arXiv subjects

J. P. Chittenden

Publications and source records attributed to J. P. Chittenden.

At least 19 recordsLinked to original sources

arXiv2026

Passive freeze-out of the Richtmyer-Meshkov instability

The Richtmyer-Meshkov instability (RMI) poses a major challenge in inertial confinement fusion (ICF) due to its role in mixing and performance degradation. We report the first experimental observation of passive freeze-out of RMI in a low-pressure surrogate regime; an instability stagnation effect induced without modifying the driving pressure pulse or the target surface geometry. Using additively manufactured sub-surface voids in a sinusoidal target, we convert a single shock into a sequence of weaker shocks that suppress instability growth upstream of the surface by over 70%. High-speed X-ray imaging and hydrodynamic simulations suggest that this suppression arises primarily from temporal shaping, with lesser contributions from spatial curvature and shock weakening. Our results demonstrate a driver-independent pathway for controlling shock-driven hydrodynamic instabilities relevant to ICF and other high energy density systems.

physics.plasm-ph↗

arXiv2025

Fusion alpha particle momentum deposition in thermonuclear burn dynamics

In inertial confinement fusion, the DT fusion alpha particles carry not only energy but also appreciable momentum that is typically neglected in models of thermonuclear burn. In the central hotspot ignition scheme, the hotspot must self-heat and propagate thermonuclear burn before disassembly. Using radiation hydrodynamics simulations with a Monte Carlo alpha particle transport model, we investigate the effect of alpha momentum deposition across sub-ignition to robustly igniting regimes by hydrodynamic scaling of current central hotspot ignition designs from the National Ignition Facility (NIF). We find that the effective alpha particle ram pressure accelerates the shell at burn, reducing hotspot compression, increasing the rate of disassembly and decreasing yield. This causes a notable (~ 30%) reduction in yield at current NIF scale, with a persistent (~ 10%) penalty at larger hydrodynamic scales. These results demonstrate that alpha momentum deposition is a significant effect for present ignition-scale implosions, necessitating its inclusion in ignition criteria, burn models, and designs for high-gain inertial confinement fusion.

physics.plasm-ph↗

arXiv2025

Magnetized ICF implosions: ignition at low laser energy using designs with more ablator mass remaining

This paper is the first work to redesign a spherical ICF implosion to best utilize the benefits of applying an external magnetic field. The sub-ignition experiment N170601 is taken as the baseline design, which used 1.57MJ of laser energy. The optimum magnetized design benefits from increasing the shell thickness by 14$μ$m and decreasing the ice thickness by 18$μ$m, resulting in a neutron yield of 8.9$\times$10$^{17}$. This is 34$\times$ greater than the unmagnetized simulation of the same design, and 18.5$\times$ the greatest unmagnetized simulation across all designs simulated. The resultant implosion velocity for the magnetized design is lower, which would also reduce ablation front instability growth. This design was found by using a simplified 1D magnetization model, then validated against full 2D extended-MHD capsule simulations with radiation asymmetries applied to correct the shape.

physics.plasm-ph↗

arXiv2024

Magnetized ICF implosions: Non-axial magnetic field topologies

This paper explores 4 different magnetic field topologies for application to spherical inertial confinement fusion implosions: axial, mirror, cusp and closed field lines. A mirror field is found to enhance the impact of magnetization over an axial field; this is because the mirror field more closely follows the hot-spot surface. A cusp field, while simple to generate, is not found to have any benefits over the tried-and-tested axial field. Closed field lines are found to be of the greatest benefit to hot-spot performance, with the simulated design undergoing a 2x increase in ion temperature before alpha-heating is considered. The plasma properties of the simulation with closed field lines are radically different from the unmagnetized counterpart, with electron temperatures in excess of 100 keV, suggesting that a fundamental redesign of the capsule implosion is possible if this method is pursued.

physics.plasm-ph↗

arXiv2024

Burn Propagation in Magnetized High-Yield Inertial Fusion

Recent experiments at the National Ignition Facility (NIF) have demonstrated ignition for the first time in an inertial confinement fusion (ICF) experiment, a major milestone allowing the possibility of high energy gain through burn propagation. Use of external magnetic fields, applied primarily to reduce thermal losses, could increase hotspot temperature and ease requirements for ignition, opening up the capsule design space for high energy gain. However, this same restriction of thermal transport has the potential to inhibit burn propagation, which is vital in the attainment of high gain. In this work, radiation-magnetohydrodynamics (MHD) simulations carried out using the code Chimera are used to investigate the effect of a pre-imposed magnetic field on ignition and burn propagation. This paper studies the propagation of burn using both an idealized planar model and in fully-integrated 2D MHD simulations of an igniting NIF capsule. A study of magnetised burn propagation in the idealized planar model identifies three regimes of magnetized burn propagation: (1) thermal conduction driven; (2) alpha transport driven; and (3) fully suppressed burn. Simulations of NIF shot N210808 with an applied 40T axial field show clear indication of burn suppression perpendicular to field lines, with rapid burn observed along field lines. Implosion shape is altered by the field, and anisotropic conduction causes significant modification to the rate of ablation during stagnation. These results highlight the fundamental changes to implosion dynamics in high yield magnetized ICF and motivate further study to better optimize future magnetized target designs for high gain.

physics.plasm-ph↗

arXiv2024

Structure and Dynamics of Magneto-Inertial, Differentially Rotating Laboratory Plasmas

We present a detailed characterization of the structure and evolution of differentially rotating plasmas driven on the MAGPIE pulsed-power generator (1.4 MA peak current, 240 ns rise-time). The experiments were designed to simulate physics relevant to accretion discs and jets on laboratory scales. A cylindrical aluminium wire array Z pinch enclosed by return posts with an overall azimuthal off-set angle was driven to produce ablation plasma flows that propagate inwards in a slightly off-radial trajectory, injecting mass, angular momentum, and confining ram pressure to a rotating plasma column on the axis. However, the plasma is free to expand axially, forming a collimated, differentially rotating axial jet that propagates at $\approx 100$ km/s. The density profile of the jet corresponds to a dense shell surrounding a low-density core, which is consistent with the centrifugal barrier effect being sustained along the jet's propagation. We show analytically that, as the rotating plasma accretes mass, conservation of mass and momentum implies plasma radial growth scaling as $r \propto t^{1/3}$. As the characteristic moment of inertia increases, the rotation velocity is predicted to decrease and settle on a characteristic value $\approx 20$ km/s. We find that both predictions are in agreement with Thomson scattering and optical self-emission imaging measurements.

physics.plasm-ph↗

arXiv2024

Extension of the SpK Atomic Physics Code to Generate Global Equation of State Data

Global microphysics models are required for the modelling of high-energy-density physics (HEDP) experiments, the improvement of which are critical to the path to inertial fusion energy. This work presents further developments to the atomic and microphysics code, SpK, part of the numerical modelling suite of Imperial College London and First Light Fusion. We extend the capabilities of SpK to allow the calculation of the equation of state (EoS). The detailed configuration accounting calculations are interpolated into finite-temperature Thomas-Fermi calculations at high coupling to form the electronic component of the model. The Cowan model provides the ionic contribution, modified to approximate the physics of diatomic molecular dissociation. By utilising bonding corrections and performing a Maxwell construction, SpK captures the EoS from states ranging from the zero-pressure solid, through the liquid-vapour coexistence region and into plasma states. This global approach offers the benefit of capturing electronic shell structure over large regions of parameter space, building highly-resolved tables in minutes on a simple desktop. We present shock Hugoniot and off-Hugoniot calculations for a number of materials, comparing SpK to other models and experimental data. We also apply EoS and opacity data generated by SpK in integrated simulations of indirectly-driven capsule implosions, highlighting physical sensitivities to the choice of EoS models.

physics.plasm-ph↗

arXiv2024

On The Structure of Plasma Jets in the Rotating Plasma Experiment

Recent pulsed-power experiments have demonstrated the formation of astrophysically-relevant, differentially rotating plasmas [1]. Key features of the plasma flows are the discovery of a quasi-Keplerian rotation curve, the launching of highly-collimated angular-momentum-transporting axial jets, and a hollow density structure sustained by the centrifugal barrier effect. In this communication we discuss several features of the plasma structure in these experiments through order-of-magnitude models. First, we show that the observed rotation velocity would produce a centrifugal force strong enough to support the hollow density profile. Second, we show that the axial jet should diverge much faster than what was observed, were it not for a magnetized halo with 3T which surrounds the jet and exerts pressure on the interface.Finally, we discuss the temperature structure in the axial jet and plasma halo.We show that a 3T magnetic field would also suppress electron heat conduction,leading to the flat profile observed experimentally. We also find that the axial jet is efficiently radiatively cooled,whereas the halo is not, which would explain the thermal decoupling between the two regions.

physics.plasm-ph↗

arXiv2024

Plasmoid formation and strong radiative cooling in a driven magnetic reconnection experiment

We present results from the first experimental study of strongly radiatively-cooled magnetic reconnection. Two exploding aluminum wire arrays, driven simultaneously by the Z machine ($I_{max} = 20 \, \text{MA}$, $t_{\text{rise}} = 300 \, \text{ns}$), generate a radiatively-cooled reconnection layer ($S_L \approx 120$) in which the total cooling rate exceeds the hydrodynamic transit rate ($τ_{\text{hydro}}/τ_{\text{cool}} > 100$). Measurements of X-ray emission from the reconnection layer using a filtered diode ($>1$ keV) show a narrow (50 ns FWHM) burst of emission at 220 ns after current start, consistent with the formation and subsequent rapid cooling of the reconnection layer. Time-gated X-ray images of the reconnection layer show fast-moving (up to 50 km/s) hotspots inside the layer, consistent with the presence of plasmoids observed in 3D resistive magnetohydrodynamic simulations. X-ray spectroscopy shows that these hotspots generate the majority of Al K-shell emission (at around 1.6 keV) prior to the onset of cooling, and exhibit temperatures of 170 eV, much greater than the temperature of the plasma inflows and the rest of the reconnection layer.

physics.plasm-ph↗

arXiv2023

Resistive Diffusion in Magnetized ICF Implosions: Reduced Magnetic Stabilization of the Richtmyer Meshkov Instability

Resistive diffusion is typically regarded to be negligible in magnetized ICF experiments, with magnetic flux effectively compressed during the implosion. In this work the Richtmyer Meshkov instability at the ice-ablator interface is taken as an example for investigating resistive effects. For a high temperature (approximately 100eV) interface with magnetic field applied perpendicular to shock propagation, perturbation growth is suppressed by magnetic tension. However, for lower temperature interfaces the resistive diffusion prevents substantial magnetic field twisting at small scales. ICF implosion simulations are then used to assess magnetic diffusivity at different interfaces; the ice-ablator interface is found to be too resistive for the magnetic fields to enhance stability. For Rayleigh-Taylor growth at the hot-spot edge, on the other hand, resistivity is estimated to only be a secondary effect, as seen in previous simulation studies.

physics.plasm-ph↗

arXiv2023

Radiative cooling effects on reverse shocks formed by magnetised supersonic plasma flows

We study the structure of reverse shocks formed by the collision of supersonic, magnetised plasma flows driven by an inverse (or exploding) wire array with a planar conducting obstacle. We observe that the structure of these reverse shocks varies dramatically with wire material, despite the similar upstream flow velocities and mass densities. For aluminium wire arrays, the shock is sharp and well defined, consistent with magneto-hydrodynamic theory. In contrast, we do not observe a well-defined shock using tungsten wires, instead, we see a broad region dominated by density fluctuations on a wide range of spatial scales. We diagnose these two very different interactions using interferometry, Thomson scattering, shadowgraphy, and a newly developed imaging refractometer which is sensitive to small deflections of the probing laser corresponding to small-scale density perturbations. We conclude that the differences in shock structure are most likely due to radiative cooling instabilities which create small-scale density perturbations elongated along magnetic field lines in the tungsten plasma. These instabilities grow more slowly and are smoothed by thermal conduction in the aluminium plasma.

physics.plasm-ph↗

arXiv2023

Plasma flows during the ablation stage of an over-massed pulsed-power-driven exploding planar wire array

We characterize the plasma flows generated during the ablation stage of an over-massed exploding planar wire array, fielded on the COBRA pulsed-power facility (1 MA peak current, 250 ns rise time). The planar wire array is designed to provide a driving magnetic field (80-100 T) and current per wire distribution (about 60 kA), similar to that in a 10 MA cylindrical exploding wire array fielded on the Z machine. Over-massing the arrays enables continuous plasma ablation over the duration of the experiment. The requirement to over-mass on the Z machine necessitates wires with diameters of 75-100 $μ$m, which are thicker than wires usually fielded on wire array experiments. To test ablation with thicker wires, we perform a parametric study by varying the initial wire diameter between 33-100 $μ$m. The largest wire diameter (100 $μ$m) array exhibits early closure of the AK gap, while the gap remains open during the duration of the experiment for wire diameters between 33-75 $μ$m. Laser plasma interferometry and time-gated XUV imaging are used to probe the plasma flows ablating from the wires. The plasma flows from the wires converge to generate a pinch, which appears as a fast-moving ($V \approx {100}$ kms$^{-1}$) column of increased plasma density ($\bar{n}_e \approx 2 \times 10^{18}$ cm$^{-3}$) and strong XUV emission. Finally, we compare the results with three-dimensional resistive-magnetohydrodynamic (MHD) simulations performed using the code GORGON, the results of which reproduce the dynamics of the experiment reasonably well.

physics.plasm-ph↗

arXiv2023

Measurements of dense fuel hydrodynamics in the NIF burning plasma experiments using backscattered neutron spectroscopy

The hydrodynamics of the dense confining fuel shell is of great importance in defining the behaviour of the burning plasma and burn propagation regimes of inertial confinement fusion experiments. However, it is difficult to probe due to its low emissivity in comparison to the central fusion core. In this work, we utilise the backscattered neutron spectroscopy technique to directly measure the hydrodynamic conditions of the dense fuel during fusion burn. Experimental data is fit to obtain dense fuel velocities and apparent ion temperatures. Trends of these inferred parameters with yield and velocity of the burning plasma are used to investigate their dependence on alpha heating and low mode drive asymmetry. It is shown that the dense fuel layer has an increased outward radial velocity as yield increases showing burn has continued into re-expansion, a key signature of hotspot ignition. Comparison with analytic and simulation models show that the observed dense fuel parameters are displaying signatures of burn propagation into the dense fuel layer, including a rapid increase in dense fuel apparent ion temperature with neutron yield.

physics.plasm-ph↗

arXiv2023

Primary Neutron Spectra in Ion Vlasov-Fokker-Planck Simulations

The energy spectra of unscattered neutrons produced by deuterium-deuterium and deuterium-tritium fusion reactions are an important diagnostic in High Energy Density Physics experiments as the spectra are sensitive to the velocities of reacting ions. Methods exist for calculating these spectra in radiation-hydrodynamic ("hydro") and Particle-in-Cell ("PiC") simulations. The spectra are particularly sensitive to the high energy tail of ion velocity distribution functions since reaction cross sections increase rapidly with the kinetic energy of a reacting ion pair at the energies achieved in laboratory experiments. This means both the hydro and PiC method may not be suitable in certain plasma regimes. The hydro method assumes that the ion velocity distribution is locally Maxwellian, while the PiC method is subject to statistical noise that makes it challenging to accurately simulate finer details of the spectra. In this work, we present a complementary approach: a method for calculating the neutron spectra in ion Vlasov-Fokker-Planck simulations in which the velocity distribution function is fully-resolved. The method is implemented in the spherically-symmetric code iFP which is used to simulate laser-driven Inertial Confinement Fusion experiments. The method is computationally intensive as it requires a five-dimensional numerical integral, but no approximations of the distribution functions or differential cross sections are required. Results show that deviations of the ion distribution functions from Maxwellian can have a noticeable effect on neutron spectra in shock-driven ICF implosions. The method should facilitate more accurate benchmarking of simulations and experiments.

physics.plasm-ph↗

arXiv2022

SpK: A fast atomic and microphysics code for the high-energy-density regime

SpK is part of the numerical codebase at Imperial College London used to model high energy density physics (HEDP) experiments. SpK is an efficient atomic and microphysics code used to perform detailed configuration accounting calculations of electronic and ionic stage populations, opacities and emissivities for use in post-processing and radiation hydrodynamics simulations. This is done using screened hydrogenic atomic data supplemented by the NIST energy level database. An extended Saha model solves for chemical equilibrium with extensions for non-ideal physics, such as ionisation potential depression, and non thermal equilibrium corrections. A tree-heap (treap) data structure is used to store spectral data, such as opacity, which is dynamic thus allowing easy insertion of points around spectral lines without a-priori knowledge of the ion stage populations. Results from SpK are compared to other codes and descriptions of radiation transport solutions which use SpK data are given. The treap data structure and SpK's computational efficiency allows inline post-processing of 3D hydrodynamics simulations with a dynamically evolving spectrum stored in a treap.

physics.plasm-ph↗

arXiv2022

Time-resolved velocity and ion sound speed measurements from simultaneous bow shock imaging and inductive probe measurements

We present a technique to measure the time-resolved velocity and ion sound speed in magnetized, supersonic high-energy-density plasmas. We place an inductive (`b-dot') probe in a supersonic pulsed-power-driven plasma flow and measure the magnetic field advected by the plasma. As the magnetic Reynolds number is large ($R_M > 10$), the plasma flow advects a magnetic field proportional to the current at the load. This enables us to estimate the plasma flow velocity as a function of time from the delay between the current at the load and the signal at the probe. The supersonic flow also generates a detached hydrodynamic bow shock around the probe, the structure of which depends on the upstream sonic Mach number. By imaging the shock around the probe with a Mach-Zehnder interferometer, we determine the upstream Mach number from the shock Mach angle, which we then use to determine the ion sound speed from the known upstream velocity. We use the measured sound speed to infer the value of $\bar{Z}T_e$, where $\bar{Z}$ is the average ionization, and $T_e$ is the electron temperature. We use this diagnostic to measure the time-resolved velocity and sound speed of a supersonic $(M_S \sim 8)$, super-Alfvénic $(M_A \sim 2)$ aluminum plasma generated during the ablation stage of an exploding wire array on the MAGPIE generator (1.4 MA, 250 ns). Velocity and $\bar{Z}T_e$ measured using this technique agree well with optical Thompson scattering measurements reported in literature, and with 3D resistive MHD simulations in GORGON.

physics.plasm-ph↗

arXiv2022

Time-Varying Magnetopause Reconnection during Sudden Commencement: Global MHD Simulations

In response to a solar wind dynamic pressure enhancement, the compression of the magnetosphere generates strong ionospheric signatures and a sharp variation in the ground magnetic field, termed sudden commencement (SC). Whilst such compressions have also been associated with a contraction of the ionospheric polar cap due to the triggering of reconnection in the magnetotail, the effect of any changes in dayside reconnection is less clear and is a key component in fully understanding the system response. In this study we explore the time-dependent nature of dayside coupling during SC by performing global simulations using the Gorgon MHD code, and impact the magnetosphere with a series of interplanetary shocks with different parameters. We identify the location and evolution of the reconnection region in each case as the shock propagates through the magnetosphere, finding strong enhancement in the dayside reconnection rate and prompt expansion of the dayside polar cap prior to the eventual triggering of tail reconnection. This effect pervades for a variety of IMF orientations, and the reconnection rate is most enhanced for events with higher dynamic pressure. We explain this by repeating the simulations with a large explicit resistivity, showing that compression of the magnetosheath plasma near the propagating shock front allows for reconnection of much greater intensity and at different locations on the dayside magnetopause than during typical solar wind conditions. The results indicate that the dynamic behaviour of dayside coupling may render steady models of reconnection inaccurate during the onset of a severe space weather event.

physics.space-ph↗

arXiv2022

Characterization of Quasi-Keplerian, Differentially Rotating, Free-Boundary Laboratory Plasmas

We present results from pulsed-power driven differentially rotating plasma experiments designed to simulate physics relevant to astrophysical disks and jets. In these experiments, angular momentum is injected by the ram pressure of the ablation flows from a wire array Z pinch. In contrast to previous liquid metal and plasma experiments, rotation is not driven by boundary forces. Axial pressure gradients launch a rotating plasma jet upwards, which is confined by a combination of ram, thermal, and magnetic pressure of a surrounding plasma halo. The jet has subsonic rotation, with a maximum rotation velocity $23 \pm 3$ km/s. The rotational velocity profile is quasi-Keplerian with a positive Rayleigh discriminant $κ^2 \propto r^{-2.8\pm0.8}$ rad$^2$/s$^2$. The plasma completes $0.5 - 2$ full rotations in the experimental time frame ($\sim 150$ ns).

physics.plasm-ph↗