Taro Konomi

High brightness and high polarization electron source using transmission photocathode with GaAs-GaAsP superlattice layers

In order to produce a high brightness and high spin polarization electron beam, a pointlike emission mechanism is required for the photocathode of a GaAs polarized electron source. For this purpose, the laser spot size on the photocathode must be minimized, which is realized by changing the direction of the injection laser light from the front side to the back side of the photocathode. Based on this concept, a 20kV gun was constructed with a transmission photocathode including an active layer of a GaAs–GaAsP superlattice layer. This system produces a laser spot diameter as small as 1.3μm for 760–810nm laser wavelength. The brightness of the polarized electron beam was ∼2.0×107Acm−2sr−1, which corresponds to a reduced brightness of ∼1.0×107Am−2sr−1V−1. The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8×108Ccm−2 was observed for an extracted current of 3μA.

Real Time Magnetic Imaging by Spin-Polarized Low Energy Electron Microscopy with Highly Spin-Polarized and High Brightness Electron Gun

We developed a spin-polarized low energy electron microscopy (SPLEEM) with a highly polarized and high brightness spin electron gun in the present study. Magnetic structures of Co/W(110) were observed with an acquisition time of 0.02 s with a field of view of 6 µm. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110).

Helical Phase Structure of Radiation from an Electron in Circular Motion

We theoretically show that a single free electron in circular/spiral motion radiates an electromagnetic wave possessing helical phase structure and carrying orbital angular momentum. We experimentally demonstrate it by double-slit diffraction on radiation from relativistic electrons in spiral motion. We show that twisted photons should be created naturally by cyclotron/synchrotron radiations or Compton scatterings in various situations in cosmic space. We propose promising laboratory vortex photon sources in various wavelengths ranging from radio wave to gamma-rays.We theoretically show that a single free electron in circular motion radiates an electromagnetic wave possessing helical phase structure, which is closely related to orbital angular momentum carried by it. We experimentally demonstrate it by interference and double-slit diffraction experiments on radiation from relativistic electrons in spiral motion. Our results indicate that photons carrying orbital angular momentum should be created naturally by cyclotron/synchrotron radiations or Compton scatterings in various situations in cosmic space. We propose promising laboratory vortex photon sources in various wavelengths ranging from radio wave to gamma-rays.

Investigation of the Electrical Field Sensitivity of Sub-μm Y–Ba–Cu–O Detectors

The behavior of submicrometer-sized thin-film YBa2Cu3O7-x (YBCO) detectors under illumination with picosecond terahertz (THz) pulses was investigated. Real-time measurements with a temporal resolution of 15 ps full width at half maximum were performed at ANKA, the synchrotron facility of Karlsruhe Institute of Technology, and the UVSOR-III facility at the Institute for Molecular Science in Okazaki, Japan. The capability of YBCO detectors to reproduce the shape of a several picosecond long THz pulse was demonstrated. Single-shot measurements adhering to a reversal of the direction of the electrical field of the THz radiation were carried out. They provided evidence for the electrical field sensitivity of the YBCO detector. Exploiting the electrical field sensitivity of the YBCO detector, the effect of microbunching was observed at UVSOR-III.

Generation of vector beam with tandem helical undulators

We propose a scheme to produce structured light in synchrotron light sources. In this scheme, light beams from two undulators are superposed by using a technique akin to the “cross undulator.” We demonstrate that a vector beam, in which the polarization direction varies with the azimuthal angle about the beam axis, is produced by superposing harmonic radiation from two helical undulators in tandem. Although this scheme is demonstrated in the ultraviolet range at the low-energy synchrotron UVSOR-III, it can be applied to high-energy synchrotrons to produce vector x-ray beams, which would open a new field in the application of synchrotron radiation.We propose a scheme to produce structured light in synchrotron light sources. In this scheme, light beams from two undulators are superposed by using a technique akin to the “cross undulator.” We demonstrate that a vector beam, in which the polarization direction varies with the azimuthal angle about the beam axis, is produced by superposing harmonic radiation from two helical undulators in tandem. Although this scheme is demonstrated in the ultraviolet range at the low-energy synchrotron UVSOR-III, it can be applied to high-energy synchrotrons to produce vector x-ray beams, which would open a new field in the application of synchrotron radiation.

Microbunching Instability in Relativistic Electron Bunches: Direct Observations of the Microstructures Using Ultrafast YBCO Detectors

Relativistic electron bunches circulating in accelerators are subjected to a dynamical instability leading to microstructures at millimeter to centimeter scale. Although this is a well-known fact, direct experimental observations of the structures, or the field that they emit, remained up to now an open problem. Here, we report the direct, shot-by-shot, time-resolved recording of the shapes (including envelope and carrier) of the pulses of coherent synchrotron radiation that are emitted, and that are a “signature” of the electron bunch microstructure. The experiments are performed on the UVSOR-III storage ring, using electrical field sensitive YBa2Cu3O7−x thin-film ultrafast detectors. The observed patterns are subjected to permanent drifts, that can be explained from a reasoning in phase space, using macroparticle simulations.

Status of 200 keV Beam Operations at Nagoya University

We have developed a 200 keV polarized electron source for generating high bunch charge of >3.2 nC within ∼1 ns bunch duration for the International Linear Collider (ILC) and >7.7 pC within 30 ps bunch duration for a basic source study of an Energy Recovery Linac (ERL). In order to improve gun operation voltage and an operational lifetime, we have employed new electrode materials of molybdenum and titanium to eliminate field emission dark current, and restructured a beam transport vacuum systems. As a beam generation for the ERL, bunch charge of up to 28 pC was generated by irradiating with sliced laser pulse trains. The details are described in this report.

High Brightness and high polarization electron source using transmission photocathode

A transmission photocathode was fabricated based on GaAs‐GaAsP strained superlattice layers on a GaP substrate and a 20 kV‐gun was built to generate the polarized electron beams with the diameter of a few micro‐meter. As the results, the reduced brightness of 1.3×107 A/cm2/sr and the polarization of 90% were achieved.

High Brightness and High Polarization Electron Source for Spin‐LEEM

A point‐like emission mechanism is required for a GaAs polarized electron source to produce an electron beam with high brightness. This is realized by changing the direction of injection laser from a front‐side to a back‐side of the photocathode. Based on this concept, a 20 keV gun (JPES‐1) was constructed and a transmission photocathode including an active layer of a GaAs‐GaAsP superlattice was installed. It produced a laser spot diameter as small as 1.3 μm for the 760∼810 nm laser wavelength, and the brightness of ∼2×107 A⋅cm−2⋅sr−1 corresponding to a reduced brightness of ∼1.0×107 A⋅cm−2⋅sr−1⋅V−1 was obtained for an extracted current of 5.3 μA. This brightness is still smaller than those of W‐field‐emitters, but one order of magnitude higher than those of LaB6 emitters. The peak polarization of ∼90% was achieved at the same time by the photocathode which has an inserted GaAs thin layer between the GaAsP substrate and the GaAaP buffer layer. It demonstrated that the strain‐control of the GaAsP buffer la...