Description
Continued developments in high-power lasers will enable intensities beyond 10^23 W/cm^2. At such intensities, a laser pulse colliding with an ultra-relativistic electron beam subjects the electrons to fields nearing the Schwinger limit, triggering strong-field QED (SFQED) processes. The dominant processes in the collision are nonlinear Compton scattering, where a charged particle emits a high-energy photon, and nonlinear Breit-Wheeler pair creation, where a photon decays into an electron-positron pair. Harnessing these processes for positron generation requires control over the processes themselves and the positrons they produce. Here, we demonstrate two approaches to controlling the properties of positrons produced during nonlinear Breit-Wheeler pair creation. In the first, the asymmetric fields of a bichromatic, circularly polarized laser pulse enable simultaneous spin-polarization and angular isolation of the created positrons, producing a positron signal that is over 50% spin-polarized and nearly devoid of electrons. In the second, the collision of a monochromatic, linearly polarized laser pulse and relativistic electron beam occurs in a plasma, where the magnetic fields excited by the laser pulse focuses the positrons and expels electrons. This focusing is energy-dependent and results in smaller energy spreads than could be achieved in a vacuum collision.