Multi-GeV electron-positron beam generation from laser-electron scattering

www.nature.com/scientificreports OPEN Received: 23 October 2017 Accepted: 5 March 2018 Published: xx xx xxxx Multi-GeV electron-positron beam generation from laser-electron scattering Marija Vranic 1,2 , Ondrej Klimo2,3, Georg Korn2 & Stefan Weber2 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 opportunity to study matter at extreme intensities in the laboratory and provides access to new physics. Here we propose to scatter GeV-class electron beams from laserplasma accelerators with a multi-PW laser at normal incidence. In this configuration, one can both create and accelerate electron-positron pairs. The new particles are generated in the laser focus and gain relativistic momentum in the direction of laser propagation. Short focal length is an advantage, as it allows the particles to be ejected from the focal region with a net energy gain in vacuum. Electronpositron beams obtained in this setup have a low divergence, are quasi-neutral and spatially separated from the initial electron beam. The pairs attain multi-GeV energies which are not limited by the maximum energy of the initial electron beam. We present an analytical model for the expected energy cutoff, supported by 2D and 3D particle-in-cell simulations. The experimental implications, such as the sensitivity to temporal synchronisation and laser duration is assessed to provide guidance for the future experiments. Generating abundance of antimatter in laboratory is of great importance both for fundamental science and potential applications. Several scenarios have been envisaged to create plasma clouds (or beams) with approximately equal number of electrons and positrons, dense enough to exhibit collective behavior. The aim is to study their self-consistent dynamics in conditions that match the ones the pair plasmas experience in space. Electron-positron pairs populate the magnetospheres of pulsars, and are believed to participate in the formation of gamma-ray bursts1. It is therefore vital to study the collective processes under extreme intensities, which may play a key role in the global dynamics of pulsar magnetospheres. Apart from laboratory astrophysics, e+e− pairs have a number of other prospective applications that span from tests of fundamental symmetries in the laws of physics and studies of antimatter gravity to the characterisation of materials2. It is thus not surprising that a strong effort is placed towards generating electron-positron plasmas using high-power laser technology. One of the most promising all-optical ways to accomplish this is via the non-linear Breit-Wheeler process3. In Breit-Wheeler (BW) pair production, a particle emits a high-energy photon, that can possibly decay into an electron-positron pair in an intense electromagnetic background. Such a strong background field can nowadays be produced by an intense laser. A proof-of-principle experiment was performed at SLAC4, where BW pairs were produced in a collision of a 46.6 GeV electron beam from a linear accelerator with a green laser at the intensity of 1018 W/cm2. Despite the resounding success of the experiment, a very small fraction of electrons composing the beam gave rise to a hard γ-ray that would eventually decay into a pair. The yield was too low to be used for a pair plasma study. On the other hand, several other laser experiments5–13 use Bethe-Heitler process for pair generation14. This is similar to the BW pair production, with one difference: a strong field background is provided by the proximity of a high-Z nucleus instead of a high-power laser pulse. Positron creation is possible also without using lasers15,16. However, a challenge in such schemes is positron accumulation and storage, in addition to the difficulty of recombining them with the electrons to later create an electron-positron flow. Multi-PW laser facilities currently under construction17–20 will access the regime of prolific pair production21,22. During scattering with such intense laser beams, relativistic electrons emit high-frequency radiation 1 GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001, Lisbon, Portugal. 2Institute of Physics of the CAS, ELI-Beamlines Project, Na Slovance 2, Prague, 182 21, Czech Republic. 3Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehova 7, Prague, 115 19, Czech Republic. Correspondence and requests for materials should be addressed to M.V. (email: ) ScientiFic RePortS | (2018) 8:4702 | DOI:10.1038/s41598-018-23126-7 1 www.nature.com/scientificreports/ Figure 1. Setup. (a) Perpendicularly moving electron beam interacts with the laser at the focus and creates new pairs; (b) Some electrons and positrons obtain a momentum component parallel to the laser propagation direction and start getting accelerated; (c) The laser defocuses shutting down the interaction; this leaves the particles with the net energy gain from the laser. (d) A fraction of the accelerated electrons and positrons distributed within the momentum space. through Compton scattering. Recent milestone all-optical experiments scattered electrons with lasers at 180 degrees, and demonstrated the potential of the state-of-the-art laser technology to generate x-rays and γ-rays23–27. A recent review on laser-wakefield acceleration-based light sources can be found in ref.28 and the most recent results on multiphoton Thompson scattering in ref.29. All these experiments were performed below the radiation reaction dominated regime, because the overall energy radiated by the interacting electrons was small compared with the initial electron energy. More recent experiments show first evidence of electron slowdown30,31 on the order consistent with the classical radiation reaction predictions for scattering an electron bunch and a laser pulse32. By using more intense laser pulses (I ∼ 1022 W/cm2) or more energetic electron beams, we will soon be able to convert a large fraction of the electron energy into radiation and access the regime of quantum radiation reaction33–40. This is expected in the next few years, as 4 GeV electron beams have already been obtained using a 16 J laser41 and the next generation of facilities is aiming to achieve laser intensities I > 1023 W/cm2. In such extreme conditions, the energetic photons produced in the scattering can decay into electron-positron pairs42. Here we propose a configuration that allows to both create and accelerate an electron-positron beam. An intense laser interacts with a relativistic electron beam at 90 degrees of incidence (setup is illustrated in Fig. 1). The pair production efficiency here is slightly lower than in a head-on collision. However, in a head-on collision the energy cutoff of the electron-positron beam is limited to the initial energy of the interacting electrons, while at 90 degrees this is not the case. A (...truncated)