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Researcher Thomas Grismaye 
GoLP 
Time: 10:00 am, May 22, 2015 
Place: LLP meeting room 
One of the most striking nonlinear quantum effect is the production of an electronpositron pair by an electric field in vacuum. The probability of this process was determined by Schwinger in 1951 and shows that the production of electronpositron pairs is exponentially small for electric fields small compared to the Schwinger field. Unfortunately, despite the tremendous progress observed in the development of ultraintense lasers in the last decades, the electric fields generated by current lasers are many orders weaker than the Schwinger field, so that many effects have exponentially small probabilities and are therefore unobservable. One can nevertheless observe certain nonlinear quantum effects in fields small compared to the Schwinger field by using ultra relativistic particles such that the field amplitude in the rest frame of the particles will be not too small or on the order of the Schwinger field. We intend to show in this presentation three types of QED effects that can nowadays be simulated in OSIRISPICQED framework: 1) QEDRadiation Reaction: we have incorporate a module which allows real photons emission from an electron (or a positron) according to the nonlinear Compton scattering. The typical electron energy sufficient to diagnose weak nonlinear quantum effects is around few GeV and such electron beams can now be generated from an alloptical source in an efficient manner: current experimental record for selfinjected electrons obtained in a laser wakefield accelerator is 4 GeV. PICQED simulations that allow us to evaluate the influence of quantum emission on the energy spread and the divergence of an electron beam colliding headon with an intense laser (I~10^21 W/cm2).Maximum attainable energy spread due to quantum stochasticity as a function of mean electron energy and the laser intensity is computed. 2) Seeded QED cascades in ultraintense counterpropagating lasers: In this configuration, the stimulated pair production is a twostep process: nonlinear Compton scattering (emission of a hard photon) + BreitWheeler (decay of a hard photon into a pair). We have investigated the pair cascades seeded by electrons in counterpropagating lasers pulses for ELI parameters. The selfconsistent modeling of these scenarios is challenging since some localized regions of ultraintense field will produce a vast number of pairs that may cause memory overflow during the simulation. To overcome this issue, we have developed a merging algorithm that allows merging a large number of particles into fewer particles with higher particle weights while conserving local particle distributions. This algorithm is crucial to investigate the laser absorption in selfgenerated pair plasmas. During the interaction, the laser energy is converted into pairs and photons and the absorption become significant when the plasma density reaches the critical density. With the results of 2D and 3D PICQED simulations, we will present the growth rates of the pair cascades and their dependence on the initial intensity of the lasers and on the polarisation. A simple analytical model for pair cascade will be compared with the numerical results. We will additionally show the respective fraction of laser energy transferred into pairs and photons for various configurations. Finally the various mechanisms contributing the laser absorption will be discussed 3) QED vacuum polarisation: Within the framework of QED, the second order process of photonphoton scattering mediated through the exchange of virtual electronpositron pairs leads to a set of corrected Maxwell's equations, effectively creating a non linear polarization and magnetization of the vacuum. To study this interaction, we incorporated a robust solver in the OSIRIS. We are thus able to self consistently solve the nonlinear Maxwell's equations for a broad set of initial conditions. Our simulations confirm the theoretical prediction of vacuum birefringence of a pulse propagating in the presence of an intense static background field. Our work also shows the generation of high harmonics when two counterpropagating pulses interact. A perturbative analysis yields an exact form for the self consistent fields in this setup which closely agrees with the simulation results obtained. Thomas Grismayer received a Master's Degree in Plasma Physics in 2003 at University Paris VI (France) with a specialization in LaserPlasma interaction. Thomas Grismayer completed a PhD degree in Plasma Physics at the Centre de Physique Théorique of Ecole Polytechnique in 2007 under the supervision of Prof. Patrick Mora. Right after in 2008, Thomas Grismayer moved to the United States to work as a postdoctoral fellow with Prof. Warren Mori, in the Plasma Simulation Group at the University of California Los Angeles (UCLA). In 2011, Thomas Grismayer received a postdoctoral fellowship from the Fundação para a Ciência e Tecnologia (FCT) and started to work in the Group for Lasers and Plasmas (GoLP) at Instituto Superior Técnico. In 2013 he was awarded a Starting Grant from FCT (Researcher) and he is currently continuing his research at GoLP. Since the beggining of his career, Thomas Grismayer has published 35 papers and proceedings in internationla peeer review journals. Research Interests: Ion acceleration in plasma expansion, particle acceleration in dense target irradiated by intense laser, parametric instabilities related to inertial fusion, nonlinear and multiple dimensional effects associated to electron plasma waves, magnetic field generation in shear instabilities and QED effects in plasma in extreme high fields. Thomas Grismayer received a Master's Degree in Plasma Physics in 2003 at University Paris VI (France) with a specialization in LaserPlasma interaction. Thomas Grismayer completed a PhD degree in Plasma Physics at the Centre de Physique Théorique of Ecole Polytechnique in 2007 under the supervision of Prof. Patrick Mora. Right after in 2008, Thomas Grismayer moved to the United States to work as a postdoctoral fellow with Prof. Warren Mori, in the Plasma Simulation Group at the University of California Los Angeles (UCLA). In 2011, Thomas Grismayer received a postdoctoral fellowship from the Fundação para a Ciência e Tecnologia (FCT) and started to work in the Group for Lasers and Plasmas (GoLP) at Instituto Superior Técnico. In 2013 he was awarded a Starting Grant from FCT (Researcher) and he is currently continuing his research at GoLP. Since the beggining of his career, Thomas Grismayer has published 35 papers and proceedings in internationla peeer review journals. Research Interests: Ion acceleration in plasma expansion, particle acceleration in dense target irradiated by intense laser, parametric instabilities related to inertial fusion, nonlinear and multiple dimensional effects associated to electron plasma waves, magnetic field generation in shear instabilities and QED effects in plasma in extreme high fields.
