High-intensity Laser-plasma Interactions Research Articles

Image plate response for conditions relevant to laser–plasma interaction experiments

We have measured the absolute response and detective quantum efficiency of image plates (IPs) for 5.9 keV x-rays using a calibrated iron-55 source. The types of IPs considered in this study are now commonly used as x-ray detectors in high-intensity laser–plasma interaction experiments, where conventional CCD fails because of the intense electromagnetic pulse that follows a high-intensity shot. Since the plates are not read out immediately after each laser shot, a detailed fading analysis of the plates is also presented. This work is important for future implementation of IPs as absolute x-ray photon detectors in large-scale laser facilities.

New ellipsoid cavity model for high-intensity laser–plasma interaction

In this work we present an ellipsoid cavity regime for the production of a bunch of quasi-monoenergetic electrons. The electron output beam is more effective than the periodic plasma wave method or the plasma-channel-guided method. A hyperbola, parabola or ellipsoid path is described for the electron trajectory motion in this model. A dense bunch of relativistic electrons with a quasi-monoenergetic spectrum is self-generated here. The obtained results show a smaller width for the electron energy spectrum in comparison with the previous results. We found that there are optimum conditions to form the ellipsoid cavity. Laser beam properties (such as the spot size, power and pulse duration) and plasma features can control the ellipsoid cavity formation. The optimum conditions for effective electron beam production can be determined from these results.

Fast electron transport in high-intensity laser-plasma interactions diagnosed by optical and ion emission

Light and proton emission from rear surface of sandwich-like targets, which are irradiated by intense femtosecond laser pulses, have been measured. It is found that both of them depend on the transport material and density inside the targets. For metal transport layers the patterns of the optical images and the proton beams are uniform. While for the transport layers of dielectric, low-density foam and vacuum gap, the patterns are broken up. This indicates that the fast electron beams are also broken up probably due to instabilities.

High-intensity laser-plasma interactions in the refluxing limit

The interaction of intense laser pulses (5×1016W∕cm2<I<2×1019W∕cm2) with small-volume copper targets (20×20×2μm3<V<500×500×50μm3) is described. Electron refluxing causes Kα yields to be independent of laser intensity at I>1018W∕cm2, where the target is heated to less than 100eV. As target volume is reduced, efficient energy transfer by the refluxing electrons heats the targets to hundreds of eV. This affects L→K and M→K electronic transitions during ion de-excitation that probe the bulk plasma conditions during this rapid isochoric heating phase. Comparison of measured Kα yields in the cold-material limit with a Kα production model, and measured Kβ∕Kα variations with numerical target-heating calculations, provides a robust method for inferring the fast-electron energy content. Bulk electron temperatures of at least 200eV are inferred for the smallest mass targets.

Reentrant cone angle dependence of the energetic electron slope temperature in high-intensity laser-plasma interactions

Energy spectra of fast electrons, generated when high-intensity laser pulses irradiated hollow conical targets, have been measured experimentally. It is shown here that the slope temperature of the fast electrons is strongly dependent on the opening angle of the cone, and has a maximum value at 25°. The data confirms optical guiding of the laser pulse, by comparison of the measured electron temperature with ray-tracing calculations that include absorption in plasmas. The enhanced energy flow and intensity induced by optical guiding of the laser pulse inside the cone as a function of the opening angle as well as the f-number of the focusing optics is discussed.

Characterization of preformed plasmas using a multi-dimensional hydrodynamic simulation code in the study of high-intensity laser–plasma interactions

We observed a preformed plasma of an aluminum slab target produced by a high-intensity Ti:sapphire laser. The expansion length of the preformed plasma at the electron density of 3 × 1018 cm−3, which was the detection limit, was around 100 μm measured with a laser interferometer. In order to characterize quantitatively and to control the preformed plasmas, we perform a two-dimensional hydrodynamic simulation. The expansion length of the preformed plasma was almost the same as the experimental result, if we assumed that the amplified spontaneous emission lasted 3.5 ns before the main pulse arrived.

Characterization of Thin-Foil Preformed Plasmas for High-Intensity Laser Plasma Interactions

We observe the electron density distribution of preformed plasmas of a titanium foil target with the 400 nm probe beam as an interferometer. The preformed plasma at the front side of the target is generated by a prepulse with a high-intensity Ti:sapphire laser. On the other hand, the preformed plasma at the rear side is not observed within the region ∼18 μm away from the original surface. The characterization of preformed plasmas will contribute to describe the high-intensity laser and thin-foil interactions.

Observation of relativistic cross-phase modulation in high-intensity laser-plasma interactions

A nonlinear optical phenomenon, relativistic cross-phase modulation, is reported. A relativistically intense light beam (I = 1.3 x 10(18) W cm(-2), lambda = 1.05 microm) is experimentally observed to cause phase modulation of a lower intensity, copropagating light beam in a plasma. The latter beam is generated when the former undergoes the stimulated Raman forward scattering instability. The bandwidth of the Raman satellite is found to be broadened from 3.8-100 nm when the pump laser power is increased from 0.45-2.4 TW. A signature of relativistic cross-phase modulation, namely, asymmetric spectral broadening of the Raman signal, is observed at a pump power of 2.4 TW. The experimental cross-phase modulated spectra compared well with theoretical calculations. Applications to generation of high-power single-cycle pulses are also discussed.

Development of time resolved x-ray spectroscopy in high intensity laser-plasma interactions

This article discusses the design of a novel time resolved von Hamos Bragg spectrometer to provide spectra in the region around the titanium K-α and He-α lines. The instrument consists of a highly oriented pyrolitic graphite mosaic crystal coupled to a picosecond x-ray streak camera. Measurements of the time dependent behavior from Ti foils illuminated with intense laser pulses can be used to improve the understanding of recombination dynamics, electron transport, and phase transitions in strongly coupled dense plasma. This is important for the modeling of the compression phase in inertial confinement fusion research and the study of astrophysical environments.

Stochastic heating in ultra high intensity laser-plasma interaction: Theory and PIC code simulations

In the first part, the theoretical model of the stochastic heating effect is presented briefly. Then, a numerical resolution of the Hamilton equations highlights the threshold of the stochastic effect. Finally, Particle-In-Cell (PIC) code simulations results, for experimentally relevant parameters, are presented in order to confirm the acceleration mechanism predicted by the one-particle theoretical model. This paper gives the conditions on the different experimental parameters in order to have an optimization of the stochastic heating.

Modelling short pulse, high intensity laser plasma interactions

Modelling the interaction of ultra-intense laser pulses with solid targets is made difficult through the large range of length and time scales involved in the transport of relativistic electrons. An implicit hybrid PIC–fluid model using the commercial code LSP (LSP is marketed by MRC (Albuquerque), New Mexico, USA) reveals a variety of complex phenomena which seem to be borne out in experiments and some existing theories.

Beam-Weibel filamentation instability in near-term and fast-ignition experiments

High intensity laser-plasma interactions accelerate electrons to suprathermal velocities. Their current is neutralized by an induced cold electron return current. These inter-penetrating and anti-parallel currents are subject to electrostatic and electromagnetic instability. Two analytical models for electron transport are used to predict the growth rates of the linear electromagnetic beam-Weibel filamentation instability in both near-term laser-solid experiments as well as in future fast-ignition experiments. Specifications and calculations of the relevant physical parameters are made. Both models predict that instability growth is significant for the fast-ignition case. Instability development in near-term experiments is also significant, but with a greater difference between the models’ predictions at low densities.

Strong kinetic effects in cavity-induced low-level saturation of stimulated Brillouin backscattering for high-intensity laser-plasma interaction

A kinetic mechanism for the saturation of Brillouin backscattering in the high-intensity regime is presented. The reflectivity goes through several characteristic phases and terminates in a low-level quasistationary state of a few percent. This low-level saturation is due to the creation of cavities in the plasma. The process responsible for cavity formation is linked to Coulomb explosions in the bulk plasma. Along with the cavity formation, particles are accelerated up to energies of 100keV. Strong electron and ion heating and acceleration are the characteristics of this saturation mechanism for stimulated Brillouin backscattering. The cavities are filled with electrostatic and electromagnetic fields. Whereas the electrostatic ones are quickly dissipated, the electromagnetic ones are stable and have intensities up to 30 times the incident laser intensity.

Low-Level Saturation of Brillouin Backscattering due to Cavity Formation in High-Intensity Laser-Plasma Interaction

Full particle-in-cell simulations of Brillouin backscattering in the high-intensity regime are presented. The final state of the strongly nonlinear evolution of the ion-acoustic wave packet consists of its collapse and the formation of a density cavity which is supported by large-amplitude localized electromagnetic fields. The cavitation manifests itself in large oscillations of the reflectivity which is terminated by a low-level saturation with strong kinetic effects. This newly discovered scenario demonstrates the importance of fully kinetic descriptions of Brillouin backscattering for high intensities.

Simulation of high-intensity laser–plasma interactions by use of the 2D Lagrangian code “ATLANT-HE”

Hot electrons may significantly influence interaction of ultrashort laser pulses with solids. Accurate consideration of resonant absorption of laser energy and hot electron generation at a critical surface was achieved through the developed physical and mathematical models. A two-dimensional (2D) ray-tracing algorithm has been developed to simulate laser beam refraction and Bremsstrahlung absorption with allowance for nonlinear influence of a strong electromagnetic field. Hot electron transport was considered as a straight-line flow weakening by a friction force calculated in the approximation of the average state of ionization. Developed models were coupled with the 2D Lagrangian gas dynamic code “ATLANT” that takes into account nonlinear heat transport. The developed program has been applied to simulate irradiation of Al foils by picosecond laser double pulses. Hot electron transport and heating resulted in thin foil explosions. The transition from the exploding foil regime to the ablative one with foil thickening has been simulated and analyzed at various values of laser light intensity. In second series of calculations we have modeled the interaction of a nanosecond iodine laser with a two-layered target.

Characterization of preformed plasmas with an interferometer for ultra-short high-intensity laser-plasma interactions

The evolution of an Al preformed plasma produced by a prepulse was observed before and after the arrival of the main pulse by an interferometer using a femtosecond probe pulse. A central density depression due to the ponderomotive force of the main laser pulse in the preformed plasma with a ∼100 μm scale length was clearly visible after the main pulse irradiation at an intensity of ∼5×1016 W/cm2. The temporal profiles of the prepulse, characterized by a cross-correlation in conjunction with a precise density profile measurement by an interferometer, contribute to the better understanding of femtosecond laser-matter interactions.

Nuclear reactions in copper induced by protons from a petawatt laser-foil interaction

High-intensity (>1019 W cm−2) laser-plasma interactions have been shown to produce large quantities of protons with energies up to several tens of MeV. A range of laser-driven proton-induced reactions in copper have been investigated and the observed reactions quantified. The energy spectrum of the accelerated protons was determined from the reactions in a single thin copper foil and found to be in agreement with that deduced from (p,n) reactions measured in a stack of copper foils. The potential applications of this diagnostic technique are discussed.

Effect of target heating on ion-induced reactions in high-intensity laser–plasma interactions

Measurements of ion-induced nuclear reactions have been used to diagnose ion acceleration from the interaction of high-intensity (>1019 W cm−2) laser light with solid targets. Nuclear activation of catcher materials surrounding the interaction region has been studied using a high-resolution germanium detector. It was found that, when a 100 μm thick Al target foil was preheated, the proton flux produced from the laser–foil interaction was considerably reduced. Observed heavy-ion-induced reactions are used with calculated reaction cross sections to quantify ion acceleration.

超短パルス超高強度レーザーと物質との相互作用

Recently, ultra-high intensity short pulse lasers have been developed quite successfully. They have been applied to the fields such as hard x-ray generation and high energy particle generation. The essence of such laser-produced plasmas is that the lasers can feed their ultra-high energy density into that of the electron motion in matter via the ponderomotive force. Then the electron energy is transferred into the atoms and ions which include excitation and ionization by the laser field and the electron collision. Ion acceleration occurs by the electrostatic potential formed by high energy electron bouncing in the finite size plasma. Subsequently, high flux x-ray with a few tens of MeV energy induces nuclear reactions, which open up the new fields of high energy plasma physics and applications. In this reviewed article, the basic ultra-short and high intensity laser-plasma interaction studies are introduced. These studies lead us to new attractive fields not only the practical applications but also the new aspects of the basic sciences such as control of a highly-nonlinear relativistic plasma.

Laser-produced protons and their application as a particle probe

One of the most exciting results recently obtained in the ultraintense interaction research area is the observation of beams of protons with energies up to several tens of megaelectron volts, generated during the interaction of ultraintense picosecond pulses with solid targets. The particular properties of these beams (high brilliance, small source size, high degree of collimation, short duration) make them of exceptional interest in view of diagnostic applications. In a series of experiments carried out at the Rutherford Appleton Laboratory (RAL) and at the Lawrence Livermore National Laboratory (LLNL), the laser-produced proton beams have been characterized in view of their application as a particle probe for high-density matter, and applied to diagnose ultraintense laser–plasma interactions. In general, the intensity cross section of a proton beam traversing matter will be modified both by collisional stopping/scattering, and deflections caused by electric/magnetic fields. With a suitable choice of irradiation geometry and target parameters, the proton probe can be made mainly sensitive to the electric field distribution in the object probed. Therefore, point projection proton imaging appears as a powerful and unique technique for electric field detection in laser-irradiated targets and plasmas. The first measurements of transient electric fields in high-intensity laser-plasma interactions have been obtained with this technique.