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The spatial configuration of the ion source generated under femtosecond laser interaction with clusters is investigated. While intense laser pulses (36 fs, 60 mJ, intensity of 4 × 10^17 W/cm^2) propagated in CO_2 cluster (~0.22 μm in diameter) media, the shape of the obtained plasma ion source was registered for the first time by means of pinhole imaging method. The remarkable decrease in fast ion yield in the vicinity of the assumed best laser focus near the gas cluster jet axis is observed. Such observed anisotropy of the ion source is suggested to originate from the influence of the laser prepulse destroying clusters in advance to the arrival of the main pulse. The assumption is confirmed by optical shadowgraphy images of the plasma channel and is important for further development of an efficient laser-plasma- based fast ion source. Following the observed geometry of the ion source, the laser intensity limit allowing to accelerate ions to ~100 keV energy range was estimated.

Direct metrology of coherent short-wavelength beamlines is important for obtaining operational beam characteristics at the experimental site. However, since beam-time limitation imposes fast metrology procedures, a multi- parametric metrology from as low as a single shot is desirable. Here a two- dimensional (2D) procedure based on high-resolution Fresnel diffraction analysis is discussed and applied, which allowed an efficient and detailed beamline characterization at the SACLA XFEL. So far, the potential of Fresnel diffraction for beamline metrology has not been fully exploited because its high- frequency fringes could be only partly resolved with ordinary pixel-limited detectors. Using the high-spatial-frequency imaging capability of an irradiated LiF crystal, 2D information of the coherence degree, beam divergence and beam quality factor M2 were retrieved from simple diffraction patterns. The developed beam metrology was validated with a laboratory reference laser, and then successfully applied at a beamline facility, in agreement with the source specifications.

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)

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

In the last two decades the regime of ultraintense (>10^19 Wcm-2) laser pulses producing and interacting with plasma has revealed unprecedented advancements in particle accelerations and as a secondary source of high-energy radiation beams. Previous experiments, developed in our group, have discovered coherent x-ray beams with high brightness (approaching that of FELs) and composed of multiple high order harmonics. The following experiments and simulations have shown that the produced x-ray sources are from tens to hundreds of nanometers in size. This novel phenomenon is known as BISER. Here, we report new experimental results concerning spatial distribution of these promising x-ray sources, which will enable a more rigorous understanding of the underpinning physics of its origin and the effect of specific laser parameters on its spatial pattern and brightness. We thank the J-KAREN-P laser operation group. We acknowledge financial support from JSPS JP 25390135, JP 26707031, SP18104, and QST Director Funds Sousei #16 and #20., Optics & Photonics International Congress 2019 (OPIC)

We overview our research on relativistic optics and BISER discovery and the reasons why it was overlooked for more than four decades, including the required driving laser parameters., International Workshop on Radiation Plasma Diagnostics In memory of Professor Anatoly Ya. Faenov

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 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 are developing a compact, ultra-bright, spatially and temporally coherent attosecond x-ray source, highly demanded in fundamental research and applications in life sciences, material sciences, nanotechnology, etc. Such source does not exist. Present-day state-of-the-art sources are lacking one or a few desirable properties, e.g., XFELs are temporally incoherent and not compact, atomic harmonics are not bright enough. \nWe have discovered BISER, which is a new promising regime of bright attosecond coherent x-ray generation by high-power (multi-TW) femtosecond lasers focused onto gas targets. In this invited presentation, we describe the experimental discovery, model and theory development and their experimental validation, and finally state the laser parameters which are required for efficient BISER x-ray generation. We also describe in detail the correct definition of important laser parameters, such as the effective pulse width and focal spot radius, the Strehl ratio, the peak power and intensity., 39th Annual Meeting of the Laser Society of Japan

High energy density physics 32, 63 - 69 (2019). doi:10.1016/j.hedp.2019.05.002, We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 10$^{19}$ W/cm$^2$, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index $(hk0)$, persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states., Published by Elsevier, Amsterdam [u.a.]

High intensity X-ray Free Electron Lasers (XFEL) are an ideal tool to heat materials directly and isochorically, which can cause them to be modified and damaged irreversibly. During XFEL-matter interactions, the energy of an XFEL beam will be mainly absorbed by photoionization, creating numerous high-energy photo- and Auger electrons. Modelling this process is quite complex since both atomic physics and plasma physics are involved. Atomic collisional-radiative (CR) codes such as FLYCHK are widely used to simulate such processes. However, the CR codes typically assume local thermodynamic equilibrium (LTE) and are limited to zero dimension. In order to understand the sample damaging mechanisms, we performed two-dimensional kinetic particle-in-cell simulations with radiation transport (PIC-RT) to retrieve the temporal processes of XFEL-matter interactions. The dynamics of XFEL-Matter interactions can roughly divide into three different time scales: 1) electron heating and photoionization by XFEL in ~ 10 fs ; 2) collisional heating and ionization by high-energy photo- and Auger electrons with several keV energy in tens of fs to sub-ps; and 3) collective hydrodynamic explosion driven by ~ TPa thermal pressure from ~100 fs to nanoseconds. The simulation results are compared to our recent experiment that a variety of samples were irradiated by Japanese XFEL SACLA with intensities on the order of 10^20 W/cm^2. The post-analysis of the irradiated samples showed that large holes with radius sizes more than one order of magnitude higher than the XFEL spot were created for metallic samples. The hole size is also much larger than the stopping range of high energy electrons. According to our PIC-RT simulations, we attribute the generation of such large holes to the micro-explosion process. Kinect simulation of the hole generation with multiple time scales is also useful and complementary to understand the change of X-Ray diffraction pattern in the experiment that infers significant material structural change on femtosecond timescales.

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

In the last two decades the regime of ultraintense (>1019 Wcm-2) laser pulses producing and interacting with plasma has revealed unprecedented advancements in particle acceleration and as a secondary source of high-energy radiation beams. Previous experiments, developed in our group, have discovered coherent x-ray beams with high brightness (approaching that of FELs1) and composed of multiple high order harmonics2,3. The following experiments and simulations have shown that the produced x-ray sources are from tens to hundreds of nanometers in size. This novel phenomenon is known as BISER4. Here, we report new experimental results concerning spatial distribution of these promising x-ray sources, which will enable a more rigorous understanding of the underpinning physics of its origin and the effect of specific laser parameters on its spatial pattern and brightness. Additionally, a potential alternative technique to measure their spatial distribution with a few hundred nanometers resolution is presented, which will enable both, the estimation of the source size and its brightness. [1] C. Pellegrini and J. Stohr, Nucl. Instrum. Meth. A, 500, 33-40, 2003. [2] A. S. Pirozhkov et al., Physical Review Letters, 108, 135004, 2012. [3] A. S. Pirozhkov et al., New Journal of Physics, 16, 093003, 2014. [4] A. S. Pirozhkov, T. Esirkepov et al., Scientific Reports, 7, 17968, 2017., 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

We have discovered a new regime of bright coherent x-ray generation by relativistic plasma singularities driven by a multi-terawatt femtosecond laser focused onto a gas jet target [1]-[3]. In this regime, the driving laser pulse propagates through the plasma, pushes electrons producing an electron-free cavity, and generates a bow wave [4]. The singularities are produced at the joining of the cavity wall and the bow wave and represent relativistically moving point-like coherent x-ray emitters. We have generalized this coherent radiation emission mechanism and discovered Burst Intensification by Singularity Emitting Radiation (BISER) [5]. \nIn previous experiments, BISER resulted from relativistic self-focusing, which is a laser beam instability. This spontaneous regime exhibited relatively large shot-to-shot fluctuations of the x-ray source position and low overall stability. However, stability is desirable for prospective applications of the new coherent x-ray source. \nBasing on simulations with the Particle-In-Cell code REMP [6], we proposed to control the BISER with a tailored plasma density profile. We demonstrated this control for the first time in experiments with the J-KAREN-P laser [7]-[9] using a sharp density jump created by a blade in the supersonic gas flow. This resulted in controllable, low-jitter x-ray source position and higher x-ray yield. \nOur results, on the one hand, represent an important step towards implementation of a next-generation bright compact coherent x-ray source, and on the other hand, demonstrate the new technique of relativistic self-focusing control and its diagnostics. \nWe thank the J-KAREN-P laser operation group. We acknowledge the financial support from JSPS Kakenhi JP 25390135, JP 26707031, and the QST Director Fund. \n[1]A. S. Pirozhkov, et al. "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes," Phys. Rev. Lett. 108, 135004 (2012). [2]A. S. Pirozhkov, et al., "High order harmonics from relativistic electron spikes," New J. Phys. 16, 093003 (2014). [3]T. Pikuz et al "High performance imaging of relativistic soft X-ray harmonics by sub-micron resolution LiF film detectors" Phys. Status Solidi C 9 2331 (2012) [4]T. Zh. Esirkepov, et al., "Bow Wave from Ultraintense Electromagnetic Pulses in Plasmas," Phys. Rev. Lett. 101, 265001 (2008). [5]A. Pirozhkov, T. Esirkepov et al., "Burst intensification by singularity emitting radiation in multi-stream flows," Sci. Rep. 7, 17968 (2017). [6]T. Esirkepov, "Exact charge conservation scheme for PIC simulation with an arbitrary form-factor," CPC 135, 144 (2001). [7]H. Kiriyama, et al., "High-Contrast, High-Intensity Petawatt-Class Laser and Applications," IEEE JSTQE 21, 1601118 (2015). [8]A. S. Pirozhkov, et al. "Approaching the diffraction-limited, bandwidth-limited Petawatt," Opt. Express 25, 20486 (2017). [9]H. Kiriyama, et al., "High-contrast high-intensity repetitive petawatt laser," Opt. Lett. 43, 2595 (2018)., ICXRL 2018

The overview of the recent results for discovery and investigations of a very exotic phenomenon – optical mirage in the X-ray spectral range – is presented. It was found that the mirage could be created in the form of coherent virtual point source, emerging in the vicinity of the second plasma in two-stage oscillator-amplifier X-ray laser. The X-ray source-mirage, rigidly phased with the initial radiation of generator, occurs only when amplification takes place in the amplifier plasma and leads to the appearance of the interference pattern in the form of concentric rings in the spatial profile of the output X-ray laser beam. The equation describing the emergence of X-ray mirage was found, numerical solution of which shows that its formation is similar to that of the optical mirages observed at propagation of light rays through an inhomogeneously heated air. Obtained results have already demonstrated novel comprehension into the physical nature of amplification of X-ray radiation, opening additional opportunities for X-ray interferometry, holography, and other applications, which require multiple rigidly phased sources of coherent radiation.