Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Stochastic transport describes physical systems in which an initially structured distribution spreads under unresolved forcing, scattering, or heterogeneous media. Useful surrogates for such systems should be probabilistic, time-resolved, and able to represent non-Gaussian distributional structure. Generative diffusion models, which corrupt data with Gaussian noise and learn a reverse flow back to structured states, have these properties. Their noise schedules, however, are usually chosen heuristically: image and audio generation-the canonical use cases-provide no physical clock. In transport, by contrast, the variance, or mean-square displacement, is often known from macroscopic theory or empirical scaling even when the full distribution is not. Here we prescribe the forward noising rate as the time derivative of this variance, turning generative time into a calibrated transport clock. The variance path is enforced by construction, while the learned score field represents how non-Gaussian structure inherited from entrance data is smoothed along that path, requiring no intermediate-time physical transport data. For ballistic-to-diffusive transport in turbulent plasmas, the surrogate matches test-particle distributions, reproduces the laboratory-measured variance scale, and tracks the simulated kurtosis evolution without schedule tuning, enabling calibrated emulation and likelihood-based inference.

Abstract:

<jats:p>Time-resolved neutron information is essential for understanding the dynamics of inertial confinement fusion (ICF) implosions, providing key indicators that distinguish igniting from non-igniting plasmas and revealing the underlying causes of shot-to-shot performance variations. The moments method framework offers a practical pathway to extract such temporally resolved quantities using only a small number of neutron time-of-flight detectors. For example, one only needs four detectors to extract the burn-averaged rate of change of ion temperature, a well-defined physically useful quantity. In this work, we evaluate and validate the moments method across several levels of complexity. We first employ a controlled toy model to test the foundational assumptions and mathematical structure of the approach. We then demonstrate that the method generalizes to realistic one-dimensional simulation data of direct drive implosions with a 25 kJ laser driver that include time-dependent ion temperature evolution and hydrodynamic burn dynamics. Together, these results establish the moments method as an experimentally accessible and robust tool for extracting unique and independent information such as the burn-averaged rates of change of fluid velocity and ion temperature with time from neutron time-of-flight measurements, broadening the diagnostic capability of existing neutron detector systems. Fielding this diagnostic on ICF campaigns would improve our understanding of failure modes, helping to determine causes for improved or reduced performance during implosions, informing future target and laser drive design.</jats:p>

Abstract:

We investigated laser-driven shock propagation in low-density foams through coordinated experiments and simulations. At the Vulcan laser facility, a nanosecond laser pulse launched a shock from a plastic ablator–titanium pusher assembly into a 0.1 g cm–3 trimethylolpropane triacrylate foam, diagnosed by side-on, time-resolved X-ray radiography using a picosecond Cu backlighter. This enabled direct tracking of the pusher motion and the shock front. Comparison with one-dimensional and two-dimensional hydrodynamic simulations (MULTI and FLASH) shows good agreement when assuming an effective on-target intensity of (3–5) × 1013 W cm–2, lower than nominal laser estimates, underscoring the influence of coupling losses. An analytical model based on impedance-matched shock relations connects the ablation pressure, pusher dynamics and transmitted shock strength. Overall, the combined results indicate that the foam behaves as an effectively homogeneous medium under these conditions, supporting modelling assumptions for foam-based fusion targets and establishing a validated platform for future shock-propagation studies in low-density materials.