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A novel method to fabricate undulator magnets of a-few-millimetre-period length is being explored. Plate-type magnets, 100 mm-long with 4 mm-period length, have been successfully fabricated. They produce an undulator field of approximately 3 kG at a gap of 1.6 mm. Prototype undulators based on this technology have been constructed. Field measurements and characterization show that the quality of the undulator field of these plate magnets is sufficient for an undulator light source, and the calculated spectrum shows that the fundamental radiation emitted from this field is quite satisfactory. Test experiments for light generation using a real electron beam have been carried out at a test accelerator at the Research Center for Electron Photon Science (ELPH), Tohoku University, Japan, which is able to realize optics conditions to accept a very short gap of 1.6 mm. First observation and characterization of blue light was successfully accomplished.

With the development Chirped Pulse Amplification (CPA) technique [1], ultra-short laser pulses with intensities over 1018 W/cm2 have been equipped in laboratories worldwide. By the incident of such intense laser pulses into underdense plasma, plasma waves with large amplitude will be stimulated in the wake of the drive laser. Electrons trapped in the wave can be accelerated to over GeV within a distance of centimeter scale. This compact laser wakefield acceleration (LWFA) [2] regime is considered to have great potentials in high energy physics and ultra-fast pump probe studies. However, due to the nonlinear interaction process between high power laser and plasma, the instability of the electron beams generated from LWFA is still an obstacle for application. To optimize the stability and beam parameters of laser wakefield accelerated electrons, single-shot beam diagnostic techniques need to be developed. Electron temporal distribution is one of the most important parameters in beam physics. The peak currents of electron bunches are partially determined by the bunch duration and related with the brightness of the electron beam. For the realization of a table-top x-ray free electron laser, this parameter plays an essential role. Yet, due to the noise of laser transmission light and fluctuations of generated electrons, single-shot non-destructive electron temporal diagnostics had been seldomly successfully conducted in LWFA. For this purpose, we for the first time introduced the electro-optic spatial decoding technique into LWFA research. In this talk, the recent development of this technique at Kansai Photon Science Institute (KPSI), National Institutes for Quantum and Radiological Science and Technology (QST) will be presented., レーザー学会第４１回学術講演会

In Japan we are conducting a laser-plasma acceleration (LPA) project aiming a compact X-ray free-electron laser (XFEL) system and a compact carbon ion injector for cancer therapy including efficient laser driver development. In this talk we will focus on the laser electron acceleration part. Our LPA electron beam platform is located in the SPring-8 Center site at RIKEN, Japan, where the prototype XFEL of SACLA was tested. In the platform a Ti:sapphire laser system is installed and it can deliver three synchronized laser pulses (~20 – 30 fs) with pulse energies of 1 J, 2 J, and 10 J. Our strategy is staged electron acceleration using these pulses by manipulating electron phase space to obtain monoenergetic electron beams. In this talk we present our recent results on ~250 MeV, reproducible, quasi-monoenergetic electron production, undulator radiation test, etc. *This work was supported by JST-Mirai Program Grant Number JPMJMI17A1, Japan., IAS Program on High Energy Physics (HEP 2021)

The upgrade status of J-KAREN is presented. The upgrade includes two pilot experiments in order to show the laser performance on target. The first experiment is to generate high-energy ions from thin-foil target. The second is the high-order harmonic at a relativistic intensity. Currently, laser acceleration of protons is being tested and we have obtained 32 MeV protons.

Laser wakefield acceleration (LWFA) has the advantage of ultrahigh acceleration gradient allowing GeV energy gain within centimeter acceleration structure. However, for application concerns, the stability of electron bunches from LWFA still need to be improved. In 2018, we utilized the ionization-injection regime and generated electron beams with charges at nC level in a single peak with the J-KAREN-P laser facility. Yet, the stability was not satisfactory. In 2019, by introducing the density transition regime with mixture gas, we observed relatively stable electron beams with energies of several hundred MeV. In addition, by using a two-gas-jet set-up, we generated electrons with energies up to 2 GeV. Beam driven plasma wakefield was considered to be responsible for the multi-GeV electron generation in the experiment., 日本物理学会 第75回年次大会

In the presentation, we show the design and our approach to achieve soft X-ray free-electron lasers (FELs) by using staged laser plasma acceleration (LPA) of electrons.The staged LPA consists of several functionalized stages such as electron injector, phase rotation, and energy booster to manipulate electron beams to satisfy the requirements for an XFEL. This is a challenging task but this scheme has a favorable scaling to higher energies and has larger tuning knobs to manipulate electron beam quality., 11th International Conference on Inertial Fusion Sciences and Applications

Burst Intensification by Singularity Emitting Radiation (BISER) is a new mechanism of bright coherent x-ray generation employing high-power femtosecond lasers to drive singularities of multi-stream relativistic plasma flows. We have recently demonstrated that the BISER source can be controlled and stabilized with a tailored plasma density profile (density jump), which is created with a blade inserted into the supersonic gas flow. In this presentation we report on new experimental results obtained with the upgraded J-KAREN-P laser. In particular, we extended the BISER emission up to keV photon energies, and increased the BISER yield in the 60 to 100 eV spectral range up to ~ 1 uJ (10^11 photons) per pulse into the detector acceptance angle of 10^-2 sr., The 11th International Conference of Inertial Fusion Sciences and Applications (IFSA-2019)

高強度・極短パルスレーザー電子加速の実験結果を報告する。我々は、関西研にある200 TW, 40 fsレーザーをf/10の光学系を用いて純ヘリウムまたはヘリウムに窒素ガス、ネオンガスを混ぜた混合ガスを用いてイオン化入射法の試験を行った。その結果、窒素ガスを用いた場合には、100 pCを超える大電荷量の数百MeV準単色電子ビームを観測し、ネオンガスを用いた場合には低発散角のビームを観測した。これらは、ガスのイオン化強度に依存した電子入射領域の体積と入射タイミングが影響していると考えられ、ガス種による電子加速の制御が行えることを示唆している。, 第16回日本加速器学会年会

We discovered a new higher order harmonic generation mechanism induced by high power femtosecond laser pulses propagating in underdense plasma. Experimentally it has been shown to be extremely bright, spatially and temporally coherent. Particle-in-cell simulations and Catastrophe theory have explained it as the generation behind the propagating laser pulse of structurally stable oscillating electron spikes at the intersection point between the wake wave and bow wave. Simulations show that the radiation emitted is of attosecond duration. This mechanism has been generalized to explain the emission of waves from a variety of media having multi-stream flows and has been named Burst Intensification by Singularity Emitting Radiation (BISER). It has been proposed that multi-stream space plasmas in relativistic cosmic jets around compact massive objects could produce extremely bright electromagnetic bursts, i.e. GRBs, FRB’s and gravitational radiation. We will present an overview of the BISER mechanism, latest experimental results using the J-KAREN-P laser showing controllability, and brief descriptions of its applications and possible relation to stellar objects., 28th ANNUAL INTERNATIONAL LASER PHYSICS WORKSHOP

Recently we successfully demonstrated controllable BISER in experiments with the J-KAREN-P laser using blade-induced shocks in supersonic gas jets. This resulted in significant improvement of the BISER source stability. Furthermore, the controllable BISER technique made an important breakthrough in the coherent x-ray source performance: the emission extended up to the keV spectral region. Additionally, the soft x-ray and XUV photon yield enhanced by orders of magnitude. In particular, we demonstrated a pulse with up to 1 uJ energy (10^11 photons) in the 60-100 eV spectral range within a 10^-2 sr acceptance angle., VII International Conference "Frontiers of Nonlinear Physics"

We describe new experimental results obtained with the recently upgraded J-KAREN-P laser. The controllable BISER technique enabled us the demonstration of an important breakthrough in the coherent x-ray source performance: the x-ray source position was stabilized, and the emission extended up to the keV spectral region. Additionally, the photon yield enhanced by an order of magnitude resulting in up to 1 µJ pulse (10^11 photons) in the 60-100 eV spectral range within a 10^-2 sr acceptance angle., OPTO-2019

Laser wakefield acceleration (LWFA) [1] has been studied intensively in the past decades. With the optimization of chirped pulse amplification technology (CPA) [2], high quality electron beams with energies over GeV [3] have been generated. The ultrahigh acceleration gradient allows for the construction of a table top x-ray free electron laser [4]. To achieve lasing in the x-ray wavelength range, GeV class electron beam with high brightness is necessary. We succeeded generating highly collimated quasi-monoenergetic electron beams with peak energies up to GeV via ionization-injection [5, 6] using the J-KAREN-P laser. By incident of 300 TW laser pulses into a 10 mm gas jet, comparison studies were performed with pure (He), (He:Ne) mixture and (He:N2) mixture gases. We observed that the divergences, energies and charges of electron beams have dependences on gas species and mixture ratios. The smallest divergence of electron beams from (He:Ne) was merely 0.18 mrad (rms), which was 5 times smaller than that from (He:N2) gas. For many shots, we got extremely high charge electron bunches at plasma denties lower than the self-injection threshold. This study shows the capabilities of the ionization injection regime manipulate and optimize the qualities of the GeV class electron beams from laser wakefield acceleration., HEDS2019

We describe properties of the BISER coherent attosecond x-ray source derived from our experiments and PIC simulations., International Workshop "Complex Systems of Charged Particles and Their Interactions with Electromagnetic Radiation" CSCPIER-2019

We describe new experimental results obtained with the recently upgraded J-KAREN-P laser, including this BISER control technique. We achieved break-through in the BISER coherent x-ray source performance, detected the BISER emission in the keV spectral region, and obtained an order of magnitude enhancement of the photon yield resulted in up to 1uJ pulse, 10^11 photons in the 60-100 eV spectral range within a 10^-2 sr acceptance angle., The 74th JPS Annual Meeting

We succeeded generating quasi-monoenergetic electron beams with peak energies up to GeV from (He:Ne) mixture gas using the J-KAREN-P laser. By incident of 300 TW laser pulses into a 10 mm gas jet, comparison studies were performed with pure (He), (He:Ne) mixture and (He:N2) mixture gases. We observed that the divergences, energies and charges of electron beams varied with different gas species and mixture ratios. The smallest divergence of electron beams from (He:Ne) was merely 0.18 mrad (rms), which was 5 times smaller than that from (He:N2) gas. For many shots, we got extremely high charge electron bunches with peak energy near GeV. This study shows the capability of the ionization injection regime in the manipulation of divergences and charges of the GeV class electron beams generated from laser wakefield acceleration., 日本物理学会 第74回年次大会（2019年）

We present new experimental results on the BISER control technique obtained with the J-KAREN-P laser at the power of 150 TW. We extended the BISER emission up to the keV spectral region and obtained an order of magnitude enhancement of the photon yield (up to 1 uJ pulse, 10^11 photons in the 60-100 eV spectral range within a 10^-2 sr acceptance angle)., The 66th JSAP Spring Meeting

We present the experimental discovery of BISER, simulations and theoretical model, experimental test of the model and its confirmation, and recently obtained new data on BISER control., 第2回QST国際シンポジウムに参加

Burst Intensification by Singularity Emitting Radiation (BISER) is emission of bright bursts of coherent high-frequency radiation by singularities formed in multi-stream flows [Scientific Reports 7 17968 (2017)]. In the case of plasma driven by intense laser, multi-stream flows of electrons (plasma waves) produce relativistic singularities of electron density. These singularities emit bright coherent emission up to the soft x-ray spectral region [PRL 108, 135004 (2012)] with the peak spectral brightness of ~10^27 photons/mm^2 mrad^2 s in 0.1% bandwidth. I will report on properties of this point-like coherent BISER x-ray source, including recent experimental data obtained with the J-KAREN-P laser [Opt. Express 25 20486 (2017)]., LXA-Workshop

Detections of the pulse durations and arrival timings of relativistic electron beams are important issues for accelerators. Electro-optic diagnostics on the Coulomb fields of electron beams have the advantages of single shot and non-destructive characteristics.1,2 We present a study of introducing the electro-optic spatial decoding technique3 to laser wakefield acceleration (LWFA)4. To determine the time scale of the observed signal, a temporal mapping relationship is necessary. By placing an electro-optic crystal very close to a gas target, we discovered that the Coulomb field of the electron bunch possessed a spherical structure that was inconsistent with the previously widely used model. The field structure was demonstrated by experimental measurement, analytic calculations and simulations. A new temporal mapping relationship with generality was derived in a geometry where the signals had spherical wavefronts5. This study could be helpful for the applications of electro-optic diagnostics in laser plasma acceleration experiments. This work was funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan)., HEDS2018

In order to control the laser accelerated electron beam, the control of the plasma wave excited by a laser pulse is necessary. In order to control the plasma wave, the wave should be measured. As a first step of the control of the laser accelerated electron beam, we measure the plasma wave. By using two chirped probe pulses, frequency domain holographic (FDH) measurement is conducted. In the case of FDH, the spectrum can be converted to time. FDH is one of the single-shot plasma wave measurements. In order to produce the plasma wave, 10 TW and 30 fs laser pulse is focused onto a Helium gas jet. The two probe pulses are parallel to the main pulse. Without plasma wave, the interference fringes are straight lines. However, with plasma wave, the fringes are shifted. From the shift, the plasma density can be calculated. As a result, single-shot plasma wave measurement is succeeded., 光・量子ビーム科学合同シンポジウム2017

Laser wakefield acceleration (LWFA)1 has been studied intensively due to the inherent ultrashort bunch duration (fs) and large acceleration gradient (GV/cm) over conventional accelerators. This regime is considered promising on construction of compact table-top X-ray free electron lasers (XFEL)2. The generation of XFEL relies on the process of self-amplification of spontaneous emission (SASE)3, which requires electron beam with extremely high current and low emittance to cause micro-bunching. During LWFA experiment, it is essential to establish electron temporal profile diagnostics as a reference to laser plasma parameter adjustment for generation of high peak current electron beams. We present LWFA study via single-shot non-destructive electro-optic (EO) sampling4 technique for the first time. The electron spectra and divergence information had been detected simultaneously with electron bunch temporal signal. In the experiment, we observed earlier electron pulse arriving time with higher plasma density. At density above 3×1019cm-3, multi-bunch signal was frequently observed. PIC simulation has been performed to study the injection process. This diagnostic proved to have superior data attainability compared with previous works5-7 and could be introduced as a real time non-destructive electron pulse duration monitor for future LWFA experiment. \nThis work was funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan)., HEDS2017

As an advanced accelerator concept, laser wakefield acceleration, with its attractive acceleration gradient, has invoked great interest worldwide. The electron beams from laser wakefield acceleration were assumed to have the advantage of ``jitter-free" or relatively small jitter, since the emergence of quasi-monoenergetic bunch structures were considered to result from the trapping of electrons into the first bucket of the plasma wave. However, the emission times of the electron bunches from laser wakefield acceleration have not been real time monitored in experiments up to now. To examine this, we introduced the electro-optic spatial decoding method into laser wakefield experiments. The relative emission times of the electrons were observed to have variations with different plasma densities when using a laser with moderate intensity. The study shows that timing issues should not be ignored in some occasions of laser wakefield acceleration experiments.

J-KAREN-Pレーザーのアップグレード後に行ったファーストライト実験の結果を報告した。, 光・量子ビーム科学合同シンポジウム2016参加