Tight focusing with very small f-numbers is necessary to achieve the highest at-focus irradiances. However, tight focusing imposes strong demands on precise target positioning in-focus to achieve the highest on-target irradiance. We describe several near-infrared, visible, ultraviolet and soft and hard X-ray diagnostics employed in a similar to 10(22) W/cm(2) laser-plasma experiment. We used nearly 10 J total energy femtosecond laser pulses focused into an approximately 1.3-mu m focal spot on 5-20 mu m thick stainless-steel targets. We discuss the applicability of these diagnostics to determine the best in-focus target position with approximately 5 mu m accuracy (i.e., around half of the short Rayleigh length) and show that several diagnostics (in particular, 3 omega reflection and on-axis hard X-rays) can ensure this accuracy. We demonstrated target positioning within several micrometers from the focus, ensuring over 80% of the ideal peak laser intensity on-target. Our approach is relatively fast (it requires 10-20 laser shots) and does not rely on the coincidence of low-power and high-power focal planes.

Gamma rays consist of high-energy photons that selectively interact with nuclei, induce and mediate nuclear reactions and elementary particle interactions, and exceed x-rays in penetrating power and thus are indispensable for analysis and modification of dense or compressed object interior. Yet, the available gamma sources lack power and brightness which, if available, would revolutionize science and technology. The predicted laser-driven Gamma Flash (GF) would be the highest-power and the brightest terrestrial gamma source with a 30-40 scattering in a laser-solid interaction at irradiance typically above 10^23W/cm^2. GF is one of the motivating goals for the most advanced laser facilities. However, till now GF remains overshadowed by simultaneously generated low-brightness Bremsstrahlung. Here we experimentally differentiate these two mechanisms and demonstrate a GF dominant regime producing several times the number of Bremsstrahlung photons. We found steep GF yield growth with the laser power and irradiance. Simulations revealed a Terawatt GF with nanometre source and an attosecond pulse train with a record brightness of 10^22photons/mm^2mrad^2s0.1%BW at up to tens of MeV. The small source size and high brightness paves the way towards spatially coherent gamma rays. At high photon energies, our regime is comparable in brightness to astrophysical Gamma Ray Bursts. We anticipate that the gamma ray source based on our findings will facilitate a breakthrough in research on future inertial fusion energy by enabling high-spatial-resolution time-resolved radiography of fuel mixing instabilities in extremely compressed targets. Such a new compact bright ultrafast gamma source could facilitate significant advances in time-resolved nuclear physics, homeland security, and nuclear waste management and non-proliferation.

Ultrafast optical probing is a widely used method of underdense plasma diagnostic. In relativistic plasma, the motion blur limits spatial resolution in the direction of motion. For many high-power lasers the initial pulse duration of 30-50 fs results in a 10-15 μm motion blur, which can be reduced by probe pulse post-compression. Here we used the Compression after Compressor Approach (CafCA) [Phys.-Usp. 62, 1096 (2019); JINST 17 P07035 (2022)], where spectral broadening is performed in thin optical plates and is followed by reflections from negative-dispersion mirrors. Our initially low-intensity probe beam was down-collimated for a more efficient spectral broadening and higher probe-to-self-emission intensity ratio. The setup is compact, fits in vacuum chamber, and can be implemented within a short experimental time slot. We proved that the compressed pulse kept high quality necessary for plasma probing.

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.

Both K-shell x-ray emission spectroscopy and fluorescence spectroscopy are well-accepted diagnostics for experimental studies of warm dense matter and hot dense matter (HDM). Until now, however, this diagnosis has been used for the study of dense matter with temperatures lower than 100 eV or with temperatures above 1 keV. In this work, we have demonstrated the possibility of using K-shell emission spectroscopy for an intermediate temperature range of 100s eV to study dense plasma. Here, we discuss an analysis of the HDM emission spectra of a solid-state copper with temperatures up to a few hundreds of electronvolts heated by laser-accelerated charged particles.

An x-ray radiography technique based upon phase contrast imaging using a lithium fluoride detector has been demonstrated for goals of high energy density physics experiments. Based on the simulation of propagation an x-ray free-electron laser beam through a test-object, the visibility of phase-contrast image depending on an object-detector distance was investigated. Additionally, the metrological capabilities of a lithium fluoride crystal as a detector were demonstrated.

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.

Imaging plates are widely used in laser-plasma interaction experiments for detection of electrons, x-ray, gamma-ray, ions, and neutrons, therefore, the calibration of imaging plates to different types of ionizing and non-ionizing radiation is vital. In this paper, a universal response function of FUJI BAS-TR imaging plate is presented using experiment and Monte-Carlo simulation with respect to high-energy photons and ions. The sensitivity of imaging plate sensitivity to x-ray radiation was examined using radioactive Co-60 isotope. Furthermore, the fading and decay mechanism affected by phosphor stimulated process were also investigated by employing the high energy gamma-rays produced in laser-plasma experiment. Finally, an ion spectrometer was designed to examine the IP response to different ion energies during a laser-driven plasma experiment. The universal relation given, can be used to evaluate the response of the imaging plate against any other types of radiation.

The interaction of high-power short lasers with solid density targets is an important application of modern solid state lasers. However, uncertainties in measurements due to lack of information on the laser pedestal-to-peak contrast limits the validity of many conclusions. We show that X-ray spectral measurements can provide a straightforward way for accessing the laser pedestal-to-peak contrast. The experiments use silicon targets and relativistic laser intensities of 3 x 10(20) W/cm(2) with a pulse duration of 1 ps. By not using or using a plasma mirror we compare low and high contrast measurements of the Ly-alpha line and its satellites to show that these lines are an effective laser contrast diagnostic. This diagnostic has potential to reduce uncertainty in future laser-solid interaction studies.

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.

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.

The diffraction method is used to study the properties of laser radiation interacting with a gas-cluster plasma. Based on the simulation of propagation a laser beam with a wavelength 800 nm through a hexagon Cu-mesh and subsequently compared with the obtained experimental diffraction pattern, the contribution of the second harmonic to the laser radiation was considered. The capability of this method to reveal the spectral composition of laser radiation is discussed. Also, the dependence of the visibility of the diffraction pattern on the intensity of the laser beam is considered. Besides, the contribution of x-ray radiation generated from the laser-cluster interaction into the observed diffraction pattern signal was estimated.

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.

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

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.