Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The optimisation of laser pulse shapes and target configurations is central to high-performance inertial confinement fusion (ICF) implosions, yet remains challenging due to the high dimensionality of the design space and the substantial computational and experimental cost of evaluation. This work presents, to our knowledge, the first comparison of GP-based Bayesian optimisation and particle swarm optimisation (PSO) frameworks augmented with physics-motivated extensions in full radiation-hydrodynamic ICF optimisation under experimentally relevant constraint handling. These methods are first applied to the reoptimisation of low-convergence-ratio (Low-CR) wetted-foam implosions, providing a benchmark against traditional sequential scan approaches. Both strategies identify improved designs, with Bayesian optimisation achieving the highest final performance using fewer simulations, while PSO converges more rapidly in wall-clock time. The PSO framework is then extended to a 16-dimensional fast ignition design problem, where Gaussian process-based Bayesian optimisation becomes computationally impractical. In this regime, PSO efficiently identifies a compressed fuel assembly with ρR ≈ 1.5 g/cm2 under strict laser intensity and energy constraints. These results demonstrate that the presented optimisation strategies outperform conventional scan-based approaches and provide a scalable platform for high-dimensional ICF optimisation. Beyond numerical design studies, the same frameworks are directly applicable to experimental optimisation campaigns on high-power laser facilities, where limited shot availability and high evaluation cost demand efficient search methodologies.

Abstract:

How momentum, energy, and magnetic fields are transported in the presence of macroscopic gradients is a fundamental question in plasma physics. Answering this question is especially challenging for weakly collisional, magnetized plasmas, where macroscopic gradients influence the plasma's microphysical structure. In this paper, we introduce thermodynamic forcing, a new method for systematically modeling how macroscopic gradients in magnetized or unmagnetized plasmas shape the distribution functions of constituent particles. In this method, we propose to apply an anomalous force to those particles inducing the anisotropy that would naturally emerge due to macroscopic gradients in weakly collisional plasmas in which thermal pressure is much larger than magnetic pressure. We implement thermodynamic forcing in particle-in-cell (TF-PIC) simulations using a modified Vay particle pusher and validate it against analytic solutions of the equations of motion. We then carry out a series of simulations of electron-proton plasmas with periodic boundary conditions using TF-PIC. First, we confirm that the properties of two electron-scale kinetic instabilities—one driven by a temperature gradient and the other by bulk-velocity gradient—are consistent with previous results. Then, we demonstrate that in the presence of both macroscopic gradients, heat-flux saturation is mediated by the bulk-velocity-gradient-driven electron firehose instability rather than the temperature-gradient-driven whistler instability. This suggests that saturation mechanisms may differ from our current understanding in the presence of multiple free energy sources. This work enables, for the first time, systematic and self-consistent transport modeling in weakly collisional plasmas, with broad applications in astrophysics, laser-plasma physics, and inertial confinement fusion.

Abstract:

Low-density foams are of significant interest in inertial confinement fusion (ICF), with potential applications as fuel carriers, ablation layers, or as a hohlraum filling material. Despite their potential, the shock response of these materials remains poorly characterised, limiting the accuracy of hydrodynamic simulations. Here we report experimental measurements of the equation of state (EOS) for 90 mg/cm3 silica (SiO2) aerogel foam under laser-driven shock compression, conducted at the GEKKO XII laser facility. Shock pressures between 50 and 160 GPa were achieved, and the corresponding states were determined using standard impedance matching techniques with a quartz reference material. Initial measurements appeared to underestimate the foam shock velocity relative to predictions by the Quotidian Equation of State (QEOS) model. Experimental diagnostics indicated the presence of a vacuum gap between the reference material and the foam. The vacuum gaps were characterised, and one-dimensional radiation-hydrodynamic simulations were conducted to estimate their impact on the measured shock velocity. After applying simulation-based corrections, the experimental Hugoniot aligns closely with QEOS predictions, supporting the model’s applicability to low-density foams.