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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

New regime of high-order harmonic generation by relativistic-irradiance multi-TW fs lasers focused onto gas jet targets is discussed. The laser induces multi-stream relativistic plasma flow resulting in the formation of density singularities. Here we analyse the dependence of harmonic yield on the focal spot quality and derive the required laser parameters for efficient harmonics generation.

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.

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

Particle acceleration driven by a high power Ti: sapphire laser has invoked great interest worldwide because of the ultrahigh acceleration gradient. For the aspect of electron acceleration, electron beams with energies over GeV have been generated using the laser wakefield acceleration mechanism. For the optimization of the electron generation process, real-time electron parameter monitors are necessary. One of the key parameters of a high energy particle beam is the temporal distribution, which is closely related with the timing resolution in a pump-probe application. Here, we introduced the electro-optic sampling method to laser wakefield acceleration. Real-time multibunch structures were observed. Careful calculations on the physical processes of signal generation in an electro-optic crystal were performed. Discussions of the methodology are elaborated in detail.

Coherent x-rays via the Burst Intensification by Singularity Emitting Radiation (BISER) mechanism are generated from relativistic plasma in helium gas target. A broad modulation of the BISER spectrum, which is significantly wider than the harmonic order, is observed and characterized. In particular, we found that the modulation period can be as large as 41 eV.

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

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

High-order harmonics from relativistic singularities are emitted when a multi-terawatt femtosecond laser irradiates a gas jet target [1, 2]. The laser pulse pushes electrons producing an electron-free cavity and generates a bow wave [3]; the singularities are produced at the joining of the cavity wall and the bow wave and represent point-like coherent x-ray emitters. The emitted spectra generally consist of high-order harmonics of optical frequency. However, the spectra often exhibit additional features such as modulations with large period (up to 10s of eV) and presence of several periods in a single spectrum. Here we report on analysis of experimental spectra with the emphasis on dependence on laser and plasma parameters, and discuss possible implications. This work was supported by JSPS KAKENHI # JP 25287103, JP 25390135, and JP 26707031., JPS Spring Annual Meeting

We have recently discovered high-order harmonics from relativistic electron spikes [1, 2] produced by multi-terawatt relativistic-irradiance (>10^18 W/cm^2) femtosecond lasers focused onto gas jet targets with typical electron densities ~ a few x10^19 cm^-3. These harmonics represent a new source of bright coherent x-rays. Our experiments with the J KAREN laser [3] demonstrated a strong dependence of the harmonics generation efficiency on the laser pulse energy and quality, in particular on the focal spot shape. \nHere we discuss laser requirements for efficient harmonics generation. In addition to the obvious need for a higher power, the focal spot quality should approach the diffraction limit, so that the Strehl ratio exceeds 0.5. We consider two typical reasons of the spot degradation. First, it is the noise-like high-frequency wavefront distortions. The above stated quality requirement corresponds to the rms wavefront error of much smaller than 100 nm. Second, it is the angular dispersion, which must be kept smaller than a fraction of the diffraction divergence. For typical 100 to 300 mm beam diameters and 50 nm bandwidths, the angular dispersion should not exceed a urad level and the angular chirp should be less than ~10^-2 urad/nm. We show our recent progress in satisfying these requirements with the J-KAREN-P laser. \n[1] A. S. Pirozhkov, et al., "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes," Phys. Rev. Lett. 108, 135004-5 (2012). \n[2] A S. Pirozhkov, et al., "High order harmonics from relativistic electron spikes," New J. Phys. 16, 093003-30 (2014). \n[3] H. Kiriyama, et al., "High-Contrast, High-Intensity Petawatt-Class Laser and Applications," IEEE J. Sel. Topics Quantum Electron. 21, 1601118-18 (2015)., The Optics & Photonics International Congress OPIC-2016, International Conference on High-Energy Density Sciences HEDS-2016

We propose nuclear experiments using gamma-ray pulses provided from high field plasma generated by high peak power laser. These gamma-ray pulses have excellent features of extremely short pulse, highly intense, and continuous energy distribution. These features are suitable for the study of explosive nucleosyntheses in novae and supernovae such as the gamma process and neutirno process. We discuss how to generate the suitable gamma-ray pulses and nuclear astrophysics involved.

Enhancement of the quality of laser wake-field accelerated (LWFA) electron beams implies the improvement and controllability of the properties of the wake waves generated by ultra-short pulse lasers in underdense plasmas. In this work we present a compendium of useful formulas giving relations between the laser and plasma target parameters allowing one to obtain basic dependences, e.g. the energy scaling of the electrons accelerated by the wake field excited in inhomogeneous media including multi-stage LWFA accelerators. Consideration of the effects of using the chirped laser pulse driver allows us to find the regimes where the chirp enhances the wake field amplitude. We present an analysis of the three-dimensional effects on the electron beam loading and on the unlimited LWFA acceleration in inhomogeneous plasmas. Using the conditions of electron trapping to the wake-field acceleration phase we analyse the multi-equal stage and multiuneven stage LWFA configurations. In the first configuration the energy of fast electrons is a linear function of the number of stages, and in the second case, the accelerated electron energy grows exponentially with the number of stages. The results of the two-dimensional particle-in-cell simulations presented here show the high quality electron acceleration in the triple stage injection–acceleration configuration.

X-ray devices are far superior to optical ones for providing nanometre spatial and attosecond temporal resolutions. Such resolution is indispensable in biology, medicine, physics, material sciences, and their applications. A bright ultrafast coherent X-ray source is highly desirable, for example, for the diffractive imaging of individual large molecules, viruses, or cells. Here we demonstrate experimentally a new compact X-ray source involving high-order harmonics produced by a relativistic-irradiance femtosecond laser in a gas target. In our first implementation using a 9 Terawatt laser, coherent soft X-rays are emitted with a comb-like spectrum reaching the 'water window' range. The generation mechanism is robust being based on phenomena inherent in relativistic laser plasmas: self-focusing, nonlinear wave generation accompanied by electron density singularities, and collective radiation by a compact electric charge. The formation of singularities (electron density spikes) is described by the elegant mathematical catastrophe theory, which explains sudden changes in various complex systems, from physics to social sciences. The new X-ray source has advantageous scalings, as the maximum harmonic order is proportional to the cube of the laser amplitude enhanced by relativistic self-focusing in plasma. This allows straightforward extension of the coherent X-ray generation to the keV and tens of keV spectral regions. The implemented X-ray source is remarkably easily accessible: the requirements for the laser can be met in a university-scale laboratory, the gas jet is a replenishable debris-free target, and the harmonics emanate directly from the gas jet without additional devices. Our results open the way to a compact coherent ultrashort brilliant X-ray source with single shot and high-repetition rate capabilities, suitable for numerous applications and diagnostics in many research fields.

Parametric X-ray generation is one of the ways to obtain a monochromatic X-ray. The X-ray is generated through the interaction between high energy electrons and a crystal. The relationship between an X-ray wavelength and an angle of emission is followed by the Bragg condition. Therefore the monochromatic energy of the X-ray can be varied continuously by rotating the crystal. This tunability of X-ray wavelength is suitable for various applications. Usually a single photon counting method is utilized for measuring of the parametric X-rays. Although this method has an advantage to obtain clear energy spectrum, it takes long time. Here, we have measured 10 keV parametric X-rays with applying a rocking curve method. In this scheme, a large number of parametric X-rays are detected simultaneously. This enables us to find and tune the parametric X-ray quickly. As a result, we could find the sharp peak from this method with the Microtron accelerator (150MeV, 20 - 30 pC) at JAEA and a Si crystal. Since the peak angle is consistent to the Bragg condition for the 10 keV parametric X-ray generation, we think 10 keV photons have been generated through the parametric X-ray mechanism., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

The pointing stability and the divergence of a quasi-monoenergetic electron bunch generated in a self-injected laser-plasma acceleration regime were investigated. Gas-jet targets have been irradiated with focused 40 fs laser pulses at the 4-TW peak power. A pointing stability of 2.4 mrad root-mean-square (RMS) and a beam divergence of 10.6 mrad (RMS) were obtained using argon gas-jet target for 50 sequential shots, while these values were about three times smaller than at the optimum condition using helium. In particular, the peak electron energy was 9 MeV using argon, which is almost three times lower than that using helium. This result implies that the formation of the wake-field is different between argon and helium, and it plays an important role in the generation of a electron bunch. This stabilization scheme is available for another gas material such as nitrogen. At nitrogen gas-jet target, the pointing stability is more improved to 1.4 times smaller (1.7 mrad (RMS)) than that in argon gas-jet target and the peak energy is increased to grater than 40 MeV. These results prove that this method not only stabilize the e-beam but also allows controlling the electron energy., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan