Ryota Kinjo

Lasing at 12 µm Mid-Infrared Free-Electron Laser in Kyoto University

Laser amplification using a 12 µm mid-infrared free-electron laser (MIR-FEL) was observed at the Institute of Advanced Energy (IAE), Kyoto University. A 25 MeV electron beam of 17 A peak current was used for the lasing experiment. A beam loading compensation method with an RF amplitude control in the thermionic RF gun was used to extend the macropulse duration against the backbombardment effect in the thermionic RF gun. As a result, an electron beam with a 4 µs duration was generated. A laser output with an intensity 50 times as high as the spontaneous emission intensity was observed. FEL gain was estimated to be 16% from the exponential growth of the laser output signal, and a cavity loss of 2.8% was estimated from the decay of the laser output signal. Three-dimensional (3D) FEL simulation was also performed to achieve the gain saturation in our FEL device.

Low-Temperature Operation of a Bulk HTSC Staggered Array Undulator

A use of bulk high-temperature superconductors (HTSs) for an undulator is attractive since a high magnetic field can be generated at low-temperatures. While potential for generation of the high magnetic field is high, in-situ magnetization of the bulk HTSs for periodic field generation is challenging issue. Recently, we proposed a new type of undulator using bulk high-Tc superconductors (HTS) and a solenoid magnet. The undulator, named Bulk HTSC staggered array undulator (Bulk HTSC SAU), consists of a stacked array of bulk HTSs and copper insulators and a solenoid magnet. A proof of principle experiment at 77 K using liquid nitrogen has been carried out. The estimated performance at about 30 K was estimated using results of property measurements for the HTS used for the Bulk HTSC SAU. The expected undulator peak field reaches to 1.08 T for undulator period length of 9.9 mm for the undulator gap of 4.0 mm. This performance is about 2 times higher than that of existing technologies.

Analysis of SNIP Algorithm for Background Estimation in Spectra Measured with LaBr3: Ce Detectors

LaBr3:Ce scintillating detectors exhibit excellent properties for γ-ray spectroscopy such as high energy resolution and operation under room temperature as well as MHz counting rates. On the other hand, sever background radiations exist due to the internal contamination of radioactive materials that are very difficult to be avoided during the manufacture. To decrease the effect of these background levels, some analytical techniques, e.g. background subtraction, should be applied. In the present work, we investigate the efficiency of the sensitive nonlinear iterative clipping peak (SNIP) method for background estimation and subtraction. Optimization of the clipping window is discussed for range of energy up to 3 MeV. Enhancement of energy resolution up to 50% was obtained.

Dependable embedded processor core for higher reliability

We are developing a 32bit embedded processor core with soft error detection and recovery mechanisms. Soft errors caused by atmospheric neutron hits or performance aging in an embedded processor core make the mission-critical embedded system to produce dangerous results like system failure. Our research goal is to investigate soft error rates in the proposed embedded processor core through fault injection tests using a neutron beam or an electromagnetic pulse generator.

Radiation-induced magnetization reversal causing a large flux loss in undulator permanent magnets

We report an unexpectedly large flux loss observed in permanent magnets in one of the undulators operated in SACLA, the x-ray free electron laser facility in Japan. Characterizations of individual magnets extracted from the relevant undulator have revealed that the flux loss was caused by a homogeneous magnetization reversal extending over a wide area, but not by demagnetization of individual magnets damaged by radiation. We show that the estimated flux-loss rate is much higher than what is reported in previous papers, and its distribution is much more localized to the upstream side. Results of numerical and experimental studies carried out to validate the magnetization reversal and quantify the flux loss are presented, together with possible countermeasures against rapid degradation of the undulator performance.

Proposal of a Bulk HTSC Staggered Array Undulator

We proposed a new type of undulator based on bulk high‐Tc superconductors (HTSC) which consists of a single solenoid and a stacked array of bulk HTSC. The main advantage of this configuration is that a mechanical structure is not required to produce and control the undulator field. In order to perform a proof of principle experiment, we have developed a prototype of bulk HTSC staggered array undulator using 11 pairs of DyBaCuO bulk superconductors and a normal conducting solenoid. Experimental results obtained by using the prototype undulator and numerical results obtained by a loop current model based on the Bean mode for a type‐II superconductor were compared.

Beam Energy Compensation in a Thermionic RF Gun by Cavity Detuning

A thermionic RF gun usually suffers from a rapid decrease of electron beam energy caused by a rapid increase of beam current due to the back-bombardment effect. The authors propose a new method named ldquocavity detuningrdquo to keep the electron beam energy constant. This method detunes the resonant frequency of the gun cavity to a frequency that is lower (by several hundred kilohertz) than the driving frequency. A proof-of-principle experiment was carried out using a 4.5 cell S-band RF gun. The beam energy was successfully kept constant during a macro-pulse duration of 7.5 mus using a cavity detuning of -590 kHz, even with a significant current increase from 240 to 660 mA. A numerical simulation was also conducted to evaluate the cathode temperature, the current density on the cathode surface, and the phase stability of the output beam. All results demonstrated that cavity detuning provides an easy and simple way to compensate for the energy decrease in thermionic RF guns.

Synthesizing high-order harmonics to generate a sub-cycle pulse in free-electron lasers

An approach is proposed to generate a quasi-isolated sub-cycle pulse in X-ray free-electron lasers. Its principle is based on the recently proposed concept of mono-cycle harmonic generation [T. Tanaka, Phys. Rev. Lett. 114, 044801 (2015)], but uses the chirped microbunch with high-order harmonic frequencies. This allows the synthesis of a sub-cycle field structure in the coherent radiation. Moreover, the tolerance in energy spread is greatly relaxed compared with the originally proposed scheme. Additionally, the practical procedure for realizing the scheme is greatly simplified. Numerical investigations show that a quasi-isolated sub-cycle pulse with a gigawatt peak power can be generated using an electron beam with a realistic energy spread as conventional accelerators for free-electron lasers.

Undulator Development for SPring-8-II

After the construction of the X-ray free electron laser facility SACLA, which achieved first lasing in 2011 [1] and has been successfully operated for nearly four years, SPring-8 has now turned to upgrading its storage ring to further enhance the light source performance in terms of brilliance. In this “SPring-8-II” project, a new lattice structure composed of five bending magnets has been chosen [2] in order to reduce the horizontal emittance, which, in turn, requires us to shorten the straight sections available for undulators by roughly 1 m. In addition, the electron energy will be reduced down to 6 GeV from the current 8 GeV for further reduction of the emittance. This upgrade plan is not necessarily compatible with the existing undulators in SPring-8 and thus most of them need to be replaced with new ones optimized for operation in the new ring in order to maximize the brilliance. This raises a number of technical challenges toward realization of SPring-8-II, including considerable reduction of the manufacturing cost and further shortening of the magnetic period of undulators. In this article, we report relevant R&D activities to overcome these challenges, together with a new concept to enable a flexible polarization control, which is one of the important options in synchrotron radiation (SR) beamlines.

Lightweight-compact variable-gap undulator with force cancellation system based on multipole monolithic magnets

A lightweight-compact variable-gap undulator (LCVGU) having the force-cancellation system based on the multipole monolithic magnets (MMMs) has been developed. The LCVGU is free from the heavy mechanical frames, which is a fundamental element specific to conventional variable-gap undulators (VGUs) because of a strong attractive force, and thus the cost and time for construction and installation are expected to be significantly reduced; the MMMs counteract the strong attractive force in a cost-effective manner. Results of mechanical tests and magnetic-field measurements of two prototype LCVGUs equipped with the proposed force cancellation system have revealed the comparable performance with the conventional VGUs.