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C形導波管は同軸管と類似の構造を持っているが、遮断周波数を持ち、内軸を効率的に冷却できるなどの特徴を持っている。これらの特徴を生かしてC形導波管型HOMカップラーの開発を進めている。空洞に装着する場合の曲げ形状の製作手順等の検討、電子ビーム溶接のための予備試験を行っており、その現状について報告する。, 第16回日本加速器学会年会

スポーク空洞は、周波数が同じならば楕円空洞よりサイズが小さく、さらにパッキングファクターにも優れている。このスポーク空洞の利点を生かしてERL 加速器を小型化し、LCS-γ/X線源を産業・学術分野への利用を図るため、 超伝導スポーク空洞の開発を進めている。現在プレス加工したハーフスポークを使ってフルスポークにするための電子ビーム溶接試験を行っている。スポーク側面は複雑な曲線形状であり、さらにプレス成型による厚さ変化もあるため、溶接条件の最適化が重要である。溶接試験の現状について報告する。, 第16回日本加速器学会年会

ニュースバル放射光施設において開催される核セキュリティワークショップにおいて、核セキュリティ補助金事業で実施した研究開発プログラムの概要について報告する。, Workshop on demonstration of nuclear resonance fluorescence technique for detection of nuclear material hidden in a shield

A mid-infrared Free Electron Laser (MIR-FEL) is a candidate to realize a high-repetition-rate attosecond hard X-ray source based on the high harmonic generation (HHG). A new project to conduct proof of principle experiment of HHG driven by a MIR-FEL has been started from November 2018. For this purpose, upgrade projects of MIR-FEL at Kyoto University (KU-FEL) has been started to achieve high peak power and ultra-short pulse operation.

共振器型自由電子レーザでは、一定の条件下で、数サイクルの極短パルスが生成できることが知られている。このようなFEL発振のシミュレーションでは、一般的なエンベロープ近似の手法が適用できない可能性がある。本講演では、数サイクルFELのシミュレーション技法について報告する。, 第26回FELとHigh-Power Radiation研究会

We present a scheme for high-harmonic-generation (HHG) driven by infrared free-electron lasers (FEL), which can be operated at MHz-repetition. Wavelength tunability of FEL enables one to explore HHG from mid-infrared pulses for higher cutoff energies above 1 keV. Plans and status of R&D are also presented.

High harmonic generation (HHG) in rare gases is now becoming a common technology to produce attosecond pulses in VUV wavelengths. So far HHG sources have been realized by femtosecond solid-state lasers, not FELs. We propose a FEL-driven HHG source to explore attosecond pulses at photon energies above 1 keV with a MHz-repetition, which is difficult with solid-state lasers [1]. A research program has been launched to establish technologies for the FEL-HHG, which covers generation and characterization of few-cycle IR pulses in a FEL oscillator, stacking of FEL pulses in an external cavity, and a seed laser for stabilization of carrier-envelope phase in a FEL oscillator. In this talk, we present the scheme of FEL-HHG and the status of the research program.

Generation of gamma photons from Compton scattering is one of the promising applications of free-electron lasers as proven by HIGS at Duke University. Monte Carlo simulations are essential for optimizing an experimental layout in a laser Compton scattered gamma source facility as well as analyzing results of gamma-ray experiments. Geant4 is a software toolkit for Monte Carlo simulations and developed continuously by world-wide collaboration. We implemented a laser Compton scattered gamma source in Geant4. The implemented model provides a primary particle source to generate gamma photons with arbitrary electron and laser beam parameters at laser Compton scattering. In this paper, benchmarking of the implemented model with other simulation codes and experimental results are presented., 39th International Free Electron Laser Conference

波長可変かつ高平均出力が可能な自由電子レーザー（FEL）で中赤外のレーザー発振を行い、これを高次高調波発生（HHG）に利用すれば、1 keV以上のアト秒X線をMHzの繰り返しで生成できる。われわれは、FEL-HHGの実現に必要な基礎基盤技術の研究を昨年度より開始した。本報では、研究計画の概要を紹介する。, 第16回日本加速器学会年会

KEKにおいてcERLをベースとした中赤外SASE型自由電子レーザーの検討・開発が進められている。現状のcERLによって生成される電子ビームのパラメータを用いてGenesisによるFEL発振のシミュレーションを行ったところ，アンジュレーター中においてレーザー光が拡がり，真空ダクトと干渉する程成長することが判明した。Genesisにおけるレーザー光の計算においては，真空ダクト境界における光の反射および吸収の効果が考慮されておらず，実際に観測されるレーザー光出力に影響が出る懸念がある。そこで，新たにGenesisにおいて光の放射場を導波管モードに展開し，真空ダクト境界面での境界条件を取り入れることにより，真空ダクトにおける光の反射・吸収を考慮したコードの開発を検討した。本発表では，導波管モードへの放射場の展開理論をまとめ，境界条件を考慮した場合としない場合との比較を報告する。, 第16回日本加速器学会年会

中赤外領域のFELで駆動する高次高調波アト秒X線パルス発生のために、キャリアエンベロープ位相が安定したシード光が必要である。そのために現在、モードロックファイバーレーザーによる差周波発生を基礎とした中赤外光源の検討と開発を進めており、本発表では、その概要と現状を報告する。, 第16回日本加速器学会年会

A compact energy-recovery linac (cERL) have been operating since 2013 at KEK to develop critical components for ERL facility. Details of design, construction and the result of initial commissioning are already reported*. This paper will describe the details of further improvements and researches to achieve higher averaged beam current of 1 mA with continuous-wave (CW) beam pattern. At first, to keep the small beam emittance produced by 500 kV DC-photocathode gun, tuning of low-energy beam transport is essential. Also, we found some components degrades the beam quality, i.e., a non-metallic mirror which disturbed the beam orbit. Other important aspects are the measurement and mitigation of the beam losses. Combination of beam collimator and tuning of the beam optics can improve the beam halo enough to operate with 1 mA stably. The cERL has been operated with beam energy at 20 MeV or 17.5 MeV and with beam rep-rate of 1300 MHz or 162.5 MHz depending on the purpose of experiments. In each operation, the efficiency of the energy recovery was confirmed to be better than 99.9 %., 10th International Particle Accelerator Conference

At the LEBRA facility of Nihon U., we have an IR-FEL oscillator to generate radiations in the range of wavelengths 1-6 um for various experiments. A research program has been established to explore the application of the IR-FEL to generate attosecond UV and X-ray pulses through the high harmonic generation (HHG) in noble gases, where the IR-FEL pulses must have a high-peak power and a short-pulse duration. The property of generated FEL pulse is affected by the cavity length detuning of FEL oscillator as well as the small signal gain and the cavity loss. The operation at a small- or zero-detuning length is necessary to generate a FEL pulse shorter than the bunch length, although it requires a long macro-pulse to reach the saturation. For the short FEL pulse generation within a limited macro-pulse length at the LEBRA LINAC, we apply a dynamical modulation to the electron bunch repetition, that is equivalent to a dynamical detuning of the FEL cavity length. We illustrate the potential performance of the IR-FEL with the dynamical detuning by time-dependent 3D FEL simulations. We also evaluate the enhancement of the FEL pulses by an external cavity stacking for the sake of the HHG application., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

We have proposed a new selective isotope transmutation method using photonuclear reactions with quasi-monochromatic gamma-ray beams. This method is based on the fact that the particle threshold of a long-lived fission product (LLFP) such as 93Zr, 107Pd, or 79Se is lower than those of stable isotopes of the same chemical element. Therefore, this method has the excellent advantage that LLFPs cannot, in principle, be produced newly even if the target materials include stable isotopes in addition to LLFPs. Furthermore, this method is effective for 126Sn, 135;137Cs, 90Sr, and 3H. The nuclear data involved and suitable gamma-ray sources are discussed. Laser Compton scattering gamma-ray sources and neutron capture gamma-rays in nuclear reactors are candidates for this method.

The Compact Energy Recovery Linac (cERL) at KEK is a test accelerator in order to develop key components to realize remarkable ERL performance as a future light source. After the beam commissioning in December 2013, the legal current limit has been increased step-by-step like 1 uA, 10 uA, and 100 uA. Survey for the source of beam losses has been conducted in each step, and the study on beam dynamics and tuning has also been carried out. As a next step, 1 mA operation is scheduled in February 2016. In parallel to the increase in beam current, a laser Compton scattering (LCS) system which can provide high-flux X-ray to a beamline has been successfully commissioned. We report recent progress in various kinds of beam tuning: improvement of electron gun performance, high bunch charge operation, mitigation of beam losses, LCS optics tuning and bunch compression for THz radiation., The 7th International Particle Accelerator Conference (IPAC2016)

The highest extraction efficiency (9.4%) of a free electron laser (FEL) oscillator has been achieved at the midinfrared FEL facility of Kyoto University. Because of the interaction between the electron beam and FEL electromagnetic field, a maximum electron energy decrease of 16% was observed. The measured energy decrease was consistent with the measured FEL spectrum. An FEL micropulse energy of ~100 μJ with the expected few-cycle pulse duration at a wavelength of 11 μm was observed. This result is an important milestone for the high-extraction-efficiency FEL oscillator and will contribute to the strong-field physics of atoms and molecules

Nuclear resonance fluorescence (NRF) is a promising technique for the non-destructive assay (NDA) of nuclear materials. Its powerfulness is apparent in the highly penetrative $\gamma$-rays emitted in an isotopic fingerprint of the NRF interactions. However; there exist other interactions that may interfere with the NRF and hence, may limit its accuracy. Of these interactions is the elastic scattering of $\gamma$-rays by atoms which needs further investigation and testing. Japan Atomic Energy Agency started in 2015 to develop a NDA system based on the NRF for nuclear non-proliferation and nuclear security purposes. One of the tasks of the current development is assessing the effect of the elastic scattering of $\gamma$-rays on NRF measurement. A new simulation code for the elastic scattering of $\gamma$-rays has recently been developed in the Geant4 environment. The present JAEA-Data/Code report provides a more detailed description of the simulation code as well as an elaborated illustration of the elastic scattering of $\gamma$-rays and its interaction cross sections. This report facilitates user feedback of the simulation code which is indispensable for reaching a stable and reliable simulation. The current report would contribute to better understanding of the elastic scattering of $\gamma$-rays. This research was implemented under the subsidiary for nuclear security promotion of MEXT., 原子核蛍光共鳴散乱(NRF)は、核物質の非破壊検査(NDA)において有用な技術である。NRFは同位体に固有のエネルギーをもったガンマ線と物質の相互作用であることから、同位体を識別したNDAを実現できる。しかしながら、NRFと競合する他の光核相互作用が存在し、そのため、NRFによるNDAの測定性能が制限される可能性がある。競合する相互作用のうち原子核によるガンマ線の弾性散乱(デルブリュック散乱)は、これまでのモンテカルロコードでは考慮されていなかった。日本原子力研究開発機構では、平成27年度からNRFに基づくNDAシステムの開発プログラムを開始し、その一環としてガンマ線の弾性散乱がNRF測定に及ぼす影響を評価するために、Geant4にガンマ線の弾性散乱を追加するためのコードを新規に開発した。今回刊行するJAEA-データ/コードレポートでは、シミュレーションコードの詳細な説明と、コード入力に用いたガンマ線弾性散乱断面積、および、コードの検証と妥当性確認に用いた計算結果を示す。このレポートを公開することで、当該コードの信頼性向上に必要なユーザーからのフィードバックの機会が得られる。本研究は文部科学省の核セキュリティ強化等推進事業費補助金の支援を受けた。

We have developed a DC gun test stand at National Institutes for Quantum Radiological Science and Technology (QST) for high current electron beam generation. The gun test stand consists of an alkali antimonide photocathode preparation chamber, a DC gun with a 250kV-50mA Cockcroft Walton high voltage power supply, and beam line with a water cooled beam dump to accommodate 1.5 kW beam power. We successfully fabricated a Cs₃Sb photocathode with quantum efficiency of 5.8 % at 532 nm wavelength and generated 150 keV beam with current up to 4.3 mA with 500 mW laser at 532 nm wavelength. Unfortunately, we encountered a vacuum incident during beam transport of high current beam and the development has been halted. We will fix the vacuum problem and restart the gun development as soon as possible., Proceedings of the 59th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, ERL17, Geneva, Switzerland

Compton scattering of a single-energy γ-ray results in a one-to-one relationship between the incident and scattered photon energies. This relationship is altered when the incident beam has a definite energy distribution because of the broadening occurring in the energy distribution upon Compton scattering. This broadening causes a change in the spectral density of the Compton-scattered spectra. To restore the spectral density, the energy distribution of the scattered radiation must be manifested as a function of the scattering kinematics. Here, we propose a simple analytic way to calculate the energy spread of the scattered photons in terms of the geometry of the scattering process and the energy spread of the incident photon beam. The predictions of the model agree with measurements of Compton scattering of quasi-monochromatic γ-ray beams, carried out at the High Intensity γ-ray Source (HIγS) facility, Duke University. As a benchmark of our method, we measured the intensity profile of energy-distributed -ray beams by direct measurements as well as by Compton scattering. We found that only when the spectral density of the scattered radiation is restored, the measured intensity profile agrees with the actual profile of the incident beam. The proposed method can continuously measure the flux of an energy-distributed γ-ray beams in real time and on a bin-by-bin basis. Such online monitoring of γ-ray beams is indispensable for in-beam measurements and applications.

The dependence of the nuclear resonance fluorescence (NRF) yield on the target thickness was studied. To this end, an NRF experiment was performed on 238U using a laser Compton back-scattering (LCS) γ-ray beam at the High Intensity γ-ray Source facility at Duke University. Various thicknesses of depleted uranium targets were irradiated by an LCS γ-ray beam with an incident beam energy of ∼2.475 MeV. The scattering NRF γ-rays were measured using an High-purity Germanium (HPGe) detector array positioned at scattering angles of 90° relative to the incident γ-beam. An analytical model for the NRF reaction yield (NRF RY model) is introduced to interpret the experimental data. Additionally, a Monte Carlo simulation using GEANT4 was performed to simulate the NRF interaction for a wide range of target thicknesses of the 238U. The measured NRF yield shows the saturation behavior. The results of both of the simulation and the analytical model can reproduce the saturation curve of the scattering NRF yield of 238U against the target thickness. In addition, we propose a method to deduce the precise integral cross section of the NRF reaction by fitting the NRF yield dependency on the target thickness without any absolute measurements.

The polarization properties of γ-rays elastically scattered by atoms have become more observable with the development of polarized photon beams. However, systematic studies are required to explore the elastic scattering in the MeV-energy range of the spectrum where Delbrück scattering becomes more significant, especially at large scattering angles. We implement a new Monte Carlo simulation to account for the polarization effects of elastic scattering. The simulation is based on explicit expressions driven in the formalism of Stokes parameters. The scattering amplitudes of Rayleigh, nuclear Thomson, and Delbrück scattering processes are superimposed onto a two orthogonal set of complex amplitudes. This set is then exploited to construct the core of the simulation in such a way that the simulation can handle arbitrary polarization states of the incoming beam and correspondingly generate polarization states of the outgoing beam. We demonstrate how the polarization of scattered photons is affected by the polarization of incoming photons. In addition, we explain the dependence of depolarization on the azimuthal angle.

Nuclear resonance fluorescence (NRF) is a promising technique for nondestructive assay (NDA) of nuclear materials. Its strength is improved when polarized $\gamma$-ray beams are used as probes because the polarized $\gamma$-rays provide an asymmetry in the intensity of the scattered photons. Nonetheless, NRF interaction takes place with other interactions such as elastic scattering of $\gamma$-rays. These interactions are unavoidable background which may limit the sensitivity of the NRF technique. Therefore, estimating polarization effects of elastic scattering is a significant factor in assessing the NRF method. Here, we implement a new Monte Carlo simulation to account for polarization effects of the elastic scattering. The simulation is based on explicit expressions driven in the Stokes parameters formalism. The scattering amplitudes of Rayleigh, nuclear Thomson, and Delbr{\"u}ck scattering processes have been superimposed into a two-orthogonal set of complex amplitudes. This set is then exploited to construct the core of the simulation in a way such that the simulation could handle arbitrary polarization states of the incoming beam and correspondingly generate polarization states for the outgoing beam. This work was supported by the subsidiary for promotion of strengthening nuclear security or the like of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

The spin and parity of the 3.004-MeV level in 27Al have been determined by measuring the angular correlation function of radiation emitted from levels populated by resonant absorption of polarized photons. The nuclear resonance fluorescence experiments were carried out at the High Intensity g-ray Source (HIgS) facility at Duke University using quasi-monoenergetic linearly polarized photon beams. The spin and parity of levels in 27Al were deduced from a comparison of the measured angular distribution ratios and azimuthal intensity asymmetries with theoretical predictions. The observed resonance properties were compared with shell model calculations using the universal sd interaction. The Monte Carlo shell model calculations were also performed to investigate low-energy structure of 27Al.

We are developing the superconducting spoke cavity for laser Compton scattered (LCS) photon sources. We adopt the superconducting spoke cavity for electron beam drivers to realize a wide use of LCS X-ray and '-ray sources in academic and industrial applications. The spoke cavity can make the accelerator more compact than an elliptical cavity because the cavity size is small at the same frequency and the packing factor is good by installing couplers on outer conductor. Though our proposal design for the photon source consists of the 325 MHz spoke cavities in 4K operation, we are fabricating the half scale model of 650 MHz spoke cavity in order to accumulate our cavity production experience by effective utilization of our limited resources. Since the spoke has more complicated structure than an elliptical cavity, we performed press forming tests for the half spoke and estimated the formed shapes with 3-dimensional measurement., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

An energy recovery linac(ERL)-based free electron laser(FEL) is a possible candidate of a high-power EUV source for lithography. The ERL can provide a high-current and high-quality electron beam for the high-power FEL and also greatly reduce the dumped beam power and activation compared to ordinary linacs. An ERL-based EUV-FEL source has been designed using available technologies and resources*. For this design, we perform Start-to-End(S2E) simulation from the electron gun to the exit of the decelerating main linac to track the electron beam parameters and to evaluate the FEL performance. The electron bunches from the injector are off-crest accelerated to 800 MeV and compressed in the 1st arc and/or chicane to obtain a high-peak current for high FEL output. After the undulator section for SASE FEL, they are decompressed in the 2nd arc and then decelerated in the main linac to optimize the energy spread or the energy recovery efficiency. This paper will present the S2E simulation for the designed EUV-FEL source., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The spoke cavity is expected to have advantages for compact ERL accelerator for X-ray source based on laser Compton scattering. We have been developing the spoke cavity under a research program of MEXT, Japan to establish the fabrication process. Since our designed shape of the spoke is complicated due to increase the RF properties, one-step press forming with one set of molds will cause so large strain to break the sheet. We designed the mold components including the process of press work. The press forming tests of the spoke cavity have been done with the various materials of sheets to check molding performance. In this paper we present status of the spoke cavity fabrication., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA