Cutting Edge Simulations of Lasers Interacting With Dense Plasmas
Intense laser pulses can quickly deposit large amounts of energy into solid materials, thus creating dense plasmas and subjecting matter to extreme temperature and pressure. This is useful in a variety of scientific applications, such as laboratory astrophysics, accelerating particles to high energies over short distances, or generating light pulses lasting just attoseconds.
Yet the lack of accurate enough computational Particle-In-Cell (PIC) codes used to model laser-matter interaction at ultra-high intensities has so far considerably hindered the understanding and development of novel particle and light sources produced by this interaction. A collaboration of scientists from LBNL and CEA Saclay has recently overcome this barrier by developing a game changer highly accurate and massively parallel 3D PIC code that enabled simulations previously out of reach of standard codes. This code uses a new type of massively parallel pseudo-spectral solvers to advance Maxwell’s equations that bring orders of magnitude in accuracy compared to Finite Difference Time Domain (FDTD) solvers. The new code nicknamed WARP+PICSAR couples the 3D PIC code WARP with the high-performance library PICSAR co-developed by LBNL and CEA Saclay. This new pseudo-spectral solver and PICSAR library are also at the heart of the new code WarpX developed with the support of DOE’s Exascale Computing Project.
Ultra-High Intensity (UHI) Physics focuses on the interaction of high-power femtosecond lasers with matter at ultra-high light intensities. UHI physics is a recent branch of physics that originated with the advent of high-power ultra-short laser pulses thanks to the Chirped Pulse Amplification (CPA) technique invented by D. Strickland and G. Mourou, who were just awarded the Nobel Prize in physics in 2018. With the onset of PetaWatt (PW) class lasers, UHI physics is attracting considerable interest from the scientific community as the understanding and control of the laser-matter interaction could provide promising particle and light sources that could solve long-standing challenges in medicine, industry and fundamental science.
Thanks to the new capabilities brought by the new Particle-In-Cell code in terms of accuracy, the collaboration significantly advanced the state-of-the-art in UHI physics by elucidating the coupling mechanisms between the ultra-intense laser light and dense plasma. This better understanding of the laser-plasma coupling mechanism is a major milestone towards the optimization of novel compact UHI particle and light sources targeting promising applications (e.g. cancer hadron therapy or Ultrafast Science).
Advancement of this UHI physics expressly requires the strong interplay between its two pillars, which are: (i) experiments with high-power lasers and (ii) first-principles Particle-In-Cell (PIC) simulations of relativistic plasmas. By giving us access to physical observables (particle orbits, radiated fields) that are very hard to get in experiments at these extremely small time and length scales, PIC simulations have played a major role in the understanding, modeling and guiding of experiments.
Unfortunately, standard PIC codes induce numerical errors that can significantly affect important UHI processes. Their mitigation demands a numerical resolution that is so high that 3D simulations required to address some long-standing challenges have not been tractable so far. The team first achieved a paradigm shift in first principles simulations of plasmas by developing a game-changer massively parallel and extremely accurate pseudo-spectral Particle-In-Cell (PIC) code WARP+PICSAR. This code enabled an accurate modeling on NERSC supercomputers of key laser-plasma processes required to address major UHI challenges. Benchmarks with WARP+PICSAR at very large scale, on Cori at NERSC and Mira at ALCF, show that the code is scalable on up to 400k cores on CORI and 800kcores on Mira and can bring up to three-order of magnitude speed-up in time to solution on problems related to UHI.
Combining experimental measurements with numerical simulations, the team demonstrated that the coupling mechanism between the laser field and the plasma depends on the steepness of the density gradient at the solid target surface, which results from the plasma expansion into vacuum. As this gradient is increased, the electron dynamics switches from periodic to chaotic behavior, and we identify clear experimental evidence of this transition. This chaotic behavior is responsible for a large laser energy deposition into the plasma by stochastic heating.
 A. Leblanc, S. Monchocé, H. Vincenti, S. Kahaly, J.-L. Vay, F. Quéré, “Spatial Properties of High-Order Harmonic Beams from Plasma Mirrors: A Ptychographic Study” Phys. Rev. Lett. 119, 155001 (2017) https://doi.org/10.1103/PhysRevLett.119.155001
 H. Vincenti, J.-L. Vay, “Ultrahigh-order maxwell solver with extreme scalability for electromagnetic pic simulations of plasmas”, Comput. Phys. Comm. 228, 22-29 (2018) https://doi.org/10.1016/j.cpc.2018.03.018
 L. Chopineau, A. Leblanc,G. Blaclard, A. Denoeud,M. Thévenet, J-L. Vay, G. Bonnaud, Ph. Martin, H. Vincenti and F. Quéré, “Identification of coupling mechanisms between ultraintense laser light and dense plasmas”, Phys. Rev. X, in press, https://arxiv.org/abs/1809.03903
About NERSC and Berkeley Lab
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