High Intensity Short Pulse Research Articles

Modelling hot electron generation in short pulse target heating experiments

Target heating experiments planned for the Orion laser facility, and electron beam driven fast ignition schemes, rely on the interaction of a short pulse high intensity laser with dense material to generate a flux of energetic electrons. It is essential that the characteristics of this electron source are well known in order to inform transport models in radiation hydrodynamics codes and allow effective evaluation of experimental results and forward modelling of future campaigns. We present results obtained with the particle in cell (PIC) code EPOCH for realistic target and laser parameters, including first and second harmonic light. The hot electron distributions are characterised and their implications for onward transport and target heating are considered with the aid of the Monte-Carlo transport code THOR.

Numerical simulations of energy transfer in two collisionless interpenetrating plasmas

Ion stream instabilities are essential for collisionless shock formation as seen in astrophysics. Weakly relativistic shocks are considered as candidates for sources of high energy cosmic rays. Laboratory experiments may provide a better understanding of this phenomenon. High intensity short pulse laser systems are opening possibilities for efficient ion acceleration to high energies. Their collision with a secondary target could be used for collisionless shock formation. In this paper, using particle-in-cell simulations we are studying interaction of a sub-relativistic, laser created proton beam with a secondary gas target. We show that the ion bunch initiates strong electron heating accompanied by the Weibel-like filamentation and ion energy losses. The energy repartition between ions, electrons and magnetic fields are investigated. This yields insight on the processes occurring in the interstellar medium (ISM) and gamma-ray burst afterglows.

Laser ion acceleration in the high laser energy and high laser intensity regimes

New laser facilities able to deliver either ultra high energy short pulses or ultra high intensity pulses are being constructed and will open new and exciting opportunities for laser ion acceleration. The interaction of a high intensity short pulse with underdense, near-critical and overdense targets has been studied using 2D Particle-In-Cell simulations in these regimes. In the ultra high energy regime, proton beams with maximum energies of hundreds of MeV and a high number of high energy protons could be accelerated using thin solid foils or low density targets. In the ultra high intensity regime, radiation losses will start affecting laser ion acceleration using thin overdense targets for intensities higher than 10 22 W/cm 2 , and produce very energetic ions.

Effect of high-intensity x-ray radiation on Bragg diffraction in silicon and diamond

We simulated the Bragg reflection of high-intensity short x-ray pulses from single-crystal silicon and diamond by coupling tight-binding-molecular dynamics with a simple atomic kinetics model. We found that even when the pulse-averaged Bragg intensity degrades significantly, the reflectivity drops only slightly at the beginning of the pulse until the lattice is disordered by non-thermal melting. These results suggest that Bragg reflectors could produce shortened x-ray pulses through temporal slicing.

Spatially resolved collective excitations of nano-plasmas via molecular dynamics simulations and fluid dynamics

Collective excitations in nano-plasmas are described by dynamical bi-local auto-correlation functions. These excitations, which are related to the plasmon excitations in bulk plasmas, arise in the classical as well as the quantum regime. Instead of the wave-vector-dependent dynamical structure factor, which is not well defined in finite systems, two different signatures are considered to characterize collective excitations: the bi-local particle density correlation function and the bi-local current density correlation function. The relation between both signatures is not as trivial as in the homogeneous case and is given here. Exemplary calculations are performed for expanding nearly spherical clusters of sodium atoms after excitation by a high-intensity short pulse laser beam. The lowest collective excitations obtained in the classical regime using molecular dynamics simulations agree well with the lowest collective excitations obtained from quantum calculations using fluid dynamics. The energy, damping and structure of the lowest collective modes are given. The dynamical bi-local correlation functions are of relevance for the optical properties, in particular the determination of photo absorption coefficients of nano-plasmas.

Prompt and Delayed Parasitic Currents Induced in Typical Insulators of Coaxial Cables by Large 14 MeV Sub-Nanosecond Neutrons Flux

Inertial Confinement Fusion (ICF) experiments performed on high-power laser facilities produce a high intensity short pulse of 14 MeV neutrons. The interaction of such an environment with coaxial cables widely used for signals transmission of diagnostic equipments induces parasitic transient currents. Depending on the magnitude and shape of those currents, measurement signals are likely to be altered and electric devices might be damaged. This paper reviews our knowledge about the generation of parasitic current in polyethylene (PE), PolyTetraFluorEthylene (PTFE) and PolyEtherEtherKetone (PEEK) insulators subjected to an ICF neutron pulse. It provides a clear evidence of the existence of a prompt neutron component and a very fast, nanosecond delayed radiation induced electromotive force (RIEMF) in insulator response to very short neutron pulses.

Collisions effect in the heating of underdense plasma by a high-intensity short laser pulse

Nonlinear heating of collisional plasma when the temporal extent of the laser pulse is smaller than the ambipolar diffusion time has been investigated. The nonlinearity in a collisionless plasma arises through the ponderomotive force, whereas in collisional plasmas ohmic nonlinearity prevails. In this case, by considering the nonrelativistic ponderomotive force effect and the variation of the collision frequency between electrons and ions due to the temperature change, the nonlinear dielectric permittivity of the plasma medium is obtained and the equation of the electromagnetic wave propagation in underdense plasma is solved. It is shown that in this case, due to the ohmic heating of electrons, the effect of the ponderomotive force in the nonrelativistic regime leads to steepening of the electron density profile and decreases the temperature of the plasma electrons noticeably. Bunches of electrons in plasma become narrower and by increasing the laser pulse strength the wavelength of density oscillations decreases. In this regime of laser–plasma interaction, the electron temperature decreases sharply with increasing the intensity of laser pulses.

A novel zirconium Kα imager for high energy density physics research

We report on the development and characterization of a zirconium Kα imager for high energy density physics research. The imager consists of a spherically bent quartz crystal operating at 15.7 keV photon energy. We compare the performance of the imager in terms of integrated reflectivity (R(int)) and temperature dependent collection efficiency (η(Te)) to that of the widely used Cu Kα imager. Our collisional-radiative simulations show that the new imager can be reliably used up to 250 eV plasma temperature. Monte Carlo simulations show that for a 25 μm thick tracer layer of zirconium, the contribution to Kα production from photo-pumping is only 2%. We present, for the first time, 2D spatially resolved images of zirconium plasmas generated by a high intensity short pulse laser interacting with Zr solid targets.

The quality of histotripsy produced tissue lesions can be remarkably insensitive to strong acoustic aberrations: Role of nonlinear phenomena.

Cavitation induced by short high intensity pulses can produce confined lesions with remarkably narrow transition zones (often bisecting individual cells). This process (histotripsy) is a nonlinear phenomenon wherein energetic bubble clouds are produced only when the incident ultrasound beam is above the cavitation threshold. It has been shown that acoustic aberrations in the path of an acoustic beam tend to rob energy from the main lobe raising surrounding side-lobes, sometimes significantly. However, in many cases, the shape of the main lobe, while diminished in amplitude, retains its original confined Gaussian-shaped cross-section. If the transducer-driving electronics has sufficient head-room, the main lobe can be boosted to preaberration above-threshold levels while maintaining the original main lobe profile. If the boosted main lobe amplitude is above the side-lobe levels, the lesion producing bubble clouds often are indistinguishable from the nonaberration case thus producing a clean confined lesion with little collateral damage even in the presence of strong aberrations. Confirmation of this strategy with porcine and polymer rib aberration phantoms is presented with a study of lesion shape as well as collateral damage with and without aberrations. Boosted intensities can be kept below thermally significant levels by adjusting pulse repetition frequency and pulse width.

Transport of hot electron currents in solid targets irradiated by high intensity short laser pulses

We have analyzed the transport of hot electrons generated in the interaction between a short-pulse, ultra-high intensity laser beam (pulse duration τ<ps, intensity Iλ > 1018 W.cm 2.um2) and a solid or dense target through the use of proton emission imaging. We used targets of different material (Cu, Al, Au) with a regularly modulated rear target surface in order to compare the electron transport in different conditions. As result, we see that the electron transport depends on the target material and on the interaction conditions.

Fast electron propagation in high-density plasmas created by 1D shock wave compression: Experiments and simulations

We present results from an experimental characterization of fast electron transport in high density plasmas created by 1D shock wave compression. The Kα fluorescence from a Cu layer embedded in Al or CH foil targets is measured. We use long laser pulses (LP) with 180 J, 1.5 ns, 0.53μm to compress the foils by shock wave propagation to 2-3 times their solid density and heat them to ∼ 4eV (close to the Fermi temperature). A counter-propagating high-intensity short laser pulse (SP), with 40 J, 1 ps, 57×1019 Wcm−2, generates intense currents of fast electrons which propagate through the deep regions of the target just before shock breakthrough. The results are compared to the uncompressed, solid density case (without the LP beam). The complete set of measurements is compared to numerical results, including a 2D hydrodynamic description of the compression and pre-pulse effects, 2D PIC simulations of the SP beam interaction and both hybrid and PIC simulations of the electron transport in the target depth and sheaths. In the case of the non-compressed targets we need to take fast electron refluxing into account to reproduce the experimental results. By exploring the domain of warm temperatures, we identify a regime for the incident fast electron current density, 1010 < jh < 1012 Acm−2, for which the collective mechanisms of electron transport differs appreciably between solid density and compressed matter.

Structure Effects in Angle-Resolved High-Order Above-Threshold Ionization of Molecules

We present energy-resolved angular distributions of photoelectrons generated in above-threshold ionization (ATI) of nonaligned diatomic molecules N2 and O2 in high-intensity short laser pulses, with emphasis on the most energetic part of the spectra. The angular distribution for photoelectrons with energy of 10U(p) (U(p): ponderomotive energy), i.e., the plateau cutoff in ATI spectra, is found to be broader in O2 than in N2. Resorting to the analyses from both an S-matrix theory and an intuitive semiclassical model, we attribute the observation to the effect of the ground state molecular orbital structure on high-energy electron emission in strong-field molecular ionization.

Dependence on pulse duration and foil thickness in high-contrast-laser proton acceleration

Experimental measurements of proton acceleration with high intensity and high-contrast short laser pulses have been carried out over an order of magnitude range in target thickness and laser pulse duration. The dependence of the maximum proton energy with these parameters is qualitatively supported by two-dimensional particle-in-cell simulations. They evidence that two regimes of proton acceleration can take place, depending on the ratio between the density gradient and the hot electron Debye length at the rear target surface. As this ratio can be affected by the target thickness, a complex interplay between pulse duration and target thickness is observed. Measurements and simulations support unexpected variations in the laser absorption and hot electron temperature with the pulse duration and laser intensity, for which density profile modification at the target front surface is the controlling parameter.

Experimental study on multiphoton ionization of Ar with high intensity short laser pulses

Experimental study on multiphoton ionization of Ar with high intensity short laser pulses

超短强激光场中Ar原子多光子电离实验研究

We present high resolution photoelectron energy spectra from multiphoton ionization (MPI) of Ar subject to laser pulses with wavelength. of 400 mn, pulse duration of 35 fs, and maximum intensity of 5x10(13) W/cm(2) Ionizations into (2)P(3/2) and (2)P(1/2) channels are observed and distinct resonance structures are found in both ionization channels The intensity dependence of the resonance structures is explained in terms of the mechanism of Freeman resonance, i.e transient resonances of alternating current, (AC) Stark-shifted Rydberg states at specific intensities within the laser pulse.

On-chip generation of high-intensity short optical pulses using dynamic microcavities

We experimentally demonstrate generation of high-intensity short optical pulses obtained by the controlled ultrafast release of the stored energy confined in silicon microcavities. This is achieved using ultrafast tuning of the coupling from a coupled-ring cavity to an external waveguide on time scales shorter than cavity photon lifetime.

X-ray spectroscopy of buried layer foils irradiated at laser intensities in excess of 1020 W/cm2

Observations of a rapid decrease in thermal temperature as a function of depth of solid targets irradiated with a short pulse, ultrahigh-intensity laser are reported. This phenomenon is investigated using the Titan short pulse laser with intensities greater than 1020 W/cm2 interacting with buried layer targets. The longitudinal temperature profile is determined by measuring K-shell spectra from a 0.4 μm copper tracer layer placed at various depths (i.e., 0–1.5 μm) within the 2.4 μm thick target. It is observed that the line ratios (He-like K-shell lines) as a function of temperature require a consideration of at least three parameters to analyze the K-shell spectra: hot electron population, time-dependent plasma conditions, and opacity. Here, the study of the effect of these three parameters on measured spectra in the short pulse high intensity laser-matter interactions using the atomic model FLYCHK [H.-K. Chung et al., High Energy Density Phys. 1, 3 (2005)] is presented.

Comparison between simulated and experimental emission spectra of photoionizing nitrogen plasma

A computational model has been developed to provide a straightforward and rapid tool for the calculation of the ionization and population distributions in photoionizing plasmas. The simulated spectral results from this code are compared with experimental data and are found to be in reasonable agreement for photoionized plasmas created with high intensity short pulse lasers.

Improvement of Proton Beam Quality from the High-intensity Short Pulse Laser Interaction with a Micro-structured Target

Target design study to improve the quality of an accelerated proton beam from the interaction of a high-intensity short pulse laser with an overdense plasma slab has been accomplished by using a two-dimensional, fully electromagnetic and relativistic particle-in-cell (PIC) simulation. The target consists of a thin core part and a thick peripheral part of equivalent plasma densities, while the ratio of the radius of the core part to the laser spot size, and the position of the peripheral part relative to the fixed core part were varied. The positive effects of this core-peripheral target structure could be expected from the knowledge of the typical target normal sheath acceleration (TNSA) mechanism in a laser-plasma interaction, and were apparently evidenced from the comparison with the case of a conventional simple planar target and the case of the transversal size reduction of the simple planar target. Improvements of the beam qualities including the collimation, the forward directionality, and the beam divergence were verified by detailed analysis of relativistic momentum, angular directionality, and the spatial density map of the accelerated protons.

Determination of electron-heated temperatures of petawatt laser-irradiated foil targets with 256 and 68 eV extreme ultraviolet imaging

Measurements of plasma temperature at the rear surface of foil targets due to heating by hot electrons, which were produced in short pulse high intensity laser matter interactions using the 150 J, 0.5 ps Titan laser, are reported. Extreme ultraviolet (XUV) imaging at 256 and 68 eV energies is used to determine spatially resolved target rear surface temperature patterns by comparing absolute intensities to radiation hydrodynamic modeling. XUV mirrors at these two energies were absolutely calibrated at the Advanced Light Source at the Lawrence Berkeley Laboratory. Temperatures deduced from both imagers are validated against each other within the range of 75-225 eV.