Advances have been made in the study of the interaction between strong lasers and matter

Spin-polarized positrons have a wide range of uses in areas such as high-energy physics, materials physics, and laboratory astrophysics. At present, the traditional polarized positron source is based on the Bethe-Heitler mechanism through circular polarized gamma light or longitudinal polarized electron bombardment of high Z solid targets, but the single-shot positron yield is only the order of the fly library (10-15 coulombs), which is difficult to meet the requirements of the naku (10-9 coulombs) and the polarized positive and negative electron plasma physics research required by the future positron and negative electron colliders. How to obtain large power and high density of polarized positrons is still a huge challenge.

Song Huaihang, PhD student of L05 Group L05, Key Laboratory of Photophysics, Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, Researcher Li Yutong, and Professor Wang Weimin, Department of Physics, Chinese Min University, conducted long-term theoretical simulations of the spin polarization effect of strong field quantum electrodynamics (QED) in laser plasma physics[PhysicalReviewA100033407(2019)]and developed the first international containing electrons/ QED-PIC Program for Positron Spin Polarization and Photon Polarization Effects (Particle Simulation)[New Journal of Physics 23, 075005 (2021)]. Recently, the team used the program to study how to generate a high-density polarized positron source, and found that using the common configuration of laser solid-state target experiments, a positron beam with a polarization rate of 30% and a charge of 30 nanograms can be generated on a 100-watt laser device, with an angular flux of 1012 sr-1 and a positive electron polarization rate of 60% collected on a specific energy segment[PhysicalReviewLetters129035001(2022)]. Since this protocol adopts the configuration commonly used in experiments (i.e., the linear polarization laser interacts with a solid target with a preplasm), without the need for a special laser and target design, it is more feasible and practical. In addition, the work shows that in future laser solid-state target experiments on 100-watt laser devices, a large number of polarized positrons and electrons will inevitably be generated, so the effects of spin and photon polarization effects of electrons/positrons must be considered in this case.

In the field of strong-field QED, high-energy electrons deflected by the field will radiate high-energy gamma photons through nonlinear Compton scattering, which in turn can be efficiently annihilated into positive and negative electron pairs by the nonlinear Breit-Wheeler process. Previous studies have shown that if an asymmetric strong laser (elliptically polarized laser or two-color line polarization laser) is used to collide with a high-energy electron beam, the positrons produced will be spin polarized. However, the Power of the GeV electron beam obtained by the laser tailfield acceleration is only tens of picos (10-12 coulombs), which results in the number of positrons produced is lower than the order of the pybank. In addition, due to the low damage threshold of the optics, it is difficult to construct a super-strong asymmetric laser field. On the other hand, the use of ten-beat-watt, 100-watt laser interaction with plasma can produce high-density positrons, but the polarization properties of these positrons are still unclear, because the current widely used QED-PIC program to study the above problems ignores the spin polarization effect. To clarify this problem, the team used the self-developed YUNIC QED-PIC program to study the effects of laser and solid targets in the 100-watt range, which used conventional linearly polarized laser pulses (the initial symmetry of the laser field) and a micron scale preplasm on the front surface of the target with laser prepulse. The simulation results show that once the laser intensity exceeds 1024 W/cm2, the positron will appear significantly polarized, and this polarization depends on the deflection angle. For positrons with an deflection angle greater than 20 degrees, the polarization rate can reach 30%, and the polarization rate can be further increased to 60% by screening positrons of specific energies (Figure 1). The polarization of positrons and electrons can be attributed to the asymmetric laser field felt when they generate and leave the laser action area. Although the laser field is symmetrical at incident, but because the laser field is strongly absorbed and reflected near the plasma skin layer on the surface of the high-density target, while positrons and electrons can freely pass through this skin layer. Thus, positrons and electrons enter the highly dense plasma after only partially passing through the laser field, which causes them to undergo a highly asymmetrical subcyclical laser field near the skin layer, resulting in angle-dependent spin polarization. In real-world experiments, since laser prepulsions are unavoidable, the simulation results of this work show that the preplasm it produces before the target will play an important role in both the polarization and yield of positrons (Figure 2). The work suggests that for future 100 PW ultra-strong lasers interacting with solid-state targets, polarized positrons will be ubiquitous. These polarized high-density positron beams can be used to study polarized positive and negative plasma physics, or they can be applied to future positron and positron colliders after subsequent acceleration.

Figure 1 (a) Scheme for the interaction of a laser solid-state target to generate polarized positrons: a beam of linearly polarized laser is incident onto a solid target with a micron density scale in front of the target. After the laser action is completed, (b) the density of positron numbers and (c) the polarization rate are two-dimensional distributions of lateral deflection angles and positron energies, and one-dimensional distributions of lateral deflection angles.

Fig. 2 Positron yield and polarization rate vary with (a) preplasm density scale and (b) laser field strength.

The results were recently published in Physiological Review Letters. The relevant research work has been supported by the National Key Research and Development Program, the National Natural Science Foundation of China, and the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences. (Source: Institute of Physics, Chinese Academy of Sciences)

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