In this work, simulations of multipetawatt lasers at irradiances ${\sim }10^{23} \ \mathrm {W}\ \mathrm {cm}^{-2}$, striking solid targets and implementing two-dimensional particle-in-cell code was used to study particle acceleration. Preformed plasma at the front surface of a solid target increases both the efficiency of particle acceleration and the reached maximum energy by the accelerated charged particles via nonlinear plasma processes. Here, we have investigated the preformed plasma scale length effects on particle acceleration in the presence and absence of nonlinear quantum electrodynamic (QED) effects, including quantum radiation reaction and multiphoton Breit–Wheeler pair production, which become important at irradiances ${\sim } 10^{23}\ \mathrm {W}\ \mathrm {cm}^{-2}$. Our results show that QED effects help particles gain higher energies with the presence of preformed plasma. In the results for all cases, preplasma leads to more efficient laser absorption and produces more energetic charged particles, as expected. In the case where QED is included, however, physical mechanisms changed and generated secondary particles ($\gamma$-rays and positrons) reversing this trend. That is, the hot electrons cool down due to QED effects, while ions gain more energy due to different acceleration methods. It is found that more energetic $\gamma$-rays and positrons are created with increasing scale length due to high laser conversion efficiency (${\sim }$24 % for $\gamma$-rays and $\sim$4 % for positrons at $L = 7\ \mathrm {\mu }\textrm {m}$ scale length), which affects the ion and electron acceleration mechanisms. It is also observed that the QED effect reduces the collimation of angular distribution of accelerated ions because the dominant ion acceleration mechanism is changing when QED is involved in the process.

A massive deuterated polyethylene target was exposed to laser intensities of about 3 × 1016 W/cm2 employing the 3-TW Prague Asterix Laser System (PALS). We achieved a yield of 2 × 108 neutrons per laser shot. Average time-of-flight signals of scintillation detectors operated in current mode reveal broad energy spectra of fusion neutrons with dominating energy of about 2.45 MeV. The energy dependence of the neutron yield shows a consistency in results of nanosecond, picosecond and sub-picosecond experiments. Here we also show that ions emitted in the backward direction from the front target surface have a multi-peak energy spectrum, which is caused by burst emission mechanisms.

Non-paraxial approximation based study of transient self-focusing of an intense short-pulse laser in plasma has been investigated by considering the effect of a transverse magnetic field. The laser with non-uniform distribution of intensity exerts a ponderomotive force on electrons and sets in an ambi-polar diffusion of the plasma. The ambient magnetic field, however, strongly inhibits the process, when the electron Larmor radius is comparable to or shorter than the laser spot size. As the plasma density is depleted, the laser beam becomes more self-focused. This study addresses a significant enhancement in the laser self-focusing rate by including the correction terms due the off-axis approximation.

To control the shape of the ultra-thin electron layer produced by directly interaction of ultrahigh contrast laser with ultra-thin foil target, we investigated the spacial distribution and temporal evolution of electron layers produced from single and double foil targets through two-dimensional particle-in-cell simulations. Results show that electron layers produced from double foil targets can fly with unperturbed velocity for a much longer time than in the single foil case, which can be explained by the integrated contribution of charge separation field from both the two foils. Further studies show that through adjusting the foil expansion, electron layers with different shapes can be obtained. Detailed studies on the forming process of layers show that electron momentum distribution evolves rapidly along with the pump laser and then the vanishing of electron transverse momentum induced by the reflected laser results in the forming of layer shape. So different foil expansion corresponds to different moments that reflected laser interact with electron layer, when the electron transverse momentum distribution is different. After the reflected laser interact with electron layer, the resultant longitude momentum distribution will finally lead to various electron layer shapes.

The effects of laser incidence angle on lateral fast electron transport at front target surface, when a plasma is preformed, irradiated by intense (>1018 W/cm2) laser pulses, are studied by Kα imaging technique and electron spectrometer. A horizontally asymmetric Kα halo, resulting from directional lateral electron transport and energy deposition, is observed for a large incidence angle (70°). Moreover, a group of MeV high energy electrons is emitted along target surface. It is believed that the deformed preplasma and the asymmetrical distribution of self-generated magnetic field, at large incidence angle, play an important role in the directional lateral electron transport.

Experiments on ion acceleration by irradiation of ultra-thin diamond-like carbon (DLC) foils, with thicknesses well below the skin depth, irradiated with laser pulses of ultra-high contrast and linear polarization, are presented. A maximum energy of 13 MeV for protons and 71 MeV for carbon ions is observed with a conversion efficiency of ~10%. Two-dimensional particle-in-cell (PIC) simulations reveal that the increase in ion energies can be attributed to a dominantly collective rather than thermal motion of the foil electrons, when the target becomes transparent for the incident laser pulse.

It is shown that the above-threshold electron de Broglie waves, generated by an intense laser pulse at a metal surface are interfering to yield attosecond electron pulses. This interference of the de Broglie waves is an analog on of the superposition of high harmonics generated from rare gas atoms, resulting in trains of attosecond light pulses. Our model is based on the Floquet analysis of the inelastic electron scattering on the oscillating double-layer potential, generated by the incoming laser field of long duration at the metal surface. Owing to the inherent kinematic dispersion, the propagation of attosecond de Broglie waves in vacuum is very different from that of attosecond light pulses, which propagate without changing shape. The clean attosecond structure of the current at the immediate vicinity of the metal surface is largely degraded due to the propagation, but it partially recovers at certain distances from the surface. Accordingly, above the metal surface, there exist “collapse bands,” where the electron current is erratic or noise-like, and there exist “revival layers,” where the electron current consist of ultrashort pulses of about 250 attosecond durations in the parameter range we considered. The maximum value of the current densities of such ultrashort electron pulses has been estimated to be on order of couple of tenth of mA/cm2. The attosecond structure of the electron photocurrent can perhaps be used for monitoring ultrafast relaxation processes in single atoms or in condensed matter.

Los Alamos National Laboratory short pulse experiments have shown using various target cleaning techniques such that heavy ion beams of different charge states can be produced. Furthermore, by controlling the thickness of light ions on the rear of the target, monoenergetic ion pulses can be generated. The spectral shape of the accelerated particles can be controlled to yield a range of distributions, from Maxwellian to ones possessing a monoenergetic peak at high energy. The key lies in understanding and utilizing target surface chemistry. Careful monitoring and control of the surface properties and induction of reactions at different temperatures allows well defined source layers to be formed, which in turn lead to the desired energy spectra in the acceleration process. Theoretical considerations provide understanding of the process of monoenergetic ion production. In addition, numerical modeling has identified a new acceleration mechanism, the laser break-out afterburner that could potentially boost particle energies by up to two orders of magnitude for the same laser parameters. This mechanism may enable application of laser-accelerated ion beams to venues such as compact accelerators, tumor therapy, and ion fast ignition.

A new version of the numerical code KALOS has been developed to solve the Vlasov-Fokker-Planck equation for electrons as well as EM wave propagation. KALOS represents the electron distribution function in momentum space by an expansion in spherical harmonics. Its unique features make possible simultaneous investigations of fast electron generation and transport and the collisional evolution of thermal particles, including the return current of cold electrons. We report here on results obtained in one spatial dimension. Absorption of 100fs, 1015 W/cm2 laser pulses has been studied at normal incidence in sharp-edged dense plasmas. We have studied the effect on absorption of energy transport into the target as well as the deviation of the electron distribution function from Maxwellian. It is shown that it is necessary to take into account collisional heat transport into the target in order to correctly model the absorption rate at the front surface.

Laser-beam interaction with expanding plasma was investigated using the PALS high-power iodine-laser system. The interaction conditions are significantly changing with the laser focus spot position. The decisive role of the laser-beam self-focusing, participating in the production of ions with the highest charge states, was proved.

Due to their particular properties (low emittance, short duration, and large number density), the beams of multi-MeV protons generated during the interaction of ultraintense (I > 1019 W/cm2) short pulses with thin solid targets are suited for use as a particle probe in laser-plasma experiments. When traversing a sample, the proton density distribution is, in general, affected by collisional stopping, scattering and deflections via electromagnetic fields, and each of these effects can be used for diagnostic purposes. In particular, in the limit of very thin targets, the proton beams represent a valuable diagnostic tool for the detection of quasi-static electromagnetic fields. The proton imaging and deflectometry techniques employ these beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser- plasma interactions, with high temporal and spatial resolution. By providing diagnostic access to electro-magnetic field distributions in dense plasmas, this novel diagnostics opens up to investigation a whole new range of unexplored phenomena. Several transient processes were investigated employing this technique, via the detection of the associated electric fields. Examples provided in this paper include the detection of pressure-gradient electric field in extended plasmas, and the study of the electrostatic fields associated to the emission of MeV proton beams in high-intensity laser-foil interactions.

This new project relies on the capabilities collocated at Los Alamos in the Trident laser facility of long-pulse laser drive, for laser-plasma formation, and high-intensity short-pulse laser drive, for relativistic laser-matter interaction experiments. Specifically, we are working to understand quantitatively the physics that underlie the generation of laser-driven MeV/nucleon ion beams, in order to extend these capabilities over a range of ion species, to optimize beam generation, and to control those beams. Furthermore, we intend to study the interaction of these novel laser-driven ion beams with dense plasmas, which are relevant to important topics such as the fast-ignition method of inertial confinement fusion (ICF), weapons physics, and planetary physics. We are interested in irradiating metallic foils with the Trident short-pulse laser to generate medium to heavy ion beams (Z = 20–45) with high efficiency. At present, target-surface impurities seem to be the main obstacle to reliable and efficient acceleration of metallic ions in the foil substrate. In order to quantify the problem, measurements of surface impurities on typical metallic-foil laser targets were made. To eliminate these impurities, we resorted to novel target-treatment techniques such as Joule-heating and laser-ablation, using a long-pulse laser intensity of ∼ 1010 W/cm2. Our progress on this promising effort is presented in this paper, along with a summary of the overall project.

This article presents a numerical and theoretical study of the generation and propagation of oscillation in the semiclassical limit ħ → 0 of the nonlinear paraxial equation. In a general setting of both dimension and nonlinearity, the essential differences between the “defocusing” and “focusing” cases are observed. Numerical comparisons of the oscillations are made between the linear (“free”) and the cubic (defocusing and focusing) cases in one dimension. The integrability of the one-dimensional cubic nonlinear paraxial equation is exploited to give a complete global characterization of the weak limits of the oscillations in the defocusing case.