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A procedure is proposed for finding a solution to the linearized kinetic equation with the Landau collision integral included for charged particles in a plasma with a high degree of ionization. This procedure is used to obtain an expression for dielectric permittivity tensor for a collisional plasma over the entire range of frequencies and wave numbers as well as the collisionality parameter. This is transformed to the known expressions in the corresponding asymptotic strongly collisional and collisionless limits. Nonlocal linear transport equations for small perturbations are also formulated for arbitrary relations between the characteristic space, time and collision scales of the plasma. [ABSTRACT FROM AUTHOR]

The return current instability of ion acoustic waves in a laser heated plasma is studied by means of a collisional particle-in-cell code and theoretical analysis in the regime of nonlocal heat transport. The physical scenario of localized, inverse Bremsstrahlung heating in a single laser hot spot, electron thermal transport, return current of cold electrons, instability of ion acoustic waves, and resulting ion acoustic turbulence are examined in a self-consistent kinetic collisional particle simulation. The observed growth of the return current instability is in excellent agreement with predictions of a linear, nonlocal theory. Ion acoustic fluctuations contribute to the inhibition of thermal transport, which leads to the enhancement of the electron temperature in the center of a hot spot. Increased electron collisionality and hot ion tail production are the dominant saturation mechanisms of the return current instability in a one-dimensional geometry. The effects of the ion acoustic turbulence on other interaction processes are also discussed. [ABSTRACT FROM AUTHOR]

The results of complex simulations using PIC-GEANT4 (particle-in-cell and Monte-Carlo) codes based on the generation of a high-energy electron bunch by a short laser pulse propagating in a relativistic self-trapping regime in a near-critical plasma has been applied to assess the possibility of medical isotope production and nuclear waste transmutation. It has been demonstrated that a 10 Hz 30 fs 4 J laser pulse is well suited to the production of therapeutic amounts of several standard medical radionuclides (111In, 123I, 103Pd, 62Cu, 64Cu). The use of direct electron irradiation has an advantage over the use of bremsstrahlung gamma radiation from the converter due to the simplification of the production scheme without loss of radionuclide yield. The study of the transmutation of long-lived fusion products showed low efficiency and the need for preliminary isotope separation. Achieving as little as 10% reduction in the activity of a 10 g sample requires the continuous operation of the next-generation laser system at a high repetition rate (1 MHzâ€"100 kHz) for (one to ten) years. [ABSTRACT FROM AUTHOR]

The concept of maximizing the D-D fusion neutron yield from the laser-heated large volume of cluster medium by matching the focal spot size and cluster plasma structural scales to the laser pulse intensity was confirmed. For this purpose, the three-dimensional particle-in-cell GEANT4 simulations have been performed by zoning of the large interaction domain. While considering a small domain of the entire interaction volume, which is partitioned into successive zones along laser propagation direction, a special algorithm was proposed allowing to reconstruct the integral spectrum of deuterons and D-D neutron yield. We demonstrate that it makes possible to specify high-performance laser–cluster neutron source following this concept. For example, for the submicron heavy water droplets heated by femtosecond laser pulse of the intensity 3 × 10 19 W/cm2 a D-D neutron yield may reach 10 7 neutrons per 1 J of deposited laser energy if the intensity contrast ratio prevents premature cluster destruction. Such yield is considerably higher than achieved to date for microstructured targets. [ABSTRACT FROM AUTHOR]

The interaction of an ultrashort (~10 fs) laser pulse with a relativistic intensity (≳1018 W/cm2) with a cluster medium characterized by a random distribution of large, submicron clusters of heavy atoms has been studied. Matching conditions for the parameters of the cluster medium and the laser pulse have been determined under which the yield of hot electrons is maximal at a given laser energy. The stochastic dynamics of laser-heated electrons in the Coulomb fields of clusters after the passage of the pulse determines the formation of a plateau with signatures of quasimonochromaticity in the energy spectrum of electrons in the ponderomotive energy range, which is important for the generation of the secondary electromagnetic radiation. [ABSTRACT FROM AUTHOR]

We have developed an analytic theory of the nonlinear resonant absorption of electromagnetic radiation in an inhomogeneous plasma with account for the relativistic nonlinearity of the plasma field being excited near the critical density of the plasma, which considerably shifts the boundary of the theoretical description of resonant absorption by the laser plasma to the region of high laser radiation intensities. The nonlinear resonant absorption coefficient as a function of four laser-plasma control parameters (laser radiation intensity, plasma inhomogeneity scale, plasma temperature, and the angle of incidence of laser radiation on the plasma) is obtained. We have demonstrated the nonlinear effect of suppression of the plasma resonance field amplitude and "switching-off" of the resonant absorption with increasing laser radiation intensity, which is a consequence of the relativistic nonlinearity of the electron plasma in the region of a critical density. [ABSTRACT FROM AUTHOR]

We develop a theoretical basis for a new method of high-intensity ultrashort laser pulse diagnostics via vacuum electron acceleration from an ultrathin foil. A laser pulse is focused by an off-axis parabolic mirror, which has practical interest for most experiments with high-intensity pulses. The field description is based on Stratton-Chu integrals, which allow covering all focusing ranges up to the diffraction limit where the six-component laser field is correctly described. The theoretical approach uses a test particle method applicable for quite thin foils and rarefied gases, where the plasma fields do not substantially affect electron acceleration. The diagnostic method is to use measurements of angular-spectrum characteristics of laser accelerated or scattered electrons to compare them with the theory developed here. The proposed method can diagnose not only the intensity but also the quality of a laser pulse. [ABSTRACT FROM AUTHOR]

Very efficient generation of a high-charge electron beam by a laser pulse propagating in a self-trapping mode in near-critical density plasma makes it possible to produce a high yield of gamma rays for radiography of samples located deep in a dense medium. The three-dimensional particle-in-cell and Monte Carlo simulations performed with end-to-end modeling from laser–plasma interaction to the final gamma-imaging of deeply shielded objects located at distances up to several meters clearly demonstrate the promise of laser pulses of several hundred TW for single-shot radiography by using a high-performance scheme of electron acceleration in the laser pulse self-trapping regime. This is illustrated by two examples with the same laser–target design used for a bremsstrahlung gamma source and an all-optical nonlinear inverse Compton source. [ABSTRACT FROM AUTHOR]

The relativistic self-trapping of a laser pulse is an efficient mechanism for the acceleration of electrons, which allows one to achieve an extreme charge of a high-energy particle beam and the corresponding conversion coefficient of laser energy. It has been shown that the compression of the femtosecond laser pulse in this regime using the innovative compression after compressor approach (CafCA) [E.A. Khazanov, S.Yu. Mironov, and G. Mourou, Phys. Usp. 62, 1096 (2019)] to extremely short durations keeping the energy of the laser beam significantly increases the efficiency of particle acceleration. This effect has been illustrated on the example of the Multitera laser facility for the project implemented at the Russian National Center for Physics and Mathematics. [ABSTRACT FROM AUTHOR]

We present a theory on harmonic generation by relativistic plasma resonance mechanism in an inhomogeneous laser plasma. We find a transverse component of the resonance–enhanced electric field and electron velocities near the plasma critical density and calculate the nonlinear current as the source of the harmonic generation in vacuum. We obtain the power-law spectra of the radiation field and discuss their characteristics depending on the laser-plasma parameters. [ABSTRACT FROM AUTHOR]

Low adiabat dynamics is necessary for efficient compression and achievement of ignition conditions in a laser fusion targets. In this case, any additional sources of target interior heating are undesirable. Parametric laser-plasma instabilities can lead to the generation of a noticeable amount of hot electrons with energies of tens to hundreds of keV, that could penetrate into the target before the front shock arrives. The paper presents a hydrodynamics consistent model for generation and propagation of such electrons. Our simulations show that up to ~2% of the laser pulse energy can go into hot electrons with a temperature of about 100 keV, which appear as a result of the two-plasmon decay instability development. In this case, despite the relatively small part of absorbed by the target electrons (the value depends on the angular distribution of hot electrons and varies in the range of 3–15%), a noticeable decrease in the neutron yield occurs. In a situation when parameters of plasma in the target are close to the ignition threshold, the effects associated with the generation of hot electrons can violate the ignition conditions. [ABSTRACT FROM AUTHOR]

Two methods of laser acceleration of protons/deuterons for initiation of nuclear reactions in the so-called pitcher-catcher scheme aimed at production of medical isotopes and obtaining short neutron pulses are studied using the combined particle-in-cell (PIC) and Monte Carlo (GEANT4) numerical simulation. It is demonstrated that the pulse repetition frequency on the order of 100 Hz is required for obtaining a medical dose of the radionuclids used in the positron-emission tomography (PET) in the case of ion acceleration from a thin foil most efficient in the forward direction by means of a femtosecond laser pulse with energy of several joules. In the case of ion acceleration from a low-density target most efficient in the transverse (radial) direction, lasers pulses of substantially higher energy and picosecond pulse duration have to be used, which allows one to increase the yield of radionuclids per unit deposited energy. In the process, the pulse repetition frequency must be at the level of 1–10 Hz. Laser generation of neutrons by means of reactions initiated by accelerated ions obtained using thin foils or targets of lower density is simulated, and the possibility of obtaining 108 neutrons per laser pulse using 3-J laser pulses is demonstrated. [ABSTRACT FROM AUTHOR]

High gradients of electron temperature appear in plasma corona under direct laser irradiation of inertial confinement fusion targets. This results in nonlocality of heat transport. Such effect influence the efficiency of laser absorption, redistribute heat fluxes and could preheat plasma ahead the front shock wave, therefore alter the compression adiabat. Ignition requires a specially tuned compression dynamics, so such an effect should be taken into account. Target simulations with nonlocal models show the decrease of compression efficiency and hot-spot parameters degradation compared to local models: Spitzer–Härm model with and without flux-limiter. [ABSTRACT FROM AUTHOR]

We study the laser-driven generation of thermonuclear neutrons from targets with a microstructured surface in the form of deuterated microwires, using three-dimensional numerical simulation with previously obtained results of large-scale structural optimization of the target, which provides its best heating by femtosecond laser pulses of moderate intensity. We show that, for modern laser technologies, femtosecond lasers of low (several mJ) energy are even better for creating a neutron source than more powerful (∼1 J) lasers because the mode of high (∼1 kHz) pulse repetition rate is practically available. Microlayers (relief) and cylindrical microholes on the irradiated side are considered as alternative microstructured targets. For the latter, we demonstrate accumulation of ions on the axis of the holes, which leads to increase in the ion density above the initial value and consequently to a possible increase in the yield of thermonuclear neutrons. [ABSTRACT FROM AUTHOR]

An Erratum to this paper has been published: https://doi.org/10.1134/S0021364022390023 [ABSTRACT FROM AUTHOR]

The Maxwell equations-based 3D-analytical solution for the terahertz (THz) half-cycle electromagnetic wave transition radiation pulse has been found. This solution describes generation and propagation of transition radiation into free space from laser-produced relativistic electron bunch which crosses a target-vacuum interface as a result of ultrashort laser pulse interaction with a thin high-conductivity target. The analytical solution found complements the theory of laser initiated transition radiation. It describes the THz wave half-cycle pulse at the arbitrary distance from a target surface including near-field zone rather than its standard far-field characterization. The analytical research has also been supplemented with the 3D simulations using the finite-difference time-domain method, which makes it possible for description of much wider spatial domain as compared to that from the particle-in-cell approach. The presented result sheds light fundamentally on the interference of the electron bunch field and the generated THz field of broadband transition radiation from laser-plasma interaction. The latter is studied for a long time in the experiments with solid density plasma and the theory developed may inspire to targeted measurements and investigations of unique super intense half-cycle THz radiation waves near the laser target.

In a dense gas plasma a short laser pulse propagates in a relativistic self-trapping mode, which enables the effective conversion of laser energy to the accelerated electrons. This regime sustains effective loading which maximizes the total charge of the accelerating electrons, that provides a large amount of betatron radiation. The three-dimensional particle-in-cell simulations demonstrate how such a regime triggers x-ray generation with 0.1-1 MeV photon energies, low divergence, and high brightness. It is shown that a 135-TW laser can be used to produce 3×10^{10} photons of >10 keV energy and a 1.2-PW laser makes it possible generating about 10^{12} photons in the same energy range. The laser-to-gamma energy conversion efficiency is up to 10^{-4} for the high-energy photons, ∼100 keV, while the conversion efficiency to the entire keV-range x rays is estimated to be a few tenths of a percent.

The use of gas cluster media as a target for an intense femtosecond laser pulses is considered to be uniquely convenient approach for the development of a compact versatile pulsed source of ionizing radiation. Also, one may consider cluster media as a nanolab to investigate fundamental issues of intense optical fields interaction with sub-wavelength scale structures. However, conventional diagnostic methods fail to register highly charged ion states from a cluster plasma because of strong recombination in the ambient gas. In the paper we introduce high-resolution X-ray spectroscopy method allowing to study energy spectra of highly charged ions created in the area of most intense laser radiation. The emission of CO 2 clusters were analyzed in experiments with 60 fs 780 nm laser pulses of 10 18  W/cm 2 intensity. Theory and according X-ray spectra modeling allows to reveal the energy spectra and yield of highly charged oxygen ions. It was found that while the laser of fundamental frequency creates commonly expected monotonic ion energy spectrum, frequency doubled laser radiation initiates energy spectra featuring of distinctive quasi-monoenergetic peaks. The later would provide definite advantage in further development of laser-plasma based compact ion accelerators.