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Breakthrough in the study of laser / plasma interactions

Breakthrough in the study of laser / plasma interactions

Large-scale simulations show that chaos is responsible for the stochastic heating of dense plasma with intense laser energy. This image shows a snapshot of a phase space for the distribution of electrons (position / momentum) from the solid plasma taken from PIC simulations illustrating the so-called "stretching and folding" mechanism responsible for the occurrence of chaos in physical systems. Sincerely, G. Blaclard, CEA Saclay

A new 3-D particle-cell simulation tool (PIC), developed by researchers from the Lawrence Berkeley and CEA Saclay National Laboratories, allows for the most advanced simulations of laser / plasma-linking mechanisms previously out of reach of the standard PIC codes used in the plasma study. A more detailed understanding of these mechanisms is critical to the development of ultra-compact particle accelerators and light sources that could address the long-standing challenges in medicine, industry and fundamental science more effectively and more effectively.

In laser plasma experiments, such as those at the Berklee laser accelerator center (BELLA) and CEA Saclay, an international research center in France, part of the French Nuclear Energy Commission, very large plasma fields in the plasma that accelerate particles higher energies at much shorter distances than existing acceleration technologies. The long-term goal of these laser-accelerators (LPAs) is one day to create high-energy collisions, but many deviations are already being developed. For example, LPAs can quickly deposit large amounts of energy into solids by creating dense plasmas and subjecting it to extreme temperatures and pressures. They also have the potential to drive lasers of free electrons that generate light pulses of only attoseconds duration. Such extremely short impulses can enable researchers to observe the interactions of molecules, atoms and even subatomic particles in extremely short terms.

Supercomputer simulations are becoming more and more critical for this research, and Berkshire Laboratory's National Research Center for Energy Research (NERSC) has become an important resource in this effort. By giving scientists access to physically observed as particle orbit and radiated fields, which are difficult to obtain in experiments at extremely small scale for time and length, PIC simulations have played a major role in understanding, modeling, and conducting high intensity experiments. But the lack of PIC codes that have sufficient computational accuracy to model the laser-material interaction at ultra-high intensities prevents the development of new particles and light sources produced by this interaction.

This challenge led Berkeley Lab / CEA Saclay's team to develop their new simulation tool called Warp + PXR, an effort that began during the first round of the NESC Exascale Science Applications (NESAP) program. The code combines the widely used 3-D PIC code Warp with the high-performance PICSAR library, developed jointly by Berkeley Lab and CEA Saclay. It also uses a new type of mass parallel pseudo-spectral solution developed jointly by Berkeley Lab and CEA Saclay, which greatly improves simulation accuracy compared to those commonly used in plasma imaging studies.

In fact, without this new, very scalable solution, "the simulations we are doing now would not be possible," said Jean-Luc Vay, a senior physicist at Berkeley Lab, who heads the accelerator modeling software in the applied physics and acceleration technology of the lab . Division. "As our team has shown in a previous study, this new FFT spectral determinant gives much more accuracy than can be achieved with FDTD, so we managed to reach some parametric spaces that do not would be available with a standard FDTD solution. "This new type of spectral determinant is also the basis of the next-generation PIC algorithm with adaptive mesh improvement that Vay and colleagues develop into the new Warp-X code as part of the Exascale Computing Project the US Secretary of State of energy.

2-D and 3-D simulations both critical

Vai also co-authored an article published on March 21st Physical Review X reported the first complete study of the laser-plasma connectors using Warp + PXR. This study combines state-of-the-art experimental measurements conducted at the UHI100 laser unit at CEA Saclay with advanced 2-D and 3-D Cori supercomputer simulations in NERSC and Mira and Theta systems in Argonne's Argonne Computer Base at the Argonne National Laboratory. These simulations have enabled the team to better understand the mechanisms of connection between ultra-intense laser light and the dense plasma it created, providing new ideas for optimizing ultra-compact particles and light sources. Warp + PXRs showed that the code is scalable up to 400,000 Cori cores and Mira's 800,000 cores, and can speed up the time to solve a whole range of problems with ultra-high intensity physics experiments. ,

"We can not repeat or reproduce what happened in the 2-D simulation experiment – 3-D is needed," said co-author Henri Vincenti, a high-intensity physics scientist at CEA Saclay. Vincenti leads the theoretical / simulation work for the new study, and Marie Curie is a post-graduate student at Berkeley Lab in Vay, where he first started working on the new code and solver. "3-D simulations were also very important in order to compare the accuracy of the new code against experiments."

For the experiment described in Physical Review X CEA Saclay's researchers use a high-power (100TW) femtosecond laser beam in the CEA's UHI100 facility, focused on a target of silica to create a dense plasma. In addition, two diagnostics – Lanex scintillation screen and extreme ultraviolet spectrometer – were used to study the laser-plasma interaction during the experiment. Diagnostic tools present additional challenges when it comes to studying time and length scales while the experiment is running, again making simulations critical of researchers' findings.

"Often in this type of experiment, you can not reach the included time and length scales, especially because in experiments you have a very intense laser field of your target, so you can not diagnose near the target," said Fabien Quare, a researcher who runs experimental program at CEA and co-authored PRX paper. "In this type of experiment, we look at things that emanate from the target, which is far away – 10, 20 cm – and it happens in real time, essentially, while the physics is on a micron or submicron scale and subfecutecond scale in time. we need simulations to decipher what is happening in the experiment. "

"The simulations of the first principles we used for this study gave us access to the complex dynamics of the interaction of the laser field with the solid target at the level of particle detail that allows us to better understand what is happening in the experiment, Vincent.

These very large simulations with ultra-high-precision spectral FFT resolver were possible thanks to the paradigm shift introduced in 2013 by Vay and associates. In a study published in the Journal of Computational Physics, they noted that in solving Maxwell's time-dependent equations, the standard FFT parallelization method (which is global and requires communication between processors throughout the simulation domain) can be replaced with a domain . decomposition with local FFT and communications limited to adjacent processors. In addition to enabling much more favorable high and low scaling in a large number of computer nodes, the new method is also more energy efficient as it reduces communications.

"In standard FFT algorithms, you have to make communication across the machine," Vai said. "The new Spectral FFT solution allows for time and energy saving on the computer, which is a great job for introducing new supercomputer architectures."

The laser drill sets a new world record for the acceleration of electrons with a laser

More information:
L. Chopineau et al., Identification of coupling mechanisms between ultra-intensive laser light and dense plasmas, Physical Review X (2019). DOI: 10.1103 / PhysRevX.9.011050

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Laurence Berkeley National Laboratory

Breakthrough in the Laser / Plasma Interaction Study (2019, April 24)
drawn up on 25 April 2019

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