontal direction. It seems that there is competition in development of radiation between the fundamental and the other higher harmonics.

Upgrade of the ISIR-FEL at Osaka University and oscillation experiments in the sub-millimeter wavelength region

Abstract The far-infrared free electron laser (FEL) at the Institute of Scientific and Industrial Research (ISIR), Osaka University has been upgraded to extend the wavelength region further to the long wavelength side. The present modification includes replacement of the optical resonator and installation of an evacuated optical transport line from the FEL to the measurement room as well as installation of a far-infrared monochromator for measuring the wavelength of FEL light. We recently started experiments using the new FEL system. The loss of light in the optical resonator was measured and it is experimentally confirmed that the diffraction loss is reduced considerably after the upgrade of the optical resonator.

L-band Photocathode RF gun at KEK-STF

The superconducting RF test facility (STF) in KEK is a facility to promote R&D of the International Linear Collider (ILC) cavities and cryomodule. L-band photocathode RF gun has been developed at KEK-STF as an electron beam source for cryomodule test scheduled in autumn of 2011. The RF cavity of the gun will be operated with a 1.3 GHz RF frequency, 1 msec RF pulse width, 5 Hz repetition rate at normal conductivity. The cavity was prepared by collaborative work with DESY and FNAL, and fabricated by FNAL. The RF conditioning of the cavity has been started since April 2010. A cesium telluride thin film as a photocathode material has been adopted, and the preparation equipment for cesium telluride has been newly designed and constructed. By using this new system, a fabrication and a performance estimation of the cesium telluride thin film as a photocathode are the next step of the research.