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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - InPairs (In Silico Pair Plasmas: from ultra intense lasers to relativistic astrophysics in the laboratory)

Teaser

Summary

The main scientific questions and the research focus of InPairs are on electron-positron pair plasmas under extreme intensities, both in laboratory and astrophysical conditions, relevant at the focus of most intense lasers and particle beams now being deployed or compact astrophysicall objects such as black holes and neutron stars. We aim to understand how the collective plasma effects impact the dynamics in these systems, and their experimental and astronomical signatures, that underlie several longstanding challenges.
The work under development takes advantage of the unique capabilities for kinetic simulation of plasmas in extreme conditions available to my team, as well as from the recent theoretical and computational breakthroughs that we have achieved in connection with a particle merging algorithm for QED PIC simulations, radiation reaction in colliding laser-beam configurations and the theoretical developments on the plasma instabilities of electron-positron plasmas. All these advances are critical to uncover the physics and the signatures of the electrodynamics of plasmas under intense fields. At the same time, we are developing the numerical tools and algorithms capable of taking advantage of the next generation of exascale supercomputers to de deployed in Europe.
The challenges to be addressed are long standing scientific questions: how pulsars radiate? How is the behavior of plasmas in the presence of ultra intense magnetic fields? Can we explore some of this physics in the laboratory with lasers? The starting points are the recent developments in PIC simulations. InPairs is opening novel opportunities to explore the laboratory conditions associated with ultra intense lasers where these environments can be reproduced, and to use these findings to address fundamental questions associated with the most exotic objects in the universe. The ERC grant InPairs is organised along five challenges. By addressing and solving these interconnected five challenges it will be possible to provide answers to the compelling key scientific questions underlying this project, to open new avenues for research in the physics of extreme conditions or high energy density science both in the laboratory and in astrophysics, and to push the technological limits in massively parallel simulations in order to tap the potential of the next generation of exascale computational resources.

Work performed

The research project InPairs is aimed at significantly advancing the understanding of electron-positron pair plasmas under extreme intensities, both in laboratory and astrophysical conditions.

The first challenge is to develop and to prepare the massively parallel infrastructure for simulations of electron-positron pair plasma dynamics in extreme astrophysical and laboratory conditions in order to tap the extreme computing power of the exascale massively parallel systems

We have not yet commissioned and the installed two (small scale) prototypes of the next generation of exascale supercomputers (now underway, to be completed in the next few months) because the technology for the next generation exascale supercomputers in Europe is still under discussion and it is critical to align the decision here with the future landscale in Europe. More importantly, we have made significant progresses on the scalability of our main numerical simulation tool, Osiris demonstrating superb scalability. Within this challenge we have successfully improved OSIRIS for deployment on petascale and pre-exascale systems:
(1) Deployment of Xeon Phi cluster;
(2) OSIRIS improvements for current Petascale HPC systems;
(3) Advanced diagnostics for large scale simulations;
(4) New algorithms and physics models;
(5) Synthetic instruments for radiation modeling.
This demonstrates that our infrastructure is building up to the realm of the next generation of supercomputers.

The second challenge is to build the multidimensional picture of the microphysics of electron-positron pairs in ultra-magnetised relativistic plasmas by identifying the plasma instabilities relevant in these extreme scenarios and how these instabilities are modified by the inclusion of radiation reaction, QED processes or nontrivial spacetime metrics.
Under this challenge we have made significant effort in the understanding and development of a novel radiation diagnostic that has been incorporated into the main simulation tool, Osiris. The research was also focused on the generation and evolution of the magnetic field in initially unmagnetized plasmas and it has been conducted mixing analytical and numerical techniques.
Finally, we have explored the magnetorotational instability (MRI) that is a crucial mechanism of angular momentum transport in a variety of astrophysical scenarios. This work focused on the study of large simulation domains and long-time evolution of a fully kinetic collisionless shear plasma. This large scale analysis was essential to identify, for the first time via a PIC code, the mechanism responsible for the generation of the 2D turbulent motion in the late nonlinear stage of the MRI in pair plasma, namely magnetic reconnection and drift- kink instability.

The third challenge is to identify and to fully characterize the inherently three-dimensional self-consistent dynamics of the electron-positron pairs generated at the focus of ultra high intensity laser pulses via QED cascades, and their feedback on the laser field, and, for even higher intensities, including the nonlinear dynamics of the electromagnetic fields in vacuum.
We have focused our work on the laser absorption in self â€“generated electron-positron pair plasmas at near-critical density, including collisionless absorption and laser frequency up-conversion. The collisionless laser absorption shows a higher absorption fraction compared with the classical electron-ion plasmas. This enhancement is due to the contribution from the positrons which respond to the laser fields as fast as the electrons.
One of the central question in this challenge is how does the growth rate of the cascade depends on the polarization and on the intensity of the laser and whether an intensity threshold can be determined. The setup with two linearly polarized lasers is more recommendable to ensure the take-off the cascade. Taking advantage of growth rates measured, we predict that the cascading process should start around I

Final results

The project focuses on using self-consistent ab initio massively parallel simulations to study electron-positron pair plasmas under extreme intensities, both in laboratory and astrophysical conditions.
We focused on acquisition and setup of an hybrid CPU/GPU machine has a first step for the development and optimization, testing and profiling of the Osiris code at node-level parallelization. Once the code is optimized at node level, the next step will be to setup a multi-node prototype in an architecture similar to the future exascale machines. It is expected until the end of the project, that weâ€™ll have installed and setup a multi-node prototype that mimics in a small scale the type of configuration found on exascale computers. This will be unique and well beyond the state-of-the-art in particle-in-cell (PIC) codes. Besides that, we have setup a new Virtual Reality system to explore novel ways for visualizing the resulting simulations with already very exciting results.

The dynamics of ultra relativistic (e-,e+) fireball beam with plasmas have also been explored in detail. We also have shown that the longitudinal energy spread, typical of plasma-based accelerated electronâ€“positron fireball beams, plays a minor role in the growth of CFI in these scenarios. Results of these works are currently being summarized for publication.
Focus has been given to collisionless laser absorption in the self-generated pair plasmas. The PIC simulation shows the laser absorption is significantly different in pair plasmas. More analyses are under way. A numerical study of the scaling with different laser intensities and pair densities is also planned before the end of this project. We have also considered high-order harmonic generation (HHG) in an electron-positron-ion plasma which is significantly changed after an electron-positron pair plasma is produced. The result in this project is summarized in a manuscript which is to be submitted.
In the framework of beam-beam colliding in high-disruption regime our PIC simulation shows the photon emission is ~20 per primary particle in the electron-positron colliding case (with disruption of D=9.6). A systematic study, including the kinetic process and the scaling of disruption parameters, is underway. This will open a new avenue for research in beam-beam physics in the high disruption regime where pair-plasma production is of significant importance.
Our work on InPairs also represents one of the very first attempts to simulate in full scale and in three dimensions the conversion of optical intense light into relativistic electron-positron-photon plasma. We have proposed new configurations to accelerate particles, and we studied experimental signatures for the radiation-dominated regime of laser-electron beam scattering. We also studied the energy transfer from electrons to hard photons due to classical / quantum radiation emission, and this has led to the participation in the design of future experiments.
We plan to study the prospects of using generated electron beams to be used as injectors for tunable table-top X-ray/gamma-ray sources. We expect these results will lead to sources of unprecedented properties of relevance for several societal applications.
We showed how the collision of relativistic particle beams (10s GeV) with high density (1021 cm-3) can access the QED regime with the production of Breit-Wheeler pairs and emission of bright Î³-rays, Del Gaudio et al. Phys. Rev. Accel. Beams 22, 023402 (2019). The beam parameters required to access this regime can be provided by the new SLAC facility FACET II and by the next generation of Laser Wakefield Accelerators.
In collaboration with SLAC, Princeton, MEPhI and DÃ¼sseldorf, we showed that the nonperturbative QED regime can be achieved by a 100 GeV-class particle collider provided a high compression and focusing of the colliding beams, V. Yakimenko et al. published in Phys. Rev. Lett. (April 2019). This paper opens a whole new resea