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Ongoing progress in laser and accelerator technology opens new possibilities in high-field science, notably for the study of the largely unexplored strong-field QED regime where electron-positron pairs can be created directly from light-matter or even light-vacuum interactions. Laserless strategies such as beam-beam collisions have also been proposed with the prospect of pushing strong-field quantum electrodynamics (SFQED) in the nonpertubative regime. Here we report on an original concept to probe strong-field QED by harnessing the interaction between an electron beam and a solid target. When a high-density, ultrarelativistic beam impinges onto an even denser plasma, the beam self fields are reflected at the plasma boundary. In the rest frame of the beam electrons, these fields can exceed the Schwinger field, leading to strong-field QED effects such as quantum nonlinear inverse Compton scattering and nonlinear Breit-Wheeler electron-positron pair creation. We show that such beam-plasma collisions can produce results similar to beam-beam collisions with the advantage of a much simpler experimental setup including the automatic overlap between the beam and the reflected fields. This scenario opens the way to precision studies of strong-field QED, with measurable clear signatures in terms of gamma-ray photon and pair production, and thus is a very promising milestone on the path towards laserless studies of nonperturbative SFQED.

International audience; We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a specially designed asymmetrically shocked gas jet. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. Particle in cell simulations confirm the role of the shock and the density transition in the electron injection mechanism. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

International audience; Simulations have been performed of the radial transport of antiprotons in positron plasmas under ambient conditions typical of those used in antihydrogen formation experiments in Penning traps. The simulations were performed using classical trajectories of both antiprotons and surrounding positrons with randomized initial conditions. In addition, friction and fluctuation forces on the antiproton due to interaction with the positron plasma were included. This gives rise to both axial and radial diffusion of the antiprotons at a rate given by the magnetic field, the positron temperature and the plasma density, and the parameter range explored includes at least two values of each. As the antiprotons travel through the positron plasma they undergo repeated cycles of antihydrogen formation and re-ionization. We find that when these effects are added to the simulations the properties of the radial diffusion alter dramatically, even changing from normal to anomalous diffusion. We attribute this to the azimuthal drift of the antiprotons in the crossed magnetic (from the trap) and electric fields (from the space charge of the plasma). When the antiprotons are neutralised through antihydrogen formation this drift is temporarily interrupted, and the antiproton (carrying a positron) continues in the tangential direction. On average this motion will give an increase of the radial position of the antiproton, relative the trap axis. At low positron plasma temperatures, repeated cycles of antihydrogen formation and destruction are the dominant source of radial (cross magnetic field) transport. On time scales of a few milliseconds (depending on plasma parameters) the antiprotons will reach the edge of the cylindrical plasma, where the radial transport will cease, and thus the antiprotons will accumulate there.

Physical review accelerators and beams 24(10), 101302 (2021). doi:10.1103/PhysRevAccelBeams.24.101302, Electron beams to be accelerated in beam-driven plasma wakes are commonly formed by a photocathode and externally injected into the wakefield of a preceding bunch. Alternatively, using the plasma itself as a cathode offers the possibility of generating ultrashort, low-emittance beams by trapping and accelerating electrons from the ambient plasma background. Here, we present a beam-driven plasma cathode realized via laser-triggered density-downramp injection, showing stable beam formation over more than a thousand consecutive events with an injection probability of 95 %. The plasma cathode is highly tunable, resulting in the injection of electron bunches of tens of pC of charge, energies of up to 79 MeV, and relative energy spreads as low as a few percent and the achieved stability allowed for the first multi-shot evaluation of the emittance of injected beams., Published by American Physical Society, College Park, MD