High-intensity Laser-matter Interactions Research Articles

Spatial Properties of High-Order Harmonic Beams from Plasma Mirrors: A Ptychographic Study.

Spatial properties of high-order harmonic beams produced by high-intensity laser-matter interactions carry rich information on the physics of the generation process, and their detailed understanding is essential for applications of these light beams. We present a thorough study of these properties in the case of harmonic generation from plasma mirrors, up to the relativistic interaction regime. Insitu ptychographic measurements of the amplitude and phase spatial profiles of the different harmonic orders in the target plane are presented, as a function of the key interaction parameters. These measurements are used to validate analytical models of the harmonic spatial phase in different generation regimes, and to benchmark ultrahigh-order Maxwell solvers of particle-in-cell simulation codes.

Spin polarization of electrons by ultraintense lasers

In a strong magnetic field, ultra-relativistic electrons or positrons undergo spin flip transitions as they radiate, preferentially spin polarizing in one direction -- the Sokolov-Ternov effect. Here we show that this effect could occur very rapidly (in less than 10 fs) in high intensity ($I\gtrsim10^{23}$ W/cm$^{2}$) laser-matter interactions, resulting in a high degree of electron spin polarization (70%-90%).

Low-noise time-resolved optical sensing of electromagnetic pulses from petawatt laser-matter interactions

We report on the development and deployment of an optical diagnostic for single-shot measurement of the electric-field components of electromagnetic pulses from high-intensity laser-matter interactions in a high-noise environment. The electro-optic Pockels effect in KDP crystals was used to measure transient electric fields using a geometry easily modifiable for magnetic field detection via Faraday rotation. Using dielectric sensors and an optical fibre-based readout ensures minimal field perturbations compared to conductive probes and greatly limits unwanted electrical pickup between probe and recording system. The device was tested at the Vulcan Petawatt facility with 1020 W cm−2 peak intensities, the first time such a diagnostic has been used in this regime. The probe crystals were located ~1.25 m from target and did not require direct view of the source plasma. The measured signals compare favourably with previously reported studies from Vulcan, in terms of the maximum measured intra-crystal field of 10.9 kV/m, signal duration and detected frequency content which was found to match the interaction chamber’s horizontal-plane fundamental harmonics of 76 and 101 MHz. Methods for improving the diagnostic for future use are also discussed in detail. Orthogonal optical probes offer a low-noise alternative for direct simultaneous measurement of each vector field component.

Sub-picosecond snapshots of fast electrons from high intensity laser-matter interactions

The interaction of a high-intensity short-pulse laser with thin solid targets produces electron jets that escape the target and positively charge it, leading to the formation of the electrostatic potential that in turn governs the ion acceleration. The typical timescale of such phenomena is on the sub-picosecond level. Here we show, for the first time, temporally-resolved measurements of the first released electrons that escaped from the target, so-called fast electrons. Their total charge, energy and temporal profile are provided by means of a diagnostics based on Electro-Optical Sampling with temporal resolution below 100 fs.

Target material dependence of positron generation from high intensity laser-matter interactions

The effective scaling of positron-electron pair production by direct, ultraintense laser-matter interaction is investigated for a range of target materials and thicknesses. An axial magnetic field, acting as a focusing lens, was employed to measure positron signals for targets with atomic numbers as low as copper (Z = 29). The pair production yield was found to be consistent with the Bethe-Heitler mechanism, where the number of positrons emitted into a 1 steradian cone angle from the target rear was found to be proportional to Z2. The unexpectedly low scaling results from Coulomb collisions that act to stop or scatter positrons into high angles. Monte Carlo simulations support the experimental results, providing a comprehensive power-law scaling relationship for all elemental materials and densities.

Femtosecond dynamics of energetic electrons in high intensity laser-matter interactions.

Highly energetic electrons are generated at the early phases of the interaction of short-pulse high-intensity lasers with solid targets. These escaping particles are identified as the essential core of picosecond-scale phenomena such as laser-based acceleration, surface manipulation, generation of intense magnetic fields and electromagnetic pulses. Increasing the number of the escaping electrons facilitate the late time processes in all cases. Up to now only indirect evidences of these important forerunners have been recorded, thus no detailed study of the governing mechanisms was possible. Here we report, for the first time, direct time-dependent measurements of energetic electrons ejected from solid targets by the interaction with a short-pulse high-intensity laser. We measured electron bunches up to 7 nanocoulombs charge, picosecond duration and 12 megaelectronvolts energy. Our ’snapshots’ capture their evolution with an unprecedented temporal resolution, demonstrat- ing a significant boost in charge and energy of escaping electrons when increasing the geometrical target curvature. These results pave the way toward significant improvement in laser acceleration of ions using shaped targets allowing the future development of small scale laser-ion accelerators.

Manifestation of anharmonic resonance in the interaction of intense ultrashort laser pulses with microstructured targets

Identification of the basic processes responsible for an efficient heating of intense laser produced plasmas is one of the important features of high intensity laser matter interaction studies. Collisionless absorption due to the anharmonicity in the self-consistent electrostatic potential of the plasma, known as anharmonic resonance (AHR), has been proposed to be a basic mechanism but a clear experimental demonstration is needed. Here, we show that microstructured targets enhance X-ray emission and the polarization dependence ascribes the enhancement to anharmonic resonance heating. It is found that p-polarized pulses of 5×1017 W/cm2 intensity bring in a 16-fold enhancement in the X-ray emission in the energy range 20–350 keV compared to s-polarized pulses with microstructured targets. This ratio is 2 for the case of polished targets under otherwise identical conditions. Particle-in-cell simulations clearly show that AHR is the key absorption mechanism responsible for this effect.

Revisiting argon cluster formation in a planar gas jet for high-intensity laser matter interaction

We determine the size of argon clusters generated with a planar nozzle, based on the optical measurements in conjunction with theoretical modelling. Using a quasi-one dimensional model for the moments of the cluster size distribution, we determine the influence of critical physical assumptions. These refer to the surface tension depending on the presence of thermal equilibrium, the mass density of clusters, and different methods to model the growth rate of the cluster radius. We show that, despite strong variation in the predicted cluster size, 〈N〉, the liquid mass ratio, g, can be determined with high trustworthiness, because g is predicted as being almost independent of the specific model assumptions. Exploiting this observation, we use the calculated value for g to retrieve the cluster size from optical measurements, i.e., calibrated Rayleigh scattering and interferometry. Based on the measurements of the cluster size vs. the nozzle stagnation pressure, we provide a new power law for the prediction of the cluster size in experiments with higher values of the Hagena parameter (Γ*>104). This range is of relevance for experiments on high-intensity laser matter interactions.

Exotic x-ray emission from dense plasmas

Exotic x-ray emission from dense matter is identified as the complex high intensity satellite emission from autoionizing states of highly charged ions. Among a vast amount of possible transitions, double K-hole hollow ion (HI) x-ray emission K0LX → K1LX−1 + hνhollow is of exceptional interest due to its advanced diagnostic potential for matter under extreme conditions where opacity and radiation fields play important roles. Transient ab initio simulations identify intense short pulse radiation fields (e.g., those emitted by x-ray free electron lasers) as possible driving mechanisms of HI x-ray emission via two distinct channels: first, successive photoionization of K-shell electrons, second, photoionization followed by resonant photoexciation among various ionic charge states that are simultaneously present in high density matter. We demonstrated that charge exchange of intermixing inhomogenous plasmas as well as collisions driven by suprathermal electrons are possible mechanisms to populate HIs to observable levels in dense plasmas, particularly in high current Z-pinch plasmas and high intensity field-ionized laser produced plasmas. Although the HI x-ray transitions were repeatedly identified in many other cases of dense optical laser produced plasmas on the basis of atomic structure calculations, their origin is far from being understood and remains one of the last holy grails of high intensity laser–matter interaction.

Guest Editorial Special Issue on Z-Pinches

There has been significant progress made in the field of pulsed power driven Z-pinches since the first Special Issue was published in IEEE T ransactions on P lasma S cience in August 1998. The community has grown significantly, with more universities getting involved in this exciting area of research, which includes fusion targets, astrophysical plasmas, intense X-ray sources, and materials science, among others. Z-pinches use a variety of loads, for example, foils, gas puffs, and wire arrays for these applications. Other pinch configurations, namely, plasma foci, X-pinches, and capillary discharges, have also been used as neutron sources, backlighters, and radiation sources. New diagnostics have been developed, that probe the Z-pinch plasma with high temporal and spatial resolution not previously possible. Examples include protons produced from shortpulse high-intensity laser–matter interactions to probe the Z-pinch plasmas and spectroscopy of Doppler shifts. Furthermore, advanced computational capabilities (Hydra, LSP, Lasnex, Alegra, Gorgon, Mach2-TCRE, Perseus) have been applied to study the underlying physics of the pinch plasmas.

High intensity laser–matter interaction and electron plasma acceleration

The basic principles of the electron acceleration in laser produced plasmas and the related secondary sources of energetic radiation with a particular attention to betatron radiation are presented.

Magnetic collimation of relativistic positrons and electrons from high intensity laser–matter interactions

Collimation of positrons produced by laser-solid interactions has been observed using an externally applied axial magnetic field. The collimation leads to a narrow divergence positron beam, with an equivalent full width at half maximum beam divergence angle of 4° vs the un-collimated divergence of about 20°. A fraction of the laser-produced relativistic electrons with energies close to those of the positrons is collimated, so the charge imbalance ratio (ne−/ne+) in the co-propagating collimated electron-positron jet is reduced from ∼100 (no collimation) to ∼2.5 (with collimation). The positron density in the collimated beam increased from 5 × 107 cm−3 to 1.9 × 109 cm−3, measured at the 0.6 m from the source. This is a significant step towards the grand challenge of making a charge neutral electron-positron pair plasma jet in the laboratory.

Development and first experimental tests of Faraday cup array

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.

Modelling gamma-ray photon emission and pair production in high-intensity laser–matter interactions

In high-intensity (>1021 Wcm−2) laser–matter interactions gamma-ray photon emission by the electrons can strongly affect the electronʼs dynamics and copious numbers of electron–positron pairs can be produced by the emitted photons. We show how these processes can be included in simulations by coupling a Monte Carlo algorithm describing the emission to a particle-in-cell code. The Monte Carlo algorithm includes quantum corrections to the photon emission, which we show must be included if the pair production rate is to be correctly determined. The accuracy, convergence and energy conservation properties of the Monte Carlo algorithm are analysed in simple test problems.

Radiation pressure-assisted acceleration of ions using multi-component foils in high-intensity laser–matter interactions

Experimental results on the acceleration of protons and carbon ions from ultra-thin polymer foils at intensities of up to 6 × 1019 W cm−2 are presented revealing quasi-monoenergetic spectral characteristics for different ion species at the same time. For carbon ions and protons, a linear correlation between the cutoff energy and the peak energy is observed when the laser intensity is increased. Particle-in-cell simulations supporting the experimental results imply an ion acceleration mechanism driven by the radiation pressure as predicted for multi-component foils at these intensities.

Dynamics of Self-Generated, Large Amplitude Magnetic Fields Following High-Intensity Laser Matter Interaction

The dynamics of magnetic fields with an amplitude of several tens of megagauss, generated at both sides of a solid target irradiated with a high-intensity (~10(19) W/cm(2)) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets.

Particle-in-cell plasma simulation on heterogeneous cluster systems

This paper offers an approach to high-performance implementation of the Particle-in-Cell (PIC) algorithm for GPU-enabled heterogeneous cluster systems. We present Picador — a tool for three dimensional plasma simulation based on the fully relativistic PIC algorithm aimed at research of high intensity laser-matter interaction. It scales up to at least 2048 CPU cores with 85% strong scaling efficiency and 512 GPUs with 64% weak scaling efficiency, achieving up to 28% of the peak performance on the CPU and 14% on the GPU, respectively.

The shaped critical surface in high intensity laser plasma interactions

This paper describes an investigation of the properties of the relativistic critical surface in a high intensity laser-plasma interaction, specifically the spatial morphology of the surface and its effect upon the divergence of the reflected light. The particle-in-cell code LSP running in two dimensions (2d3v) was used to model the formation of the critical surface and to show that it resides at a varying depth into the material that is dependent on both the intensity radial dependence of the laser focus as well as the shape of the longitudinal vacuum-material interface. The result is a shaped “mirror” surface that creates a reflected beam with phase and amplitude information informed by the extent of the preplasma present before the intense laser pulse arrived. A robust, highly effective means of experimentally determining the preplasma conditions for any high intensity laser-matter interaction is proposed using this effect. The important physics is elucidated with a simplified model that, within reasonable intensity bounds, recasts the effect of the complex laser-plasma interaction on the reflected beam into a standard Gaussian optics calculation.

SU-GG-T-462: Observation of Quasi-Monoenergetic Laser Accelerated Proton and Carbon Beams

Purpose: To perform preliminary experiments to achieve the Directed Coulomb Explosion (DCE) regime of proton acceleration to therapeutic energies in high-intensity laser-matter interactions. Method and Materials: Particle-in-Cell (PIC) simulations of the planned experiments at HERCULES laser at the University of Michigan have predicted a new regime of attainable laser-target interactions for proton acceleration. The laser was recently upgraded to 300 TW with Amplified Spontaneous Emission (ASE) intensity contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. Dual plasma mirrors have been installed and characterized to reduce the prepulse at < 30 ps (from the uncompensated dispersion of optical elements during the pulse compression) before the main pulse providing 3 orders of magnitude contrast improvement. This allowed experiments on thin foil membranes (50 nm) with 50TW temporally clean pulses without compromising the target. Results: We found for the first time that for all target thicknesses proton spectra exhibit quasi-monoenergetic features, which are more pronounced for ultra-thin (50 nm Si3N4) targets resulted in AE/E∼30%. Moreover for these Si3N4 targets spectra for all the charge states of carbon ions C3+-C6+ are also found to be quasi-monoenergetic. Maximum proton energy drops from 6 MeV for 1 ⌈ m Mylar foil to 4 MeV for 50 nm Si3N4 membranes. Conclusion: Implementation of dual plasma mirrors substantially improved laser contrast and created more favorable proton and carbon flux-energy distributions. Further improvements to the plasma mirrors are required, using better antireflection coatings on glass substrates, to achieve the DCE regime of proton acceleration.

High-intensity lasers as radiation sources

In the last decade or so, an evolution in experimental relativistic laser-plasma physics has led to highly sophisticated lasers which are now capable of generating ultra-short pulses and can be focused to intensities in excess of 1021 W cm-2. The laser interaction with solid or gas targets can generate collimated beams of highly energetic electrons, protons and ions. These high-intensity laser systems, therefore, turn out to be versatile and powerful sources of radiation and high-energy particles, without recourse to large-scale facilities such as nuclear reactors or particle accelerators. The potential to induce various kinds of nuclear reactions with laser-induced radiation fields has been demonstrated at several laboratories in recent years. The present paper lays out a comprehensive overview of nuclear reactions induced by high-intensity laser matter interactions. Mechanisms for electron, proton and ion acceleration, in addition to secondary bremsstrahlung, positron and neutron production, are addressed, with a focus on the types of nuclear reactions that are possible and potential applications. Discussion of the extrapolation of these processes and applications to the next generation of table-top lasers under construction is also presented.