Burst Intensification by Singularity Emitting Radiation (BISER) in underdense relativistic laser plasma is a bright source of coherent extreme ultraviolet (XUV) and x-ray radiation. In contrast to all harmonic generation mechanisms, high-resolution experimental BISER spectra in the XUV region contain spectral fringes with separation much finer (down to 0.12 eV) than the initial driving laser frequency (~1.5 eV). We show that these fringe separations result from two main factors: laser frequency downshift (redshift) due to the quasi-adiabatic energy loss to the plasma waves, and spectral interference of different harmonic orders from different emission moments, i.e. alloharmonics [Pirozhkova et al., arXiv:2306.01018]

High-order harmonics are ubiquitous in nature and present in electromagnetic, acoustic, and gravitational waves. They are generated by periodic nonlinear processes or periodic high-frequency pulses. However, this periodicity is often inexact, such as that in chirped (frequency-swept) optical waveforms or interactions with nonstationary matter -- for instance, reflection from accelerating mirrors. Spectra observed in such cases contain complicated sets of harmonic-like fringes. We encountered such fringes in our experiment on coherent extreme ultraviolet generation via BISER, and could not interpret them using currently available knowledge. Here, we present a comprehensive theory based on interference of harmonics with different orders fully explaining the formation of these fringes, which we call alloharmonics. Like atomic spectra, the complex alloharmonic spectra depend on several integer numbers and bear a unique imprint of the emission process, which the theory can decipher, avoiding confusion or misinterpretation. We also demonstrate the alloharmonics in simulations of gravitational waves emitted by binary black hole mergers. Further, we predict the presence of alloharmonics in the radio spectra of pulsars and in optical frequency combs, and propose their use for measurement of extremely small accelerations necessary for testing gravity theories. The alloharmonics phenomenon generalizes classical harmonics and is critical in research fields such as laser mode locking, frequency comb generation, attosecond pulse generation, pulsar studies, and future gravitational wave spectroscopy.

Study of warm dense matter remains a very important task for understanding of many unique phenomena observing as in astrophysical research as in inertial fusion and fast ignition. In this work, we studied the parameters of plasma created by 1.7 ps laser pulses of relativistic intensity of 7 × 10 18 W/cm 2 in a specially designed Al–Cu wire-shape target, in comparison with a flat Cu and Al foil targets. We observed the strong emission of neutral or virtually neutral Cu K α line from both Cu foil and Cu wire part of targets, which indicates the creation of a dense state exposed to the intense flow of hot electrons. Parameters of the plasma were evaluated by comparison of experimental spectra with the results of modeling by collisional-radiative kinetic code PrismSpec under the plasma zone approach. The using of Al foil in front of Cu wire part of target allowed avoiding the direct heating of Cu-wire and acquiring spectra of Cu K-shell emission evidently belonging to emission of warm dense matter (WDM) state. The upper estimate for the electron temperature in WDM region was found to be below 80 eV.

Density effects have great importance for studying the state of matter at high energy densities both for astrophysical and laboratory objects. At the moment, the question remains as to which model should be used for the description of the effect of ionization potential depression (IPD) in plasma with densities up to solid-state ones. Recent X-ray free-electron laser based researches claimed the adequacy of an early IPD model of Ecker and Kroll over the well-accepted model of Stewart and Pyatt, provided that these two models give significantly different predictions. Subsequent researches performed with optical lasers have received the opposite result. This work is intended to resolve the existing contradiction in conclusions of mentioned experiments. For the first time, the IPD effect for Si XIII ions of plasma generated by an ultrarelativistic optical laser pulse was described and quantified. We demonstrate the "disappearance" of electronic states up to the state with a principal quantum number of n = 4 (inclusively); the plasma density was varied from the critical to the near-solid one. A radiation-collision code PrismSPECT was used to distinguish different IPD models.

Electron heating and ion acceleration in ultrarelativistic laser-solid interactions N. P. Dover1, M. Nishiuchi1,2, H. Sakaki1, A. Kon1, Ko. Kondo1, H. F. Lowe1, M. A. Alkhimova3, E. J. Ditter4, O. C. Ettlinger4, A. Ya. Faenov5,3, M. Hata6, G. S. Hicks4, N. Iwata6, H. Kiriyama1, J. K. Koga1, T. Miyahara1,7, T. Miyatake1,7, Z. Najmudin4, T. A. Pikuz5,3, A. S. Pirozhkov1, T. Pueschel8, A. Sagisaka1, U. Schramm8, Y. Sentoku6, K. Shiokawa1,7, Y. Watanabe7, T. Ziegler8, K. Zeil8, M. Kando1, K. Kondo1 1) Kansai Photon Science Institute, QST, 2) PRESTO, JST, 3) Joint Institute for High Temperatures, Russian Academy of Sciences, 4) John Adams Institute for Accelerator Science, Imperial College London, 5) Open and Transdisciplinary Research Initiative, Osaka University, 6) Institute of Laser Engineering, Osaka University, 7) Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 8) Helmholtz-Zentrum Dresden-Rossendorf e-mail (speaker): nickdover@gmail.com

Acceleration of particles from the interaction of ultraintense laser pulses up to 5×10^{21}  W cm^{-2} with thin foils is investigated experimentally. The electron beam parameters varied with decreasing spot size, not just laser intensity, resulting in reduced temperatures and divergence. In particular, the temperature saturated due to insufficient acceleration length in the tightly focused spot. These dependencies affected the sheath-accelerated protons, which showed poorer spot-size scaling than widely used scaling laws. It is therefore shown that maximizing laser intensity by using very small foci has reducing returns for some applications.

High power laser systems are an attractive driver for compact energetic ion sources. We demonstrate repetitive acceleration at 0.1 Hz of proton beams up to 40 MeV from a reeled tape target irradiated by ultra-high intensities up to 5 x 10(21) Wcm(-2) and laser energies approximate to 15 J using the J-KAREN-P laser system. We investigate the stability of the source and its behaviour with laser spot focal size. We compare the scaling of proton energy with laser energy to a recently developed analytical model, and also demonstrate that it is possible to reach energies up to 50 MeV on a single shot with a lower laser energy approximate to 10 J by using a thinner target, motivating development of high repetition targetry suitable for thinner targets.

X-ray absorption spectroscopy is a well-accepted diagnostic for experimental studies of warm dense matter. It requires a short-lived X-ray source of sufficiently high emissivity and without characteristic lines in the spectral range of interest. In the present work, we discuss how to choose an optimum material and thickness to get a bright source in the wavelength range 2 Å–6 Å (∼2 keV to 6 keV) by considering relatively low-Z elements. We demonstrate that the highest emissivity of solid aluminum and silicon foil targets irradiated with a 1-ps high-contrast sub-kJ laser pulse is achieved when the target thickness is close to 10 µm. An outer plastic layer can increase the emissivity even further.

This paper describes recent progress of the study of short wavelength coherent radiation and laser particle acceleration by the use of ultra-intense lasers in our institute. In laser-driven soft x-ray laser research, the 18.8 nm laser has been demonstrated with the repetition rate of 10 Hz, and new driver laser with higher repetition rate is under development. Generation of higher order harmonics from relativistic plasmas is conducted by using J-KAREN-P, and the source size together with the spatial coherence are investigated. We also show the progress in laser-driven heavy ion acceleration study, which is done as a part of the development of compact cancer therapy machine, and the proposal of alternative method toward high average power coherent attosecond x-ray generation.

The formation of high energy density matter occurs in inertial confinement fusion, astrophysical, and geophysical systems. In this context, it is important to couple as much energy as possible into a target while maintaining high density. A recent experimental campaign, using buried layer (or "sandwich" type) targets and the ultrahigh laser contrast Vulcan petawatt laser facility, resulted in 500 Mbar pressures in solid density plasmas (which corresponds to about 4.6×10^{7}J/cm^{3} energy density). The densities and temperatures of the generated plasma were measured based on the analysis of x-ray spectral line profiles and relative intensities.

Motivated by the development of next-generation heavy-ion sources, we have investigated the ionization and acceleration dynamics of an ultraintense laser-driven high-Z silver target, experimentally, numerically, and analytically. Using a novel ion measurement technique allowing us to uniquely identify silver ions, we experimentally demonstrate generation of highly charged silver ions (Z* = 45(-2)(+2)) with energies of >20 MeV/nucleon (>2.2 GeV) from submicron silver targets driven by a laser with intensity 5 x 10(21) W/cm(2), with increasing ion energy and charge state for decreasing target thickness. We show that although target pre-expansion by the unavoidable rising edge of state-of-the-art high-power lasers can limit proton energies, it is advantageous for heavy-ion acceleration. Two-dimensional particle-in-cell simulations show that the Joule heating in the target bulk results in a high temperature (similar to 10 keV) solid density plasma, leading to the generation of high flux highly charged ions (Z* = 40(-2)(+2), greater than or similar to 10 MeV/nucleon) via electron collisional ionization, which are extracted and accelerated with a small divergence by an extreme sheath field at the target rear. However, with reduced target thickness this favorable acceleration is degraded due to the target deformation via laser hole boring, which accompanies higher energy ions with higher charge states but in an uncontrollable manner. Our elucidation of the fundamental processes of high-intensity laser-driven ionization and ion acceleration provides a path for improving the control and parameters of laser-driven heavy-ion sources, a key component for next-generation heavy-ion accelerators.

The use of targets with surface structures for laser-driven particle acceleration has potential to significantly boost the particle and radiation energies because of enhanced laser absorption. We investigate, via experiment and particle-in-cell simulations, the impact of micron-scale surface-structured targets on the spectrum of electrons and protons accelerated by a picosecond laser pulse at relativistic intensity. Our results show that, compared with flat-surfaced targets, structures on this scale give rise to a significant enhancement in particle and radiation emission over a wide range of laser–target interaction parameters. This is due to the longer plasma scale length when using micro-structures on the target front surface. We do not observe an increase in the proton cutoff energy with our microstructured targets, and this is due to the large volume of the relief.

In the present report, we discussed our recent results concerning the investigations of high energy density matter. By using of high-resolution X-ray spectroscopic measurements and kinetic simulations, we demonstrate that the energy of femtosecond laser pulses is efficiently converted to X-ray radiation and produces exotic states in solid density plasma regions. It is shown that exotic hollow multicharged ions states can be obtained and studied not only by ultra-intense X-ray radiation of XFEL lasers, but also upon using optical laser technology. We have concluded that observable features of the hollow-ion spectra are sensitive to such plasma parameters as density, temperature, hot-electron fraction, and intensity of the X-ray pumping radiation and could be used for effective diagnostics of warm dense matter parameters.

Study of radiation properties of solid dense plasma irradiated by ultraintense lasers has a great interest both from fundamental physics and different application point of views. Recently upgraded petawatt J-KAREN-P laser together with precise focusing technique delivers 35 fs laser pulses of 10 22 W/cm 2 intensity into a micron-size focal spot on target. For such unprecedented intensities the application of high-resolution Xray spectroscopy allows to investigate the ionization mechanisms and to measure the parameters of relativistic plasma from front and rear sides of moderate (Al) and high Z (Ti, Fe,) thin foil targets. Kinetic modeling of the spectra is used to estimate electron plasma density and temperature, demonstrating T e ∼2 keV for Ne ∼5e22 cm −3 in the hottest emission region. Thus, it is experimentally demonstrated for the first time that the laser pulse of over 1e21 W/cm 2 intensity is absorbed neither in the solid density plasma nor in a pre-plasma of a common critical density, but in the matter of so called relativistic critical density. It is revealed how even small displacement of the target out of the optimal laser focus, as well the decrease in temporal contrast of the laser pulse, strongly reduce both the intensity of X-ray radiation and degree of plasma ionization. 2D PIC code simulations of femtosecond laser interaction with various materials are provided and compared with experimental results.

We modified a soft X-ray laser (SXRL) interferometer synchronized with a Ti:sapphire laser to observe a single-shot image of the nanoscale structure dynamics of materials induced by an optical laser pulse. The lateral resolution on the sample surface was improved to 0.7 μm using precise imaging optics. Using this system, we succeeded in observing thin film structures above the solid (or liquid) surface in the femtosecond laser ablation processFemtosecond laser ablation process of metals (Au). The thin film worked as soft X-ray beam splitter. This result shows a thin film was smooth and dense (with a roughness of a few nanometers and near sold density). Furthermore, it gave rise to the possibility of generating novel transient soft X-ray opticsTransient soft X-ray optics .