The new generation of laser facilities is expected to deliver short (10 fs -100 fs) laser pulses with 10 - 100 PW of peak power. This opens an opportunityto study matter at extreme intensities in the laboratory and provides access tonew physics. Here we propose to scatter GeV-class electron beams fromlaser-plasma accelerators with a multi-PW laser at normal incidence. In thisconfiguration, one can both create and accelerate electron-positron pairs. Thenew particles are generated in the laser focus and gain relativistic momentumin the direction of laser propagation. Short focal length is an advantage, asit allows the particles to be ejected from the focal region with a net energygain in vacuum. Electron-positron beams obtained in this setup have a lowdivergence, are quasi-neutral and spatially separated from the initial electronbeam. The pairs attain multi-GeV energies which are not limited by the maximumenergy of the initial electron beam. We present an analytical model for theexpected energy cutoff, supported by 2D and 3D particle-in-cell simulations.The experimental implications, such as the sensitivity to temporalsynchronisation and laser duration is assessed to provide guidance for thefuture experiments.
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