The interaction of multipetawatt lasers with plasma is a complex multiparameter problem, providing a wide field for fundamental research and opening up great opportunities for creating unique sources of high-energy electrons and positrons, dense pair plasma, and gamma-photons. However, to achieve the high efficiency of such a source, it is necessary to use targets with optimized parameters, primarily density and size, for the given laser parameters. With the use of 3D QED-PIC modeling, it is shown that, when targets whose size is comparable with the laser wavelength are irradiated by laser beams with a total power of several tens of PW, the total initial number of target electrons may be regarded as the similarity parameter. In practice, this can significantly simplify the selection of the targets needed for controlling the interaction and, accordingly, for achieving the specified parameters of the developed electron-positron plasma and gamma-photon sources. Based on the similarity parameter, various laser-plasma interaction modes are identified, the necessary conditions for their launch are determined, and the properties of the pair particle and gamma-photon source are revealed. Moreover, qualitative estimates of the quantitative and energy characteristics of such a source are obtained, allowing it to be optimized for various laser beam configurations. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).

Numerical simulation of strong-field quantum electrodynamics (SFQED) processes is an essential step towards current and future high-intensity laser experiments. The complexity of SFQED phenomena and their stochastic nature make them extremely computationally challenging, requiring the use of supercomputers for realistic simulations. Recently, we have presented a novel approach to numerical simulation of SFQED processes based on an accurate approximation of precomputed rates, which minimizes the number of rate calculations per QED event. The current paper is focused on the high-performance implementation of this method, including vectorization of resource-intensive kernels and improvement of parallel computing efficiency. Using two codes, PICADOR and Hi-. (the latter being free and publicly available), we demonstrate significant reduction in computation time due to these improvements. We hope that the proposed approach can be applied in other codes for the numerical simulation of SFQED processes.

In this work we investigate vacuum breakdown in multipetawatt laser fields of the multibeam configuration corresponding to a converging magnetic dipole wave. Three-dimensional numerical simulation via the particle-in-cell method was used to determine the threshold total laser radiation power required for vacuum breakdown depending on the number of laser beams. It is shown that the minimal vacuum breakdown threshold power of 14 PW is attained when there are 12 beams while for a lower number of beams (from two to six) the threshold increases to 18 PW. The dependence of the growth rate of the quantum-electrodynamic cascade on the number of laser beams is determined in a wide range of total radiation power. Fluctuations in the power of each of the beams are shown to have little effect on both the breakdown threshold and the cascade growth rate.

We found experimentally that Cherenkov-type terahertz radiation produced by optical rectification of ultrashort laser pulses in LiNbO3 can experience strong spectral broadening in the regime of multiphoton laser absorption. The broadening is attributed to the terahertz emission from a surge current of the optically generated carriers. The effect can be used to improve the bandwidth of optical-to-terahertz converters based on optical rectification.

Probabilistic generation of photons and electron-positron pairs due to the processes of strong-field quantum electrodynamics (SFQED) is often the most resource-intensive part of the kinetic simulations required in order to model current and future experimental studies at high-intensity laser facilities. To reduce its computational demands one can exploit tabulation of the precomputed rates, time-step sub-cycling, dynamic down-sampling of particle/photon ensembles and other approaches. As the culmination of previous improvements, the method described here provides the opportunity to make the minimal possible number of rate computations per QED event and, therefore, this method can increase performance by more than an order of magnitude. The computational routine is publicly available as a part of the open-source framework hi-$\chi$ designed as a Python-controlled toolbox for collaborative development.

The possibility of developing an ultra-high-brightness photon source with energies up to several GeV based on promising multipetawatt multibeam laser systems have been studied. For a maximum photon energy to be reached and for a directed photon beam to be generated, it is proposed to use an electric dipole field structure as the main configuration, which maximizes the amplitude of the electric field in the focus region. Dependences of the characteristics of the generated gamma radiation on the number of laser beams irradiating the plasma target and their focusing are investigated. It is shown that for the laser system parameters close to the stated XCELS facility parameters and a total power of 36 PW, the maximum energy of gamma photons can reach 2 GeV, the efficiency of conversion of laser radiation into photons with an energy of more than 1 MeV is 40

Currently, machine learning (ML) methods are widely used to process the results of physical experiments. In some cases, due to the limited amount of real experimental data, ML-models can be pre-trained on synthetic data simulated based on the analytical theory and then fine-tuned using real experimental data. A limitation of this approach is the presence of the latent parameters of the analytical model, which values are difficult or impossible to estimate. Setting these parameters incorrectly may induce a dataset shift even when applied to synthetic data. To overcome this problem, we train the ML-model on a dataset with randomly varied latent parameters of the analythical model to force the ML-model to concentrate on more general patterns that depend weakly on the latent parameters. We applied this approach to the problem of tight focusing of a laser pulse with the complex structure of the wavefront. We observed good accuracy of reconstructing of the tilt parameters when training and testing the ML-model on datasets generated for different values of the latent parameters. This confirms that the ML-model was able to select relevant information without over-fitting for specific features inherent in certain values of the latent parameters. We believe that this approach will enrich possible applications of ML-methods to an experimental diagnostics of laser pulses.

Free carrier generation is commonly viewed as detrimental to the optical-to-terahertz conversion efficiency in electro-optic crystals due to free carrier absorption of terahertz radiation. Contrary to this view, we show by numerical simulation that free carrier generation can have a positive effect on the Cherenkov-type terahertz emission from ultrashort laser pulses by strongly broadening the radiation bandwidth. The broadening is produced by terahertz emission from a surge current of the optically generated carriers, which are accelerated by the rectified electric field. Specific results are obtained for GaP pumped at a 1.7-mu m wavelength.

The article deals with the problem of taking into account radiation losses in the description of the dynamics of a charged particle in electromagnetic fields. The solution of this fundamental problem is becoming more and more in demand due to the development of ultrahigh-power lasers. Since the generation of radiation by a charged particle depends both on the magnitude of the fields in which it moves and on their structure, the developed multipetawatt multibeam laser systems, such as XCELS, can open up unique ways to solve this problem. This paper proposes one of such ways based on using a magnetic dipole field configuration formed by several laser pulses. We show by means of numerical simulations that when a solid target in the form of a nanowire is irradiated with several petawatt laser pulses that maximize the magnetic field in focus, there may appear an anomalous radiative trapping regime. The formed distributions of accelerated electrons and generated gamma photons in this regime have distinctive properties on the basis of which it is possible to experimentally determine the applicability of various theoretical approaches to the description of radiation losses.

It is proposed to use multipetawatt multibeam systems that reproduce dipole waves of electric configuration to maximize the electric field in the interaction region and the energy of accelerated particles. The breakdown of vacuum in fields of this configuration leads to the formation of beams of electrons and positrons accelerated to several GeV. It is shown that when use is made of 12 laser pulses with a duration of 30 fs and a total power of 36 PW, the total charge of accelerated electrons/positrons can reach 250 to 270 nC and the charge of particles with energies above 1 GeV can exceed 1 nC. It is demonstrated that the electric dipole structure of the fields makes it possible to obtain an extremely narrow (several mrad) distribution of particles over the polar angle measured from the field symmetry axis. It is also shown that the distribution of particles over the azimuthal angle can be used to diagnose interaction modes during vacuum breakdown. The obtained results can be useful in preparing experiments on the promising XCELS laser system.

We explore, both theoretically and experimentally, the potential of semiconductor materials for the Cherenkov scheme of terahertz generation on the example of GaP crystal pumped by femtosecond laser pulses of 1.54 μm wavelength. We use a convenient scheme with a focused-to-a-line laser beam which is introduced into the crystal by oblique incidence at the Brewster angle on the crystal front face and propagates in the crystal at the Cherenkov angle to its normal. A half of the generated Cherenkov wedge impinges normally the rear face of the crystal and generates an output terahertz beam with a plane wavefront. The whole generation process is simulated by FDTD method. In experiments, the laser pulses of 140 fs duration and 10 μJ energy were converted to wideband (~2.5 THz bandwidth) terahertz radiation with the efficiency of ~3 × 10−5, which exceeds the efficiency of the standard collinear scheme by at least an order of magnitude.

In studies of interaction of matter with laser fields of extreme intensity there are two limiting cases of a multibeam setup maximizing either the electric field or the magnetic field. In this work attention is paid to the optimal configuration of laser beams in the form of an m-dipole wave, which maximizes the magnetic field. We consider in such highly inhomogeneous fields the advantages and specific features of laser-matter interaction, which stem from individual particle trajectories that are strongly affected by gamma photon emission. It is shown that in this field mode qualitatively different scenarios of particle dynamics take place in comparison with the mode that maximizes the electric field. A detailed map of possible regimes of particle motion (ponderomotive trapping, normal radiative trapping, radial, and axial anomalous radiative trapping), as well as angular and energy distributions of particles and gamma photons, is obtained in a wide range of laser powers up to 300 PW, and it reveals signatures of radiation losses experimentally detectable even with subpetawatt lasers.

It was recently predicted [Phys. Rev. A95(6), 063817 (2017) 10.1103/PhysRevA.95.063817] that an intense femtosecond laser pulse propagating in an electro-optic crystal and producing free carriers via multiphoton ionization can generate a unipolar electromagnetic precursor propagating ahead of the laser pulse. Here we report the experimental observation of this phenomenon in a GaP crystal excited by an amplified Ti:sapphire laser.

The vacuum breakdown by 10-PW-class lasers is studied in the optimal configuration of laser beams in the form of an m-dipole wave, which maximizes the magnetic field. Using 3D PIC simulations we calculated the threshold of vacuum breakdown, which is about 10 PW. We examined in detail the dynamics of particles and identified particle trajectories which contribute the most to vacuum breakdown in such highly inhomogeneous fields. We analyzed the dynamics of the electron-positron plasma distribution on the avalanche stage. It is shown that the forming plasma structures represent concentric toroidal layers and the interplay between particle ensembles from different spatial regions favors vacuum breakdown. Based on the angular distribution of charged particles and gamma photons a way to experimentally identify the process of vacuum breakdown is proposed.

Particles moving in current sheets under extreme conditions, such as those in the vicinity of pulsars or those predicted on upcoming multipetawatt laser facilities, may be subject to significant radiation losses. We present an analysis of particle motion in model fields of a relativistic neutral electron-positron current sheet in the case when radiative effects must be accounted for. In the Landau-Lifshitz radiation reaction force model, when quantum effects are negligible, an analytical solution for particle trajectories is derived. Based on this solution, for the case when quantum effects are significant an averaged quantum solution in the semiclassical approach is obtained. The applicability region of the solutions is determined and analytical trajectories are found to be in good agreement with those of numerical simulations which account for radiative effects. Based on these results we demonstrate that radiation reaction itself can provide a mechanism of pinching even within a given field consideration.

The power of machine learning (ML) in feature identification can be harnessed for determining quantities in experiments that are difficult to measure directly. However, if an ML model is trained on simulated data, rather than experimental results, the differences between the two can pose an obstacle to reliable data extraction. Here we report on the development of ML-based diagnostics for experiments on high-intensity laser–matter interactions. With the intention to accentuate robust, physics-governed features, the presence of which is tolerant to such differences, we test the application of principal component analysis, data augmentation and training with data that has superimposed noise of gradually increasing amplitude. Using synthetic data of simulated experiments, we identify that the approach based on the noise of increasing amplitude yields the most accurate ML models and thus is likely to be useful in similar projects on ML-based diagnostics.

When studying the interaction of matter with extreme fields using multipetawatt lasers, there are two limiting cases maximizing either the electric field or the magnetic field. Here, the main attention is paid to the optimal configuration of laser beams in the form of an m-dipole wave, which maximizes the magnetic field, and the corresponding production of pair plasma via a QED cascade using 10-PW class lasers. We show that the threshold of vacuum breakdown with respect to avalanche-like pair generation is about 10 PW. Using 3D PIC modeling in the specified fields, we go deeper into the physics of vacuum breakdown, i.e. we examined in detail the individual trajectories of particles produced in inhomogeneous electric and magnetic fields, the space-time distributions of pair densities on the avalanche stage, and the energy distributions of charged particles and gamma photons. The forming plasma structures represent concentric rings around the central magnetic axis, which can result in significant change of laser-plasma interaction in comparison with the case of an e-dipole wave.

We present an analysis of particle motion in fields of a relativistic neutral electron-positron current sheet in the case when radiative effects must be accounted for. In the Landau-Lifshitz radiation reaction force model, when quantum effects are negligible, a high-precision analytical solution for particle trajectories is derived. Based on this solution, for the case when quantum effects are significant an averaged quantum solution in the semiclassical approach is obtained. The applicability region of the solutions is determined and analytical trajectories are found to be in good agreement with those of numerical simulations with account for radiative effects.

In particle-in-cell simulations, excessive or even unfeasible computational demands can be caused by the growth of the number of particles in the course of prolific ionization or cascaded pair production due to the effects of quantum electrodynamics. Here we discuss how one can organize a dynamic rearrangement of the ensemble to reduce the number of macroparticles, while maintaining acceptable sampling of an arbitrary particle distribution. The approaches of merging and thinning as well as their variants are discussed and the aspects of use are considered.

When a pulsed, few-cycle electromagnetic wave is focused by optics with f-number smaller than two, the frequency components it contains are focused to different regions of space, building up a complex electromagnetic field structure. Accurate numerical computation of this structure is essential for many applications such as the analysis, diagnostics, and control of high-intensity laser-matter interactions. However, straightforward use of finite-difference methods can impose unacceptably high demands on computational resources, owing to the necessity of resolving far-field and near-field zones at sufficiently high resolution to overcome numerical dispersion effects. Here, we present a procedure for fast computation of tight focusing by mapping a spherically curved far-field region to periodic space, where the field can be advanced by a dispersion-free spectral solver. In many cases of interest, the mapping reduces both run time and memory requirements by a factor of order 10, making it possible to carry out simulations on a desktop machine or a single node of a supercomputer. We provide an open-source C++ implementation with Python bindings and demonstrate its use for a desktop machine, where the routine provides the opportunity to use the resolution sufficient for handling the pulses with spectra spanning over several octaves. The described approach can facilitate the stability analysis of theoretical proposals, the studies based on statistical inferences, as well as the overall development and analysis of experiments with tightly-focused short laser pulses.