We review common extensions of particle-in-cell (PIC) schemes which account for strong field phenomena in laser-plasma interactions. After describing the physical processes of interest and their numerical implementation, we provide solutions for several associated methodological and algorithmic problems. We propose a modified event generator that precisely models the entire spectrum of incoherent particle emission without any low-energy cutoff, and which imposes close to the weakest possible demands on the numerical time step. Based on this, we also develop an adaptive event generator that subdivides the time step for locally resolving QED events, allowing for efficient simulation of cascades. Further, we present a new and unified technical interface for including the processes of interest in different PIC implementations. Two PIC codes which support this interface, PICADOR and ELMIS, are also briefly reviewed. CONTENTS

One aim of upcoming high-intensity laser facilities [1][2][3] is to provide new high-flux gammaray sources [4]. Electromagnetic cascades [5-9] may serve for this, but are known to limit both field strengths and particle energies [10], restricting efficient production of photons to sub-GeV energies [11][12][13]. Here we show how to create a directed GeV photon source, enabled by a controlled interplay between the cascade and anomalous radiative trapping [14]. Using advanced 3D QED particle-in-cell (PIC) simulations [15] and analytic estimates, we show that the concept is feasible for planned [3] peak powers of 10 PW level. A higher peak power of 40 PW can provide 10 9 photons with GeV energies in a well-collimated 3 fs beam, achieving peak brilliance 9 × 10 24 ph s −1 mrad −2 mm −2 /0.1%BW. Such a source would be a powerful tool for studying fundamental electromagnetic [16] and nuclear processes [1, 17,18].Advances in high-intensity laser science offers opportunities for creating a new kind of high flux gamma-ray source, based on the use of strong laser fields to accelerate particles and stimulate emission within a single optical cycle [11-13, 19, 20]. However, from a naive consideration of particle dynamics one would expect particles to be expelled from the temporal and spatial regions which are optimal for energy gain (i.e. the electric field antinodes). Furthermore, for intensities above 10 24 W/cm 2 radiation losses prevent particles from reaching their potential maximum energy (during a single phase of acceleration). This limits the effective generation of photons to sub-GeV energies. [10,21] Our aim here is to find the optimal strategy for source creation. To do so we exploit the anomalous radiative trapping [14] (ART) of electrons and positrons in a dipole wave [22], the latter being the field configuration which provides the highest possible field strength for a given peak power arXiv:1610.06404v1 [physics.plasm-ph]

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Triggering vacuum breakdown at laser facility is expected to provide rapid electron-positron pair production for studies in laboratory astrophysics and fundamental physics. However, the density of the produced plasma may cease to increase at a relativistic critical density, when the plasma becomes opaque. Here, we identify the opportunity of breaking this limit using optimal beam configuration of petawatt-class lasers. Tightly focused laser fields allow generating plasma in a small focal volume much less than λ3 and creating extreme plasma states in terms of density and produced currents. These states can be regarded to be a new object of nonlinear plasma physics. Using 3D QED-PIC simulations we demonstrate a possibility of reaching densities over 1025 cm−3, which is an order of magnitude higher than expected earlier. Controlling the process via initial target parameters provides an opportunity to reach the discovered plasma states at the upcoming laser facilities.

a b s t r a c tThis paper concerns the development of a high-performance implementation of the Particle-in-Cell method for plasma simulation on Intel Xeon Phi coprocessors. We discuss the suitability of the method for Xeon Phi architecture and present our experience in the porting and optimization of the existing parallel Particle-in-Cell code PICADOR. Direct porting without code modification gives performance on Xeon Phi close to that of an 8-core CPU on a benchmark problem with 50 particles per cell. We demonstrate step-by-step optimization techniques, such as improving data locality, enhancing parallelization efficiency and vectorization leading to an overall 4.2× speedup on CPU and 7.5× on Xeon Phi compared to the baseline version. The optimized version achieves 16.9 ns per particle update on an Intel Xeon E5-2660 CPU and 9.3 ns per particle update on an Intel Xeon Phi 5110P. For a real problem of laser ion acceleration in targets with surface grating, where a large number of macroparticles per cell is required, the speedup of Xeon Phi compared to CPU is 1.6×.

The cascaded production and dynamics of electron-positron plasma in ultimately focused laser fields of extreme intensity are studied by 3D particle-in-cell simulations with the account for the relevant processes of quantum electrodynamics (QED). We show that, if the laser facility provides a total power above 20 PW, it is possible to trigger not only a QED cascade but also pinching in the produced electron-positron plasma. The plasma self-compression in this case leads to an abrupt rise of the peak density and magnetic (electric) field up to at least 10 28 cm −3 and 1/20 (1/40) of the Schwinger field, respectively. Determining the actual limits and physics of this process might require quantum treatment beyond the used standard semiclassical approach. The proposed setup can thus provide extreme conditions for probing and exploring fundamental physics of the matter and vacuum.

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