Ultraintense Laser-solid Interactions Research Articles

Density effect on proton acceleration from carbon-containing high-density thin foils irradiated by high-intensity laser pulses

The acceleration of protons in dense plastic foils irradiated by ultrahigh intensity laser pulses is simulated using a two-dimensional hybrid particle-in-cell scheme. For the chosen parameters of the overdense foils of densities ρ=0.2, 1, and 3 g∕cm3 and of an ultrahigh intensity (2×1020 W∕cm2) laser pulse, our simulations illustrate that a high-density target is favorable to high collimation of the target-normal-sheath acceleration protons but less energy for a short acceleration time (<100 fs). In particular, the difference of strong local heating of the carbon ion for different plasma densities is clearly observed at both the front and rear surfaces of thin solid targets, suggesting that the effect of the density and composition of the targets are also important for correctly simulating energetic ion generation in ultraintense laser-solid interactions.

Short pulse laser produced energetic electron and positron measurements

Very energetic (>MeV) electrons are commonly produced from ultraintense (>1019W∕cm2) laser-solid interactions. Diagnosing these so-called hot electrons is of fundamental importance in understanding the detailed laser plasma physics present in these interactions, as well as in potential applications. Interestingly, positrons can also be produced in these interactions if the number and effective temperature of hot electrons exceed a threshold value. This additional information may help to better determine the effective electron temperature that exists in these targets. We have designed and constructed a compact spectrometer that can simultaneously measure the energy spectra of both electrons and positrons. Presented here are the principles of the spectrometer together with its experimental results obtained on the ultraintense (>1019W∕cm2) lasers at Lawrence Livermore National Laboratory and the Rutherford Appleton Laboratory Vulcan petawatt laser.

Evaluation of Transmutation of 137Cs(γ,n) 136Cs Using Ultra-Intense Lasers in Solid Targets

Relativistic electrons produced in ultra-intense laser–solidinteraction generate highly collimated γ-ray beams throughBremsstrahlung that can be used to induce photonuclear reactions.Photonuclear transmutation (of (γ,n) type) of 137Cs, oneof the hazardous nuclear wastes with half-life of 30.17 years whichcannot be transmuted practically with neutron bombardment due to itsvery low neutron capture cross section, has been considered. Nuclearactivity of produced 136Cs with half-life of 13.16 days has beenevaluated analytically using available experimental data. Withirradiating a 137Cs sample by p-polarized laser light of1020 Wcm−2 and the repetition rate of 10 Hz for 30 min,the activity of 0.24 Bq is obtained. It is found that intensity has alarge effect in yield around 1021 Wcm−2. For similarlaser with intensity of 5×1021 Wcm−2, the activityincreases with a factor of 105.

Laser Accelerated, High Quality Ion Beams

Intense beams of protons and heavy ions have been observed in ultra-intense laser-solid interaction experiments. Thereby, a considerable fraction of the laser energy is transferred to collimated beams of energetic ions (e.g. up to 50 MeV protons; 100 MeV fluorine), which makes these beams highly interesting for various applications. Experimental results indicate very short pulse duration and an excellent beam quality, leading to beam intensities in the TW range. To characterize the beam quality and its dependence on laser parameters and target conditions we performed experiments at several high-power laser systems. We found a strong dependence on the target rear surface conditions allowing to tailor the ion beam by an appropriate target design. We also succeeded in the generation of heavy ion beams by suppressing the proton amount at the target surface.

Effect of Target Composition on Proton Energy Spectra in Ultraintense Laser-Solid Interactions

We study how the proton density in a target irradiated by an ultraintense laser affects the proton spectrum, with analytical models and Vlasov simulations. A low relative proton density gives rise to peaks in the energy spectrum. Furthermore, a target with the protons confined to a thin, low density layer produces a quasimonoenergetic spectrum. This is a simple technique for producing proton beams with a narrow energy spread for proton radiography of laser-plasma interactions.

Evidence of enhanced effective hot electron temperatures in ultraintense laser-solid interactions due to reflexing

It is shown that the effective hot electron temperature, Thot, associated with the energetic electrons produced during the interaction of an ultra-intense laser with thin solid targets is dependent on the thickness of the target. We report the first direct experimental observations of electron energy spectra obtained from laser-solid interactions that indicates the reflexing of electrons in thin targets results in higher electron temperatures than those obtained in thick target interactions. This can occur for targets whose thickness, xt, is less than about half the range of an electron at the energy associated with the initial effective electron temperature, provided the laser pulse length is at least cτp > 2xt. A simple theoretical model that demonstrates the physical mechanism behind this enhanced heating is presented and the results of computer simulations are used to verify the model.

One-dimensional model of the electrostatic ion acceleration in the ultraintense laser–solid interaction

Effective ion acceleration of picosecond-duration well-collimated bunches in the strong relativistic interaction of a short laser pulse with a thin solid target has been experimentally demonstrated. In this work, with reference to the sharp rear solid–vacuum interface, where ion energization takes place, the one-dimensional Poisson–Boltzmann equation is analytically solved on a finite spatial interval whose extension is determined by requiring electron energy conservation, resulting in the consistent spatial distributions of the hot electrons created by the laser and of the corresponding electrostatic potential. Then, the equation of motions for an ensemble of test ions, initially distributed in a thin layer of the rear target surface, with different initial conditions, is solved and the energy spectrum corresponding to a given initial ion distribution is determined.

Effects of Pre-plasma on Energetic Electrons Generation in Ultra-short Ultra-intense Laser–Solid Interaction

The interactions between an ultra-short ultra-intense laser pulse and a solid target with and without a low density pre-plasma are examined. The electrons in the pre-plasma can be trapped and accelerated by the ponderomotive force of a propagating laser pulse, and escape from the pulse as the latter reflects from the target surface, preserving high energy. While in laser interaction with a target without pre-plasma, the maximum energy achieved is much lower. In this sense, the pre-plasma is advantageous for energetic electron generation in the ultra-short ultra-intense laser–solid interaction.

The generation of high-quality, intense ion beams by ultra-intense lasers

Intense beams of protons and heavy ions have been observed in ultra-intense laser–solid interaction experiments. Thereby, a considerable fraction of the laser energy is transferred to collimated beams of energetic ions (e.g. up to 50 MeV protons; 100 MeV fluorine), which makes these beams highly interesting for various applications. Experimental results indicate a very short-pulse duration and an excellent beam quality, leading to beam intensities in the TW range. To characterize the beam quality and its dependence on laser parameters and target conditions we performed experiments using the 100 TW laser system at Laboratoire pour l'Utilisation des Lasers Intenses at the Ecole Polytechnique, France, with focused intensities exceeding 1019 W cm−2. We found a strong dependence on the target rear surface conditions allowing to tailor the ion beam by an appropriate target design. We also succeeded in the generation of heavy ion beams by suppressing the proton amount at the target surface. We will present recent experimental results demonstrating a transverse beam emittance far superior to the accelerator based ion beams. Finally, we will discuss the prospect of laser accelerated ion beams as new diagnostics in laser–solid interaction experiments. Special fields of interest are proton radiography, electric field imaging, and relativistic electron transport inside the target.

Energetic proton generation in ultra-intense laser–solid interactions

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Photonuclear fission from high energy electrons from ultraintense laser-solid interactions

A new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic, with energies extending well above the threshold for nuclear processes, is studied using a petawatt laser system. In solid target experiments with focused intensities exceeding 10(20) W/cm(2), high energy electron generation, hard bremsstrahlung, and nuclear phenomena have been observed. We report here a quantitative comparison of the high energy electrons and the bremsstrahlung spectrum, as measured by photonuclear reaction yields, including the photoinduced fission of 238U.

High energy electrons, nuclear phenomena and heating in petawatt laser-solid experiments

The Petawatt laser at Lawrence Livermore National Laboratory (LLNL) has opened a new regime of laser matter interactions in which the quiver motion of plasma electrons is fully relativistic with energies extending well above the threshold for nuclear processes. In addition to ∼few MeV ponderomotive electrons produced in ultra intense laser-solid interactions, we have found a high energy component of electrons extending to ∼100 MeV apparently from relativistic selffocusing and plasma acceleration in the underdense preformed plasma. The generation of hard bremsstrahlung, photonuclear reactions, and preliminary evidence for positron-electron pair production will be discussed.