Laser-solid Interactions Research Articles

Real time reconstruction of the fast electron spectrum from high intensity laser plasma interaction using gamma counting technique

X-ray and gamma fluxes from the high intensity laser-plasma interaction are extremely short, well beyond temporal resolution of any detectors. If laser pulses come repetitively, the single photon counting technique allows to accumulate the photon spectra, however, its relation to the spectrum of the initial fast electron population in plasma is not straightforward. We present efficient and fast approach based on the Geant4 package that significantly reduces computer time needed to re-construct the high energy tail of electron spectrum from experimental data accounting for the pileup effect. Here, we first tabulate gamma spectrum from monoenergetic electron bunches of different energy for a given experimental setup, and then compose the simulated spectrum. To account for the pileups, we derive analytical formula to reverse the data. We also consider errors coming from the approximation of the initial electron spectrum by the sum of monoenergetic impacts, the finite range of the electron spectrum, etc. and give estimates on how to choose modelling parameters to minimize the approximation errors. Finally, we present an example of the experimental data treatment for the case of laser-solid interaction using 50 fs laser pulse with intensity above 1018 W/cm2.

Significant intensity dependence of anisotropic two-photon absorption in ZnSe and GaP

Using polarized femtosecond laser transmission measurements toward (100), (110), (111) ZnSe, and (100) GaP, we show that in a wide laser irradiation intensity range over two orders of magnitude before the laser damage threshold, a periodic amplitude modulation always appears clearly in the angle-dependent two-photon absorption (TPA) curves for (100), (110) ZnSe, and (100) GaP. It also exhibits a close connection between the orientation-dependent TPA and the ionic bonding symmetrical characteristics of specific crystal plane, thus revealing the origin of the orientation dependence of nonlinear absorption in the strong field laser-solid interaction. Interestingly, the modulation amplitudes of orientation-dependent TPA exhibit significant intensity dependence, with the maxima occurring around 1011 W cm−2. For the orientation-dependent nonlinear absorption in solids, such a nonmonotonic relationship between the irradiation intensity and the modulation amplitude has not been revealed previously in the near-damage-threshold regime by direct transmission measurements. In particular, with the intensity approaching the laser damage threshold, the significant descending trend of modulation amplitude confirms the consistent characteristics of saturation or even attenuation for the orientation dependence of TPA in the two crystals, which might be considered as a product of absorption saturation phenomenon of TPA at high irradiation intensity and would provide new understanding about the orientation-dependent TPA in the near-damage-threshold regime.

Transient Laser-Induced Breakdown of Dielectrics in Ultrarelativistic Laser-Solid Interactions

For high-intensity laser-solid interactions, the absolute density and surface density gradients of the target at the arrival of the ultrarelativistic laser peak are critical parameters. Accurate modeling of the leading edge-driven target preexpansion is desired to strengthen the predictive power of associated computer simulations. The transition from an initial solid state to a plasma state, i.e., the breakdown of the solid, defines the starting point of the subsequent target preexpansion. In this work, we report on the time-resolved observation of transient laser-induced breakdown (LIB) during the leading edge of high-intensity petawatt-class laser pulses with peak intensities of up to 5.7×1021W/cm2 in interaction with dielectric cryogenic hydrogen jet targets. LIB occurs much earlier than what is typically expected following the concept of barrier suppression ionization. The observation is explained by comparing a characterization study of target-specific LIB thresholds with laser contrast measurements. The results demonstrate the relevance of the laser pulse duration dependence of LIB for high-intensity laser-solid interactions. We provide an effective approach to determine the onset of LIB and thereby the starting point of target preexpansion in other laser-target systems.Received 28 July 2022Revised 2 December 2022Accepted 16 December 2022DOI:https://doi.org/10.1103/PhysRevApplied.19.014070Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsLight-matter interactionPlasma production & heating by laser beams, laser-foil, laser-clusterPhysical SystemsDielectricsRelativistic plasmasTechniquesFemtosecond laser irradiationLaser ablationOptical plasma measurementsPlasma PhysicsCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Effects of laser waveform on the generation of fast electrons in laser–solid interactions

In the scheme of fast ignition of inertial confinement fusion, the fuel temperature mainly relies on fast electrons, which act as an energy carrier, transferring the laser energy to the fuel. Both conversion efficiency from the laser to the fast electron and the energy spectrum of the fast electron are essentially important to achieve highly effective heating. In this study, a two-dimensional particle in cell simulation is applied to study the generation of fast electrons from solid-density plasmas with different laser waveforms. The results have shown that the slope of the rising edge has a significant effect on fast electron generation and energy absorption. For the negative skew pulse with a relatively slow rising edge, the J×B mechanism can most effectively accelerate the electrons. The overall absorption efficiency of the laser energy is optimized, and the fast electron yield in the middle- and low-energy range is also improved.

Optimization and control of synchrotron emission in ultraintense laser–solid interactions using machine learning

Abstract The optimum parameters for the generation of synchrotron radiation in ultraintense laser pulse interactions with planar foils are investigated with the application of Bayesian optimization, via Gaussian process regression, to 2D particle-in-cell simulations. Individual properties of the synchrotron emission, such as the yield, are maximized, and simultaneous mitigation of bremsstrahlung emission is achieved with multi-variate objective functions. The angle-of-incidence of the laser pulse onto the target is shown to strongly influence the synchrotron yield and angular profile, with oblique incidence producing the optimal results. This is further explored in 3D simulations, in which additional control of the spatial profile of synchrotron emission is demonstrated by varying the polarization of the laser light. The results demonstrate the utility of applying a machine learning-based optimization approach and provide new insights into the physics of radiation generation in laser–foil interactions, which will inform the design of experiments in the quantum electrodynamics (QED)-plasma regime.

Collimated terahertz radiation through a laser irradiating on a T-type target

A scheme, applying a linearly polarized laser irradiating on a T-type target which contains a longitudinal target followed by a transverse target, is proposed to collimate the terahertz radiation. The results show that the interaction between the laser and the longitudinal target can effectively reduce the electron transverse divergence while increasing the electron maximum cut-off energy. In this way, the terahertz radiation can be well collimated while enhancing its intensity. By using two-dimensional particle-in-cell simulations, we show that the pointing angle is about −13.6° and 17.5° when the length and thickness of the longitudianl target are 90.0 μm and 4.0 μm, respectively. In addition, the impact of the laser and target parameters on the collimation of terahertz radiation are also investigated separately. Such a scheme may pave a new way for enhancing and collimating the terahertz radiation via laser-solid interactions.

A non-collinear autocorrelator for single-shot characterization of ultrabroadband terahertz pulses.

Conventional terahertz (THz) waveform or spectral diagnostics mainly employ the electro-optic-based techniques or the multi-shot Michelson interferometer. Simultaneously, single-shot, ultrabroadband THz spectral measurements remain challenging. In this paper, a novel probe-free scheme based on the non-collinear autocorrelation technique is proposed to characterize the ultrabroadband THz spectrum at a single-shot mode. The non-collinear autocorrelator is a modified beam-division interferometer, in which the two beams are recombined non-collinearly onto a camera. The temporal or spectral resolution and range depend on the noncollinear configuration and camera parameters. This simple approach has been applied experimentally to characterize the ultrashort THz pulse generated from ultraintense laser-solid interactions, demonstrating the capability of single-shot ultrabroadband measurements without an auxiliary ultrafast laser probe. The proposed non-collinear autocorrelator here would be much useful for characterization and applications of low-repetition-rate intense THz sources and could also be extended to other frequency bands.

On the control of electron heating for optimal laser radiation pressure ion acceleration

We study the onset of electron heating in intense laser–solid interactions and its impact on the spectral quality of radiation pressure accelerated ions in both hole boring and light sail regimes. Two- and three-dimensional particle-in-cell (PIC) simulations are performed over a wide range of laser and target parameters and reveal how the pulse duration, profile, polarization and target surface stability control the electron heating, the dominant ion acceleration mechanisms and the ion spectra. We find that the onset of strong electron heating is associated with the growth of the Rayleigh–Taylor-like instability at the front surface and must be controlled to produce high-quality ion beams, even when circularly polarized lasers are employed. We define a threshold condition for the maximum duration of the laser pulse that allows mitigation of electron heating and radiation pressure acceleration of narrow energy spread ion beams. The model is validated by three-dimensional PIC simulations, and the few experimental studies that reported low energy spread radiation pressure accelerated ion beams appear to meet the derived criteria. The understanding provided by our work will be important in guiding future experimental developments, for example for the ultrashort laser pulses becoming available at state-of-the-art laser facilities, for which we predict that proton beams with $\sim$ 150–250 MeV, $\sim$ 30% energy spread, and a total laser-to-proton conversion efficiency of $\sim$ 20% can be produced.

Comparison study of the energy and instability of ion-acoustic solitary waves in magnetized electron–positron–ion quantum plasma

Notably, solitary waves that emerge from the nonlinear properties of plasmas are the main focus of many current studies of localized disturbances in both laboratory and astrophysical plasmas. By applying the reductive perturbation method, we derive the nonlinear homogeneous quantum Zakharov–Kuznetsov (QZK) equation in three-component collisionless quantum plasma consisting of electrons, positrons, and ions in the presence of an external static magnetic field. The solitary wave structures are dependent on the Bohm potential, magnetic field, obliqueness, species Fermi temperatures, and densities. The soliton’s electric field and energy are also derived and investigated, which were found to be reduced as the magnetic field increases. The instability growth rate is also derived by using the small-k perturbation expansion method. The previous parameters affect the instability growth rate as well. A comparison of the energy and instability growth rate behaviour against system parameters is carried out. Large energy and large instability growth rate occur at large values of positron density or lower values of ion density. At zero or small rotation angle, both decrease as the magnetic field increases. Our findings could help us understand the dynamics of magnetic white dwarfs, pulsar magnetospheres, semiconductor plasma, and high-intensity laser-solid matter interaction experiments where e-p-i plasma exists.

Laser ion acceleration from tailored solid targets with micron-scale channels

Laser ion acceleration is a promising concept for the generation of fast ions using a compact laser-solid interaction setup. In this study, we theoretically investigate the feasibility of ion acceleration from the interaction of petawatt-scale laser pulses with a structured target that embodies a micron-scale channel filled with relativistically transparent plasma. Using 2D and 3D particle-in-cell (PIC) simulations and theoretical estimates, we show that it is possible to generate GeV protons with high volumetric charge and quasimonoenergetic feature in the energy spectrum. Optimal parameters of the target are obtained using 2D PIC simulations and interpreted on a basis of an analytical two-stage ion acceleration model. 3D PIC simulations and realistic preplasma profile runs with 2D PIC show the feasibility of the presented laser ion acceleration scheme for the experimental implementation at the currently available petawatt laser facilities.

Negative energy dust acoustic waves evolution in a dense magnetized quantum Thomas–Fermi plasma

Propagation of nonlinear waves in the magnetized quantum Thomas–Fermi dense plasma is analyzed. The Zakharov–Kuznetsov–Burgers equation is derived by using the theory of reductive perturbation. The exact solution contains both solitary and shock terms. Also, it is shown that rarefactive waves propagate in most cases. Both the associated electric field and the wave energy have been derived. The effects of dust and electrons temperature, dust density, magnetic field magnitude, and direction besides the effect of the kinematic viscosity on the amplitude, width, and energy of the formed waves are discussed. It is shown that the negative energy wave is formed and its value is enhanced due to the increase of the kinematic viscosity and the ambient magnetic field which lead to an increase in the instability. The present results are helpful in controlling the stabilization of confined Thomas–Fermi dense magnetoplasma that are found in white dwarfs and in the high-intensity laser-solid matter interaction experiments.

Homogeneous, Micron-Scale High-Energy-Density Matter Generated by Relativistic Laser-Solid Interactions.

Short-pulse, laser-solid interactions provide a unique platform for studying complex high-energy-density matter. We present the first demonstration of solid-density, micron-scale keV plasmas uniformly heated by a high-contrast, 400nm wavelength laser at intensities up to 2×10^{21} W/cm^{2}. High-resolution spectral analysis of x-ray emission reveals uniform heating up to 3.0keV over 1 μm depths. Particle-in-cell simulations indicate the production of a uniformly heated keV plasma to depths of 2 μm. The significant bulk heating and presence of highly ionized ions deep within the target are attributed to the few MeV hot electrons that become trapped and undergo refluxing within the target sheath fields. These conditions enabled the differentiation of atomic physics models of ionization potential depression in high-energy-density environments.

Mapping the power-law decay of high-harmonic spectra from few-cycle laser–solid interactions

Visible or near infrared light can be manipulated to produce bursts of coherent extreme ultraviolet or x rays via the relativistic high-order harmonic generation process when a laser irradiates a solid plasma target. The intensity of the spectral components of the reflected signal decays with the increase in harmonic order and the efficiency of this non-linear process largely hinges on how prompt this decay is. This is governed by the conditions of the laser–plasma interaction for which various models have been proposed. At relativistic intensities, a spectrum exhibiting a power-law decay with an exponent of 8/3 or 4/3 is often stated. Here, we analyze the dependence of this exponent on interaction parameters, including the angle of incidence, the carrier envelope phase, intensity of the laser, and the pre-plasma length, and discuss opportunities for optimization. Our simulations show that, rather than there being one universal exponent, the spectral decay is a continuous function of the laser–plasma interaction parameters.

Ultrafast time-resolved two-dimensional velocity mapping of the hot-dense plasmas generated by intense-laser pulses

Experimental measurements of spatially resolved ultrafast dynamics of the critical surface in ultra-intense laser–solid interactions are essential for a proper understanding of the physical mechanism of the interaction. Resolving ultrafast motion at both the relevant length scales (micrometers) and timescales (femtoseconds) simultaneously has been a challenging task. Here, we demonstrate a novel technique for mapping the spatiotemporal dynamics of hot and solid dense plasma created by high contrast (picosecond contrast ∼10−9) femtosecond relativistic intensity laser pulses. This pump–probe Doppler spectrometry technique offers hundreds of femtosecond temporal resolution, together with a few micrometer spatial resolution across the transverse profile of the plasma. We present the evolution of the plasma surface critical for the probe pulse at the target front as well as the rear. Early time measurements (< 5 ps) using this technique provide very important information about shock wave generation and propagation and the state of the target rear.

Laser–solid interaction studies enabled by the new capabilities of the iP2 BELLA PW beamline

The new capabilities of the short focal length, high intensity beamline, named iP2, at the BELLA Center will extend the reach of research in high energy density science, including accessing new regimes of high gradient ion acceleration and their applications. This 1 Hz system will provide an on-target peak intensity beyond 1021 W/cm2 with a temporal contrast ratio of <10−14 that will be enabled by the addition of an on-demand double plasma mirror setup. An overview of the beamline design and the main available diagnostics are presented in this paper as well as a selection of accessible research areas. As a demonstration of the iP2 beamline's capabilities, we present 3D particle-in-cell simulations of ion acceleration in the magnetic vortex acceleration regime. The simulations were performed with pure hydrogen targets and multi-species targets. Proton beams with energy up to 125 MeV and an approximately 12° full angle emission are observed as preplasma scale length and target tilt are varied. The number of accelerated protons is on the order of 109/MeV/sr for energies above 60 MeV.

Nanoscale subsurface dynamics of solids upon high-intensity femtosecond laser irradiation observed by grazing-incidence x-ray scattering

Observing ultrafast laser-induced structural changes in nanoscale systems is essential for understanding the dynamics of intense light-matter interactions. For laser intensities on the order of 1014W/cm2, highly collisional plasmas are generated at and below the surface. Subsequent transport processes such as heat conduction, electron-ion thermalization, surface ablation, and resolidification occur at picosecond and nanosecond timescales. Imaging methods, e.g., using x-ray free-electron lasers (XFEL), were hitherto unable to measure the depth-resolved subsurface dynamics of laser-solid interactions with appropriate temporal and spatial resolution. Here we demonstrate picosecond grazing-incidence small-angle x-ray scattering (GISAXS) from laser-produced plasmas using XFEL pulses. Using multilayer (ML) samples, both the surface ablation and subsurface density dynamics are measured with nanometer depth resolution. Our experimental data challenges the state-of-the-art modeling of matter under extreme conditions and opens new perspectives for laser material processing and high-energy density science.Received 11 December 2021Accepted 23 May 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.033038Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsHigh-energy-density plasmasPhysical SystemsPlasma sheaths & surfacesStrongly-coupled plasmasWarm-dense matterTechniquesHydrodynamicsLaser ablationParticle-in-cell methodsX-ray reflectivityX-ray scatteringPlasma PhysicsCondensed Matter, Materials & Applied Physics

Dense Polarized Positrons from Laser-Irradiated Foil Targets in the QED Regime.

Many works have shown that dense positrons can be effectively generated from laser-solid interactions in the strong-field quantum electrodynamics (QED) regime. Whether these positrons are polarized has not yet been reported, limiting their potential applications. Here, by polarized QED particle-in-cell simulations including electron-positron spin and photon polarization effects, we investigate a typical laser-solid setup that an ultraintense linearly polarized laser irradiates a foil target with micrometer-scale-length preplasmas. We find that once the positron yield becomes appreciable with the laser intensity exceeding 10^{24} W/cm^{2}, the positrons are obviously polarized. Around 30nC positrons can acquire >30% polarization degree with a flux of 10^{12} sr^{-1}. The angle-dependent polarization is attributed to the asymmetrical laser fields that positrons undergo near the skin layer of overdense plasmas, where radiative spin flip and radiation reaction play significant roles. The polarization mechanism is robust and could generally appear in future 100-PW-class laser-solid experiments.

Off-harmonic optical probing of high intensity laser plasma expansion dynamics in solid density hydrogen jets

Due to the non-linear nature of relativistic laser induced plasma processes, the development of laser-plasma accelerators requires precise numerical modeling. Especially high intensity laser-solid interactions are sensitive to the temporal laser rising edge and the predictive capability of simulations suffers from incomplete information on the plasma state at the onset of the relativistic interaction. Experimental diagnostics utilizing ultra-fast optical backlighters can help to ease this challenge by providing temporally resolved inside into the plasma density evolution. We present the successful implementation of an off-harmonic optical probe laser setup to investigate the interaction of a high-intensity laser at 5.4times 10^{21},hbox {W/cm}^{2} peak intensity with a solid-density cylindrical cryogenic hydrogen jet target of {5},{upmu }mathrm{m} diameter as a target test bed. The temporal synchronization of pump and probe laser, spectral filtering and spectrally resolved data of the parasitic plasma self-emission are discussed. The probing technique mitigates detector saturation by self-emission and allowed to record a temporal scan of shadowgraphy data revealing details of the target ionization and expansion dynamics that were so far not accessible for the given laser intensity. Plasma expansion speeds of up to (2.3 pm 0.4)times 10^{7},hbox {m/s} followed by full target transparency at {1.4},{mathrm{ps}} after the high intensity laser peak are observed. A three dimensional particle-in-cell simulation initiated with the diagnosed target pre-expansion at {-0.2},{mathrm{ps}} and post processed by ray tracing simulations supports the experimental observations and demonstrates the capability of time resolved optical diagnostics to provide quantitative input and feedback to the numerical treatment within the time frame of the relativistic laser-plasma interaction.

Collimation of fast electron beams by strong external magnetic field in laser–solid interaction using Monte Carlo simulation

In this paper, the effect of the external magnetic field on the fast electron beam angular spread and spatial divergence in laser–solid interaction were investigated and energy deposition of these electrons into the high-density fuel was evaluated using Monte Carlo simulation by Geant4 simulation toolkit. In our simulation, two types of the energy distribution function, exponential energy distribution, and quasi-two temperature energy distribution function, for electrons were considered. Our simulations in the presence of an external magnetic field show that the dynamics and spatial divergence of the high energy electrons are strongly affected by the external magnetic field. It was shown that in the case with the two-temperature energy distribution, the number of electrons (electron number density) with a large angular spread in the momentum space which were trapped by the magnetic fields lines and collimated toward the fuel were more populated than that of the exponential function. Comparison of the results indicate that the acceptable condition is obtained for the density in the presence of a magnetic field of about with a laser wavelength and laser intensity . It was presented that in the case with the sufficiently strong external fields, , it seems that the optimal results for suppression of electron spatial divergence, electron beam collimation, and energy deposition rate are obtained.

Optimized laser-assisted electron injection into a quasilinear plasma wakefield.

We present an electron injection scheme for plasma wakefield acceleration. The method is based on a recently proposed technique of fast electron generation via laser-solid interaction: a femtosecond laser pulse with the energy of tens of mJ hitting a dense plasma target at 45^{∘} angle expels a well collimated bunch of electrons and accelerates these close to the specular direction up to several MeVs. We study trapping of these fast electrons by a quasilinear wakefield excited by an external beam driver in a surrounding low density plasma. This configuration can be relevant to the AWAKE experiment at CERN. We vary different injection parameters: the phase and angle of injection, the laser pulse energy. An approximate trapping condition is derived for a linear axisymmetric wake. It is used to optimize the trapped charge and is verified by three-dimensional particle-in-cell simulations. It is shown that a quasilinear plasma wave with the accelerating field ∼ 2.5 GV/m can trap electron bunches with ∼ 100 pC charge, ∼60μm transverse normalized emittance and accelerate them to energies of several GeV with the spread ≲ 1% after 10 m..