A. Mochihashi

Intense Terahertz Synchrotron Radiation by Laser Bunch Slicing at UVSOR-II Electron Storage Ring

A laser bunch slicing system has been constructed at the UVSOR-II electron storage ring to produce a submillimeter-sized microstructure on an electron bunch. As its first result, terahertz coherent synchrotron radiation (THz CSR) was successfully produced by the dense structure of the electron bunch. Its intensity is higher by four or five orders of magnitude than that of normal synchrotron radiation. The intensity is proportional to the square of the peak current of the electron bunch, as expected for coherent emission. It is also demonstrated that the spectral shape of the THz CSR can be controlled by changing the slicing laser pulse duration.

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.

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 ...

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.

Full Energy Injection and Top-up Operation at UVSOR-II

Top‐up operation was successfully demonstrated at UVSOR‐II, which will remove the short beam‐lifetime problem caused by strong Touschek effect due to the small emittance, 27 nm‐rad, and the low electron energy, 750 MeV. In these years, we have improved the accelerators, step by step, towards top‐up operation. The radiation shielding wall was reconstructed. The energy of the booster synchrotron and the beam transport line were successfully upgraded from 600 MeV to 750 MeV, by replacing the magnet power supplies in 2006. Soon after, we succeeded in injecting the electron beam at the full energy. We improved the radiation safety system and constructed a injection control system. In autumn, 2008, we succeeded in operating the ring for 12 hours as keeping the beam current quasi‐constant at 300 mA. Currently, we operated the ring in the top‐up mode for 12 hours on every Thursday night, to check the effects on the users’ experiments. In 2009, we have succeeded in the top‐up operation in single bunch mode. Free e...

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.

Status and Prospects of User Applications of the UVSOR Storage Ring Free Electron Laser

Because of in addition to the high out‐coupled power and a good coherence, the temporal feature that the laser pulse is naturally synchronized with the SR pulse, the SRFEL is a powerful tool for application experiment. At UVSOR, we have performed an application experiment of the SRFEL on the two‐photon double‐resonant excitation of the Xe. The storage ring is temporary shutdown now to upgrade its performance. The improve performance of the electron beam is of great advantage for the SRFEL and its application.

Suppression of Longitudinal Coupled Bunch Instability by Harmonic Cavity in UVSOR Electron Storage Ring

In the UVSOR electron storage ring [1], which is dedicated for a VUV synchrotron radiation light source, a longitudinal coupled bunch instability (LCBI) is observed in multi-bunch operation. To suppress the LCBI, we routinely operate a third harmonic cavity (HCV) in a passive mode. By properly tuning the HCV, the instability is almost completely suppressed. Because of the lower beam energy (750 MeV) and brilliant beam emittance (17.5 nm-rad), the Touschek effect becomes severe in the UVSOR. To guarantee enough beam lifetime, we also apply the HCV for lengthening the bunch. The suppression of the instability and increasing the beam lifetime are crucial benefits by the HCV for the UVSOR. However, not only the origin of the LCBI but also the Landau damping effect by the HCV has not been understood systematically yet. We have noticed that one of the HOMs at the HCV itself could cause the LCBI and observed the behavior of the instability, which strongly depends on the beam current. From the experiment we have discussed the cause of the instability with the HOM theory. We have also tried to observe synchrotron tune spread and discussed a competition between the Landau damping and the instability growth.