Creation and direct laser acceleration of positrons in a single stage

Relativistic positron beams are required for fundamental research in nonlinear strong field QED, plasma physics, and laboratory astrophysics. Positrons are difficult to create and manipulate due to their short lifetime, and their energy gain is limited by the accelerator size in conventional facilities. Alternative compact accelerator concepts in plasmas are becoming more and more mature for electrons, but positron generation and acceleration remain an outstanding challenge. Here, we propose a new setup where we can generate, inject, and accelerate them in a single stage during the propagation of an intense laser in a plasma channel. The positrons are created from a laser-electron collision at 90°, where the injection and guiding are made possible by an 800-nC electron beam loading which reverses the sign of the background electrostatic field. We obtain a 17-fC positron beam, with GeV-level central energy within 0.5 mm of plasma.

Figure 1

(a) Setup: electron-positron pairs (in green/orange) are created during the interaction of an intense laser pulse (blue/red) with a relativistic electron beam (green) at a 90° incidence angle. A fraction of the pairs is injected and propagates with the pulse through the plasma channel, experiencing direct laser acceleration. (b) Quasi-3D modeling of this setup. A slab of $\gamma$-ray photons from the electron-laser collision is initialized at $t=0$.

Figure 2

Positron guiding and injection. (a) Electron beam loading after $280\mathrm{\mu }\mathrm{m}$ of propagation. Upper panel: channel fields contribution to the transverse Lorentz force on the positrons ($m=0$). Lower panel: net plasma charge density. (b) The number of generated positrons; (c) The fraction of positrons injected in the laser propagation direction as a function of the electron beam energy for a laser field amplitude $5\times {10}^{23}\mathrm{W}/{\mathrm{cm}}^2$ (circles). Panel (b) shows the number of positrons generated using an electron beam of 10 pC and can be directly scaled to different beams (e.g., $Q_{e-}=100\mathrm{pC}$ would give 10 times more positrons).

Figure 3

Positron direct laser acceleration. (a) Transverse electric field ($x$ direction). White circles represent a sample of injected positrons. Mode $m=1$ is associated with the laser component, while $m=0$ represents the channel field. (b) Longitudinal electric field ($z$ direction). (c) Energy gain of injected positrons in transverse ($W_{\perp }$) and longitudinal directions ($W_{\parallel }$). (d) Energy gain of noninjected positrons. The energy gains $W$ represent the accumulated work in the channel fields ($m=0$, red squares) and the laser fields ($m=1$, blue circles) after a propagation distance of $340\mathrm{\mu }\mathrm{m}$.

Figure 4

The number of injected positrons as a function of the laser propagation distance for different channel sizes. The black circle marks the central interaction point of the laser pulse with the photon beam. Inset: Positron energy spectrum for $r_c=26\mathrm{\mu }\mathrm{m}$ at creation time (black). The total charge is 1.3 pC and $N_{\max }=6\mathrm{pC}/\mathrm{GeV}$. Positron spectrum at $z=340\mathrm{\mu }\mathrm{m}$. The total charge is 17 fC and $N_{\max }=12\mathrm{fC}/\mathrm{GeV}$. The normalization facilitates the scaling of the result to a higher electron beam charge.