Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Here we present results from an experiment performed at the GSI Helmholtz Center for Heavy Ion Research. A mono-energetic beam of chromium ions with initial energies of  ~ 450 MeV was fired through a magnetized interaction region formed by the collision of two counter-propagating laser-ablated plasma jets. While laser interferometry revealed the absence of strong fluid-scale turbulence, acceleration and diffusion of the beam ions was driven by wave-particle interactions. A possible mechanism is particle acceleration by electrostatic, short scale length kinetic turbulence, such as the lower-hybrid drift instability.

Abstract:

We introduce a time-embedded convolutional neural network (TCNN) for modeling spatiotemporal heat transport in plasmas, particularly under strongly nonlocal conditions. In our earlier work, the Luciani-Mora-Virmont (LMV) Informed Neural Network (LINN) (Luo , ) combined prior knowledge from the LMV model with kinetic Particle-in-Cell (PIC) data to improve kernel-based heat-flux predictions. While effective under moderately nonlocal conditions, LINN produced physically inconsistent kernels in strongly time-dependent regimes due to its reliance on the quasistationary LMV formulation. To overcome this limitation, TCNN is designed to capture the coupled evolution of both the normalized heat flux and the characteristic nonlocality parameter using a unified neural architecture informed by underlying physical principles. Trained on fully kinetic PIC simulations, TCNN accurately reproduces nonlocal dynamics across a broad range of collisionalities. Our results demonstrate that the combination of time modulation, coupled prediction, and convolutional depth significantly enhances predictive performance, offering a data-driven yet physically consistent framework for multiscale plasma transport problems.

Abstract:

Many laser-based high-energy-density science (HEDS) facilities have one or more short-pulse (sub- to few-picosecond) laser beams for diagnostics. For the past decade, we have been developing a novel x-ray probing capability using such picosecond lasers interacting with an underdense plasma to produce relativistic electrons. The ultimate goal of these experiments is to demonstrate a new type of x-ray backlighter using the short-pulse ARC laser at the National Ignition Facility (NIF). Before this diagnostic is fielded at the NIF, it is critical to demonstrate the viability and reproducibility of the x-ray source on comparable high-power short-pulse laser systems. We present experiments that were carried out with the OMEGA EP laser at the University of Rochester’s laboratory for laser energetics. In these experiments, high-energy electrons are produced through a combination of the self-modulation instability and direct laser acceleration in an underdense gas jet. These electrons generate directional x-rays with characteristic energies up to several tens of keV as they execute betatron motion in the ion channel, and the inverse Compton scattering process generates even harder x-rays, with characteristic photon energies of 60–240 keV. When implemented on the OMEGA EP laser(s), this x-ray source yields results that are comparable to those obtained recently on the short-pulse Titan laser at the Jupiter Laser Facility at Lawrence Livermore National Laboratory, after accounting for differences in laser energy, peak intensity, focusing f/#, and plasma source. Applications of such an x-ray source for HEDS experiments are discussed.