A programming library was developed, based on Stratton-Chu diffraction integrals for calculating reflected optical fields. Dipole-type focusing schemes with a tunable number of beams and mirror placements were studied, considering the influence of phase distortion and aberrations. The intensity above 3×1026 W/c m 2 was found theoretically attainable in a system of 12 beams of 50 PW each, with about 90% of that value realistically achievable.

Context. Numerical studies as well as scaled laboratory experiments suggest that bipolar outflows arising from young stellar objects (YSOs) could be collimated into narrow and stable jets as a result of their interaction with a poloidal magnetic field. However, this magnetic collimation mechanism was demonstrated only for the simplified topology of the uniform poloidal magnetic field. Aims. We have extended the experimental studies to the case of a plasma outflow expanding in a region of strong poloidal magnetic field and then propagating through divergent magnetic field lines. In this case the magnetic field distribution is closer to the hourglass magnetic field distribution expected near YSOs. Our aim was to find out whether (and under what conditions) magnetic collimation is possible in such a strongly nonuniform B-field configuration. Methods. The experiments were carried out on the PEARL high-power laser facility. The laser produced plasma outflow was embedded in a strong (~10 T ) magnetic field generated by our unique magnetic system. The morphology and dynamics of the plasma were diagnosed with a Mach-Zehnder interferometer. Results. Laboratory experiments and 3D numerical modeling allow us to reveal the various stages of plasma jet formation in a divergent poloidal magnetic field. The results show (i) that there is a fundamental possibility for magnetic collimation of a plasma outflow in a divergent magnetic field; (ii) that there is good scalability of astrophysical and laboratory flows; (iii) that the conditions for the formation of a magnetic nozzle, hence collimation by poloidal magnetic field, have been met; and (iv) that the propagation of the jet proceeds unimpeded through the region of weak and strongly divergent magnetic fields, maintaining a high aspect ratio. Conclusions. Since we have verified that the laboratory plasma scales favorably to YSO jets and outflows, our laboratory modeling hints at the possibility of the YSO jet collimation in a divergent poloidal magnetic field.

We consider a possibility of generating mid-IR radiation due to the Doppler effect upon reflection of relativistically intense radiation produced by the planned XCELS laser facility from a near-critical density plasma. It is shown that the highest efficiency on the order of tenths of a percent is observed at the shortest possible duration of the incident laser pulse and its ultratight focusing into a spot with a diameter of less than 2 μm. The use of two pulses allows the efficiency to be increased by 1.5–2 times. Thus, it is possible to generate radiation pulses in the region above 3 μm with an energy of several joules.

We study generation of Čerenkov radiation by an ultrashort laser pulse propagating in the magnetized plasma across an external magnetic field basing on the analysis of the dispersion relation and numerical simulation in the case where the cyclotron frequency exceeds the plasma frequency. It is shown that the radiation with a frequency in the range from the plasma frequency to the cyclotron one and with an angle between the wavevector and propagation direction below 45° can escape the plasma. The radiation power increases with the magnetic field before reaching the maximum at the cyclotron frequency being of the order of the inverse of the laser pulse duration and starts decreasing afterwards. It is also observed that, with an increase in the field, the pattern of the radiation escaping from the plasma narrows, the generated pulse is shortened, and the pulse spectrum widens.

It is shown that when relativistically intense ultrashort laser pulses are reflected from the boundary of a plasma with a near-critical density, the Doppler frequency shift leads to generation of intense radiation in both the high-frequency (up to the x-ray) and low-frequency (mid-infrared) ranges. The efficiency of energy conversion into the wavelength range above 3 µm can reach several percent, which makes it possible to obtain relativistically intense pulses in the mid-infrared range. These pulses are synchronized with high harmonics in the ultraviolet and x-ray ranges, which opens up opportunities for high-precision pump–probe measurements, in particular, laser-induced electron diffraction and transient absorption spectroscopy.

The brightness of betatron radiation generated by laser wakefield accelerated electrons can be increased by utilizing the laser driver with shorter duration at the same energy. Such shortening is possible by pulse compression after its nonlinear self-phase modulation in a thin plate. However, this method can lead to a rather complex femtosecond-scale time structure of the pulse. In this work, the results of numerical simulations show that the presence of prepulses containing a few percent of the main pulse energy can significantly alter the acceleration process and lead to either lower or higher energies of accelerated electrons and generated photons, depending on the prepulse parameters. Simultaneously, the presence of a pedestal inhibits the acceleration process lowering the brightness of the betatron source. Furthermore, postpulses following the main pulse are not found to have a significant effect on betatron radiation.

The possibility of creating conditions for observing relativistic magnetic reconnection using two laser pulses of the XCELS facility being designed is analyzed. The necessary conditions are supposed to be created in a scheme, where magnetized plasma flows are generated on the back surface of thin solid targets due to beams of high-energy electrons injected into the depth of the target by the laser pulse fields and forming a strong current on the system symmetry axis. It is shown that in this case it is possible to obtain dense plasma bunches with a relatively low (of the order of several megaelectronvolts) temperature and a frozen-in magnetic field of the order of tens of kilotesla, so that the magnetization parameter will be several units.

The eXawatt Center for Extreme Light Studies project aimed to create a large scientific infrastructure based on lasers with giant peak power. The project relies on the significant progress achieved in the last decade. The planned infrastructure will incorporate a unique light source with a pulse power of 600 PW using optical parametric chirped pulse amplification in large-aperture KD2PO4, deuterated potassium dihydrogen phosphate crystals. The interaction of such laser radiation with matter represents a completely new fundamental physics. The direct study of the space-time structure of vacuums and other unknown phenomena at the frontier of high-energy physics and the physics of superstrong fields will be challenged. Expected applications will include the development of compact particle accelerators, the generation of ultrashort pulses of hard X-ray and gamma radiation for material science enabling one to probe material samples with unprecedented spatial and temporal resolution, the development of new radiation and particle sources, etc. The paper is translation from Russian [Kvantovaya Elektronika 53, 95 (2023)].

The possibility of generating a substance with a high energy density during the deceleration of protons, which are accelerated by the laser-plasma method in a layer of a substance with a high charge, gold, is analyzed for the parameters of laser pulses that are planned to be obtained at the XCELS installation. It is shown that in this case, the formation of a substance is possible with record pressure values of more than 1 Gbar at a solid-state density, which corresponds to a specific energy release of tens of megajoules per gram. A specific feature of the deceleration process with such a high specific energy release is that the properties of the decelerating substance change during interaction. It is demonstrated that the ionization multiplicity of gold ions reaches 40–50, and the temperature of the resulting plasma is 1 keV. This leads to a decrease in the slowing-down power of the substance and to a modified curve of the specific energy release.

We numerically investigate the process of generating magnetic fields from temperature anisotropy of electrons in collisionless initially uniform plasmas. We use a fully kinetic modeling and compare it against a hybrid modeling which treats ions kinetically and use ten-moment fluid model for electrons. The results of the one-to-one comparison show a good agreement in terms of the maximal magnitude of the self-generated magnetic field and similar trends during the non-linear stage of the instability. Additionally, we performed hybrid modelling of the instability without resolving electron spatial scales. In this case the results are only qualitatively the same however it shows that hydrodynamical approach can be used to some extent for the simulation of the Weibel instability in large-scale systems, including astrophysical environments and laser-produced plasmas.

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was investigated experimentally and explained theoretically. We found that emitted photons are polarized predominantly perpendicular to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. Comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate a momentum isotropization time for electrons in graphene to be ~25 fs and roughly reconstruct the distribution function evolution in k-space.

Context. The equatorial accretion scenario, caused by the development of the Rayleigh-Taylor (RT) instability at the disk edge, was suggested by accurate three-dimensional magnetohydrodynamic (MHD) modelling, but no observational or experimental confirmation of such phenomena has been evidenced yet. Aims. We studied the propagation of a laterally extended laser-generated plasma stream across a magnetic field and investigated if this kind of structure can be scaled to the case of equatorial 'tongue' accretion channels in young stellar objects (YSOs); if so, this would support the possibility of equatorial accretion in young accreting stars. Methods. We conducted a scaled laboratory experiment at the PEARL laser facility. The experiment consists in an optical laser pulse that is focused onto the surface of a Teflon target. The irradiation of the target leads to the expansion of a hot plasma stream into the vacuum, perpendicularly to an externally applied magnetic field. We used a Mach-Zehnder interferometer to diagnose the plasma stream propagation along two axes, to obtain the three-dimensional distribution of the plasma stream. Results. The laboratory experiment shows the propagation of a laterally extended laser-generated plasma stream across a magnetic field. We demonstrate that: (i) such a stream is subject to the development of the RT instability, and (ii) the stream, decomposed into tongues, is able to efficiently propagate perpendicular to the magnetic field. Based on numerical simulations, we show that the origin of the development of the instability in the laboratory is similar to that observed in MHD models of equatorial tongue accretion in YSOs. Conclusions. As we verify that the laboratory plasma scales favourably to accretion inflows of YSOs, our laboratory results support the argument in favour of the possibility of the RT-instability-caused equatorial tongue accretion scenario in the astrophysical case.

A scheme to generate magnetized relativistic plasmas in a laboratory setting is proposed. It is based on the interaction of ultra-high-intensity sub-picosecond laser pulses with few-micron-thick foils or films. By means of Particle-In-Cell simulations, it is shown that energetic electrons produced by the laser and evacuated at the rear of the target trigger an expansion of the target, building up a strong azimuthal magnetic field. It is shown that in the expanding plasma sheath, a ratio of the magnetic pressure and the electron rest-mass energy density exceeds unity, whereas the plasma pressure is lower than the magnetic pressure and the electron gyroradius is lower than the plasma dimension. This scheme can be utilized to study astrophysical extreme phenomena such as relativistic magnetic reconnection in laboratory.

We investigate the time evolution of the six-component electron pressure tensor in a hybrid code studying consequences for the two-dimensional reconnection process in an initially perturbed Harris sheet. We put forward that two tensor components (a diagonal and a non-diagonal one) grow in an unstable way unless an isotropization operator is considered. This isotropization term is physically associated with an electron heat flux. As a consequence, we put forward that an enhanced value of a diagonal component is observed in the very middle of field reversal at sub-ion scale. Because of the increase of the kinetic pressure, the magnetic field is decreased in this electron layer, hence increasing the associated out-of-plane current at its edges and leading to its bifurcation. The bifurcation mechanism is based on the presence of electron pressure anisotropy, related to the gradient of inflow electron bulk velocity. The gradient in the inflow direction of the enhanced diagonal electron pressure tensor component results in the deceleration of the ions entering the X-point region. We suggest that bifurcated current sheets resulting from the anisotropies/agyrotropies of the six-component electron pressure tensor correspond to smaller reconnection rates comparing to non-bifurcated ones.

We show that a hole-boring radiation pressure acceleration is unstable in near-criticaldensity plasmas and that a threshold electron density increases with an ion mass-to-charge ratio.

We consider the effect of ion mobility on the threshold of relativistic self-induced transparency under irradiation of thin plasma layers by circularly polarised laser radiation. An analytical model of the motion of ions during the removal of electrons from the layer surface by the ponderomotive force of laser radiation is constructed. The model is used to analyse the motion of probe electrons in the resulting electromagnetic field. It is shown that the higher the ion mobility and the longer the laser pulse, the more stable the plasma layer to longitudinal perturbations of a finite value and the higher the threshold of self-induced transparency. These conclusions are verified by one-dimensional numerical simulation of a complete system of kinetic equations for plasma and Maxwell's equations.

We show that in back-reflection of an ultra-intense laser radiation from a near-critical density plasma with a sharp edge a generation of few-cycles mid-infrared pulses with relativistic intensities is feasible.

Recent laser-produced plasmas experiments open up new opportunities for the so-called laboratory astrophysics, allowing observation and studying a number of fundamental physical processes relevant to magnetized plasmas, such as thermo-magnetic instabilities leading to magnetic field generation, magnetic reconnection, collisionless shocks. In order to supplement those experiments with full-scale numerical simulations we develop a code AKA52 (Arbitrary-Kinetic-Algorithm) implementing a hybrid model that includes the dynamics of magnetic fields: advection by the ion flow and Hall effect, magnetic field generation by the Biermann battery effect and Weibel instability. The fully-parallelized high-performance hybrid algorithm includes Particle-in-Cell (PIC) formalism for ions and a 10-moment fluid model for electrons that are described by density, bulk velocity and the six-component pressure tensor evolution equation. Laser-plasma interaction is simulated by means of an ablation operator which imitates laser ionization and heating at critical density surface. As an example, we chose a problem of plasma expansion in the externally applied magnetic field perpendicular to the flow that is related to a number of recent laser-plasma experiments.

We show that almost isochoric heating to high temperatures (300 eV) of $\mu $m-thick solid-density foil can be achieved in a compact and efficient manner using an ultrahigh contrast, high-power, ultra-short duration laser.