Z. Rusiniak

We experimentally demonstrate the generation of quasi-monoenergetic electron bunch at the end of the electron energy distribution. The experiment was conducted at the Prague Asterix Laser System where iodine laser supplies laser energy up to 600 J at the fundamental wavelength of 1.315 μm with a pulse duration of 350 ps. The thickness of different target materials (Cu, Sn, Ta, Pb) was varied between 10 and 25 μm. The energy spectrum of electrons was measured using an array of electron spectrometers at different angular directions with respect to the laser axis. Three frame femtosecond interferometry driven by a Ti:Sa laser was implemented to scan the changes in electron density during plasma ignition by the iodine laser. The experimental results indicate the generation of well-defined quasi-monoenergetic peaks in the electron energy distribution. The energy range of the measured quasi-monoenergetic peaks lies between 0.6 and 1.5 MeV; however, the energy spread of the distribution varies between 4% and 12%. These features were observed consistently in the electron spectrum and illustrate the quasi-monoenergetic electron beam generation by interaction of a sub-nanosecond laser beam with plasma. In addition, a comparison of electron spectra from spectrometers located in opposite directions relative to the position of the laser focus indicates the splitting of the electron cloud into plasma blocks during the acceleration of hot electrons by Coulomb repulsion. These findings can be applicable in fast electron beam-driven radiation sources, electromagnetic pulse generation, charge particle acceleration, and inertial fusion studies.

The paper presents a comparison of shots with high and low neutron yield at deuterium filling. Shots with high neutron yield are characterized by a higher energy produced hard x rays and neutrons, a larger volume of plasmoid-like compact structure, and lower plasma density in the pinched column. The discrepancy was interpreted by different magnetic energies stored in the pinched column. The model of distribution of magnetic energy in a plasmoid assumes the existence of a poloidal component of magnetic field in the plasma, the formation of compact structures with closed poloidal and toroidal current components, filamentary distribution of the current, and development of MHD instabilities along the filaments. The model meets the characteristics of observed sources of fast deuterons, such as submillimeter dimensions, various directions of path with narrow directivity, and radial and azimuthal configuration of spots. The presented mechanism of acceleration of fast deuterons is based on the model of magnetic reconnection, in which part of the disappearing magnetic field induces a strong electric field sufficient for obtaining hundreds of keV energy.

We present measurements of magnetic fields generated in laser-driven coil targets irradiated by laser pulses of nanosecond duration, 1.053 μm wavelength, 500 J energy, and ∼1015 W/cm2 intensity, at the LULI2000 facility. Using two perpendicular probing axes, proton deflectometry is used to characterize the coil current and static charge at different times. Results reveal various deflection features that can be unambiguously linked to a looping quasi-steady current of well-understood polarity or to a static charging of the coil surface. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped circuit model, supporting the quasi-steady assessment of the discharges. Peak magnetic fields of ∼50 T at the center of 500-μm-diameter coils, obtained at the moderate laser intensity, open up the use of such laser-driven coil targets at facilities worldwide to study numerous phenomena in magnetized high-energy-density plasmas, and its potential applications.

Expanding laser plasmas, produced by high energy laser radiation, possess both high thermal and magnetic field energy densities. Characterization of such plasma is challenging but may provide essential information needed for understanding its physical behavior. Among the standard experimental techniques used for plasma diagnostics, conventional interferometry is one of the most convenient, informative, and accurate. Attempts to extract more information from each laser shot on large facilities have led to development of complex interferometry, which allows us to reconstruct both plasma electron density and magnetic field distributions from a single data object. However, such a benefit requires more accurate processing, critically important in some situations. This work focuses on quasi-axisymmetric interaction geometry. Starting from basic principles, we present a general analysis, consider main error sources, and obtain plasma density and magnetic field distributions with their derived error bars. A regularization procedure, significantly decreasing an error near the plasma symmetry axis, is proposed and analyzed in detail. With use of synthetic datasets, the presented analysis is generally universal for quasi-axisymmetric plasmas.

Multidimensional modeling of phenomena and processes occurring during the expansion of the laser-produced plasma for different irradiation conditions related to both the laser beam parameters and the target constructions is a very complex issue, especially when modeling requires consideration of kinetic processes associated with the development of various types of microscopic instability. Multidimensional PIC codes create such a possibility, but their use is limited to modeling phenomena even in a very narrow timescale due to the limited computational capabilities of current supercomputers. For this reason, the paper attempts to interpret the results of the spontaneous magnetic field (SMF) measurements obtained during the PALS (Prague Asterix Laser System) experiment [Pisarczyk et al., AIP Adv. 10, 115201 (2020); Pisarczyk et al., Phys. Plasmas 22, 102706 (2015)] based on the 2D magneto-hydrodynamic (MHD) model [Jach et al., Computer Modeling of Dynamic Interaction of Bodies by Free Point Method (PWN, Warsaw, 2011)]. The MHD equations were used with included arbitrary (i) current of hot electrons treating it as an additional external current and (ii) ion-sound instability responsible for the increase in anomalous resistance in areas with high temperature and low-density plasma. The spatial distribution of magnetic fields and current density obtained from 2D modeling are in acceptable agreement with the experimental results [Pisarczyk et al., Plasma Phys. Controlled Fusion 62, 115020 (2020); Zaraś-Szydłowska et al., AIP Adv. 10, 115201 (2020); Pisarczyk et al., Phys. Plasmas 22, 102706 (2015)]. The inclusion of temporal changes in anomalous resistance in modeling allowed us to explain the persistence of high SMF amplitude at the level of several megagauss after the laser pulse ended due to the effect of magnetic field freezing.

This paper describes design, development, and implementation of a multi-channel magnetic electron spectrometer for the application in laser–plasma interaction experiments carried out at the Prague Asterix Laser System. Modular design of the spectrometer allows the setup in variable configurations to evaluate the angular distribution of hot electron emission. The angular array configuration of the electron spectrometers consists of 16 channels mounted around the target. The modules incorporate a plastic electron collimator designed to suppress the secondary radiation by absorbing the wide angle scattered electrons and photons inside the collimator. The compact model of the spectrometer measures electron energies in the range from 50 keV to 1.5MeV using ferrite magnets and from 250 keV to 5MeV using stronger neodymium magnets. An extended model of the spectrometer increases the measured energy range up to 21MeV or 35MeV using ferrite or neodymium magnets, respectively. Position to energy calibration was obtained using the particle tracking simulations. The experimental results show the measured angularly resolved electron energy distribution functions from interaction with solid targets. The angular distribution of hot electron temperature, the total flux, and the maximum electron energy show a directional dependence. The measured values of these quantities increase toward the target normal. For a copper target, the average amount of measured electron flux is 1.36 × 10¹¹, which corresponds to the total charge of about 21 nC.

Generation of spontaneous magnetic fields (SMFs) is one of the most interesting phenomena accompanying an intense laser–matter interaction. One method of credible SMFs measurements is based on the magneto-optical Faraday effect, which requires simultaneous measurements of an angle of polarization plane rotation of a probe wave and plasma electron density. In classical polaro-interferometry, these values are provided independently by polarimetric and interferometric images. Complex interferometry is an innovative approach in SMF measurement, obtaining information on SMF directly from a phase–amplitude analysis of an image called a complex interferogram. Although the theoretical basis of complex interferometry has been well known for many years, this approach has not been effectively employed in laser plasma research until recently; this approach has been successfully implemented in SMF measurement at the Prague Asterix Laser System (PALS). In this paper, proprietary construction solutions of polaro-interferometers are presented; they allow us to register high-quality complex interferograms in practical experiments, which undergo quantitative analysis (with an original software) to obtain information on the electron density and SMFs distributions in an examined plasma. The theoretical foundations of polaro-interferometric measurement, in particular, complex-interferometry, are presented. The main part of the paper details the methodology of the amplitude–phase analysis of complex interferograms. This includes software testing and examples of the electron density and SMF distribution of a laser ablative plasma generated by irradiating Cu thick planar targets with an iodine PALS laser at an intensity above about 10¹⁶ W/cm².

The laser-induced Cavity Pressure Acceleration (CPA) scheme [S. Borodziuk et al., Appl. Phys. Lett. 95, 231501 (2009)] allows for effective transformation of the laser energy into the kinetic energy of plasma streams and dense plasma objects. It has been proven that using long-wavelength laser beams, with relatively low energies (up to 500 J for λ₁ = 1.315 µm and FWHM = 350 ps), it is possible to accelerate macroparticles to very high velocities (above 10⁷ cm/s). The study of neutron yield showed the benefit of CPA in delivering ion temperatures and density sufficient to reach the thermonuclear region.