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We propose a positron acceleration scheme in which a laser-driven positron beam is injected into a solid target hit by a laser beam and accelerated in the sheath field on the target back side. The positron beam injection and acceleration in the target have been investigated with numerical simulation. The feasibility of such an acceleration scheme was proved according to the simulation results, which show that a 10 MeV positron beam can be accelerated up to 30 MeV. The dependency of the positron beam properties on the positron injection location, injection time, and target thickness was studied. Related acceleration details were obtained and analyzed. The acceleration scheme provides a method in positron energy controlling and its related applications. [ABSTRACT FROM AUTHOR]

Spin-polarized positron beams have widely been utilized in applications ranging from fundamental physical studies to material processing. Preparing highly polarized positron beams for accurate probing is a long-standing issue. Here, we put forward a method to produce ultra-relativistic polarized positrons with unprecedented purity in a femtosecond timescale employing a few-PW circularly polarized laser pulse. The fully spin-resolved QED Monte Carlo method is used for simulating the two successive QED processes during the interaction, i.e., nonlinear Compton scattering and nonlinear Breit–Wheeler pair production. As the photons emitted in a circularly polarized laser field are symmetrically polarized, the polarization of the intermediate gamma photon beam averages out to zero, which is advantageous for improving the polarization of positrons. Meanwhile, the moderate laser intensity suppresses the depolarization of the new-born positrons induced by radiation reaction effect. As a result, the polarization of the positrons can reach up to ≳ 90%, the highest among the laser-driven polarization schemes conceived hitherto. Furthermore, our method relaxes the requirement on laser intensity to few-PW level, offering a promising way of preparing polarized positrons with current-generation laser facilities. [ABSTRACT FROM AUTHOR]

Preserving the quality of a positron beam in a plasma-based accelerator, where a wakefield suitable for positron transport and acceleration is generated by means of an electron filament, is challenging. This is due to the nature of the wakefields, characterized by focusing fields that vary nonlinearly in the transverse direction, and by accelerating fields that are non-uniform. These fields also change slice-by-slice along the beam. Maintaining a high beam quality is pivotal for application of positron beams in a plasma-based collider. In this paper, we show that an initial background plasma temperature can help mitigate the positron beam quality degradation in plasma-based accelerators that rely on electron filaments. We show that temperature effects broaden the electron filament and smooth radially both the non-linear transverse and the non-uniform longitudinal wakefields. Using warm plasmas opens up new possibilities to improve beam quality in several plasma-based positron acceleration concepts. [ABSTRACT FROM AUTHOR]

The stability of plasma-based accelerators against transverse misalignments and asymmetries of the drive beam is crucial for their applicability. Without stabilizing mechanisms, even small initial offsets of the drive beam centroid can couple coherently to the plasma wake, grow, and ultimately lead to emittance degradation or beam loss for a trailing witness beam. In this work, we demonstrate the intrinsic stability of a beam propagating in a plasma column. This result is relevant in the context of plasma-based positron acceleration, where a wakefield suitable for the transport and acceleration of a positron witness beam is generated in a plasma column by means of an electron drive beam. The stable propagation of the drive beam is a necessary condition for the experimental implementation of this scheme. The differences and similarities of stabilizing mechanisms in a plasma column compared to a homogeneous plasma are identified via theory and particle-in-cell simulations. Experimental tolerances are given, demonstrating the experimental feasibility of the scheme. [ABSTRACT FROM AUTHOR]

Particle-in-cell simulations of jets of electrons and positrons in an ambient electron–proton plasma have revealed an acceleration of positrons at the expense of electron kinetic energy. We show that a filamentation instability, between an unmagnetized ambient electron–proton plasma at rest and a beam of pair plasma that moves through it at a non-relativistic speed, indeed results in preferential positron acceleration. Filaments form that are filled predominantly with particles with the same direction of their electric current vector. Positron filaments are separated by electromagnetic fields from beam electron filaments. Some particles can cross the field boundary and enter the filament of the other species. Positron filaments can neutralize their net charge by collecting the electrons of the ambient plasma, while protons cannot easily follow the beam electron filaments. Positron filaments can thus be compressed to a higher density and temperature than the beam electron filaments. Filament mergers, which take place after the exponential growth phase of the instability has ended, lead to an expansion of the beam electron filaments, which amplifies the magnetic field they generate and induces an electric field in this filament. Beam electrons lose a substantial fraction of their kinetic energy to the electric field. Some positrons in the beam electron filament are accelerated by the induced electric field to almost twice their initial speed. The simulations show that a weaker electric field is induced in the positron filament and particles in this filament hardly change their speed. [ABSTRACT FROM AUTHOR]

Compact short pulse positron source in the energy range of a few tens of MeV driven by laser plasma accelerator is a promising tool for many applications, such as materials science and source of colliders. A collection and transmission system based on laser plasma accelerators is proposed to intensify and narrow the beam divergence for positrons generated from converters. The system is composed of a converter, a collimator, a solenoid, quadrupoles, and a slit in the exit. Numerical simulations were performed to demonstrate the feasibility and robustness of the design. The designed system can work in the center energy range from 15 to 40 MeV within ±10% energy spread (FWHM) and greatly improve the quality of collected positron beams with a divergence of around 50 mrad. [ABSTRACT FROM AUTHOR]

The interaction occurring between two counterstreaming rotating electron-positron beams and an inhomogeneous magnetized electron-ion plasma is studied with the focus of research on either positrons or electrons propagating in the direction of the magnetic field. Using the Vlasov theory along with geometrical optics, the linear eikonal equation corresponds to the gradient drift wave which is extracted in the background plasma, taking into account the beam contribution. The results reveal that the gradient drift instability is experienced where the gradients of density and temperature of electrons stand in the opposite directions, and in addition, the gradients act as destabilization effects. Regarding the beam contribution, when the electron beams propagate in the direction of the magnetic field, the parallel and perpendicular components of velocity and the Langmuir frequency of the rotating beams can induce stabilization effects on the unstable inhomogeneous configuration. However, as a considerable achievement, the mentioned stabilization effects vanish for the perpendicular velocity component lower than a certain threshold value. In addition, the destabilization effects of the characteristic parameters of the counterstreaming beams are observed as well, when the positron beams propagate in the direction of the magnetic field. [ABSTRACT FROM AUTHOR]

Described here are simulations of pulsed, magnetically guided positron beams formed by ejection from Penning-Malmberg-style traps. In a previous paper [M. R. Natisin et al., Phys. Plasmas 22, 033501 (2015)], simulations were developed and used to describe the operation of an existing trap-based beam system and provided good agreement with experimental measurements. These techniques are used here to study the processes underlying beam formation in more detail and under more general conditions, therefore further optimizing system design. The focus is on lowenergy beams (∼eV) with the lowest possible spread in energies (<10 meV), while maintaining microsecond pulse durations. The simulations begin with positrons trapped within a potential well and subsequently ejected by raising the bottom of the trapping well, forcing the particles over an end-gate potential barrier. Under typical conditions, the beam formation process is intrinsically dynamical, with the positron dynamics near the well lip, just before ejection, particularly crucial to setting beam quality. In addition to an investigation of the effects of beam formation on beam quality under typical conditions, two other regimes are discussed; one occurring at low positron temperatures in which significantly lower energy and temporal spreads may be obtained, and a second in cases where the positrons are ejected on time scales significantly faster than the axial bounce time, which results in the ejection process being essentially non-dynamical. [ABSTRACT FROM AUTHOR]

By using two-dimensional particle-in-cell simulations, we demonstrate high-flux dense positrons generation by irradiating an ultra-intense laser pulse onto a tapered hollow target. By using a laser with an intensity of 4 ×1023W/cm², it is shown that the Breit-Wheeler process dominates the positron production during the laser-target interaction and a positron beam with a total number >1015 is obtained, which is increased by five orders of magnitude than in the previous work at the same laser intensity. Due to the focusing effect of the transverse electric fields formed in the hollow cone wall, the divergence angle of the positron beam effectively decreases to ~15° with an effective temperature of ~674 MeV. When the laser intensity is doubled, both the positron flux (>1016) and temperature (963 MeV) increase, while the divergence angle gets smaller (~13°). The obtained high-flux low-divergence positron beam may have diverse applications in science, medicine, and engineering. [ABSTRACT FROM AUTHOR]

Plasma wakefield acceleration of positron beams in the wake of a dense electron beam (in the blowout regime) is numerically analyzed. The acceleration is possible only if the energy content of the wakefield is not very high. This is in contrast to electron acceleration, for which the optimum performance requires driver currents and wave energies to be as high as possible. For positrons, the efficiency of plasma-to-witness energy exchange can amount to several tens percent, but high efficiencies require precise location of the positron beam and sophisticated beam shapes. Unlike an electron witness, the positron always gets an energy spread of about several percent caused by the transverse inhomogeneity of the accelerating field. [ABSTRACT FROM AUTHOR]