Coherent x-ray radiation induced by high-current breakdown on a ferrite surface

We observe that at the initial stage of a high-current discharge, a low-divergence short x-ray pulse ($\approx\thinspace$$0.5^{\circ}$, $\thinspace$$500$ eV) with the energy of $\sim$$21\mu$J is formed over a ferrite surface, which propagates parallel to the surface in the anode direction. The high directionality of the radiation points to its coherent nature. We propose that the radiation is due to the short-lived magnetization of the ferrite surface excited by a high-power electromagnetic pulse. The radiation is coherent due to the equivalent excitation conditions for all emitters. The excitation pulse and the radiation it generates move at the same speed ($\sim$$c$). Thereby, the emitted waves propagating parallel to the ferrite surface are phase-matched, providing the high radiant intensity of the radiation

X ray diagnostics of hot dense plasma created by the action of 10 PW ultra-intense laser pulses on micron-scale cluster targets

Combined with X-ray spectral plasma diagnostics, studies on the interaction of intense laser pulses with cluster targets provide essential information for the basic properties of matter under extreme conditions. The X-ray radiation spectra of high-temperature plasmas are formed primarily due to atomic processes occurring in it, like electron–ion collisions and the radiative or autoionization decay of ion levels. Therefore, X-ray diagnostics are employed primarily for evaluating the electron density and the temperature of the transient nonequilibrium plasmas, as well as the nonlinear processes occurring in the relativistic laser plasmas. One practical outcome is the capability to estimate the effect of laser prepulses: Without the accurate knowledge of the temporal structure of a laser pulse, theoretical calculations based on atomic and kinetic models can give a rough estimation of the cluster evolution under the influence of the laser prepulses and the main pulse by comparing the profiles of the resonance spectral lines and the hollow ion spectra. Here we propose to introduce the focusing spectrometer with spatial resolution (FSSR) equipped with spherically bent crystals and the micron-scale cluster target, both of which have been successfully applied simultaneously in several HEDP experiments with high-intensity lasers at QST-KPSI and CEA Saclay. The introduction of these two devices promises to give absolutely unique research environment if combined with the ELI-NP 10 PW laser facility: We will start with Argon clusters to know how well the laser system performs at laser intensity of around 10^18 W/cm^2. With the increase of intensity for 1-2 orders of magnitude (around 10^20 W/cm^2), there will be a lot of H-like Argons, never observed before. With the higher laser contrasts, the spectral lines of hollow ions could be clearly identified. An observation of hollow ion spectra suggests the existence of a dense core at the moment of arrival of the main pulse, i.e. the laser contrast is extremely high enough. For the unexplored laser intensities over 10^22 W/cm^2, we try to look on Krypton K-shell spectra from Krypton clusters to examine the higher temperature and density plasmas, and try to find a way to define laser intensity from X ray spectroscopy.1st ELI-NP User Worksho

The Role of Stepwise Photoionization in Measurements of the Ionization Potentials in Dense Plasma

The interaction of high-contrast high-intensity laser radiation with solids allows us to create hot or warm plasma of solid or even over-solid density, such as in the case of inertial fusion particularly. The multicharged ions contained in it can no longer be considered isolated. As a result, this leads to a decrease in the ionization potentials and to the disappearance of a number of bound ionic states. To describe the ionization potential depression, two major approaches are now used predominantly, where the key parameter is either average interelectronic or interionic distance. Since neither of the approaches can be substantiated purely theoretically, their applicability can only be established by comparison with experimental results. In recent experiments with X-ray free-electron lasers, it was concluded that the ionization potential depression rather depends on the interelectronic distance. However, when measuring ionization potentials, it was assumed that the main role in ionization processes is played by the direct photoionization of the ion ground state. In the present paper, we show that stepwise photoionization processes should play a significant role in dense plasma, disrupting a straight correspondence between the threshold in direct photoionization by X-ray laser photons and the actual ionization potential of multicharged ions. It means that the measurement results mentioned above are not correct, and the main conclusion about the importance of the interelectronic distance for depression of the ionization potential is not correct

Analysis of Lyα Dielectronic Satellites to Characterize Temporal Profile of Intense Femtosecond Laser Pulses

In the paper, an X-ray spectroscopy-based approach on laser pulse temporal profile characterization is described. The structure of dielectronic satellites to H-like Lyα lines strongly depends on a plasma electron density, so it can be applied for diagnostics. These spectral lines are mainly emitted during initial stage of laser plasma expansion. It means that plasma parameters obtained via them characterizes matter conditions in a region surrounding a spot of laser-matter interaction. In the case when a laser contrast is high enough, the radiation interacts with cold matter, which had not been preliminary perturbed by a laser prepulse, and the satellites structure shape corresponding to high densities should be observed. It allows us to consider the satellites as a diagnostic tool for the laser temporal profile quality. In the paper dependencies of the dielectronic satellites structure on electron densities obtained from detailed kinetic calculations in the wide range of plasma parameter for different elements are under discussion. Fundamental theoretical aspects of plasma diagnostic based on the feature of satellite structures shape in hot dense plasma, which led to development of the proposed method, are also explained

Analysis of Lyα dielectronic satellites to characterize temporal profile of intense femtosecond laser pulses

In the paper, an X-ray spectroscopy-based approach on laser pulse temporal profile characterization is described. The structure of dielectronic satellites to H-like Lyα lines strongly depends on a plasma electron density, so it can be applied for diagnostics. These spectral lines are mainly emitted during initial stage of laser plasma expansion. It means that plasma parameters obtained via them characterizes matter conditions in a region surrounding a spot of laser-matter interaction. In the case when a laser contrast is high enough, the radiation interacts with cold matter, which had not been preliminary perturbed by a laser prepulse, and the satellites structure shape corresponding to high densities should be observed. It allows us to consider the satellites as a diagnostic tool for the laser temporal profile quality. In the paper dependencies of the dielectronic satellites structure on electron densities obtained from detailed kinetic calculations in the wide range of plasma parameter for different elements are under discussion. Fundamental theoretical aspects of plasma diagnostic based on the feature of satellite structures shape in hot dense plasma, which led to development of the proposed method, are also explained

Analysis of Ly_α Dielectronic Satellites to Characterize Temporal Profile of Intense Femtosecond Laser Pulses

In the paper, an X-ray spectroscopy-based approach on laser pulse temporal profile characterization is described. The structure of dielectronic satellites to H-like Lyα lines strongly depends on a plasma electron density, so it can be applied for diagnostics. These spectral lines are mainly emitted during initial stage of laser plasma expansion. It means that plasma parameters obtained via them characterizes matter conditions in a region surrounding a spot of laser-matter interaction. In the case when a laser contrast is high enough, the radiation interacts with cold matter, which had not been preliminary perturbed by a laser prepulse, and the satellites structure shape corresponding to high densities should be observed. It allows us to consider the satellites as a diagnostic tool for the laser temporal profile quality. In the paper dependencies of the dielectronic satellites structure on electron densities obtained from detailed kinetic calculations in the wide range of plasma parameter for different elements are under discussion. Fundamental theoretical aspects of plasma diagnostic based on the feature of satellite structures shape in hot dense plasma, which led to development of the proposed method, are also explained

Soft X-ray diffraction patterns measured by a LiF detector with sub-micrometre resolution and an ultimate dynamic range

The unique diagnostic possibilities of X-ray diffraction, small X-ray scattering and phase-contrast imaging techniques applied with high-intensity coherent X-ray synchrotron and X-ray free-electron laser radiation can only be fully realized if a sufficient dynamic range and/or spatial resolution of the detector is available. In this work, it is demonstrated that the use of lithium fluoride (LiF) as a photoluminescence (PL) imaging detector allows measuring of an X-ray diffraction image with a dynamic range of ∼107 within the sub-micrometre spatial resolution. At the PETRA III facility, the diffraction pattern created behind a circular aperture with a diameter of 5 µm irradiated by a beam with a photon energy of 500 eV was recorded on a LiF crystal. In the diffraction pattern, the accumulated dose was varied from 1.7 × 10$^5$ J cm$^{−3}$ in the central maximum to 2 × 10$^{−2}$ J cm$^{−3}$ in the 16th maximum of diffraction fringes. The period of the last fringe was measured with 0.8 µm width. The PL response of the LiF crystal being used as a detector on the irradiation dose of 500 eV photons was evaluated. For the particular model of laser-scanning confocal microscope Carl Zeiss LSM700, used for the readout of the PL signal, the calibration dependencies on the intensity of photopumping (excitation) radiation (λ = 488 nm) and the gain have been obtained

Structural Evolution and Formation of High-Pressure Plasmas in X Pinches

International audienceTwo- and three-dimensional MHD simulations are used to provide a theoretical description of 2 wire molybdenum X-pinch experiments. The initial evolution from solid wires to the formation of supersonic jets and a central micro-Z pinch is found to result from the slow rate of wire ablation and from the distribution of the Lorentz force. The growth of m=0 instabilities triggers the formation of micron sized regions of intense x-ray emission with plasma pressures in the Gbar range. A simple analytical model is used to predict how the maximum density and temperature scale with material and current

Direct imaging of shock wave splitting in diamond at Mbar pressure

International audienceUnderstanding the behavior of matter at extreme pressures of the order of a megabar (Mbar) is essential to gain insight into various physical phenomena at macroscales—the formation of planets, young stars, and the cores of super-Earths, and at microscales—damage to ceramic materials and high-pressure plastic transformation and phase transitions in solids. Under dynamic compression of solids up to Mbar pressures, even a solid with high strength exhibits plastic properties, causing the induced shock wave to split in two: an elastic precursor and a plastic shock wave. This phenomenon is described by theoretical models based on indirect measurements of material response. The advent of x-ray free-electron lasers (XFELs) has made it possible to use their ultrashort pulses for direct observations of the propagation of shock waves in solid materials by the method of phase-contrast radiography. However, there is still a lack of comprehensive data for verification of theoretical models of different solids. Here, we present the results of an experiment in which the evolution of the coupled elastic–plastic wave structure in diamond was directly observed and studied with submicrometer spatial resolution, using the unique capabilities of the x-ray free-electron laser (XFEL). The direct measurements allowed, for the first time, the fitting and validation of the 2D failure model for diamond in the range of several Mbar. Our experimental approach opens new possibilities for the direct verification and construction of equations of state of matter in the ultra-high-stress range, which are relevant to solving a variety of problems in high-energy-density physics