###### Abstract  
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. relativistic plasmas; magnetized plasmas ultra-high-intensity lasers; laboratory astrophysics; particle-in-cell simulations +
Footnote †: journal: Applied Sci. 0000-0001-6000-0001-6000-0001-6000-0001-6000-0001-6000-0001-60001-60001-  
of a parameter \(\beta=\mu_{0}n_{e}kT_{e}/B^{2}\), equal to the ratio of the electron kinetic pressure to the magnetic pressure (here, \(k\) is the Boltzmann constant and \(T_{e}\) is the electron temperature). In the experiment, its value was estimated at \(\beta\sim 50\). At the same time, in the aspect of astrophysical applications, a cold plasma with a dominant role of a magnetic field and \(\beta\ll 1\) is usually considered. The second method for achieving high degrees of magnetization in a laser-plasma experiment is based on the use of a picosecond laser pulse of a kilojoule energy level to generate a magnetic field in a microcoil [11]. When a coil was irradiated with laser radiation at opposite ends, counterstreaming currents of the order of 1 MA were generated in it, which led to the creation of antiparallel fields of magnitude above 1 kT. According to estimates, the experiment reached \(\sigma\sim 20\)-100 at \(\beta\sim 0.01\)-0.1. The main disadvantage of such experiments is a hard accessibility of such laser systems. Other methods of organizing relativistic magnetic reconnection have been theoretically proposed as alternative approaches. For example, in [12; 13; 14; 15; 16; 17] it is proposed to observe an antiparallel configuration of magnetic fields during a parallel propagation of two ultrashort superintense laser pulses in transparent plasma. In plasma wakefields generated by them, the azimuthal magnetic field is observed. When two wakefields come into contact, for example, when they gradually expand in plasma with a negative density gradient, those fields begin to annihilate. The disadvantage of this method is that the electrons in the interaction region are relativistic, and their Larmor radii exceed a size of the region of localization of magnetic fields. That is the reconnection is observed only in the electron diffusion region in the very vicinity of an X-point. Finally, in [18], the authors propose using micro-slabs irradiated with laser pulses along a surface to observe relativistic magnetic reconnection. It was shown numerically that when using a femtosecond laser pulse of multi-terawatt power, \(\sigma\sim 1\) can be achieved at \(\beta\sim 0.1\), and when the power is increased to a petawatt level, \(\sigma>100\) can be achieved. The main difficulty in the experimental implementation of such a scheme is the need to use high-contrast laser pulses and precise targeting of a relatively tightly focused pulse to the end of a slab of submicron thickness. In this paper, we propose another method for generating cold (\(\beta\sim 0.1\)) magnetized (\(\sigma\sim 10\)) relativistic plasma, which is based on the interaction of powerful subpicosecond pulses with thin targets. As is known, the interaction of relativistically intense radiation with the surface leads to efficient generation of beams of energetic electrons injected by the field into the target [19; 20; 21].