Electron-positron plasmas are found near the most exotic objects in our Universe, such as black holes [1], neutron stars [2], quasars [3], etc. These astrophysical objects are extremely distant and cannot be directly probed, so our knowledge of the key physical processes occurring within them is very limited. Further expanding and testing our understanding of these processes requires reproducing similar conditions in a controlled laboratory environment. Significant efforts are currently being made to create experimental facilities capable of reproducing such extreme conditions. The most common method for producing electron-positron pairs in the laboratory is to pass a high-energy electron beam from a classical accelerator through a thick layer of a material with a high charge number. In this process, electrons either produce high-energy photons via bremsstrahlung, which in turn decay into electron-positron pairs via the Bethe-Heitler process, or produce electron-positron pairs directly in the so-called trident process [4]. Despite the relative simplicity of this method, numerous attempts at its practical implementation have demonstrated the impossibility of producing an electron-positron beam with a density sufficient to exhibit plasma effects, with the exception of a single experiment recently conducted at CERN in a slightly modified configuration [5].

Next-generation multi-petawatt laser systems may offer a promising alternative for laboratory generation of relativistically hot electron-positron plasma. One way to produce electron-positron plasma using such laser pulses is through the nonlinear quantum electrodynamic (QED) effects that occur when strong laser fields interact with fast particles [6]. One of these effects is a shift in the peak of the electron synchrotron radiation spectrum toward higher energies, leading to the emission of extremely hard photons by the electrons. The second effect is the decay of these photons into electron-positron pairs via the nonlinear Breit-Wheeler process. The successive emission of hard photons by electrons and positrons and the decay of the former into new pairs can lead to an avalanche-like increase in the total number of particles – a QED cascade [7] – and the formation of a dense electron-positron plasma.

This work proposes a scheme for generating relativistic electron-positron beams, similar to those proposed in [8; 9], which involves the interaction of two counterpropagating laser pulses with a solid target with two conical cavities. This scheme is adapted for the first stage of the XCELS project, XCELS-100, which envisions the creation of a two-channel setup with a radiation power of up to 50 PW in each channel. Using full-scale three-dimensional PIC simulations taking into account QED processes, it is demonstrated that interaction in such a configuration leads, firstly, to the generation of a quasi-static azimuthal magnetic field with an amplitude of the order of ten GGs due to the current of electrons accelerated along the conical surfaces, and, secondly, to the formation of over 10^11^ electron-positron pairs with a peak density exceeding 5 × 10^24^ cm^-3^. The generated magnetic field turns out to be strong enough to confine produced plasma in transverse direction which results in plasma parameters remaining almost constant on a timescale of hundreds of femtoseconds.

The proposed scheme ultimately addresses three critical challenges in producing laboratory pair plasma: the production of high-energy seed particles, the conversion of seed particles into dense pair plasma, and, finally, the confinement of the produced plasma for a sufficiently long duration. The latter, in particular, represents a significant advantage since it allows to further observe the processes in the produced plasma on its own characteristic timescales, independent from the laser scale. Thus, the proposed scheme in future could offer a versatile tool for scale modeling of extreme astrophysical environments in a laboratory.

The research is supported by RSCF grant № 25-12-00336.