In simulations of a 10PW laser striking a solid we demonstrate the possibility of producing a pure electron-positron plasma by the same processes as those thought to operate in high-energy astrophysical environments. A maximum positron density of 10 26 m −3 can be achieved, seven orders of magnitude greater than achieved in previous experiments. Additionally, 35% of the laser energy is converted to a burst of gamma-rays of intensity 10 22 Wcm −2 , potentially the most intense gammaray source available in the laboratory. This absorption results in a strong feedback between both pair and γ-ray production and classical plasma physics in the new 'QED-plasma' regime.Electron-positron (e − e + ) plasmas are a prominent feature of the winds from pulsars and black holes [1,2]. They result from the presence of electromagnetic fields strong enough to cause non-linear quantum electrodynamics (QED) reactions [3] in these environments leading to a cascade of e − e + pair production [4]. These fields can be much lower than the Schwinger field for vacuum breakdown [5] if they interact with highly relativistic electrons (γ >> 1) [3]. Non-linear QED has been probed experimentally with lasers in two complementary ways:(1) with a particle accelerator accelerating electrons to the necessary γ and a laser supplying the fields [6-8]; or (2) with a laser accelerating the electrons and goldnuclei supplying the fields [9][10][11]. An alternative configuration, using next-generation high-intensity lasers to provide both the acceleration and the fields [12], has the potential to generate dense e − e + plasmas. Analytical calculations and simulations exploring this configuration have shown that an overdense e − e + plasma can be generated from a single electron by counter-propagating 100PW lasers [12][13][14][15]. Here we will show that such a plasma can be generated with an order of magnitude less laser power by firing the laser at a solid target, putting such experiments in reach of next-generation 10PW lasers [16].The dominant non-linear QED effects in 10PW laserplasma interactions are: synchrotron gamma-ray photon (γ h ) emission from electrons in the laser's electromagnetic fields; and pair-production by the multiphoton Breit-Wheeler process, γ h + nγ l → e − + e + , where γ l is a laser photon [3,17,18]. Each reaction is a strongly multiphoton process, the former process being non-linear Compton scattering, e − + mγ l → e − + γ h [19,20], in the limit m → ∞. Therefore, these reactions only become important at the ultra-high intensities reached in 10PW laser-plasma interactions. The importance of synchrotron emission is determined by the parameter η. This depends on the ratio of the electric and magnetic fields in the plasma to the Schwinger field [5] (E s = 1.3 × 10 18 Vm −1 ). For ultra-relativistic particles 17,18]. γ is the Lorentz factor of the emitting electron or positron, β is the corresponding velocity normalised to c and E ⊥ is the electric field perpendicular to its motion. As η approaches unity each emitted photon takes a ...

In high-intensity (> 10 21 Wcm −2 ) laser-matter interactions gamma-ray photon emission by the electrons can strongly effect the electron's dynamics and copious numbers of electron-positron pairs can be produced by the emitted photons. We show how these processes can be included in simulations by coupling a Monte-Carlo algorithm describing the emission to a particle-in-cell code. The Monte-Carlo algorithm includes quantum corrections to the photon emission, which we show must be included if the pair production rate is to be correctly determined. The accuracy, convergence and energy conservation properties of the Monte-Carlo algorithm are analysed in simple test problems.

Abstract.Gamma-ray and electron-positron pair production will figure prominently in laser-plasma experiments with next generation lasers. Using a Monte Carlo approach we show that straggling effects arising from the finite recoil an electron experiences when it emits a high energy photon, increase the number of pairs produced on further interaction with the laser fields.

Radiation reaction in the interaction of ultrarelativistic electrons with a strong external electromagnetic field is investigated using a kinetic approach in the nonlinear moderately quantum regime. Three complementary descriptions are discussed considering arbitrary geometries of interaction: a deterministic one relying on the quantum-corrected radiation reaction force in the Landau and Lifschitz (LL) form, a linear Boltzmann equation for the electron distribution function, and a Fokker-Planck (FP) expansion in the limit where the emitted photon energies are small with respect to that of the emitting electrons. The latter description is equivalent to a stochastic differential equation where the effect of the radiation reaction appears in the form of the deterministic term corresponding to the quantum-corrected LL friction force, and by a diffusion term accounting for the stochastic nature of photon emission. By studying the evolution of the energy moments of the electron distribution function with the three models, we are able to show that all three descriptions provide similar predictions on the temporal evolution of the average energy of an electron population in various physical situations of interest, even for large values of the quantum parameter χ. The FP and full linear Boltzmann descriptions also allow us to correctly describe the evolution of the energy variance (second-order moment) of the distribution function, while higher-order moments are in general correctly captured with the full linear Boltzmann description only. A general criterion for the limit of validity of each description is proposed, as well as a numerical scheme for the inclusion of the FP description in particle-in-cell codes. This work, not limited to the configuration of a monoenergetic electron beam colliding with a laser pulse, allows further insight into the relative importance of various effects of radiation reaction and in particular of the discrete and stochastic nature of high-energy photon emission and its back-reaction in the deformation of the particle distribution function.