Electron-positron pair generation using a single kJ-class laser pulse in a foam-reflector setup

We investigate the process of creating electron-positron pairs from laser-matter interaction in pre-ionised foam targets using particle-in-cell simulations. A high-intensity laser pulse drives electrons via direct laser acceleration up to a cone-shaped reflector. The high-energy electrons interact with the reflected laser pulse, generating abundant pairs. The effects of the plasma-channel shape on the propagation of the laser pulse and subsequent pair production is studied. The results show that the number of Compton emission and Breit-Wheeler pair creation events is highly sensitive to the diffraction of the laser due to its interaction with the foam.

💡 Research Summary

The paper proposes and numerically investigates a single‑laser scheme for generating large numbers of electron‑positron pairs using a kilojoule‑class, ultra‑intense laser pulse (1.5 kJ, a₀≈194, λ=800 nm). The key idea is to let the same laser pulse act both as a direct‑laser‑acceleration (DLA) driver for electrons in a pre‑ionized, near‑critical density foam and, after passing through the foam, as a colliding pulse after being reflected by a cone‑shaped gold reflector placed behind the foam.

In the foam, a Gaussian plasma channel (inner density n₀, outer density n₁, width σ_C) guides the laser and enables DLA, accelerating electrons to multi‑GeV energies (γ up to ~10⁴) over a few hundred laser wavelengths. The accelerated electrons travel together with the laser until they encounter the reflector. The cone geometry focuses the reflected pulse, increasing the local field amplitude (B_z up to ~1.5 B₀) and creating a region where the quantum non‑linearity parameter χ = γ E/E_cr becomes of order unity or larger. In this regime, nonlinear Compton scattering produces hard photons, which then undergo the nonlinear Breit‑Wheeler process, yielding electron‑positron pairs.

The authors employ the fully electromagnetic 3‑D PIC code VLPL with stochastic QED Monte‑Carlo modules to capture photon emission and pair creation. Because the photon‑to‑pair conversion probability is extremely low for χ ≪ 1, they introduce a sub‑sampling technique: low‑energy photons are discarded, while high‑energy photons are split into many virtual sub‑particles to improve statistical resolution without prohibitive computational cost.

A systematic 2‑D parameter scan complements the expensive 3‑D runs, allowing the authors to map out the dependence of pair yield on plasma and reflector parameters:

• Inner density (n₀) – Too low (≤0.2 n_c) leads to rapid laser diffraction and weak field amplification; too high (≥0.6 n_c) causes early laser depletion. The optimum lies around 0.3–0.5 n_c, where both electron acceleration and field strength are balanced.
• Channel width (σ_C) – Values between 5 λ and 10 λ give the best trade‑off. Narrower channels accelerate electrons faster initially but limit total energy gain; wider channels reduce guiding, causing the laser to spread and χ to drop.
• Reflector opening angle (α) – An angle of ≈5° maximizes the magnetic field at the focal spot of the reflected pulse. The optimal angle also depends on the reflector’s axial position; the authors find the best performance when the reflector is placed at x₀≈150 λ from the foam entrance.
• Foam length (L) – Counter‑intuitively, the highest pair numbers are obtained for relatively short interaction lengths (L ≲ 200 λ). In this regime electrons reach γ≈2000, sufficient for χ ≈ 1 when the reflected field is strong, and the laser maintains a high quality beam. Longer foams increase electron energy but cause severe laser depletion, reducing the field at the collision point and thus the overall pair yield.

Under the optimal configuration (n₀≈0.35 n_c, n₁≈n_c, σ_C≈8 λ, α≈5°, x₀≈150 λ, L≈200 λ), the simulations predict the generation of on the order of 10⁸–10⁹ electron‑positron pairs per shot. This represents a substantial improvement over traditional two‑stage schemes, where separate acceleration and collision stages often suffer from synchronization losses and lower overall efficiency.

The study demonstrates that a single, high‑energy laser pulse can simultaneously provide the necessary high‑energy electron beam and the intense counter‑propagating field required for strong‑field QED processes. It also offers concrete design guidelines for future experiments at upcoming multi‑PW facilities (e.g., ELI, Apollon). By reducing experimental complexity and highlighting the importance of laser guiding, reflector geometry, and interaction length, the work paves the way toward laboratory studies of astrophysically relevant electron‑positron plasmas, such as those found in gamma‑ray bursts and pulsar wind nebulae.