Search

We report the development of a multipurpose differential x-ray calorimeter with a broad energy bandwidth. The absorber architecture is combined with a Bayesian unfolding algorithm to unfold high energy x-ray spectra generated in high-intensity laser–matter interactions. Particularly, we show how to extract absolute energy spectra and how our unfolding algorithm can reconstruct features not included in the initial guess. The performance of the calorimeter is evaluated via Monte Carlo generated data. The method accuracy to reconstruct electron temperatures from bremsstrahlung is shown to be 5% for electron temperatures from 1 to 50 MeV. We study bremsstrahlung generated in solid target interaction showing an electron temperature of 0.56 ± 0.04 MeV for a 700 μm Ti titanium target and 0.53 ± 0.03 MeV for a 50 μm target. We investigate bremsstrahlung from a target irradiated by laser-wakefield accelerated electrons showing an endpoint energy of 551 ± 5 MeV, inverse Compton generated x rays with a peak energy of 1.1 MeV, and calibrated radioactive sources. The total energy range covered by all these sources ranges from 10 keV to 551 MeV. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of hot refluxing electrons in the interaction of an ultra-short relativistic laser pulse with a thin foil target via particle-in-cell (PIC) simulations, which is governed by the multidimensional spatiotemporal evolution of a self-generated sheath field. The comparison of time-integrated energy spectra of refluxing and escaping electrons indicates the refluxing efficiency is higher than 95% in average for each bounce. The characteristics of wide transverse spatial distribution and energy-resolved angular distribution caused by the refluxing electrons show a direct correlation with the angular-dependent photon yield of bremsstrahlung emission, as verified by the hybrid simulations of coupling the PIC results with Monte Carlo particle transport code. We further clarify the energy dissipation mechanisms of refluxing electrons through the recirculation in the thin target under the electron-refluxing dominated regime and conclude that the self-generated sheath field plays a dominant role over the competing processes, such as the radiation loss, collisional stopping, and anomalous inhibition via the resistive field. The lifetime of recirculation is calculated to be a few hundred femtoseconds, that is, one order of magnitude shorter than the timescale of collisional dissipation, while it is one order of magnitude longer than the laser pulse duration. The results could provide useful insights to understand the hot electron transport and stopping, secondary radiation generation, and ion acceleration in the high energy density plasmas. [ABSTRACT FROM AUTHOR]

Small angle x-ray scattering (SAXS) is a well established technique to detect nanometer scale structures in matter. In a typical setup, this diagnostic uses a detector with a direct line of sight to the scattering target. However, in the harsh environment of high intensity laser interaction, intense secondary radiation and high-energy particles are generated. Such a setup would therefore suffer a significant increase of noise due to this background, which could eventually prevent such measurements. In this paper, we present a novel tool consisting of a mosaic graphite crystal that works as a mirror for the SAXS signal and allows us to position the detector behind appropriate shielding. This paper studies the performance of this mirror both by experiment at the European XFEL (X-Ray Free-Electron Laser Facility) laboratory and by simulations. [ABSTRACT FROM AUTHOR]

Various nominally undoped, hydrothermally or melt grown (MG) ZnO single crystals have been investigated by standard positron lifetime measurements. Furthermore, optical transmission measurements and structural characterizations have been performed; the content of hydrogen in the bound state was determined by nuclear reaction analysis. A positron lifetime of 165-167 ps, measured for a brownish MG ZnO sample containing (0.30 ± 0.03) at.-% of bound hydrogen, matches perfectly the value found for colorless MG ZnO crystals. The edge shift, observed in the 'blue light domain' of the optical absorption for the former sample with respect to the latter samples, is estimated to be 0.70 eV, and found equal to a value reported previously. The possible role of zinc interstitials is considered and discussed. Microstructure analysis by X-ray diffraction and transmission electron microscopy revealed the presence of stacking faults in MG crystals in a high concentration, which suggests these defects to be responsible for the observed positron lifetime. [ABSTRACT FROM AUTHOR]

Laser-accelerated beams of Mega-electron volt protons have been produced at the Los Alamos Trident laser facility and used for high-resolution point-projection proton radiography of Au grids. The effective proton source size affords an inherent resolution of 2-3 µm in the object plane. The proton beam is characterized by ion time-of-flight Faraday cup measurements and nuclear particle track detectors. Laser-driven proton radiography appears promising as a valuable research tool for probing plasmas or modest density objects. [ABSTRACT FROM AUTHOR]

Intense and energetic electron currents can be generated by ultra-intense lasers interacting with solid density targets. Specifically, for ultra-short laser pulses, their temporal evolution needs to be taken into account for many non-linear processes as instantaneous values may differ significantly from the average. Hence, a dynamic model including the temporal variation of the electron currents—which goes beyond a simple bunching with twice the laser frequency but otherwise constant current—is needed. Here, we present a time-dependent solution to describe the laser generated currents and obtain simple expressions for the electron spectrum, temporal evolution, and resulting correction of average values. To exemplify the semi-empiric model and its predictive capabilities, we show the impact of temporal evolution, spectral distribution, and spatial modulations on Ohmic heating of the bulk target material. [ABSTRACT FROM AUTHOR]

Diamond formation in polystyrene (C8H8)n, which is laser-compressed and heated to conditions around 150 GPa and 5000 K, has recently been demonstrated in the laboratory [Kraus et al., Nat. Astron. 1, 606–611 (2017)]. Here, we show an extended analysis and comparison to first-principles simulations of the acquired data and their implications for planetary physics and inertial confinement fusion. Moreover, we discuss the advanced diagnostic capabilities of adding high-quality small angle X-ray scattering and spectrally resolved X-ray scattering to the platform, which shows great prospects of precisely studying the kinetics of chemical reactions in dense plasma environments at pressures exceeding 100 GPa. [ABSTRACT FROM AUTHOR]

When an intense laser accelerated electron beam, with large current density on the order of 1012 A/cm², enters a solid density plasma, it is well-known to be subject to a number of different types of instabilities that cause it to filament. In this work, we investigate the transport instability of a fast electron beam that is imprinted on the self-generated magnetic filaments inside the solid density plasmas using particle-in-cell simulations. By varying collisional ionization models, our simulations show that the atomic ionization process is crucial to determine the structure of the magnetic filaments. We further attribute the generation of bulk magnetic filaments to Weibel-like instability mechanism caused by counter-propagating hot forward-bulk return current streams and counter-propagating hot forward-reflux current streams. It is found that the magnetic fields in the filament channels near the rear surface are around one order of magnitude higher than those near the front surface of the thin solid target. This asymmetry is likely induced by the very different properties of bulk electron stream and hot reflux electron stream in terms of density and velocity distribution. Finally, we propose to probe the magnetic fields inside the solid density plasmas by X-Ray polarimetry via Faraday rotation using X-Ray free electron lasers (XFELs). The synthetic simulations show that XFELs are capable to detect the magnetic fields from relativistic laser-solid interactions. [ABSTRACT FROM AUTHOR]

The dynamics of bulk heating and ionization is investigated both in simulations and theory, which determines the crucial plasma parameters such as plasma temperature and density in ultra-short relativistic laser-solid target interactions. During laser-plasma interactions, the solid density plasma absorbs a fraction of laser energy and converts it into kinetic energy of electrons. A portion of the electrons with relativistic kinetic energy goes through the solid density plasma and transfers energy into the bulk electrons, which results in bulk electron heating. The bulk electron heating is finally translated into the processes of bulk collisional ionization inside the solid target. A simple model based on the Ohmic heating mechanism indicates that the local and temporal profile of bulk return current is essential to determine the temporal evolution of bulk electron temperature. A series of particle-in-cell simulations showing the local heating model is robust in the cases of target with a preplasma and without a preplasma. Predicting the bulk electron heating is then benefit for understanding the collisional ionization dynamics inside the solid targets. The connection of the heating and ionization inside the solid target is further studied using Thomas-Fermi model. [ABSTRACT FROM AUTHOR]

Here, we propose to exploit the low energy bandwidth, small wavelength, and penetration power of ultrashort pulses from XFELs for resonant Small Angle Scattering (SAXS) on plasma structures in laser excited plasmas. Small angle scattering allows to detect nanoscale density fluctuations in forward scattering direction. Typically, the SAXS signal from laser excited plasmas is expected to be dominated by the free electron distribution. We propose that the ionic scattering signal becomes visible when the X-ray energy is in resonance with an electron transition between two bound states (resonant coherent X-ray diffraction). In this case, the scattering cross-section dramatically increases so that the signal of X-ray scattering from ions silhouettes against the free electron scattering background which allows to measure the opacity and derived quantities with high spatial and temporal resolution, being fundamentally limited only by the X-ray wavelength and timing. Deriving quantities such as ion spatial distribution, charge state distribution, and plasma temperature with such high spatial and temporal resolution will make a vast number of processes in shortpulse laser-solid interaction accessible for direct experimental observation, e.g., hole-boring and shock propagation, filamentation and instability dynamics, electron transport, heating, and ultrafast ionization dynamics. [ABSTRACT FROM AUTHOR]

Laser proton acceleration can be enhanced by using target ablation, due to the energetic electrons generated in the ablation preplasma. When the ablation pulse matches main pulse, the enhancement gets optimized because the electrons' energy density is highest. A scaling law between the ablation pulse and main pulse is confirmed by the simulation, showing that for given CPA pulse and target, proton energy improvement can be achieved several times by adjusting the target ablation. [ABSTRACT FROM AUTHOR]

The laser-driven acceleration of protons from thin foils irradiated by hollow high-intensity laser beams in the regime of target normal sheath acceleration (TNSA) is reported for the first time. The use of hollow beams aims at reducing the initial emission solid angle of the TNSA source, due to a flattening of the electron sheath at the target rear side. The experiments were conducted at the PHELIX laser facility at the GSI Helmholtzzentrum für Schwerionenforschung GmbH with laser intensities in the range from 1018 Wcm-2 to 1020W cm-2 We observed an average reduction of the half opening angle by (3.07 ±.42)° or (13.2±2.0)% when the targets have a thickness between 12 μ and 14 μm. In addition, the highest proton energies were achieved with the hollow laser beam in comparison to the typical Gaussian focal spot. [ABSTRACT FROM AUTHOR]

We demonstrate the potential of X-ray free-electron lasers (XFEL) to advance the understanding of complex plasma dynamics by allowing for the first time nanometer and femtosecond resolution at the same time in plasma diagnostics. Plasma phenomena on such short timescales are of high relevance for many fields of physics, in particular in the ultra-intense ultra-short laser interaction with matter. Highly relevant yet only partially understood phenomena become directly accessible in experiment. These include relativistic laser absorption at solid targets, creation of energetic electrons and electron transport in warm dense matter, including the seeding and development of surface and beam instabilities, ambipolar expansion, shock formation, and dynamics at the surfaces or at buried layers. In this paper, we focus on XFEL plasma probing for high power laser matter interactions based on quantitative calculations using synthesized data and evaluate the feasibility of various imaging and scattering techniques with special focus on the small angle X-ray scattering technique. [ABSTRACT FROM AUTHOR]

A new type of Faraday cup, capable of detecting high energy charged particles produced in a high intensity laser-matter interaction environment, has recently been developed and demonstrated as a real-time detector based on the time-of-flight technique. An array of these Faraday cups was designed and constructed to cover different observation angles with respect to the target normal direction. Thus, it allows reconstruction of the spatial distribution of ion current density in the subcritical plasma region and the ability to visualise its time evolution through time-of-flight measurements, which cannot be achieved with standard laser optical interferometry. This is a unique method for two-dimensional visualisation of ion currents from laser-generated plasmas. A technical description of the new type of Faraday cup is introduced along with an ad hoc data analysis procedure. Experimental results obtained during campaigns at the Petawatt High-Energy Laser for Heavy Ion Experiments (GSI, Darmstadt) and at the Prague Asterix Laser System (AS CR) are presented. Advantages and limitations of the used diagnostic system are discussed. [ABSTRACT FROM AUTHOR]

We present experimental results showing a laser-accelerated proton beam maximum energy cutoff of 67.5 MeV, with more than 5 × 106 protons per MeV at that energy, using flat-top hollow microcone targets. This result was obtained with a modest laser energy of ∼80 J, on the high-contrast Trident laser at Los Alamos National Laboratory. From 2D particle-in-cell simulations, we attribute the source of these enhanced proton energies to direct laser-light-pressure acceleration of electrons along the inner cone wall surface, where the laser light wave accelerates electrons just outside the surface critical density, in a potential well created by a shift of the electrostatic field maximum with respect to that of the magnetic field maximum. Simulations show that for an increasing acceleration length, the continuous loading of electrons into the accelerating phase of the laser field yields an increase in high-energy electrons. [ABSTRACT FROM AUTHOR]

This paper reports on simulations of solid mass-limited targets (MLT) via electrodynamic two-dimensional, three velocity component particle-in-cell simulations. The interaction with long (300 fs) high intensity (1020 W/cm2) laser pulses with targets of diameter down to 1 μm is described in detail with respect to electron dynamics and proton and ion acceleration. Depending on the foil diameter, different effects consecutively arise. Electrons laterally recirculate within the target, smoothening the target rear accelerating sheath and increasing the hot electron density and temperature. Our results suggest that the most significant ion energy enhancement should be expected for MLT with diameter below the laser focal spot size. The spread of energetic protons is decreased for medium sized foils while it is greatly increased for foils of size near the focal spot size. [ABSTRACT FROM AUTHOR]

We report on new charge calibrations and linearity tests with high-dynamic range for eight different scintillating screens typically used for the detection of relativistic electrons from laser-plasma based acceleration schemes. The absolute charge calibration was done with picosecond electron bunches at the ELBE linear accelerator in Dresden. The lower detection limit in our setup for the most sensitive scintillating screen (KODAK Biomax MS) was 10 fC/mm2. The screens showed a linear photon-to-charge dependency over several orders of magnitude. An onset of saturation effects starting around 10–100 pC/mm2 was found for some of the screens. Additionally, a constant light source was employed as a luminosity reference to simplify the transfer of a one-time absolute calibration to different experimental setups. [ABSTRACT FROM AUTHOR]

Energetic proton acceleration from interaction of intense short circularly polarized laser pulse with a sandwich target is investigated using two-dimensional particle-in-cell simulation. The sandwich target consists of a hydrogen-plasma layer surrounded by carbon-plasma layers. It is found that the transverse electric fields generated at the plasma layer interfaces efficiently confine the longitudinally accelerated protons to within the hydrogen-plasma layer such that they are collimated and have smaller energy spread compared with a pure proton layer target. The proton energy spectrum can be controlled by adjusting the target parameters, in particular the width of the hydrogen-plasma layer and the density of the carbon-plasma layer. [ABSTRACT FROM AUTHOR]

X-ray production from the interaction of femtosecond laser pulses focused to relativistic intensity into re-entrant targets etched into silicon has been investigated. Kα and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though Kα is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

We study ultrafast heating of thin plastic foils by intense laser irradiation theoretically using collisional two-dimensional particle-in-cell simulations. We find that the laser-generated hot electrons are confined laterally by self-generated resistive magnetic fields, heating the laser focal area beyond keV electron temperatures isochorically in a few picoseconds. Using this confinement one can excite shock waves that compress the plasma beyond solid density and achieve keV thermal plasmas before the plasma disassembles. Such shocks can be launched at material interfaces inside the target where jumps in the average ionization state and thus electron density lead to gigabar pressure. They propagate stably over picoseconds accompanied by multi-megagauss magnetic fields, and thus have a potential for various applications in high energy density physics. [ABSTRACT FROM AUTHOR]

The effect of current prepulse on the initiation of Al wire arrays, ablation dynamics and x-ray production was investigated on the 1-MA ZEBRA accelerator (University of Nevada, Reno). It is shown that increasing the number of wires lowers the temperature of the wire cores at the time of breakdown. Al arrays with cold wire cores demonstrate long and inhomogeneous ablation, and a less intense, wider x-ray pulse. Shortening the current prepulse by a flashover switch causes an increased wire-core temperature, symmetrization and synchronization of the wires’ ablation, and improvement of the amplitude and shape of the x-ray pulse. Application of a vacuum flashover switch can be important for shortening the current prepulse on the upcoming 28-MA ZR-accelerator at Sandia National Laboratories to optimize the x-ray production and shot-to-shot reproducibility from wire-array Z pinches. [ABSTRACT FROM AUTHOR]

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850 fs), and for high laser intensities [(1-6)×1019 W/cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018 W cm-2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100 fs) or longer (1–10 ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous analytical model of rear-surface acceleration [J. Fuchs et al., Nat. Phys. 2, 48 (2006)] to adapt it to the very short pulse durations. Finally, it appears, for the explored parameters, that rear-surface acceleration is the dominant mechanism. [ABSTRACT FROM AUTHOR]

Experiments on the 1-MA ZEBRA accelerator with reduced current prepulse duration, using a flashover switch, demonstrate a significant increase of initial energy deposition into the tungsten wire array before breakdown, and of total radiation energy from the Z pinch. Shorter current prepulse raises the current rate through each individual wire in the array and results in an increase of the energy deposition into wire cores before breakdown. In our experiments, the inferred tungsten wire temperature increases from ∼800 K (with 250 ns prepulse) up to ∼3700 K (with 60 ns prepulse). Total radiation energy increases from 12 to 16 kJ. Our experimental results relate wire-array initiation to heating of the individual array wires up to the time of breakdown. [ABSTRACT FROM AUTHOR]

Ablation and implosion dynamics were investigated by optical probing in linear wire arrays of different geometry. Formation of ablation jets begins on the outermost wires. In the beginning of implosion plasma bubbles arise in breaks on the outer wires. Implosion bubbles move to the next wire in the array and hit the plasma column with the speed >250 km/s. Imploding plasma moves to the center of the array cascading from wire to wire. Configuration of magnetic fields in the linear array can be changed by variation of wire spacing. The regimes of ablation and implosion in the wire arrays are found to differ with different wire spacing. [ABSTRACT FROM AUTHOR]

Relativistic electrons are efficiently generated when multiterawatt lasers focused to ultrahigh intensities >=1019 W/cm2 illuminate the surface of dense plasma targets. A theoretical study finds that during typical picosecond pulse widths, significant amounts of Dreicer produced runaway electrons can build up due to the high axial electric field driving the neutralizing return current. An important consequence is that there will be a conversion of plasma current to runaway electron current, which is maximized at some optimum value of the beam-to-plasma density ratio Nb=nb/ne, depending on the plasma collisionality. At collisionalities representative of solid target experiments, complete conversion to runaway electrons can only take place over a certain range of Nb values. At higher collisionalities and pulse widths, applicable to the fast ignition concept for inertial confinement fusion, it was found that conversion to runaways has a peak at ∼90% around Nb∼0.06. Significant lessening of target material heating by Joule current dissipation is also possible, since part of the beam energy loss is transferred through the electric field directly to the formation of energetic runaways. Implications for beam transport inhibition by the electric field are also discussed. [ABSTRACT FROM AUTHOR]

The occurrence of saturation in CR39 solid state nuclear track detectors has been systematically studied as a function of the incident ion (α particles and laser-accelerated protons) fluence and the etching time. When overexposed (i.e., for fluences above ∼108 particles/cm2) and/or overetched, the CR39 detectors enter a saturated regime where direct track counting is not possible anymore. In this regime, optical measurements of saturated CR39 detectors become unreliable as well, since the optical response of the saturated detectors with respect to the ion fluence is highly nonlinear. This nonlinear optical response is likely due to scattering from the surface of irregular clumping patterns which have a diameter ∼20 μm, i.e., ten times larger than the diameter of individual tracks. These patterns, which aggregate many individual tracks, are observed to develop in highly saturated regimes. For fluences typical of high energy short pulse laser experiments, saturation occurs, inducing the appearance of artifact ringlike structures. By careful microscopic analysis, these artifact ring patterns can be distinguished from the genuine rings occurring below saturation and characteristic of low energy laser accelerated proton beams. [ABSTRACT FROM AUTHOR]

Implosion of wire arrays was investigated at the 1 MA Zebra accelerator by multiframe laser probing and gated x-ray self-emission diagnostics. Different regimes of implosion were observed in Al and Cu wire arrays. Implosion of Al loads with masses of 33–37 μg/cm produces a dense pinch 1–1.5 mm in diameter. Strong instabilities are observed in the Z pinch at the time of stagnation. Implosion of “overmassed” loads produces a plasma column 3–4 mm in diameter with a core. The plasma column does not collapse during the x-ray pulse. The core of the plasma column is not subjected to the kink instability and transforms to a chain of dense spots in the later stage. Different regimes of implosion were observed in Al 8×15 μm loads presumably due to variations in the current pulse and load conditions. Observed regimes are compared to three-dimensional hybrid simulation of ideal and nonideal magnetohydrodynamics modes of implosion. [ABSTRACT FROM AUTHOR]

Collimated jets of beryllium, carbon, oxygen, fluorine, and palladium ions with >1 MeV/nucleon energies are observed from the rear surface of thin foils irradiated with laser intensities of up to 5×1019 W/cm2. The normally dominant proton acceleration is suppressed when the target is subjected to Joule heating to remove hydrogen-bearing contaminant. This inhibits screening effects and permits effective energy transfer to and acceleration of heavier ion species. The influence of remnant protons on the spectral shape of the next highest charge-to-mass ratio species is shown. Particle-in-cell simulations confirming the experimental findings are presented. [ABSTRACT FROM AUTHOR]

Electron transport in femtosecond laser-irradiated solid targets is investigated by means of one-dimensional particle-in-cell simulations that include a model of collisional ionization, binary collisions and field ionization, while treating ions as individual particles. In particular, heat and particle fluxes in conductor and insulator targets are compared at the onset of relativistic laser intensities, i.e., at I=1017 W/cm2. Simulations show that fast electrons generate a longitudinal electric field of the order 1011 V/m in the bulk material that acts to inhibit heat flux in insulators, while the electric fields observed in metals are weaker and electrons penetrate deeper into the target. The bulk heat transport is found to be similar in both materials and mainly Spitzer-like, with a noticeable contribution by fast electrons. [ABSTRACT FROM AUTHOR]

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]