Plasma Physics (physics.plasm-ph)

Abstract: Electron acceleration mechanisms near the counterstreaming interface of a relativistic collisionless shock (RCS) are investigated using particle-in-cell (PIC) simulations. We identify a slingshot-like injection process induced by the drifting electric field sustained by the flowing focus of backwards-moving electrons, which is distinct from the well-known stochastic acceleration. The flowing focus signifies the plasma kinetic transition from a preturbulent laminar motion to a chaotic turbulence. We find a characteristic correlation between the electron dynamics in the slingshot acceleration and the photon emission features. In particular, the integrated radiation from the RCS exhibits a counterintuitive non-monotonic dependence of the photon polarization degree on the photon energy, which originates from a polarization degradation of relatively high-energy photons emitted by the slingshot-injected electrons. Our results demonstrate the potential of photon polarization as an essential information source in exploring intricate dynamics in RCSs with relevance for earth-based plasma and astrophysical scenarios.

Abstract: The resonant external field-assisted Breit-Wheeler process (Oleinik resonances) for strong electromagnetic fields with intensities less than the critical Schwinger field has been theoretically studied. The resonant kinematics has been studied in detail. The case of high-energy initial gamma quanta and emerging ultrarelativistic electron-positron pairs is studied. The resonant differential cross section is obtained. The generation of narrow beams of ultrarelativistic positrons (for Channel A) and electrons (for Channel B) is predicted with a probability significantly exceeding corresponding to the non-resonant process.