Sushil Kumar Singh, Ph.D.

The generation of intense radio-frequency and microwave electromagnetic pulses (EMPs) by the interaction of a high-power laser with a target is an interesting phenomenon, the exact mechanisms of which remain inadequately explained. In this paper we present a detailed characterization of the EMP emission at a sub-nanosecond kilojoule laser facility, the Prague Asterix Laser System. The EMPs were detected using a comprehensive set of broadband diagnostics including B-dot and D-dot probes, various antennas, target current and voltage probes and oscilloscopes with 100 and 128 GS/s sampling. Measurements show that the EMP spectrum was strongly dependent on the laser energy: the maximum frequency of the spectrum and the frequency of the spectrum centroid increased with increasing laser beam energy in the signals from all detectors used. The highest observed frequencies exceeded 9 GHz. The amplitude and energy of the detected EMP signals were scaled as a function of laser energy, power and number of emitted electrons.

A large laser spark was produced in a homogeneous sulphur hexafluoride gas (pressures ranged from 10.7 to 101.3 kPa) by a focused high-power laser pulse (350 ps, 125 J, 1315.2 nm). Magnetic fields, electromagnetic pulses (EMP), optical emission spectra (OES) and chemical changes associated with the laser-induced dielectric breakdown (LIDB) in the SF6 gas were investigated. During the laser interaction, hot electrons escaping the plasma kernel produced EMP and spontaneous magnetic field, the frequency spectrum of which contains three bands around 1.15, 2.1 and 3 GHz, while the EMP frequency band appeared around 1.1 GHz. The EMP emission from a laser spark was very weak in a comparison to those generated at a solid target. Gas chromatography revealed the formation of only a limited number of products and a low degree of sulphur hexafluoride (SF6) conversion. OES diagnosed the LIDB plasma in phase of its formation as well as during its recombination.

Cavity Pressure Acceleration (CPA) is a technique for accelerating dense plasma streams by utilizing laser-generated plasma pressure within a spatially confined region. This approach has been proposed as an alternative to the classical ablative acceleration of plasma. Initially, the primary goal of this approach was to create a dense plasma stream (a theoretical macroparticle delivering energy/momentum) suitable for experiments related to Impact Fast Ignition. In recent experimental sessions, we used targets equipped with cavities lined with deuterated polyethylene ([Formula: see text]) foils and powder. These targets were irradiated with a [Formula: see text] PALS sub-kilojoule, low-contrast laser beam ([Formula: see text]), focused to an intensity of [Formula: see text] within a [Formula: see text] pulse. The scheme has proven to be highly efficient in converting laser energy into high-energy interaction products, such as high-density plasma streams and protons. We observed neutron yields among the highest achieved to date in Deuterium-Deuterium laser-induced experiments, even when compared to facilities with lasers operating at significantly higher energies and intensities. 1D hydrodynamic code used to simulate plasma parameters in the targets confirmed the high potential of the method, regardless of the driving laser wavelength.

Recent experiments at the National Ignition Facility and theoretical modeling suggest that side stimulated Raman scattering (SSRS) instability could reduce laser-plasma coupling and generate considerable fluxes of suprathermal hot electrons under interaction conditions envisaged for direct-drive schemes for inertial confinement fusion. Nonetheless, SSRS remains to date one of the least understood parametric instabilities. Here, we report the first angularly and spectrally resolved measurements of scattered light at laser intensities relevant for the shock ignition scheme (I similar to 10(16) W/cm(2)), showing significant SSRS growth in the direction perpendicular to the laser polarization. Modification of the focal spot shape and orientation, obtained by using two different random phase plates, and of the density gradient of the plasma, by utilizing exploding foil targets of different thicknesses, clearly reveals a different dependence of backward SRS (BSRS) and SSRS on experimental parameters. While convective BSRS scales with plasma density scale length, as expected by linear theory, the growth of SSRS depends on the spot extension in the direction perpendicular to laser polarization. Our analysis therefore demonstrates that under current experimental conditions, with density scale lengths L-n approximate to 60-120 mu m and spot sizes FWHM approximate to 40-100 mu m, SSRS is limited by laser beam size rather than by the density scale length of the plasma.

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 mu m with a pulse duration of 350 ps. The thickness of different target materials (Cu, Sn, Ta, Pb) was varied between 10 and 25 mu m. 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. (c) 025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

This article provides an up-to-date overview of the problems associated with the detection of hot electrons escaping from laser-produced plasma and corresponding return current flowing from the ground to the target, which neutralises the positive charge occurring on the target due to the escaped electrons. In addition, the target holder system acts as an antenna emitting an electromagnetic pulse (EMP), which is powered by the return target. If the amount of positive charge generated on the target is equal to the amount of charge carried away from the plasma by the escaping electrons, the measurement of the return current makes it possible to determine this charge, and thus also the number of escaped electrons. Methods of return current detection in the mA–10 kA range is presented, and the corresponding charge is compared to the charge determined using calibrated magnetic electron energy analysers. The influence of grounded and insulated targets on the number of escaped electrons and EMP intensity is discussed. In addition to EMP detection, mapping of the electrical potential near the target is mentioned.

In laser-plasma interaction experiments, a significant amount of incoming laser energy is converted into hot electrons and energetic bremsstrahlung radiation. An accurate measurement of bremsstrahlung spectrum is crucial for understanding the formation and dynamics of hot dense plasmas produced by intense laser radiation. An optimized filter (attenuator) stack spectrometer has been developed to measure bremsstrahlung emission of plasmas generated at the Prague Asterix Laser System (PALS) facility. Aluminium, copper and lead slabs with thicknesses ranging from 0.5 mm to 2 mm were stacked alternating imaging plates (IPs) placed in a box made of Aluminium. This compact device makes it possible to determine a spectrum of highly penetrating bremsstrahlung radiation using a technique of multiple attenuators (filters). After an exposure to hard x-ray emission of hot dense plasmas, imaging plates are removed from the spectrometer to be read out in IPs’ reader. Integrated IPs’ signals are subjected to numerical treatment using FLUktuierende KAskade (FLUKA) simulation and unfolding algorithm. FLUKA simulation will be implemented to determine the response matrix of filter stack spectrometer however signal will be reconstructed using unfolding algorithm such as Gravel or a Bayesian algorithm. This diagnostic represents a useful tool for determining an energy distribution of hard x-ray photons which can improve our understanding of laser-plasma interaction phenomena.

The interaction of focused high power laser beam with solid targets leads to acceleration of charged particles among other by non-linear effects in the plasma. In this experiment, the hot electrons are characterized from the interaction of sub-nanosecond and kilo-joule class laser pulse with thin metal foil targets (Cu, Ta, Ti, Sn, Pb). The energy distribution functions of electrons were measured by angularly resolved multichannel electron spectrometer. The hot electron temperatures were observed in range from 30 to 80 keV for laser intensities between 10^15 and 3x10^16 Wcm^-2. The measured energy distribution and electron temperature were compared with published results and known scaling laws at higher laser intensities. For foil targets of different materials, the temperature and flux of hot electrons were scaled with target thickness in the range of 1-100 um from low Z to high Z materials where Z is the atomic number. The profile of conversion efficiency from laser energy to hot electrons is discussed in the energy range from 100 to 600 J. For the given laser and target parameters, the nonlinear behaviour of conversion efficiency and relevant physics are also described in detail.

We report on an experiment performed at the FLASH2 free-electron laser (FEL) aimed at producing warm dense matter via soft x-ray isochoric heating. In the experiment, we focus on study of the ions emitted during the soft x-ray ablation process using time-of-flight electron multipliers and a shifted Maxwell-Boltzmann velocity distribution model. We find that most emitted ions are thermal, but that some impurities chemisorbed on the target surface, such as protons, are accelerated by the electrostatic field created in the plasma by escaped electrons. The morphology of the complex crater structure indicates the presence of several ion groups with varying temperatures. We find that the ion sound velocity is controlled by the ion temperature and show how the ion yield depends on the FEL radiation attenuation length in different materials.

We present a topical review of the detailed experimental investigations of the Electron Temperature Gradient (ETG) instability-induced turbulence and associated transport that have been carried out in the Large Volume Plasma Device (LVPD) at the Institute for Plasma Research. These results pertaining to a high beta plasma are supported by theoretical modeling and their significance with relation to earlier ETG investigations are discussed. The removal of non-thermal electrons and control of electron temperature gradient, ∇Te are achieved in a finite beta plasma (β ∼ 0.6) by making use of an Electron Energy Filter (EEF). It divides the plasma into source, EEF and target regions. In the core region ( x ≤ 50 cm) of target plasma, the observed electromagnetic instability exhibits fluctuations in the lower hybrid range of frequencies ( f = 1 − 80kHz ), with a broad band spectra having its peak power at wave number, k⟂ = (0.1 − 0.2) cm− 1 and frequency, f ~ 10 kHz satisfying condition k⟂Re ≤ 1 and k∥∕k⟂ < 1, where Re is the electron Larmor radius and k⟂ and k∥ are the perpendicular and parallel wave numbers, respectively. It was demonstrated successfully that when ∇Te is made significant such that𝜂e ≈ Ln∕LTe > 2∕3 , where Ln is density scale length and LTe is electron temperature scale length, the ETG scale turbulence gets excited in the presence of pre-excited Whistler mode. A linear theory of coupled Whistler-Electron Temperature Gradient (W-ETG) mode is developed using a two-fluid model applicable for the LVPD plasma. A comparison of the experimental and numerical results is found to be in good agreement. Upon identification of the ETG turbulence, the electrostatic and electromagnetic particle fluxes and energy flux induced by it in the target region of LVPD were investigated. We observed an inward directed radial particle flux, but total heat flux remained radially outward. The particle and thermal fluxes are compared with the numerically obtained values and are found to be in good agreement in the core plasma of LVPD.

We report on ion emission from plasma produced on thick targets irradiated with nanosecond and femtosecond pulses delivered by mid-ultraviolet and soft x-ray lasers, respectively. To distinguish between different ion acceleration mechanisms, the maximum kinetic energy of ions produced under different interaction conditions is plotted versus laser fluence. The transformation of the time-of-flight detector signal into ion charge density distance-of-flight spectra makes it possible to determine the mean kinetic energy of the fastest ion groups based on the influence of the acoustic velocity of ion expansion. This allows obtaining additional characteristics of the ion production. The final energy of the group of fast ions determined using the ion sound velocity model is an order of magnitude larger in the fs-XFEL interaction than in the ns-UV one. On the contrary, the ablation yield of ions in our experiment is seven orders of magnitude greater when applying ns-UV laser pulses, not only due to higher energies of UV laser pulses, but also due to a significant difference in interaction and ion formation mechanisms.