X-ray harmonic comb from relativistic electron spikes

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

A Case Study of a Nursing Home in Nagano, Japan: Field Survey on Thermal Comfort and Building Energy Simulation for Future Climate Change

With an increase in the aging population in many countries worldwide, much attention is being paid to the study of thermal comfort for the elderly. Because the elderly spend most of their time indoors, the demand for air conditioning is expected to increase, and it is important to study the thermal comfort of the elderly and appropriate operation plans for air conditioning. In this study, we conducted a field survey of thermal comfort and building energy simulation for an air-conditioned nursing home in Nagano, Japan. The field survey was conducted between June 2020 and June 2021. Over 80% of the subjects were satisfied with the indoor thermal environment. The thermal neutral temperature of the elderly was 25.9 °C in summer and 23.8 °C in winter. Future weather data was used to predict the future heating and cooling loads of the nursing home. The results showed that the total heat load may not change significantly, as the decrease in heating load compensates for the increase in cooling load. This study will serve as a useful reference for a wide range of stakeholders, including managers and designers of nursing homes

High-Order Harmonic Generation by Relativistic Plasma Singularities: The Driving Laser Requirements

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

Required Laser Properties for Efficient High-Order Harmonic Generation from Relativistic Electron Spikes

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-201