Compact sources of high-energy particles and secondary radiation based on the interaction of high-power laser pulses with plasma targets are of interest due to a possibility of their application in a wide range of practical problems. One of the ways to optimize such sources is related to the possibility of controlling the preplasma of a solid target by changing the density gradient of the expanding plasma plume on the irradiated side. The problem of accelerating charged particles by a short laser pulse from targets with a preplasma generated by a nanosecond prepulse (additional pulse) of different intensity and duration is considered. Using self-consistent hydrodynamic/kinetic simulation, the efficiency of electron acceleration is shown to depend not only on the gradient of the preplasma density profile, but also on the focusing point of the laser pulse on this profile.

abstract-The concept of maximizing synchrotron X rays and DD neutron yield from laser-heated large volume of microdroplet target by matching the focal spot size and structural scales of target to the laser pulse intensity was confirmed.

In this work, we study plasma that arises during the interaction of laser radiation with a target. Depending on the parameters of the laser pulse, the resulting plasma can have a significant impact on the efficiency of acceleration of charged particles.

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 3×1019 W/cm2 a D-D neutron yield may reach 107 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.

Femtosecond laser pulse propagation in a relativistic self-trapping (RST) regime in a near-critical density plasma makes it possible to maximize the total charge of the accelerating electrons and laser-to-electrons conversion rate, that can be used to provide a large amount of the terahertz range coherent transition radiation. The three-dimensional particle-in-cell simulations demonstrate how such transition radiation generates when electrons escape into vacuum either from the low-density target itself, or after passing through a thin foil located at the target end. The advantage of the RST regime for the generation of terahertz pulses is clearly demonstrated as compared to laser irradiation of such a standard target as a foil with preplasma on its front side. The simulation performed has shown that for the optimized laser-target matching a 2-J femtosecond laser pulse is able to produce quasiunipolar terahertz pulses with energy exceeding 100 mJ.

The PIC simulations results on generation of high-energy electron bunch by a short laser pulse propagating in strongly nonlinear relativistic self-trapping regime in a near-critical plasma have been addressed to the production of super-intense THz pulse. It has been demonstrated that a 30 fs 4J laser pulse is able to produce quasi-unipolar Thz pulses with energy of about 100 mJ.

The high energy of the XCELS laser allows for obtaining a large number of laser-heated/accelerated particles and products of nuclear reactions initiated by them in a large-volume transparent microstructured medium. As an example, the mode of laser–plasma interaction was studied at a moderately relativistic heating pulse intensity of ~10 18 W/cm 2 in a sufficiently large volume of a microcluster medium, which does not require sharp focusing of a powerful laser beam(s), simplifying the experiment. It has already been shown earlier that, for a laser pulse with an energy of ~1 J, under certain conditions for the geometric and compositional parameters of a deuterium-containing cluster target, it is possible to maximize the yield of hot superponderomotive electrons and explosively accelerated deuterons. In this study, this approach was extended to a femtosecond laser driver with an energy hundreds of times greater (300–400 J). Recommendations were developed for obtaining a record number of laser-heated deuterons of moderate energies (0.2–2 MeV) in a large volume of a cluster medium (heavy water spray) at the level of 10 15 particles per shot and for creating a superbright source of thermonuclear DD neutrons with an expected peak flux of ~10 18 neutrons cm –2 s –1 .

At present, a significant part of experiments on the interaction of high-power short laser pulses with solid targets is faced with the problem of target modification under the action of a nanosecond prepulse. In this paper, a series of hydrodynamic calculations of target irradiation with a nanosecond laser pulse is performed, which describes the plasma density profiles arising during the target expansion as a function of the parameters of the laser prepulse/additional pulse. The results obtained can make it possible to improve the efficiency of ion acceleration in the interaction of short laser pulses with a profiled plasma target.

The effect of a high-power short laser pulse from the Exawatt Center for Extreme Light Studies (XCELS) facility on solid-state metal targets with different constructions and sizes can yield high-power THz pulses with an unprecedentedly high energy. Their further application requires focusing and transport, which calls for the development of corresponding control units. In this context, it is of interest to use targets, which, on the one hand, are elements of the radiation source and, on the other hand, can collimate and transfer the THz radiation energy. A target in the form of a thin wire appears promising for this purpose. The process of THz pulse generation upon interaction between XCELS laser pulses and a metal cylindrical target (microwire) has been numerically simulated. It is shown that THz radiation is generated in a unique form (as a unipolar pulse) and the microwire target allows to concentrate a significant part of the radiation near its surface and transfer it (in the form of a unipolar surface pulse as well) with the speed of light along the wire to large distances with weak damping.

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

Modern XCELS-type laser facilities under development can be a good addition to traditional meson factories, leading to high momentum fluxes of elementary particles produced by irradiation of converter targets with a laser-accelerated electron or proton beam. This is substantiated by the numerical simulation of the generation of high-current charged particle beams under the action of a femtosecond laser pulse from the XCELS [1] facility either on a near-critical density plasma or on a denser (but still low-density) plasma, and the subsequent calculation of the meson yield from the converter target. The end-to-end simulation by the particle-in-cell method (PIC codes) and the Monte Carlo method (GEANT4 code) quantitatively characterizes the generation of pions, mesons, kaons, and even more exotic elementary particles. For example, the number of produced pions for the parameters of the XCELS laser pulse is predicted to be about 10 8 particles per shot. The calculation of the yield of fast neutrons is also performed, the number of which exceeds 10 11 particles.

Super-power sources of THz radiation are of interest in various fields of science and technology, which stimulates corresponding studies on the Exawatt Center for Extreme Light Studies (XCELS) infrastructure. In this context, design-theoretical justification is given for the possibility of achieving record-high parameters of laser-initiated THz pulses, generated upon interaction of an XCELS laser beam with a flat metal target (foil). These pulses are due to transition radiation of electrons leaving the target from the rear side after acceleration by the laser pulse field. This is accompanied by the generation of unique (unipolar) high-intensity THz pulses in vacuum with the power and energy limited only by the power of the initial laser pulse. It is shown theoretically and by numerical simulation how the interaction between a laser pulse of the XCELS facility and a target can provide generation of high-intensity THz pulses with unprecedentedly high power (10–50 TW) and energy (1–5 J), propagating both in the environment and over the conducting target surface.

The XCELS [1] infrastructure is capable of providing a breakthrough in creating a record-breaking high-power source of gamma radiation using laser-accelerated electron beams, which is substantiated by the numerical simulation of the action of a short XCELS laser pulse on low-density targets, and by calculating the bremsstrahlung generated by an electron bunch in a converter target to produce a high-power gamma-ray pulse. The high efficiency of generating a record number of multi-MeV gamma quanta with a huge peak gamma flux is due to the use of relativistic self-trapping of a laser pulse as a driver of such wakefield acceleration of electrons, which ensures the achievement of a maximum charge of electrons accelerated to a multi-MeV level and a maximum conversion efficiency of laser energy in near-critical density targets. The possibility of converting up to 8% of laser energy into the energy of a beam of gamma-ray quanta (with an energy of more than 1 MeV) and the prospects for using the resulting source for deep gamma radiography in a single laser shot are demonstrated. The latter is also substantiated by a numerical experiment on obtaining gamma-ray images of dense hidden objects with a currently record-breaking shielding thickness (up to 400 mm of iron, which corresponds to a linear density of 320 g/cm 2 ) with good contrast (high spatial resolution).

Still unreachable sub-GeV scale laser-accelerated ion energies per nucleon, necessary for a number of practical applications, can be obtained using a new generation ultrashort-pulse laser XCELS. To accelerate ions to such energies, it is proposed to use low-density targets obtained, for example, as a result of pre-irradiation of a solid-state foil with an additional, longer laser pulse. Targets with controlled preplasma on the front side make it possible to significantly increase the efficiency of electron heating and subsequent acceleration of ions by the charge separation field from the rear side of the target. This, in general, classical acceleration mechanism is compared with the recently proposed mechanism of synchronized acceleration of ions by slow light. The PIC simulation of laser acceleration of protons is supplemented by hydrodynamic calculations to find the optimal preplasma profile, which allows high-energy particles to be most efficiently generated. The possibility of generation of a large number of protons with a 1 GeV energy-scale is demonstrated.

The interaction of ultrashort laser pulses of subrel-ativistic intensity with large nanoclusters is studied. The cluster target parameters were optimized based on 3D particle-in-cell simulations (PIC) to find a laser plasma regime that maximizes the number of high-energy deuterons and fusion neutrons. The considered design of the laser target has the potential to be used in a compact short laser driven source of neutrons.

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 ( 111 In, 123 I, 103 Pd, 62 Cu, 64 Cu). 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 MHz–100 kHz) for (one to ten) years.

The interaction of an ultrashort laser pulse of moderately relativistic intensity ( $$ \gtrsim $$ 1018 W/cm2) with cluster plasma of spherical submicrometer droplets of heavy water is studied using three-dimensional numerical simulation. Optimization calculations are performed for determining the irradiated medium parameters providing its best heating by such laser pulses and a maximum yield of fast deuterons initiating DD reactions in microdroplet plasma. The thermonuclear neutron yield is estimated. It is shown that the yield of DD neutrons can reach ~107 neutr./(s sr) due to the practically available mode of a sufficiently high (10 Hz) repetition rate of femtosecond laser pulses of moderate energy (~1 J).

A three-dimensional analytical solution for the near field for a terahertz unipolar pulse based on Maxwell's equations is shown. It is calculated in a transition sample in free space by a laser bunch of relativistic electrons as a result of radiation of a thin foil by a femtosecond laser pulse. The paper shows the theoretical obtaining of unipolar pulses from the Maxwell equations, as well as numerical simulations that confirm the theoretical conclusions. The temporal and spatial characteristics of the system are estimated, and the coefficient of conversion of laser radiation into unipolar terahertz radiation is also obtained.