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Oct 1

Moments in time: Numerical analysis of a method for time-resolved neutron spectroscopy

C. B. Stuart, B. Appelbe, A. J. Crilly, C. Forrest, A. DeVault, M. Gatu Johnson, B. J. Lahmann, D. Schlossberg, S. P. Regan, G. Gregori

Physics of Plasmas4 days agoControl & Diagnostics AI, Modeling & Simulation Inertial Fusion & HEDP

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.

Massachusetts Institute of TechnologyUniversity of RochesterLawrence Livermore National LaboratoryImperial College LondonUniversity of Oxford

Sep 30

High-beta runaway transitions in a fluid model of electromagnetic ion-temperature-gradient turbulence

Y. Zhang, M. Barnes, A. A. Schekochihin, P. G. Ivanov, T. Adkins

Plasma Physics and Controlled Fusion5 days agoPlasma & Confinement AI, Modeling & Simulation

Gyrokinetic simulations of tokamak turbulence indicate that fluctuation levels increase abruptly and dramatically when the plasma beta exceeds a certain critical value. This increase in fluctuation levels coincides with a transition from a state dominated by zonal flow to one in which turbulent eddies form radially-elongated `streamers'. Here we derive from gyrokinetics a minimal fluid model for electromagnetic ion-temperature-gradient (ITG) turbulence that captures the key features of this transition. Due to the relative simplicity of the model, we are able to conduct a detailed numerical study of the interplay between the turbulence and the zonal flow across a broad range of values of the plasma beta and the ITG. We find that the transition occurs when the Reynolds stress, which tends to strengthen zonal flows, is overwhelmed by the Maxwell and diamagnetic stresses, which tend to weaken them. Power-law scalings of the stress ratios with plasma beta and ITG are obtained, indicating a possible means by which the location of the transition could be predicted with minimal computational cost.

Princeton Plasma Physics LaboratoryUniversity of Oxford

Sep 21

Electronic-Entropy-Driven Phase Transitions in Compressed Iron Oxides

S. Azadi, S. M. Vinko, C. Crepisson, A. Principi, T. D. Kuehne, M. S. Bahramy

arXiv (cond-mat.mtrl-sci)Sep 21, 2026Inertial Fusion & HEDP

Electronic entropy is usually treated as a secondary correction to structural stability, but under strong electronic excitation it can become a primary thermodynamic driving force. Here we show that electronic entropy can drive both polymorphic and stoichiometric phase transformations in compressed iron oxides. Using finite-temperature density functional theory, we calculate the electronic-temperature-dependent Gibbs free energies of Fe$_2$O, FeO, Fe$_4$O$_5$, Fe$_3$O$_4$, and multiple Fe$_2$O$_3$ polymorphs, including $α$-, $ι$-, $ζ$-, $η$-, and $θ$-Fe$_2$O$_3$, over the pressure range 60--260 GPa. At 60-140 GPa, electronic excitation mainly reorganizes the relative stability of Fe$_2$O$_3$ polymorphs, driving transitions from $ι$-Fe$_2$O$_3$ to $η$-Fe$_2$O$_3$. At 180 GPa, the free-energy landscape becomes strongly competitive as FeO is stabilized over an intermediate range of electronic temperature, while $η$-Fe$_2$O$_3$ becomes favourable at higher T. At 220-260 GPa, the lowest-free-energy phase at low T is the Fe-rich compound Fe$_2$O, but increasing electronic temperature stabilizes FeO. These results demonstrate that electronic entropy can control not only the relative stability of crystal structures at fixed composition, but also the competition between different iron-oxide stoichiometries. The predicted electronic-entropy-driven phase boundaries provide a route to nonthermal structural transformations in ultrafast and high-energy-density experiments.

University of ManchesterUniversity of Oxford

Sep 7

Two-Phase Structure of Synchrotron-Cooling-Unstable Relativistic Plasma

Agnieszka Wierzchucka, Pablo J. Bilbao, Robert J. Ewart, Dmitri A. Uzdensky, Alexander A. Schekochihin

arXiv (astro-ph.HE)Sep 7, 2026AI, Modeling & Simulation

Using analytic theory, radiative particle-in-cell (PIC) simulations, and fluid simulations, we show that relativistic, synchrotron-cooling, collisionless, high-$β$ pair plasmas filament into a two-phase medium. This process occurs through the interplay of the synchrotron cooling instability (SCI) with the synchrotron firehose instability (SFHI). One phase has high plasma~$β$ and is infested with small-scale firehose fluctuations, which scatter particles and pin the pressure anisotropy to the firehose-marginal level. The other phase has much lower~$β$, causing the suppression of firehose modes and thus allowing large pressure anisotropies. We propose a fluid model for this two-phase plasma, which we use to study the linear and nonlinear evolution of the SCI and SFHI, and to predict the emergence time of the two-phase structure.

University of Oxford

Sep 2

Hall-MHD in driven turbulence FLASH simulations

A. Mohapatra, E. C. Hansen, A. Reyes, A. F. A. Bott, E. G. Blackman, P. Tzeferacos

arXiv (physics.plasm-ph)Sep 2, 2026AI, Modeling & Simulation Inertial Fusion & HEDP

The origin of magnetic fields in turbulent astrophysical systems has long been a central problem in plasma astrophysics. Fluctuation dynamos are a class of field amplification mechanisms that occur in turbulent magnetohydrodynamics whereby stochastically forced motions of plasma at sufficiently high magnetic Reynolds numbers exponentially amplify magnetic energy. For steady forcing, such dynamos saturate with magnetic energies at a sizable fraction of the turbulent kinetic energy. Although fluctuation dynamo is widely studied within the framework of resistive magnetohydrodynamics (MHD) and driven-turbulence numerical simulations, this work explores the difference when the Hall term is included in the magneto-fluid's generalized Ohm's law. The inclusion is motivated in part by recent high energy-density plasma experiments studying fluctuation dynamo that are governed by an extended magnetohydrodynamics (xMHD) ansatz, which includes the Hall term. We first discuss the details of the Hall-MHD implementation in the FLASH code, the tool we use to model xMHD fluctuation dynamo. We then investigate the influence of the Hall term on the fluctuation dynamo in a three-dimensional periodic box, driven with stochastic forcing at the box scale. We compare cases with a Hall term of varying magnitude to no-Hall cases with respect to the magnetic field growth rate, saturation level, and magnetic field structure. The Hall-MHD fluctuation dynamo is found to saturate at lower magnetic energies and with fewer small-scale magnetic structures than the no-Hall cases. Both findings are consistent with the interpretation that the Hall term acts as an additional, non-linear transport term, akin to an enhanced turbulent diffusivity.

University of OxfordUniversity of RochesterPacific Fusion

Sep 1

Generative Diffusion Surrogates with Analytical Variance Schedule

Patrick Reichherzer, Gianluca Gregori, David N. Hosking, Subir Sarkar

arXiv (cs.LG)Sep 1, 2026AI, Modeling & Simulation

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.

University of Oxford

Aug 19

Informing spectral models for dense plasmas with K-edge absorption measurements of warm dense copper

T. Cordova, E. V. Marley, D. A. Chin, R. A. London, S. B. Hansen, S. M. Vinko, J. E. Pask, H. A. Scott, H. P. Le, D. Aberg, et al.

arXiv (physics.plasm-ph)Aug 19, 2026Inertial Fusion & HEDP

Warm dense matter remains a challenging regime to characterize experimentally and to model with predictive accuracy. Recent experimental platforms have been developed to generate, characterize, and diagnose uniform warm dense matter, enabling detailed comparisons with models. Here, we present experiments conducted at the OMEGA laser facility that compress and heat a buried layer target to warm dense matter conditions, where the targets are heated to temperatures of approximately 20 eV and compressed to densities of 25 g/cm^3. We probe the warm dense plasma using x-ray absorption spectroscopy, using the K-edge and bound-bound absorption features to constrain the temperature and charge state distribution of the plasma. We compare these measurements with two types of models: collisional-radiative models with detailed electronic structure and ad-hoc density effects, and a multi-ion model based on density functional theory in combination with excited-state projector augmented-wave potentials. Neither approach fully reproduces the observed data, We show that the broad structure and position of the K-edge region can be modeled using density functional theory in combination with excited-state projector augmented-wave potentials. The density functional theory results are contrasted with a collisional-radiative model approach that incorporates ad-hoc density effects, which show incomplete agreement with the experimental observations, highlighting a need for improved density-dependent atomic modeling in warm dense plasmas.

University of California San DiegoUniversity of OxfordUniversity of RochesterSandia National LaboratoryLawrence Livermore National Laboratory

Aug 17

Assessing the Projector Augmented-Wave Method for Stopping Power Calculations

Bryn Lloyd, Dirk O. Gericke, Gilles Rodway-Gant, Gianluca Gregori

arXiv (physics.plasm-ph)Aug 17, 2026

The stopping power of charged particles is investigated using time-dependent density functional theory (TDDFT). Such simulations are made possible by recent advances in computational resources and numerical implementations of this first-principles method. In practice, DFT simulations widely employ the projector augmented-wave (PAW) method to approximate all-electron behaviour, but the implications of the PAW approximation for non-adiabatic stopping simulations remain insufficiently explored. Here, the suitability of the PAW method for stopping power simulations is evaluated. A workflow for generating and selecting PAW datasets tailored to these simulations is developed, enabling systematic optimisation of augmentation radii and projector constructions. The approach is applied to proton stopping in FCC aluminium, demonstrating how dataset design influences stopping predictions, and enabling an investigation of crystal channelling effects on charged-particle transport.

Alpha RingUniversity of Oxford

Aug 3

Inertial fusion by fast ignition with mirroring

Anthony R Bell

Plasma Physics and Controlled FusionAug 3, 2026Inertial Fusion & HEDP

We outline a potential route to laser-driven Inertial Confinement Fusion (ICF) by a variant of Fast Ignition (FI) in which energetic hot electrons are confined in a magneto-electric bottle between a strong magnetic field in the dense core and the electric field and ponderomotive pressure in the corona. The magnetic field is amplified by compression during the implosion. The converging magnetic field lines act as tramlines to guide the hot electrons into the dense core. The hot electrons make many transits of the bottle which acts as an electron hohlraum in which the hot electrons preferentially deposit energy in the dense core. With a suitable choice of parameters, about 50% of 50kJ given to hot electrons heats a small dense portion of the core to fusion temperatures and meets the Lawson condition for ignition. The paper concludes by listing further work needed on this preliminary proposal.

University of Oxford

Jul 28

Transport in high-performance plasmas of the TJ-II stellarator: From first-principles simulations to experimental validation

José Manuel García-Regaña, Daniel Alegre, Arturo Alonso, Régulo Anton, Enrique Ascasibar, Alfonso Baciero, Alejandro Banon Navarro, Jose-Miguel Barcala, Michael Barnes, M. Scherezade Barquero Balsera, et al.

Nuclear FusionJul 28, 2026Plasma & Confinement AI, Modeling & Simulation

We provide an overview of activities carried out at the TJ-II stellarator aimed at understanding transport from first principles and power balance analysis. These tasks include gyrokinetic simulations with the codes stella and EUTERPE, neoclassical simulations with the code SFINCS, particle deposition calculations with the code HPI2, and heat source estimates with ASCOT5. All these numerical simulation efforts converge, together with transport analyses, to address the transport mechanisms in plasmas with improved confinement through pellet injection—a scenario studied during the past few TJ-II campaigns bearing resemblance to the pellet-fueled high-performance plasma scenarios of observed in W7-X.

National Institute for Fusion ScienceCentre for Energy, Environmental and Technological ResearchUniversity of OxfordMax Planck Institute for Plasma Physics

Jul 13

Collisionless whistler heat-flux instability in ultra-high-$β$ plasmas

Rhisiart Davies, Prakriti Pal Choudhury, Archie F. A. Bott

arXiv (physics.plasm-ph)Jul 13, 2026AI, Modeling & Simulation

Kinetic instabilities, notably the whistler heat-flux instability (WHFI), are known to suppress thermal transport significantly in the moderate- to high-$β$ plasmas relevant to many astrophysical systems. This paper explores WHFI-regulated heat transport in a new regime: ultra-high-$β$ plasmas with $β_{e} \gtrsim L_{\mathrm{T}}/ρ_e$. Extrapolating previous theories of the WHFI to ultra-high-$β$ plasmas, we propose that the magnetic energy in unstable whistler fluctuations becomes comparable to that of the background magnetic field at saturation. We corroborate this hypothesis using 1D3V and 2D3V kinetic simulations using the particle-in-cell code OSIRIS. We find that, in ultra-high-$β$ plasmas, the heat flux is localised and no longer regulated primarily by resonant pitch-angle scattering of electrons; instead, thermal energy is transported predominantly by advection at the whistler phase velocity. Heat-flux suppression is observed in 1D3V and 2D3V simulations; however, we show that the saturation of the WHFI and the regulation of heat flux are sensitive to dimensionality in the ultra-high-$β$ regime. The amplitude and phase velocity of the heat-flux-regulating whistler waves scale differently with $β_e$, yielding parallel heat fluxes, normalised to the free-streaming value, of $q_{e\parallel} / q_\mathrm{fs} \approx 4.7 β_{e}^{-1}$ and $q_{e\parallel} / q_\mathrm{fs} \approx 0.3 β_{e}^{-1/2}$ in 2D3V and 1D3V simulations, respectively. We perform 2D3V simulations with background magnetic fields inclined to the temperature gradient, showing cross-field heat transport remains negligible. We develop a heuristic theory from kinetic equations that explains these phenomena. Our work extends our understanding of how the WHFI modifies thermal transport to regimes applicable to high-energy-density physics and the reionised intergalactic medium.

University of Oxford

Zonal-flow generation and saturation of electromagnetic ion-scale turbulence in tokamaks

Y. Zhang, T. Adkins, M. Barnes, A. V. Dudkovskaia, M. R. Hardman, P. G. Ivanov, D. Kennedy, A. A. Schekochihin

arXiv (physics.plasm-ph)Jul 13, 2026Plasma & Confinement AI, Modeling & Simulation

Local flux-tube gyrokinetic simulations of ion-scale turbulence in tokamak plasmas at finite plasma beta are conducted to investigate the generation of zonal flows via turbulent stresses. A parameter scan in the safety factor $q$ and electron beta $β_e$ reveals a transition from low- to high-transport states when $β_{\mathrm{eff}} \equiv q^2β_e$ exceeds a certain critical value $C_{\mathrm{nl}}$. While the linear stability limits for kinetic and ideal ballooning modes also scale as $β_e \propto 1/q^2$, they lie above the observed transition, indicating that the effect is not due to linear instabilities but to nonlinear dynamics. At low $β_{\mathrm{eff}}$, Reynolds stress dominates and drives zonal flows. At higher values, Maxwell stress becomes comparable, suppressing zonal-flow formation and leading to divergent transport. This nonlinear-transition boundary is determined for both the Cyclone Base Case and a spherical tokamak (ST40) configuration, suggesting that the relation $β_{\mathrm{eff}} = C_{\mathrm{nl}}$ may have broader applicability, though $C_{\mathrm{nl}}$ appears to be configuration-dependent. For the Cyclone Base Case, the ratio of energy transfer rates into zonal flows due to Maxwell and Reynolds stresses is observed empirically to scale as $β_e$ for $β_e$ below a critical value $β_{e,\mathrm{sb}}$ (scaling breakdown). The value of $β_{e,\mathrm{sb}}$ is found to increase with decreasing aspect ratio, suggesting that the linear scaling remains valid over a wider range of $β_e$ for more compact magnetic equilibria. This low-$β_e$ scaling provides the basis for a practical method to predict the nonlinear-transition threshold with minimal reliance on highly electromagnetic nonlinear simulations.

United Kingdom Atomic Energy AuthorityUniversity of OxfordPrinceton Plasma Physics Laboratory