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A breakthrough in the study of laser / plasma interactions



 Large-scale simulation demonstrating that chaos is responsible for stochastic heating of dense plasma by intense laser energy. This image shows a snapshot of electron distribution phase space (position / momentum) from the dense plasma taken from PIC simulation, illustrating the so-called "stretching and folding" mechanism responsible for the emergence of chaos in physical systems. Credit: G. Blaclard, CEA Saclay
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<p> A new 3-D particle-in-cell (PIC) simulation tool was developed by Lawrence Berkeley National Laboratory and CEA Saclay codes used in plasma research. More detailed Understanding of the Mechanisms for the Development of Ultra-Compact Particle Accelerators and Light Sources that Could Lead to Long-standing Challenges in Medicine, Industry, and Fundamental Science more efficiently and cost effectively.<br />
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In laser-plasma experiments at the Berkeley Lab Laser Accelerator Center (BELLA) and at the CEA Saclay-an international research facility in France that is part of the French Atomic Energy Commission-very large electric fields within that accelerate particle beams to high energy over much shorter distances when compared to existing accelerator technologies. The long-term goal of these laser-plasma accelerators (LPAs) is to build one-day colliders for high-energy research. For instance, LPAs can rapidly deposit large amounts of energy into solid materials, creating dense plasmas and subject matter to extreme temperatures and pressures. They also hold the potential for driving free-electron lasers that generate light pulses lasting just attoseconds.

Supercomputer simulations have become increasingly critical to this research, and Berkeley Lab's National Energy Research Scientific Computing Centers (NERSC) has become an important resource in this effort. PIC simulations have taken a major role in understanding, modeling, and guiding high-intensity physics experiments. This challenge led to the creation of the Berkeley Lab / CEA Saclay team.

This challenge led to the creation of the Berkeley Lab / CEA Saclay team dubbed Warp + PXR, an effort launched during the first round of the NERSC Exascale Science Applications Program (NESAP). The code combines the 3-D PIC code Warp with the high performance library PICSAR co-developed by Berkeley Lab and CEA Saclay. Berkeley Lab and CEA Saclay have dramatically improved the accuracy of the simulation compared to the solvers typically used in plasma research.

In fact, without this new, highly scalable solver, "said Jean-Luc Vay, a senior physicist at Berkeley Lab who heads the Accelerator Modeling Program in the Lab's Applied Physics and Accelerator Technologies Division. "As our team showed in a previous study, this new FFT spectral solver allows much higher precision than can be done with finite difference time domain (FDTD) solvers, so we were able to reach some parameter spaces that would not have been accessible with standard FDTD solvers. " PIC algorithm with adaptive mesh refinement that Vay and colleagues are developing in the new Warp-X code as part of the United States. Department of Energy's Exascale Computing Project.

2-D and 3-D Simulation Both Critical

Vay is also co-authored on a March 21 paper in Physical Review X reports on the first comprehensive study of the laser-plasma coupling mechanisms using Warp + PXR. The study combines the state-of-the-art experimental measurements of the UHI100 laser facility at CEA Saclay with cutting-edge 2-D and 3-D simulations on the Cori supercomputer at NERSC and the Mira and Theta systems at the Argonne Leadership Computing Facility at Argonne National Laboratory. The simulation of the interaction between the ultra-intense laser light and the plasma creates it, provides new insight into how to optimize the ultra-compact particle and light sources. Benchmarks with Warp + PXR showed that it can be scaled up to 400,000 cores on Cori and 800,000 cores on the way to solve the problem of ultra-high-intensity physics experiments

"We can not repeat or reproduce what happened in the experiment with 2-D simulation-we need 3-D for this," said co-author Henri Vincenti, a scientist in the high-intensity physics group at CEA Saclay , Vincenti is a theoretical researcher at the Berkeley Lab in Vay's group, where he started working on the new code and solver. "The 3-D simulations were thus really important to be able to benchmark the accuracy brought by the new code against experiments."

For the experiment outlined in the Physical Review X paper, the CEA Saclay researchers CEA's UHI100 facility uses a high-power (100TW) femtosecond laser beam focused on a silica target to create a dense plasma. In addition, two diagnostics-a Lanex scintillating screen and extreme-ultraviolet spectrometer were applied to study the laser-plasma interaction during the experiment.

"Often in this kind of experiment you can not access the time and length scales involved, especially because in the experiment you have a very intense laser field on your target, "said Fabien Quéré, a research scientist who leads the experimental program at CEA and is a co Author of the PRX paper. In this sort of experiment we are looking far away-10, 20 cm-and happening in real time, essentially, while the physics are on the micron or submicron scale and sub-femtosecond scale in time

"The first-principles simulation we use for this research gave us access to the complex dynamics of laser field interaction, with the solid target at the level of detailing individual particle orbits, "Vincenti added."

These very large simulations with ultrahigh-precision spectral FFT solutions were made possible by a paradigm shift in 2013 by Vay and collaborators , In a study published in the Journal of Computational Physics, they find that when Maxwell's equations, the standard FFT parallelization method (which is global and requires communication between processors across the entire simulation domain), were replaced with a domain decomposition with local FFTs and communications limited to neighboring processors.

"With standard FFT algorithms you need to do all the communication across the entire machine," "Vay said. "But the new spectral FFT solver enables savings in both computer time and energy, which is a big deal for new supercomputing architectures."
                                                                                                                        


Laser 'drill' sets a new world record in laser-driven electron acceleration


More information:
Chopineau et al., Identification of Coupling Mechanisms between Ultrasonic Laser Light and Dense Plasmas, Physical Review X (2019). DOI: 10.1103 / PhysRevX.9.011050

Provided by
Lawrence Berkeley National Laboratory




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                                                 A breakthrough in the study of laser / plasma interactions (2019, April 24)
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