Researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) and CEA Saclay simulated laser/plasma coupling mechanisms using a new 3D particle-in-cell simulation tool. A more detailed understanding of laser-plasma interactions could contribute to the development of novel particle and light sources for application in medicine, industry, and fundamental science. The simulation tool, dubbed Warp+PXR, leverages a new type of massively parallel pseudo-spectral solver developed by the researchers. Researcher Jean-Luc Vay said that the new spectral FFT solver enabled much higher precision than finite difference time domain (FDTD) solvers, allowing the team to reach parameter spaces that would not have been accessible with standard FDTD solvers. Using the Warp+PXR, the team performed a comprehensive study of laser-plasma coupling mechanisms that combined experimental measurements conducted at CEA Saclay with 2D and 3D simulations conducted at the National Energy Research Scientific Computing Center (NERSC) and the Argonne National Laboratory (ANL). These simulations enabled the team to better understand the coupling mechanisms between ultra-intense laser light and the dense plasma it created, providing new insights into how to optimize ultracompact particle and light sources. Benchmarks with the Warp+PXR tool showed that the code was scalable up to 400,000 cores on the Cori supercomputer at NERSC and up to 800,000 cores on the Mira system at ANL. Benchmarks additionally showed that the new tool could speed up the time to solution by as much as three orders of magnitude on problems related to ultrahigh-intensity physics experiments. Large-scale simulations demonstrate 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 particle-in-cell simulations, illustrating the so-called stretching and folding mechanism responsible for the emergence of chaos in physical systems. Courtesy of G. Blaclard, CEA Saclay. For the experiment, the CEA Saclay researchers used a high-power (100 TW) femtosecond laser beam focused on a silica target to create a dense plasma. 2D and 3D simulations were both critical, according to the researchers. “We cannot consistently repeat or reproduce what happened in the experiment with 2D simulations — we need 3D for this,” said researcher Henri Vincenti. “The 3D simulations were also really important to be able to benchmark the accuracy brought by the new code against experiments.” Two diagnostics — a Lanex scintillating screen and an extreme-UV spectrometer — were applied to study the laser-plasma interaction during the experiment. The diagnostic tools presented challenges when it came to studying time and length scales while the experiment was running, making the simulations critical to the researchers’ findings. “Often in this kind of experiment you cannot access the time and length scales involved, especially because in the experiments you have a very intense laser field on your target, so you can’t put any diagnostic close to the target,” said researcher Fabien Quéré. “In this sort of experiment we are looking at things emitted by the target that is far away — 10, 20 cm — and happening in real time, essentially, while the physics are on the micron or submicron scale and subfemtosecond scale in time. So we need the simulations to decipher what is going on in the experiment.” Vincenti said that the first-principles simulations used for the experiment gave the researchers access to the complex dynamics of the laser field interaction, allowing them to better understand what was happening in the experiment. In addition to enabling more favorable strong and weak scaling across a large number of computer nodes, the new simulation method is more energy-efficient because it reduces communications. “With standard FFT algorithms you need to do communications 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 the new supercomputing architectures being introduced.” The research was published in Physical Review X (https://doi.org/10.1103/PhysRevX.9.011050).