Identifying the electron–positron cascade regimes in high-intensity laser-matter interactions

Abstract

Strong-field quantum electrodynamics predicts electron-seeded electron-positron pair cascades when the electric field in the rest-frame of the seed electron approaches the Sauter-Schwinger field, i.e. h =Ẽ E 1 S RF . Electrons in the focus of next generation multi-PW lasers are expected to reach this threshold. We identify three distinct cascading regimes in the interaction of counter-propagating, circularly-polarised laser pulses with a thin foil by performing a comprehensive scan over the laser intensity (from 10 23 to 5×10 24 W cm −2 ) and initial foil target density (from 10 26 to 10 31 m −3 ). For low densities and intensities the number of pairs grows exponentially. If the intensity and target density are high enough the number density of created pairs reaches the relativistically-corrected critical density, the pair plasma efficiently absorbs the laser energy (through radiation reaction) and the cascade saturates. If the initial density is too high, such that the initial target is overdense, the cascade is suppressed by the skin effect. We derive a semi-analytical model which predicts that dense pair plasmas are endemic features of these interactions for intensities above 10 24 W cm −2 provided the target's relativistic skin-depth is longer than the laser wavelength. Further, it shows that pair production is maximised in near-critical-density targets, providing a guide for near-term experiments.S RF . We can reach η∼1 for laser fields much weaker than E S as the fields themselves can rapidly accelerate electrons to high Lorentz factor, resulting in a strong Lorentz boost to E RF . Pair production by the multi-photon Breit-Wheeler process can occur if the photons emitted during Compton scattering satisfy a similar condition on their quantum efficiency parameter (defined below) χ∼1. An electromagnetic cascade can ensue if many generations of electrons and positrons can be generated by the fields. Usually this occurs via a two-step process whereby the electrons and positrons produced by the Breit-Wheeler process radiate photons by nonlinear Compton scattering which subsequently decay to further pairs and so on.Upcoming facilities, like several of those comprising the extreme light infrastructure (ELI) [1,2], are expected to reach laser intensities of > -

Particle trajectories, gamma-ray emission, and anomalous radiative trapping effects in magnetic dipole wave

“…In order to generate pair plasma by incoming laser pulses we must prepare a seed in the focal region to trigger vacuum breakdown. Here the seed can be an electron beam, a plasma or solid target, or a gamma ray (see for an example [35][36][37][38][39][40]). In general, this problem looks like the well-known air breakdown in a focused laser beam.…”

Investigation of QED Effects With Varying Z in Thin Foil Targets

“…Similar study by Luo et al [19] gives an estimate of the QED cascade saturation. Based on a semi-analytical scaling for the pair plasma density and corresponding PIC simulations, the dependency of QED cascade on initial target density has been presented by Slade-Lowther et al [20] . In fact, they have also shown that there exist three different cascade regimes with respect to initial target density in high intensity laser-thin foil interaction.…”

Onset of electron-seeded cascades in generic electromagnetic fields

“…Further simulations, however, demonstrated generation of cascades in more realistic settings, e.g. by irradiating dilute gas or electron cloud [35][36][37][38], near-critical-density plasma [39,40], or dense solid [41][42][43][44][45][46] targets. Note that in the latter case it was enough to irradiate a target from one side, since due to reflection a counterpropagating wave was formed by itself.…”