# Prof. Jeremy Chittenden

Faculty of Natural SciencesDepartment of Physics

Professor of Plasma Physics

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### Contact

+44 (0)20 7594 7654j.chittenden

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### Location

744Blackett LaboratorySouth Kensington Campus

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## Publications

Publication Type
Year
to

327 results found

Eggington J, Coxon J, Shore R, Desai R, Mejnertsen L, Chittenden J, Eastwood Jet al., 2022, Response Timescales of the Magnetotail Current Sheet during a Geomagnetic Storm: Global MHD Simulations, Frontiers in Astronomy and Space Sciences, ISSN: 2296-987X

Journal article

Crilly AJ, Appelbe BD, Mannion OM, Forrest CJ, Knauer JP, Schlossberg DJ, Hartouni EP, Moore AS, Chittenden JPet al., 2022, Neutron backscatter edges as a diagnostic of burn propagation, PHYSICS OF PLASMAS, Vol: 29, ISSN: 1070-664X

Journal article

Moody JD, Pollock BB, Sio H, Strozzi DJ, Ho DD-M, Walsh C, Kemp GE, Kucheyev SO, Kozioziemski B, Carroll EG, Kroll J, Yanagisawa DK, Angus J, Bhandarkar SD, Bude JD, Divol L, Ferguson B, Fry J, Hagler L, Hartouni E, Herrmann MC, Hsing W, Holunga DM, Javedani J, Johnson A, Kalantar D, Kohut T, Logan BG, Masters N, Nikroo A, Orsi N, Piston K, Provencher C, Rowe A, Sater J, Skulina K, Stygar WA, Tang V, Winters SE, Chittenden JP, Appelbe B, Boxall A, Crilly A, O'Neill S, Davies J, Peebles J, Fujioka Set al., 2022, The Magnetized Indirect Drive Project on the National Ignition Facility, JOURNAL OF FUSION ENERGY, Vol: 41, ISSN: 0164-0313

Journal article

Mannion OM, Crilly AJ, Forrest CJ, Appelbe BD, Betti R, Glebov VY, Gopalaswamy V, Knauer JP, Mohamed ZL, Stoeckl C, Chittenden JP, Regan SPet al., 2022, Measurements of the temperature and velocity of the dense fuel layer in inertial confinement fusion experiments, PHYSICAL REVIEW E, Vol: 105, ISSN: 2470-0045

Journal article

Bose A, Peebles J, Walsh CA, Frenje JA, V Kabadi N, Adrian PJ, Sutcliffe GD, Johnson MG, Frank CA, Davies JR, Betti R, Glebov VY, Marshall FJ, Regan SP, Stoeckl C, Campbell EM, Sio H, Moody J, Crilly A, Appelbe BD, Chittenden JP, Atzeni S, Barbato F, Forte A, Li CK, Seguin FH, Petrasso RDet al., 2022, Effect of Strongly Magnetized Electrons and Ions on Heat Flow and Symmetry of Inertial Fusion Implosions, PHYSICAL REVIEW LETTERS, Vol: 128, ISSN: 0031-9007

Journal article

Halliday JWD, Crilly A, Chittenden J, Mancini RC, Merlini S, Rose S, Russell DR, Suttle LG, Valenzuela-Villaseca V, Bland SN, Lebedev SVet al., 2022, Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator, Physics of Plasmas, Vol: 29, Pages: 1-13, ISSN: 1070-664X

We present first results from a novel experimental platform which is able toaccess physics relevant to topics including indirect-drive magnetised ICF;laser energy deposition; various topics in atomic physics; and laboratoryastrophysics (for example the penetration of B-fields into HED plasmas). Thisplatform uses the X-Rays from a wire array Z-Pinch to irradiate a silicontarget, producing an outflow of ablated plasma. The ablated plasma expands intoambient, dynamically significant B-fields (~5 T) which are supported by thecurrent flowing through the Z-Pinch. The outflows have a well-defined(quasi-1D) morphology, enabling the study of fundamental processes typicallyonly available in more complex, integrated schemes. Experiments were fielded onthe MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). On this machine awire array Z-Pinch produces an X-Ray pulse carrying a total energy of ~15 kJover ~30 ns. This equates to an average brightness temperature of around 10 eVon-target.

Journal article

Eggington J, Desai R, Mejnertsen L, Chittenden J, Eastwood Jet al., 2022, Time-varying magnetopause reconnection during sudden commencement: global MHD simulations, Journal of Geophysical Research: Space Physics, Vol: 127, ISSN: 2169-9380

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 evolu tion 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.

Journal article

Walsh CA, O'Neill S, Chittenden JP, Crilly AJ, Appelbe B, Strozzi DJ, Ho D, Sio H, Pollock B, Divol L, Hartouni E, Rosen M, Logan BG, Moody JDet al., 2022, Magnetized ICF implosions: Scaling of temperature and yield enhancement, PHYSICS OF PLASMAS, Vol: 29, ISSN: 1070-664X

Journal article

Farrow G, Chittenden JP, Kagan G, 2022, Self-similar solutions for resistive diffusion, Ohmic heating, and Ettingshausen effects in plasmas of arbitrary beta, PHYSICS OF PLASMAS, Vol: 29, ISSN: 1070-664X

Journal article

Walsh CA, Florido R, Bailly-Grandvaux M, Suzuki-Vidal F, Chittenden JP, Crilly AJ, Gigosos MA, Mancini RC, Perez-Callejo G, Vlachos C, McGuffey C, Beg FN, Santos JJet al., 2022, Exploring extreme magnetization phenomena in directly driven imploding cylindrical targets, Plasma Physics and Controlled Fusion, Vol: 64, Pages: 1-19, ISSN: 0741-3335

This paper uses extended-magnetohydrodynamics (MHD) simulations to explore an extreme magnetized plasma regime realizable by cylindrical implosions on the OMEGA laser facility. This regime is characterized by highly compressed magnetic fields (greater than 10 kT across the fuel), which contain a significant proportion of the implosion energy and induce large electrical currents in the plasma. Parameters governing the different magnetization processes such as Ohmic dissipation and suppression of instabilities by magnetic tension are presented, allowing for optimization of experiments to study specific phenomena. For instance, a dopant added to the target gas-fill can enhance magnetic flux compression while enabling spectroscopic diagnosis of the imploding core. In particular, the use of Ar K-shell spectroscopy is investigated by performing detailed non-LTE atomic kinetics and radiative transfer calculations on the MHD data. Direct measurement of the core electron density and temperature would be possible, allowing for both the impact of magnetization on the final temperature and thermal pressure to be obtained. By assuming the magnetic field is frozen into the plasma motion, which is shown to be a good approximation for highly magnetized implosions, spectroscopic diagnosis could be used to estimate which magnetization processes are ruling the implosion dynamics; for example, a relation is given for inferring whether thermally driven or current-driven transport is dominating.

Journal article

Campbell PT, Walsh CA, Russell BK, Chittenden JP, Crilly A, Fiksel G, Gao L, Igumenshchev IV, Nilson PM, Thomas AGR, Krushelnick K, Willingale Let al., 2022, Measuring magnetic flux suppression in high-power laser-plasma interactions, Physics of Plasmas, Vol: 29, ISSN: 1070-664X

Biermann battery magnetic field generation driven by high power laser–solid interactions is explored in experiments performed with theOMEGA EP laser system. Proton deflectometry captures changes to the strength, spatial profile, and temporal dynamics of the self-generatedmagnetic fields as the target material or laser intensity is varied. Measurements of the magnetic flux during the interaction are used to helpvalidate extended magnetohydrodynamic (MHD) simulations. Results suggest that kinetic effects cause suppression of the Biermann batterymechanism in laser–plasma interactions relevant to both direct and indirect-drive inertial confinement fusion. Experiments also find thatmore magnetic flux is generated as the target atomic number is increased, which is counter to a standard MHD understanding.

Journal article

Halliday JWD, Crilly A, Chittenden J, Merlini S, Rose S, Russell D, Suttle LG, Mancini RC, Valenzuela-Villaseca V, Bland SN, Lebedev SVet al., 2022, An Experimental Study of Magnetic Flux Penetration in Radiatively Driven Plasma Flows, ISSN: 0730-9244

In this talk we present measurements from a novel platform in which the X-Rays from a wire-array Z-Pinch irradiate a silicon target, producing an outflow of ablated silicon plasma. This ablated plasma expands into ambient, dynamically significant magnetic fields (B ∼ 5 T) which are supported by the current flowing through the Z-Pinch.

Conference paper

Mejnertsen L, Eastwood J, Chittenden J, 2021, Control of magnetopause flux rope topology by non-local reconnection, Frontiers in Astronomy and Space Sciences, Vol: 8, Pages: 1-15, ISSN: 2296-987X

Dayside magnetic reconnection between the interplanetary magnetic field and the Earth’s magnetic field is the primary mechanism enabling mass and energy entry into the magnetosphere. During favorable solar wind conditions, multiple reconnection X-lines can form on the dayside magnetopause, potentially forming flux ropes. These flux ropes move tailward, but their evolution and fate in the tail is not fully understood. Whilst flux ropes may constitute a class of flux transfer events, the extent to which they add flux to the tail depends on their topology, which can only be measured in situ by satellites providing local observations. Global simulations allow the entire magnetospheric system to be captured at an instant in time, and thus reveal the interconnection between different plasma regions and dynamics on large scales. Using the Gorgon MHD code, we analyze the formation and evolution of flux ropes on the dayside magnetopause during a simulation of a real solar wind event. With a relatively strong solar wind dynamic pressure and southward interplanetary magnetic field, the dayside region becomes very dynamic with evidence of multiple reconnection events. The resulting flux ropes transit around the flank of the magnetosphere before eventually dissipating due to non-local reconnection. This shows that non-local effects may be important in controlling the topology of flux ropes and is a complicating factor in attempts to establish the overall contribution that flux ropes make in the general circulation of magnetic flux through the magnetosphere.

Journal article

Desai R, Eastwood J, Horne R, Allison H, Allanson O, Watt C, Eggington J, Glauert S, Meredith N, Archer M, Staples F, Mejnertsen L, Tong J, Chittenden Jet al., 2021, Drift orbit bifurcations and cross-field transport in the outer radiation belt: global MHD and integrated test-particle simulations, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-14, ISSN: 2169-9380

Energetic particle fluxes in the outer magnetosphere present a significant challenge to modellingefforts as they can vary by orders of magnitude in response to solar wind driving conditions. In thisarticle, we demonstrate the ability to propagate test particles through global MHD simulations to ahigh level of precision and use this to map the cross-field radial transport associated with relativisticelectrons undergoing drift orbit bifurcations (DOBs). The simulations predict DOBs primarily occurwithin an Earth radius of the magnetopause loss cone and appears significantly different for southwardand northward interplanetary magnetic field orientations. The changes to the second invariant areshown to manifest as a dropout in particle fluxes with pitch angles close to 90◦and indicate DOBsare a cause of butterfly pitch angle distributions within the night-time sector. The convective electricfield, not included in previous DOB studies, is found to have a significant effect on the resultant longterm transport, and losses to the magnetopause and atmosphere are identified as a potential methodfor incorporating DOBs within Fokker-Planck transport models.

Journal article

Desai RT, Freeman M, Eastwood J, Eggington J, Archer M, Shprits Y, Meredith N, Staples F, Ian R, Hietala H, Mejnertsen L, Chittenden J, Horne Ret al., 2021, Interplanetary shock-induced magnetopause motion: Comparison between theory and global magnetohydrodynamic simulations, Geophysical Research Letters, Vol: 48, Pages: 1-11, ISSN: 0094-8276

The magnetopause marks the outer edge of the Earth’s magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this letter, we use global magneto-hydrodynamic simulations to examine the response of the terrestrial magnetopause to fast-forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterised by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance travelled, and large-scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991.

Journal article

Chapman DA, Pecover JD, Chaturvedi N, Niasse N, Read MP, Vassilev DH, Chittenden JP, Hawker N, Joiner Net al., 2021, A preliminary assessment of the sensitivity of uniaxially driven fusion targets to flux-limited thermal conduction modeling, Physics of Plasmas, Vol: 28, Pages: 1-15, ISSN: 1070-664X

The role of flux-limited thermal conduction on the fusion performance of the uniaxially driven targets studied by Derentowicz et al. [J. Tech. Phys. 18, 465 (1977) and J. Tech. Phys. 25, 135 (1977)] is explored as part of a wider effort to understand and quantify uncertainties in inertial confinement fusion (ICF) systems sharing similarities with First Light Fusion's projectile-driven concept. We examine the role of uncertainties in plasma microphysics and different choices for the numerical implementation of the conduction operator on simple metrics encapsulating the target performance. The results indicate that choices that affect the description of ionic heat flow between the heated fusion fuel and the gold anvil used to contain it are the most important. The electronic contribution is found to be robustly described by local diffusion. The sensitivities found suggest a prevalent role for quasi-nonlocal ionic transport, especially in the treatment of conduction across material interfaces with strong gradients in temperature and conductivity. We note that none of the simulations produce neutron yields that substantiate those reported by Derentowicz et al. [J. Tech. Phys. 25, 135 (1977)], leaving open future studies aimed at more fully understanding this class of ICF systems.

Journal article

Suzuki-Vidal F, Clayson T, Stehlé C, Chaulagain U, Halliday JWD, Sun M, Ren L, Kang N, Liu H, Zhu B, Zhu J, Rossi CDA, Mihailescu T, Velarde P, Cotelo M, Foster JM, Danson CN, Spindloe C, Chittenden JP, Kuranz Cet al., 2021, First radiative shock experiments on the SG-II laser, High Power Laser Science and Engineering, Vol: 9, ISSN: 2095-4719

We report on the design and first results from experiments looking at theformation of radiative shocks on the Shenguang-II (SG-II) laser at the ShanghaiInstitute of Optics and Fine Mechanics in China. Laser-heating of a two-layerCH/CH-Br foil drives a $\sim$40 km/s shock inside a gas-cell filled with argonat an initial pressure of 1 bar. The use of gas-cell targets with large(several mm) lateral and axial extent allows the shock to propagate freelywithout any wall interactions, and permits a large field of view to imagesingle and colliding counter-propagating shocks with time resolved,point-projection X-ray backlighting ($\sim20$ $\mu$m source size, 4.3 keVphoton energy). Single shocks were imaged up to 100 ns after the onset of thelaser drive allowing to probe the growth of spatial non-uniformities in theshock apex. These results are compared with experiments looking atcounter-propagating shocks, showing a symmetric drive which leads to acollision and stagnation from $\sim$40 ns onward. We present a preliminarycomparison with numerical simulations with the radiation hydrodynamics codeARWEN, which provides expected plasma parameters for the design of futureexperiments in this facility.

Journal article

Sio H, Moody JD, Ho DD, Pollock BB, Walsh CA, Lahmann B, Strozzi DJ, Kemp GE, Hsing WW, Crilly A, Chittenden JP, Appelbe Bet al., 2021, Diagnosing plasma magnetization in inertial confinement fusion implosions using secondary deuterium-tritium reactions, Review of Scientific Instruments, Vol: 92, Pages: 1-5, ISSN: 0034-6748

Diagnosing plasma magnetization in inertial confinement fusion implosions is important for understanding how magnetic fields affect implosion dynamics and to assess plasma conditions in magnetized implosion experiments. Secondary deuterium–tritium (DT) reactions provide two diagnostic signatures to infer neutron-averaged magnetization. Magnetically confining fusion tritons from deuterium–deuterium (DD) reactions in the hot spot increases their path lengths and energy loss, leading to an increase in the secondary DT reaction yield. In addition, the distribution of magnetically confined DD-triton is anisotropic, and this drives anisotropy in the secondary DT neutron spectra along different lines of sight. Implosion parameter space as well as sensitivity to the applied B-field, fuel ρR, temperature, and hot-spot shape will be examined using Monte Carlo and 2D radiation-magnetohydrodynamic simulations.

Journal article

Appelbe B, Velikovich AL, Sherlock M, Walsh C, Crilly A, O' Neill S, Chittenden Jet al., 2021, Magnetic field transport in propagating thermonuclear burn, Physics of Plasmas, Vol: 28, Pages: 1-9, ISSN: 1070-664X

High energy gain in inertial fusion schemes requires the propagation of a thermonuclear burn wave from hot to cold fuel. We consider the problem of burn propagation when a magnetic field is orthogonal to the burn wave. Using an extended-MHD model with a magnetized α energy transport equation, we find that the magnetic field can reduce the rate of burn propagation by suppressing electron thermal conduction and α particle flux. Magnetic field transport during burn propagation is subject to competing effects: the field can be advected from cold to hot regions by ablation of cold fuel, while the Nernst and α particle flux effects transport the field from hot to cold fuel. These effects, combined with the temperature increase due to burn, can cause the electron Hall parameter to grow rapidly at the burn front. This results in the formation of a self-insulating layer between hot and cold fuel, which reduces electron thermal conductivity and α transport, increases the temperature gradient, and reduces the rate of burn propagation.

Journal article

Hare J, Burdiak G, Merlini S, Chittenden J, Clayson T, Crilly A, Halliday J, Russell D, Smith R, Stuart N, Suttle L, Lebedev Set al., 2021, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Review of Scientific Instruments, Vol: 92, ISSN: 0034-6748

We report on a recently developed laser-probing diagnostic which allows direct measurements of ray-deflection anglesin one axis, whilst retaining imaging capabilities in the other axis. This allows us to measure the spectrum of angulardeflections from a laser beam which passes though a turbulent high-energy-density plasma. This spectrum containsinformation about the density fluctuations within the plasma, which deflect the probing laser over a range of angles. Wecreate synthetic diagnostics using ray-tracing to compare this new diagnostic with standard shadowgraphy and schlierenimaging approaches, which demonstrates the enhanced sensitivity of this new diagnostic over standard techniques. Wepresent experimental data from turbulence behind a reverse shock in a plasma and demonstrate that this technique canmeasure angular deflections between 0.06 and 34 mrad, corresponding to a dynamic range of over 500.

Journal article

Crilly AJ, Appelbe BD, Mannion OM, Forrest CJ, Chittenden JPet al., 2021, The effect of areal density asymmetries on scattered neutron spectra in ICF implosions, Physics of Plasmas, Vol: 28, Pages: 1-9, ISSN: 1070-664X

Scattered neutron spectroscopy is a diagnostic technique commonly used to measure areal density in inertial confinement fusion experiments. Deleterious areal density asymmetries modify the shape of the scattered neutron spectrum. In this work, a novel analysis is developed, which can be used to fit the shape change. This will allow experimental scattered neutron spectroscopy to directly infer the amplitude and mode of the areal density asymmetries, with little sensitivity to confounding factors that affect other diagnostics for areal density. The model is tested on spectra produced by a neutron transport calculation with both isotropic and anisotropic primary fusion neutron sources. Multiple lines of sight are required to infer the areal density distribution over the whole sphere—we investigate the error propagation and optimal detector arrangement associated with the inference of mode 1 asymmetries.

Journal article

Walsh CA, Crilly AJ, Chittenden JP, 2020, Magnetized directly-driven ICF capsules: increased instability growth from non-uniform laser drive, Nuclear Fusion, Vol: 60, Pages: 1-8, ISSN: 0029-5515

Simulations anticipate increased perturbation growth from non-uniform laser heating for magnetized direct-drive implosions. At the capsule pole, where the magnetic field is normal to the ablator surface, the field remains in the conduction zone and suppresses non-radial thermal conduction; in unmagnetized implosions this non-radial heat-flow is crucial in mitigating laser heating imbalances. Single-mode simulations show the magnetic field particularly amplifying short wavelength perturbations, whose behavior is dominated by thermal conduction. The most unstable wavelength can also become shorter. 3D multi-mode simulations of the capsule pole reinforce these findings, with increased perturbation growth anticipated across a wide range of scales. The results indicate that high-gain spherical direct-drive implosions require greater constraints on the laser heating uniformity when magnetized.

Journal article

Yanuka D, Theocharous S, Chittenden JP, Bland SNet al., 2020, High velocity outflows along the axis of pulsed power driven rod z-pinches, AIP Advances, Vol: 10, Pages: 1-9, ISSN: 2158-3226

We report on initial observations of high velocity outflows from the ends of a rod compressed using pulsed power. 1 mm and 2 mm diameter copper rods were placed in a water bath and driven by ∼0.6 MA currents with rise times of ∼700 ns. Laser backlit framing images and streak photography showed an outflow of the material from the ends of each rod, of the initial velocity of up to 7 km/s, which began ∼500 ns after the start of the current pulse and continued throughout the experiment. Ballistics gel was used to help separate low density gas/plasma from any solid/liquid component in the outflow, successfully capturing the material from larger diameter rods (enabling an estimate of its energy) and tracing the path of the material that passed straight through the gel with smaller rods. Experimental results were compared to 1D and 2D MHD simulations performed with the Gorgon code. These suggested that the outflow had two different components, resulting from two different physical processes. Differences in the resistivity between the copper rod and stainless steel anode result in the opening of a small gap between them and ablated stainless steel being projected above the rod, which is captured in framing and streak images. Later in time, a dense copper material, pinched by the magnetic pressure, is launched—explaining the ballistics gel results. The simulations also suggest that the tamped explosion of the rod surface plays a small role in any outflow.

Journal article

Campbell PT, Walsh CA, Russell BK, Chittenden JP, Crilly A, Fiksel G, Nilson PM, Thomas AGR, Krushelnick K, Willingale Let al., 2020, Magnetic signatures of radiation-driven double ablation fronts, Physical Review Letters, Vol: 125, Pages: 145001 – 1-145001 – 6, ISSN: 0031-9007

In experiments performed with the OMEGA EP laser system, magnetic field generation in double ablation fronts was observed. Proton radiography measured the strength, spatial profile, and temporal dynamics of self-generated magnetic fields as the target material was varied between plastic, aluminum, copper, and gold. Two distinct regions of magnetic field are generated in mid-Z targets—one produced by gradients from electron thermal transport and the second from radiation-driven gradients. Extended magnetohydrodynamic simulations including radiation transport reproduced key aspects of the experiment, including field generation and double ablation front formation.

Journal article

Eggington JWB, Eastwood JP, Mejnertsen L, Desai RT, Chittenden JPet al., 2020, Dipole tilt effect on magnetopause reconnection and the steady‐state magnetosphere‐ionosphere system: global MHD simulation, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-17, ISSN: 2169-9380

The Earth’s dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere‐ionosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying IMF orientation and non‐uniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle‐dependence, we perform MHD simulations of the steady‐state magnetosphere‐ionosphere system under purely southward IMF conditions for tilt angles from 0°‐90°. We identify the location of the magnetic separator in each case, and find that an increasing tilt angle shifts the 3‐D X‐line southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50° tilt angle, characteristic of regular FTE generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90°. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region‐I field‐aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north‐south asymmetry in magnetospheric convection, thus driving more FAC in the northern (sunward‐facing) hemisphere for large tilt angles than in the south independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.

Journal article

Hare JD, Burdiak GC, Merlini S, Chittenden JP, Clayson T, Crilly AJ, Halliday JWD, Russell DR, Smith RA, Stuart N, Suttle LG, Lebedev SVet al., 2020, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Publisher: arXiv

We report on a recently developed laser-based diagnostic which allows directmeasurements of ray-deflection angles in one axis, whilst retaining imagingcapabilities in the other axis. This allows us to measure the spectrum ofangular deflections from a laser beam which passes though a turbulenthigh-energy-density plasma. This spectrum contains information about thedensity fluctuations within the plasma, which deflect the probing laser over arange of angles. The principle of this diagnostic is described, along with ourspecific experimental realisation. We create synthetic diagnostics usingray-tracing to compare this new diagnostic with standard shadowgraphy andschlieren imaging approaches, which demonstrates the enhanced sensitivity ofthis new diagnostic over standard techniques. We present experimental data fromturbulence behind a reverse shock in a plasma and demonstrate that thistechnique can measure angular deflections between 0.05 and 34 mrad,corresponding to a dynamic range of over 500.

Working paper

Gatu Johnson M, Adrian PJ, Anderson KS, Appelbe BD, Chittenden JP, Crilly AJ, Edgell D, Forrest CJ, Frenje JA, Glebov VY, Haines BM, Igumenshchev I, Jacobs-Perkins D, Janezic R, Kabadi NV, Knauer JP, Lahmann B, Mannion OM, Marshall FJ, Michel T, Seguin FH, Shah R, Stoeckl C, Walsh CA, Petrasso RDet al., 2020, Impact of stalk on directly driven inertial confinement fusion implosions, Physics of Plasmas, Vol: 27, Pages: 1-18, ISSN: 1070-664X

Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion (ICF) implosions. In direct-drive ICF, an important potential seed of such asymmetries is the capsule stalk mount, the impact of which has remained a contentious question. In this paper, we describe the results from an experiment on the OMEGA laser with intentional offsets at varying angles to the capsule stalk mount, which clearly demonstrates the impact of the stalk mount on implosion dynamics. The angle between stalk and offset is found to significantly impact observables. Specifically, a larger directional flow is observed in neutron spectrum measurements when the offset is toward rather than away from the stalk, while an offset at 42° to the stalk gives minimal directional flow but still generates a large flow field in the implosion. No significant directional flow is seen due to stalk only. Time-integrated x-ray images support these flow observations. A trend is also seen in implosion yield, with lower yield obtained for offsets with a smaller angle than with a larger angle toward the stalk. Radiation hydrodynamic simulations using 2D DRACO and 2D/3D Chimera not including the stalk mount and using 2D xRAGE including the stalk mount are brought to bear on the data. The yield trend, the minimal directional flow with stalk only, and the larger flow enhancement observed with the offset toward the stalk are all reproduced in the xRAGE simulations. The results strongly indicate that the stalk impact must be considered and mitigated to achieve high-performing implosions.

Journal article

Volegov PL, Batha SH, Geppert-Kleinrath V, Danly CR, Merrill FE, Wilde CH, Wilson DC, Casey DT, Fittinghoff D, Appelbe B, Chittenden JP, Crilly AJ, McGlinchey Ket al., 2020, Density determination of the thermonuclear fuel region in inertial confinement fusion implosions, Journal of Applied Physics, Vol: 127, Pages: 1-10, ISSN: 0021-8979

Understanding of the thermonuclear burn in an inertial confinement fusion implosion requires knowledge of the local deuterium–tritium (DT) fuel density. Neutron imaging of the core now provides this previously unavailable information. Two types of neutron images are required. The first is an image of the primary 14-MeV neutrons produced by the D + T fusion reaction. The second is an image of the 14-MeV neutrons that leave the implosion hot spot and are downscattered to lower energy by elastic and inelastic collisions in the fuel. These neutrons are measured by gating the detector to record the 6–12 MeV neutrons. Using the reconstructed primary image as a nonuniform source, a set of linear equations is derived that describes the contribution of each voxel of the DT fuel region to a pixel in the downscattered image. Using the measured intensity of the 14-MeV neutrons and downscattered images, the set of equations is solved for the density distribution in the fuel region. The method is validated against test problems and simulations of high-yield implosions. The calculated DT density distribution from one experiment is presented.

Journal article

Walsh CA, Chittenden JP, Hill DW, Ridgers Cet al., 2020, Extended-magnetohydrodynamics in under-dense plasmas, Physics of Plasmas, Vol: 27, Pages: 022103-022103, ISSN: 1070-664X

Extended-magnetohydrodynamics (MHD) transports magnetic flux and electron energy in high-energy-density experiments, but individual transport effects remain unobserved experimentally. Two factors are responsible in defining the transport: electron temperature and electron current. Each electron energy transport term has a direct analog in magnetic flux transport. To measure the thermally driven transport of magnetic flux and electron energy, a simple experimental configuration is explored computationally using a laser-heated pre-magnetized under-dense plasma. Changes to the laser heating profile precipitate clear diagnostic signatures from the Nernst, cross-gradient-Nernst, anisotropic conduction, and Righi-Leduc heat-flow. With a wide operating parameter range, this configuration can be used in both small and large scale facilities to benchmark MHD and kinetic transport in collisional/semi-collisional, local/non-local, and magnetized/unmagnetized regimes.

Journal article

Crilly AJ, Appelbe BD, Mannion OM, Forrest CJ, Gopalaswamy V, Walsh CA, Chittenden JPet al., 2020, Neutron backscatter edge: A measure of the hydrodynamic properties of the dense DT fuel at stagnation in ICF experiments, Physics of Plasmas, Vol: 27, Pages: 012701-1-012701-11, ISSN: 1070-664X

The kinematic lower bound for the single scattering of neutrons produced in deuterium-tritium (DT) fusion reactions produces a backscatter edge in the measured neutron spectrum. The energy spectrum of backscattered neutrons is dependent on the scattering ion velocity distribution. As the neutrons preferentially scatter in the densest regions of the capsule, the neutron backscatter edge presents a unique measurement of the hydrodynamic conditions in the dense DT fuel. It is shown that the spectral shape of the edge is determined by the scattering rate weighted fluid velocity and temperature of the dense DT fuel layer during neutron production. In order to fit the neutron spectrum, a model for the various backgrounds around the backscatter edge is developed and tested on synthetic data produced from hydrodynamic simulations of OMEGA implosions. It is determined that the analysis could be utilized on current inertial confinement fusion experiments in order to measure the dense fuel properties.

Journal article

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