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We have demonstrated a full noncontact laser technology to measure the velocity of ultrasonic waves and their spectra propagated through concrete samples exposed to specified high-temperature conditions with specified durations as models of concrete structures in a severe accident at the Fukushima Daiichi Nuclear Power Station. The velocities and spectra of the ultrasonic waves were strongly dependent on the exposed temperature, i.e. at a high-temperature condition of 400��C, the velocity was 3700 m/s, and at room temperature, the velocity was 5000 m/s. Experimental results are almost comparable to those obtained by the contact ultrasonic technique.

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.

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 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

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

*Abstract:* We discuss the new regime of high-order harmonic generation by relativistic-irradiance multi-terawatt femtosecond lasers focused onto gas jet targets [PRL 108, 135004 (2012); NJP 16, 093003 (2014)]. The laser induces multi-stream relativistic plasma flow resulting in the formation of density singularities: structurally stable, oscillating electron spikes coherently emitting high-frequency radiation. In this presentation we analyze the dependence of the harmonic yield on the laser power and focal spot quality, and derive the required laser parameters for efficient harmonics generation. We show the status of the J-KAREN-P laser and report on the progress towards satisfying these requirements. \nIn the new regime of high-order harmonic generation, intense (>10^18 W/cm^2) high-power (multi-TW) femtosecond (30-50 fs) laser pulses focused onto gas targets induce multi-stream relativistic flows in underdense plasma (ne ~ 10^19 cm^-3). This results in the formation of density singularities, which are structurally stable, oscillating electron spikes coherently emitting high-frequency radiation with spectra comprising high-order harmonics [1, 2]. The experiments with the J-KAREN laser [3] demonstrated strong dependence of the harmonics yield on the laser pulse energy and focal spot quality. \nIn this presentation we analyze the obtained dependences and derive laser parameters required for efficient harmonics generation. Apart from higher power, it turns out that the focal spot quality should approach the diffraction limit, i.e. the Strehl ratio should exceed 0.5. The focal spots of high-power lasers typically suffer from wavefront distortions and angular dispersion. For noise-like wavefront distortions, the above stated requirement corresponds to an rms wavefront error, The 15th International Conference on X-Ray Lasers (ICXRL2016)

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