Single-laser scheme for reaching strong field QED regime via direct laser acceleration

TL;DR

This work demonstrates a single-laser, direct laser acceleration (DLA) scheme in which electrons are first accelerated in an underdense gas and then collide head-on with the reflected laser pulse from an overdense mirror, enabling strong-field QED processes. By combining analytical scaling laws for nonlinear Compton scattering and nonlinear Breit–Wheeler pair production with quasi-3D PIC simulations that include QED effects, the authors show that a 2 PW laser can reach the SFQED regime ($\nchi_e>1$) and that higher powers yield rapid, nonlinear increases in positron yield (e.g., >2 nC at 10 PW). A semi-analytical model for positron production, validated against simulations, estimates yields across parameter regimes and highlights the impact of laser depletion and mirror positioning. The results indicate that this all-optical, single-laser approach is experimentally feasible with current multi-Petawatt facilities and provides a practical platform to study SFQED phenomena and related plasma-positron physics.

Abstract

We investigate a single-laser scheme for reaching the strong-field QED regime based on direct laser acceleration (DLA) of electrons followed by their head-on collision with the same laser pulse reflected from an overdense foil. In this configuration, electrons are first accelerated inside an underdense plasma by a relativistic laser pulse and subsequently interact with the reflected laser field, emitting high-energy photons via nonlinear Compton scattering which decay into electron-positron pairs through the nonlinear Breit-Wheeler process. Using analytical scalings supported by quasi-3D particle-in-cell simulations including QED effects, we demonstrate that a laser pulse with power as low as 2 PW is sufficient to reach the quantum regime characterized by $χ_e> 1$ . For higher powers, we observe a rapid nonlinear increase in the number of generated positrons, reaching more than 2 nC for a 10 PW laser pulse with energy of approximately 1.1 kJ. A semi-analytical model is employed to estimate the positron yield, showing good agreement with simulation results. We further study the influence of laser depletion and the positioning of the reflecting foil on the efficiency of pair production. The presented scheme provides an experimentally feasible platform for probing strong-field QED effects using currently available multi-petawatt laser systems.

Figures (10)

• Figure 1: Illustration of the different stages studied: from single-laser DLA towards reaching the strong field QED regime. At first, electrons are accelerated inside the gas target by the relativistic laser pulse propagating for several millimetres. After the electrons are accelerated, an overdense foil is placed on the optical axes, to reflect the remainder of the laser pulse backwards. After the reflection, the electrons emit photons via non-linear Compton scattering. These photons then decay into electron-positron pairs via Breit-Wheeler process in the strong field environment provided by the laser pulse.
• Figure 2: Temporal evolution of the: (a) Transverse component of the laser field (b) Electron plasma density. For each timestep shown, a lineout is presented considering a simulation window moving at the speed of light. In the beginning, the front etching velocity matches the theoretical prediction, slowing down after the propagation for $\approx$ 800 microns. The data shown is taken from quasi-3D PIC simulations, with a 6 PW laser propagating through the plasma with the density of $n_e/n_c = 0.02$.
• Figure 3: The dynamics of a 6 PW-laser propagating through the constant density plasma at 0.02 $n_c$ . In panel (a), the laser is initialized in vacuum. Panels (b-d) demonstrate the laser propagation in self-guided regime for 2.5 mm during which electrons get accelerated.
• Figure 4: Lineouts of the laser transverse electric field and the electron plasma density after different distances of propagation inside the plasma. A 6 PW laser propagates through the plasma with the density $n_e/n_c = 0.02$. A steep density spike at the laser front causes photon deceleration, visible as a long-wavelength laser field at the back. Laser front steepening due to local pump depletion is also present.
• Figure 5: Comparison of initial laser pulse shape with the laser pulse shape immediately before the reflection from the overdense target for different laser powers.