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.

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.

The dynamics of charged particles in the self-generated quasistatic fields of channels produced by the interaction of an ultrashort relativistically intense laser pulse with near-critical density plasmas has been studied numerically and analytically. We identify distinctive mechanisms of drift, diffusion, and acceleration of electrons in quasi-static fields generated in the laser plasma channel produced by ultrashort intense laser pulses.

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 .

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 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).

Relativistic collisionless shocks are considered responsible for particle energization mechanisms leading to particle acceleration. While electron energization in shock front region of electron/ion collisionless shocks are the most studied, the mechanism of electron energization in interaction with self-generated magnetic vortices (MVs) in the upstream region is still unclear. We investigate electron energization mechanism in the upstream region of electron/ion relativistic collisionless shocks, using two dimensional particle-in-cell (PIC) simulations. We discuss mechanism of electron energization which takes place in the upstream region of the shock, where the counter stream particles interact with incoming flow. The energy gain of electrons happens during their interaction with evolving fields of self-generated magnetic vortices in this region. Three Fermi-like electron energization scenarios are discussed. Stochastic acceleration of electrons in interaction with fields of MV leads to anisotropic heating of fast electrons due to diffusion in the momentum space of electrons and, finally, synergetic effect of evolving fields of MVs leads to the formation of a power-law tail of supra-thermal particles.

We presented the results of a comprehensive simulation of PIC-GEANT4 (particle-in-cell and Monte-Carlo) codes for the generation of high-energy electron or ion bunches by a short laser pulse and their application to assess the possibility of medical isotope production and nuclear waste transmutation.

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.

An increase in the yield of fast neutrons is experimentally demonstrated by exciting a nuclear DD reaction in the interaction of a relativistically intense (over 10(18) W cm(-2)) ultrashort laser pulse with a deuterated low-average-density target volume-structured at the wavelength scale. It is shown that decreasing the average target density from 0.78 to 0.35 g cm(-3) doubles the neutron flux, which reaches 7 x 10(4) particles per I .J of input energy. The effect may be associated with an increase in the number of accelerated deuterium ions due to the three dimensional expansion of individual elements of the target structure.

The interaction of an ultrashort laser pulse of moderate intensity (greater than or similar to 10(18) W/cm(2)) with an innovative high-average-density target consisting of numerous sub-microwires was examined using 3D PIC simulations. We geometrically optimize the target to increase the absorption of laser light and energy (temperature) and the number of hot electrons. To explain the production of super-ponderomotive electrons (with energies significantly exceeding the ponderomotive energy), we use the test particle approach and stochastic dynamics methods to study the electron dynamics in a complicated EM field consisting of incident and reflected laser waves, the Coulomb field of charge separation at wire-vacuum interfaces, and the quasistatic magnetic field. The research demonstrates the stochastic nature of high-energy electron production in such a complicated EM field using Lyapunov exponent analysis. We describe the practical fabrication of such targets in the context of possible experiments. The considered laser-target design has a potential for use in compact laser-based neutron, x-ray sources.

IV International Conference on Ultrafast Optical Science (UltrafastLight-2020) was held on September 28-October 02, 2020 in P.N. Lebedev Physical Institute (LPI) in Moscow, Russia. The mixed (offline and online) Conference format was chosen due to the current quarantine measures and travel restrictions, nonetheless it did not affect the highest quality of the talks given by all the speakers registered at the Conference. The possibility of broadcasting through Zoom drastically increased the number of reports made by foreign participants which enabled more detailed outlook of the present problems and development of the optical science field. IV International Conference on Ultrafast Optical Science aimed at experience exchange between scientists and researchers from different countries and united the specialists presenting even different continents. Succession ensurance and its maintenance being the sufficient part of our mission was reflected in practice by inviting not only scientists, but also students specialized in optics and laser physics. List of Scientific Program Committee, Local organizing committee, Conference Sections, Plenary Speakers, Invited Speakers, Sponsors, are available in this pdf.

The interaction of an ultrashort laser pulse of sub-relativistic intensity with an innovative high-average-density targets consisting of numerous sub-microwires or sheets was examined. The research demonstrates the stochastic nature of high-energy electron production in complicated EM field in such targets where volumetric laser energy absorption occurs. The considered laser-target design has a potential for use in compact laser-based neutron sources and warm dense matter research.

Efficient electron acceleration and heating is demonstrated in a multimode structure created by interference of several laser beams of a relativistic intensity and a picosecond duration near a sharp target boundary. Electron energization proceeds in two steps, with a slow stochastic heating followed by a fast regular acceleration in a resonance interaction with one of wave packets. It results in formation of a population of energetic electrons with an exponential distribution in energy characterized by a high effective temperature and a sharp cutoff. Hot electron characteristics depend on the number of crossing laser beams and their respective angles. This process is an example of efficient electron heating in vacuum by electromagnetic fields without participation of electrostatic plasma waves. It might contribute to creation of a superthermal particle population with an effective temperature significantly exceeding the common ponderomotive scaling.

A novel technique of peak intensity evaluation of tightly focused femtosecond laser pulse is proposed. The method is based on numerical and experimental studies of electrons angular distribution at acceleration in the field of laser radiation interacting with low density (<10 16 cm −3 ) Helium. The possibility of laser pulse peak intensity estimation in each shot in range from 10 18 to 10 20 W/cm 2 is demonstrated.