Y. Takashima

Construction and Commissioning of UVSOR‐II

UVSOR, a 2nd generation synchrotron light source of 750 MeV, was converted to UVSOR‐II, which has eight straight sections and small emittance of 27 nm‐rad. The magnetic lattice was modified without changing the circumference of 53.2 m. All the magnets and their beam ducts except for the bending magnets were replaced. An undulator and a super‐conducting wiggler were replaced with two new in‐vacuum undulators. Some parts of the injector were replaced and upgraded. Some beam‐lines were reconstructed. All the reconstruction works were completed within three months, from April to June 2003. In July, UVSOR‐II was successfully commissioned. Some preliminary measurements on the beam parameters suggested that the design goal of the emittance, 27 nm‐rad, was likely achieved. Vacuum conditioning with beams are in progress. Users experiments will start in September.

Observation of Intense Bursts of Terahertz Synchrotron Radiation at UVSOR-II

We have detected very intense bursts of terahertz synchrotron radiation at the UVSOR-II electron storage ring operated in single bunch mode. The bursts were observed in the wavelength range from 0.2 to 3.0 mm by using a liquid-helium-cooled InSb hot-electron bolometer. The typical duration and interval of the bursts were about 200 µs and 10–15 ms, respectively. Each burst shows the quasi-periodic structure of about 30 µs. The peak intensity of the bursts was about 10000 times larger than that of ordinary synchrotron radiation in the same wavelength region. The extremely high intensity strongly suggests that the bursts are coherent synchrotron radiation, although the radiation wavelength was much shorter than the electron bunch length.

Angular Momentum of Twisted Radiation from an Electron in Spiral Motion

We theoretically demonstrate for the first time that a single free electron in circular or spiral motion emits twisted photons carrying well-defined orbital angular momentum along the axis of the electron circulation, in adding to spin angular momentum. We show that, when the electron velocity is relativistic, the radiation field contains harmonic components and the photons of lth harmonic carry lℏ total angular momentum for each. This work indicates that twisted photons are naturally emitted by free electrons and are more ubiquitous in laboratories and in nature than ever thought.

Q-switching operation of the UVSOR-FEL

Q-switching operation of the storage ring FEL provides high peak power and is therefore very attractive to application experiments. On the UVSOR, the Q-switching is performed by modulation of an RF frequency. Collective synchrotron oscillation of the electron bunch is excited in the actual operation. However, it is found that damping time of the oscillation is 100 times faster than the one expected from synchrotron radiation, so the influence of the collective oscillation on the lasing is relatively small. Analysis reveals that the phenomenon is explained with Robinson damping.

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.

Coherent Terahertz Radiation at UVSOR‐II

Development of intense terahertz radiation source is progressing at UVSOR‐II, based on the mechanism of coherent synchrotron radiation (CSR). The terahertz CSR has successfully been produced by two methods. When the storage ring is operated in the single bunch mode with a sufficiently high beam current, intense bursts of terahertz radiation are emitted. Micro‐structures in the longitudinal density distribution of the electron bunches created by a beam instability may be the origin of the radiation. The duration of the bursts is typically 100 micro‐seconds. The peak intensity is 10000 times higher than that of the normal synchrotron radiation. The bursts appear chaotically or quasi‐periodically depending on the beam current with a typical interval of 10 milli‐seconds. It has been also demonstrated that the terahertz CSR could be produced by the laser‐bunch slicing method. The density modulation produced on the electron bunch by the laser is the origin of CSR. The repetition rate of the terahertz pulses is ...

In‐vacuum undulators in UVSOR electron storage ring

In‐vacuum undulators have been installed in the UVSOR‐BL3U and BL7U. Due to the improvement of the UVSOR, the insertion devices with narrow magnetic gaps such as the in‐vacuum undulators have become available. The undulators have the period length of 36/38mm and the minimum gap of 15mm, which results in wide tunability in VUV and soft X‐ray region. Influence on the stored beam, especially for the resistive wall instability, has been estimated and it has found that the disturbance for the stored beam can be negligible even in the minimum gap height.

Ion Trapping Phenomenon in UVSOR Electron Storage Ring

A vertical betatron tune shift depending on beam current under multibunch condition was observed in the UVSOR electron storage ring. Vertical tune increased as beam current decreased, and the slope of the tune shift depended on the condition of the vacuum in the ring. Such a change in vertical tune was explained by a change in the stability condition of trapped ions with beam current. The experimental results were discussed with analytic and tracking calculations.

Central Japan Synchrotron Radiation Research Facility Project-(II)

A synchrotron radiation facility that is used not only for basic research, but also for engineering and industrial research and development has been proposed to be constructed in the Central area of Japan. The key equipment of this facility is a compact electron storage ring that is able to supply hard X‐rays. The circumference of the storage ring is 72 m with the energy of 1.2 GeV, the beam current of 300 mA, and the natural emittance of about 53 nm‐rad. The configuration of the storage ring is based on four triple bend cells, and four of the twelve bending magnets are 5 T superconducting ones. The bending angle and critical energy are 12 degree and 4.8 keV, respectively. For the top‐up operation, the electron beam will be injected from a booster synchrotron with the full energy. Currently, six beamlines are planned for the first phase starting from 2012.

Local heating induced by Coherent Harmonic Generation on electron beam dynamics in storage ring

We investigate the dynamical response of an electron bunch in a storage ring to a brief and intense excitation of an external laser. We present a model that simulates the evolution of the electronic distribution in the Coherent Harmonic Generation (CHG) configuration: the energy exchange between photons and electrons occurs within an undulator magnetic field, and further leads to a coherent radiation process. Using those simulations and experimental results on the UVSOR-II CHG free electron laser, we demonstrate that the electronic distribution reaches an equilibrium regime thanks to energy spread relaxation in between two laser injections. We also show that saturation is driven by laser-induced local heating, leading to global effect over the whole bunch.