309 results found
Sio H, Moody JD, Ho DD, et al., 2021, Diagnosing plasma magnetization in inertial confinement fusion implosions using secondary deuterium-tritium reactions, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 92, ISSN: 0034-6748
Appelbe B, Velikovich AL, Sherlock M, et al., 2021, Magnetic field transport in propagating thermonuclear burn, PHYSICS OF PLASMAS, Vol: 28, ISSN: 1070-664X
Hare J, Burdiak G, Merlini S, et al., 2021, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Review of Scientific Instruments, 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.
Crilly AJ, Appelbe BD, Mannion OM, et al., 2021, The effect of areal density asymmetries on scattered neutron spectra in ICF implosions, PHYSICS OF PLASMAS, Vol: 28, ISSN: 1070-664X
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.
Yanuka D, Theocharous S, Chittenden JP, et al., 2020, High velocity outflows along the axis of pulsed power driven rod z-pinches, AIP ADVANCES, Vol: 10
Campbell PT, Walsh CA, Russell BK, et 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.
Eggington JWB, Eastwood JP, Mejnertsen L, et 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.
Hare JD, Burdiak GC, Merlini S, et 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.
Gatu Johnson M, Adrian PJ, Anderson KS, et 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.
Volegov PL, Batha SH, Geppert-Kleinrath V, et 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.
Walsh CA, Chittenden JP, Hill DW, et 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.
Crilly AJ, Appelbe BD, Mannion OM, et 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.
Appelbe B, Sherlock M, El-Amiri O, et al., 2019, Modification of classical electron transport due to collisions between electrons and fast ions, Physics of Plasmas, Vol: 26, Pages: 102704-1-102704-12, ISSN: 1070-664X
A Fokker-Planck model for the interaction of fast ions with the thermal electrons in a quasineutral plasma is developed. When the fast ion population has a net flux (i.e., the distribution of fast ions is anisotropic in velocity space), the electron distribution function is perturbed from Maxwellian by collisions with the fast ions, even if the fast ion density is orders of magnitude smaller than the electron density. The Fokker-Planck model is used to derive classical electron transport equations (a generalized Ohm's law and a heat flow equation) that include the effects of the electron-fast ion collisions. It is found that these collisions result in a collisionally induced current term in the transport equations which can be significant. The new transport equations are analyzed in the context of a number of scenarios including α particle heating in inertial confinement fusion and magnetoinertial fusion plasmas as well as ion beam heating of dense plasmas.
Tong JK, McGlinchey K, Appelbe BD, et al., 2019, Burn regimes in the hydrodynamic scaling of perturbed inertial confinement fusion hotspots, Nuclear Fusion, Vol: 59, Pages: 1-16, ISSN: 0029-5515
We present simulations of ignition and burn based on the Highfoot and high-density carbon indirect drive designs of the National Ignition Facility for three regimes of alpha-heating—self-heating, robust ignition and propagating burn—exploring hotspot power balance, perturbations and hydrodynamic scaling. A Monte-Carlo particle-in-cell charged particle transport package for the radiation-magnetohydrodynamics code Chimera was developed for this purpose, using a linked-list type data structure.The hotspot power balance between alpha-heating, electron thermal conduction and radiation was investigated in 1D for the three burn regimes. Stronger alpha-heating levels alter the hydrodynamics: sharper temperature and density gradients at hotspot edge; and increased hotspot pressures which further compress the shell, increase hotspot size and induce faster re-expansion. The impact of perturbations on this power balance is explored in 3D using a single Rayleigh–Taylor spike. Heat flow into the perturbation from thermal conduction and alpha-heating increases by factors of , due to sharper temperature gradients and increased proximity of the cold, dense material to the main fusion regions respectively. The radiative contribution remains largely unaffected in magnitude.Hydrodynamic scaling with capsule size and laser energy of different perturbation scenarios (a short-wavelength multi-mode and a long-wavelength radiation asymmetry) is explored in 3D, demonstrating the differing hydrodynamic evolution of the three alpha-heating regimes. The multi-mode yield increases faster with scale factor due to more synchronous compression producing higher temperatures and densities, and therefore stronger bootstrapping of alpha-heating. The perturbed implosions exhibit differences in hydrodynamic evolution due to alpha-heating in addition to the 1D effects, including: reduced perturbation growth due to ablation from both fire-polishing and stronger thermal conduction; and fa
Walsh CA, McGlinchey K, Tong JK, et al., 2019, Perturbation modifications by pre-magnetisation of inertial confinement fusion implosions, Physics of Plasmas, Vol: 26, ISSN: 1070-664X
Pre-magnetisation of inertial confinement fusion implosions on the National Ignition Facility has the potential to raise current high-performing targets into the ignition regime [Perkins et al. "The potential of imposed magnetic fields for enhancing ignition probability and fusion energy yield in indirect-drive inertial confinement fusion," Phys. Plasmas 24, 062708 (2017)]. A key concern with this method is that the application of a magnetic field inherently increases asymmetry. This paper uses 3-D extended-magnetohydrodynamics Gorgon simulations to investigate how thermal conduction suppression, the Lorentz force, and α-particle magnetisation affect three hot-spot perturbation scenarios: a cold fuel spike, a time-dependent radiation drive asymmetry, and a multi-mode perturbation. For moderate magnetisations (B0 = 5 T), the single spike penetrates deeper into the hot-spot, as thermal ablative stabilisation is reduced. However, at higher magnetisations (B0 = 50 T), magnetic tension acts to stabilise the spike. While magnetisation of α-particle orbits increases the peak hot-spot temperature, no impact on the perturbation penetration depth is observed. The P4-dominated radiation drive asymmetry demonstrates the anisotropic nature of the thermal ablative stabilisation modifications, with perturbations perpendicular to the magnetic field penetrating deeper and perturbations parallel to the field being preferentially stabilised by increased heat-flows. Moderate magnetisations also increase the prevalence of high modes, while magnetic tension reduces vorticity at the hot-spot edge for larger magnetisations. For a simulated high-foot experiment, the yield doubles through the application of a 50 T magnetic field-an amplification which is expected to be larger for higher-performing configurations.
Johnson MG, Appelbe BD, Chittenden JP, et al., 2019, Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions, Physics of Plasmas, Vol: 26, ISSN: 1070-664X
Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion implosions. These asymmetries seed flows in the implosions, which will manifest as modifications to the measured ion temperature (Tion) as inferred from the broadening of primary neutron spectra. The effects are important to understand (i) to learn to control and mitigate low-mode asymmetries and (ii) to experimentally more closely capture thermal Tion used as input in implosion performance metric calculations. In this paper, results from and simulations of a set of experiments with a seeded mode 2 in the laser drive are described. The goal of this intentionally asymmetrically driven experiment was to test our capability to predict and measure the signatures of flows seeded by the low-mode asymmetry. The results from these experiments [first discussed in M. Gatu Johnson et al., Phys. Rev. E 98, 051201(R) (2018)] demonstrate the importance of interplay of flows seeded by various asymmetry seeds. In particular, measured Tion and self-emission x-ray asymmetries are expected to be well captured by interplay between flows seeded by the imposed mode 2 and the capsule stalk mount. Measurements of areal density asymmetry also indicate the importance of the stalk mount as an asymmetry seed in these implosions. The simulations brought to bear on the problem (1D LILAC, 2D xRAGE, 3D ASTER, and 3D Chimera) show how thermal Tion is expected to be significantly lower than Tion as inferred from the broadening of measured neutron spectra. They also show that the electron temperature is not expected to be the same as Tion for these implosions.
Crilly AJ, Appelbe BD, McGlinchey K, et al., 2018, Synthetic nuclear diagnostics for inferring plasma properties of inertial confinement fusion implosions, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
A suite of synthetic nuclear diagnostics has been developed to post-process radiation hydrodynamics simulations performed with the code Chimera. These provide experimental observables based on simulated capsule properties and are used to assess alternative experimental and data analysis techniques. These diagnostics include neutron spectroscopy, primary and scattered neutron imaging, neutron activation, γ-ray time histories and carbon γ-ray imaging. Novel features of the neutron spectrum have been analysed to infer plasma parameters. The nT and nD backscatter edges have been shown to provide a shell velocity measurement. Areal density asymmetries created by low mode perturbations have been inferred from the slope of the downscatter spectrum down to 10 MeV. Neutron activation diagnostics showed significant aliasing of high mode areal density asymmetries when observing a capsule implosion with 3D multimode perturbations applied. Carbon γ-ray imaging could be used to image the ablator at a high convergence ratio. Time histories of both the fusion and carbon γ signals showed a greater time difference between peak intensities for the perturbed case when compared to a symmetric simulation.
McGlinchey K, Appelbe BD, Crilly AJ, et al., 2018, Diagnostic signatures of performance degrading perturbations in inertial confinement fusion implosions, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
We present 3D radiation-hydrodynamics simulations of indirect-drive inertial confinement fusion experiments performed at the National Ignition Facility (NIF). The simulations are carried out on two shots from different NIF experimental campaigns: N130927 from the high foot series and N161023 from the ongoing high density carbon series. Applying representative perturbation sources from each implosion, synthetic nuclear diagnostics are used to post-process the simulations to infer the stagnation parameters. The underlying physical mechanisms that produce the observed signatures are then explored. We find that the radiation asymmetry and tent scar perturbations extend the nuclear burn width; this is due to an asymmetric stagnation of the shell that causes the delivery of mechanical PdV work to be extended compared to an idealised implosion. Radiation asymmetries seed directed flow patterns that can result in a difference in the inferred ion temperature ranging from 80 eV to 230 eV depending on the magnitude and orientation of the asymmetry considered in the simulation; the tent scar shows no such temperature difference. For N130927, radiation asymmetries dominate the yield and inferred ion temperature and the tent scar has the largest influence on the neutron burnwidth. For N161023, the fill tube decreases the burn width by injecting mix into the hot spot, leading to a smaller hot spot and increased energy losses. Both the radiation asymmetry and the fill tube generate directed flows that lead to an anisotropic inferred temperature distribution. Through existing and novel synthetic neutron imaging techniques, we can observe the hot spot and shell shape to a degree that accurately captures the perturbations present.
Gatu Johnson M, Appelbe BD, Chittenden JP, et al., 2018, Impact of asymmetries on fuel performance in inertial confinement fusion, Physical Review E, Vol: 98, ISSN: 2470-0045
Low-mode asymmetries prevent effective compression, confinement, and heating of the fuel in inertial confinement fusion (ICF) implosions, and their control is essential to achieving ignition. Ion temperatures (Tion) in ICF experiments are inferred from the broadening of primary neutron spectra. Directional motion (flow) of the fuel at burn also impacts broadening and will lead to artificially inflated "Tion" values. Flow due to low-mode asymmetries is expected to give rise to line-of-sight variations in measured Tion. We report on intentionally asymmetrically driven experiments at the OMEGA laser facility designed to test the ability to accurately predict and measure line-of-sight differences in apparent Tion due to low-mode asymmetry-seeded flows. Contrasted to chimera and xrage simulations, the measurements demonstrate how all asymmetry seeds have to be considered to fully capture the flow field in an implosion. In particular, flow induced by the stalk that holds the target is found to interfere with the seeded asymmetry. A substantial stalk-seeded asymmetry in the areal density of the implosion is also observed.
Suttle LG, Hare JD, Lebedev SV, et al., 2018, Ion heating and magnetic flux pile-up in a magnetic reconnection experiment with super-Alfvenic plasma inflows, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
This work presents a magnetic reconnection experiment in which the kinetic, magnetic, and thermal properties of the plasma each play an important role in the overall energy balance and structure of the generated reconnection layer. Magnetic reconnection occurs during the interaction of continuous and steady flows of super-Alfvénic, magnetized, aluminum plasma, which collide in a geometry with two-dimensional symmetry, producing a stable and long-lasting reconnection layer. Optical Thomson scattering measurements show that when the layer forms, ions inside the layer are more strongly heated than electrons, reaching temperatures of Ti∼Z⎯⎯⎯Te≳300 eV—much greater than can be expected from strong shock and viscous heating alone. Later in time, as the plasma density in the layer increases, the electron and ion temperatures are found to equilibrate, and a constant plasma temperature is achieved through a balance of the heating mechanisms and radiative losses of the plasma. Measurements from Faraday rotation polarimetry also indicate the presence of significant magnetic field pile-up occurring at the boundary of the reconnection region, which is consistent with the super-Alfvénic velocity of the inflows.
Hare J, Suttle L, Lebedev S, et al., 2018, An experimental platform for pulsed-power driven magnetic reconnection, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
We describe a versatile pulsed-power driven platform for magnetic reconnection experiments, based on the exploding wire arrays driven in parallel [Suttle et al., Phys. Rev. Lett. 116, 225001 (2016)]. This platform produces inherently magnetised plasma flows for the duration of the generator current pulse (250 ns), resulting in a long-lasting reconnection layer. The layer exists for long enough to allow the evolution of complex processes such as plasmoid formation and movement to be diagnosed by a suite of high spatial and temporal resolution laser-based diagnostics. We can access a wide range of magnetic reconnection regimes by changing the wire material or moving the electrodes inside the wire arrays. We present results with aluminium and carbon wires, in which the parameters of the inflows and the layer that forms are significantly different. By moving the electrodes inside the wire arrays, we change how strongly the inflows are driven. This enables us to study both symmetric reconnection in a range of different regimes and asymmetric reconnection.
Mejnertsen L, Eastwood J, Hietala H, et al., 2017, Global MHD simulations of the Earth's bow shock shape and motion under variable solar wind conditions, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 259-271, ISSN: 2169-9380
Empirical models of the Earth's bow shock are often used to place in situ measurements in context and to understand the global behavior of the foreshock/bow shock system. They are derived statistically from spacecraft bow shock crossings and typically treat the shock surface as a conic section parameterized according to a uniform solar wind ram pressure, although more complex models exist. Here a global magnetohydrodynamic simulation is used to analyze the variability of the Earth's bow shock under real solar wind conditions. The shape and location of the bow shock is found as a function of time, and this is used to calculate the shock velocity over the shock surface. The results are compared to existing empirical models. Good agreement is found in the variability of the subsolar shock location. However, empirical models fail to reproduce the two-dimensional shape of the shock in the simulation. This is because significant solar wind variability occurs on timescales less than the transit time of a single solar wind phase front over the curved shock surface. Empirical models must therefore be used with care when interpreting spacecraft data, especially when observations are made far from the Sun-Earth line. Further analysis reveals a bias to higher shock speeds when measured by virtual spacecraft. This is attributed to the fact that the spacecraft only observes the shock when it is in motion. This must be accounted for when studying bow shock motion and variability with spacecraft data.
Hare JD, Lebedev SV, Suttle LG, et al., 2017, Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments, Physics of Plasmas, Vol: 24, ISSN: 1070-664X
We describe magnetic reconnection experiments using a new, pulsed-powerdriven experimental platform in which the inflows are super-sonic butsub-Alfv\'enic.The intrinsically magnetised plasma flows are long lasting,producing a well-defined reconnection layer that persists over manyhydrodynamic time scales.The layer is diagnosed using a suite of highresolution laser based diagnostics which provide measurements of the electrondensity, reconnecting magnetic field, inflow and outflow velocities and theelectron and ion temperatures.Using these measurements we observe a balancebetween the power flow into and out of the layer, and we find that the heatingrates for the electrons and ions are significantly in excess of the classicalpredictions. The formation of plasmoids is observed in laser interferometry andoptical self-emission, and the magnetic O-point structure of these plasmoids isconfirmed using magnetic probes.
Burdiak GC, Lebedev SV, Bland SN, et al., 2017, The structure of bow shocks formed by the interaction of pulsed-power driven magnetised plasma flows with conducting obstacles, PHYSICS OF PLASMAS, Vol: 24, ISSN: 1070-664X
We present an experimental study of the development and structure of bow shocks produced by the interaction of a magnetised, collisional, super-Alfvénic plasma flow with conducting cylindrical obstacles. The plasma flow with an embedded, frozen-in magnetic field (ReM ∼ 20) is produced by the current-driven ablation of fine aluminium wires in an inverse, exploding wire array z-pinch. We show that the orientation of the embedded field with respect to the obstacles has a dramatic effect on the bow shock structure. When the field is aligned with the obstacle, a sharp bow shock is formed with a global structure that is determined simply by the fast magneto-sonic Mach number. When the field is orthogonal to the obstacle, magnetic draping occurs. This leads to the growth of a magnetic precursor and the subsequent development of a magnetised bow shock that is mediated by two-fluid effects, with an opening angle and a stand-off distance, that are both many times larger than in the parallel geometry. By changing the field orientation, we change the fluid regime and physical mechanisms that are responsible for the development of the bow shocks. MHD simulations show good agreement with the structure of well-developed bow shocks. However, collisionless, two-fluid effects will need to be included within models to accurately reproduce the development of the shock with an orthogonal B-field.
Walsh C, Chittenden JP, McGlinchey K, et al., 2017, Self-Generated magnetic fields in the stagnation phase of indirect-drive implosions on the national ignition facility, Physical Review Letters, Vol: 118, ISSN: 1079-7114
Three-dimensional extended-magnetohydrodynamic simulations of the stagnation phase of inertial confinement fusion implosion experiments at the National Ignition Facility are presented, showing self-generated magnetic fields over 10^4 T. Angular high mode-number perturbations develop large magnetic fields, but are localized to the cold, dense hot-spot surface, which is hard to magnetize. When low-mode perturbations are also present, the magnetic fields are injected into the hot core, reaching significant magnetizations, with peak local thermal conductivity reductions greater than 90%. However, Righi-Leduc heat transport effectively cools the hot spot and lowers the neutron spectra-inferred ion temperatures compared to the unmagnetized case. The Nernst effect qualitatively changes the results by demagnetizing the hot-spot core, while increasing magnetizations at the edge and near regions of large heat loss.
Hare JD, Suttle L, Lebedev SV, et al., 2017, Anomalous heating and plasmoid formation in a driven magnetic reconnection experiment, Physical Review Letters, Vol: 118, ISSN: 0031-9007
We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields (B=3 T), advected by supersonic, sub-Alfvénic carbon plasma flows (Vin=50 km/s), are brought together and mutually annihilate inside a thin current layer (δ=0.6 mm). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures (Te=100 eV, Ti=600 eV), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory.
Appelbe B, Pecover J, Chittenden J, 2017, The effects of magnetic field topology on secondary neutron spectra in magnetized liner inertial fusion, High Energy Density Physics, Vol: 22, Pages: 27-36, ISSN: 1878-0563
The Magnetized Liner Inertial Fusion (MagLIF) concept involves the compression of a magnetized fuel such that the stagnated fuel contains a magnetic field that can suppress heat flow losses and confine α particles. Magnetic confinement of α particles reduces the fuel ρR required for ignition. Recent work [1,2] has demonstrated that the magnitude of the magnetic field in deuterium fuel can be inferred from the yields and spectra of secondary DT neutrons. In this work we investigate the potential for using the shape of the secondary neutron spectra to diagnose the magnetic field topology in the stagnated fuel. Three different field topologies that could possibly occur in MagLIF experiments are studied: (1) a cylindrical fuel column containing axial and azimuthal magnetic field components, (2) a fuel column which is pinched at the ends to form a magnetic mirror and (3) a fuel column that has a helical tube shape with magnetic field lines following the helical path of the tube’s axis. Each topology is motivated by observations from experimental or simulated MagLIF data. For each topology we use a multi-physics model to investigate the shapes of the secondary neutron spectra emitted from a steady-state stagnated fuel column. It is found that the azimuthal and helical topologies are more suitable than the mirror topology for reproducing an asymmetry in the axial spectra that was observed in experiments. Gorgon MHD simulations of the MagLIF implosion in 1D are also carried out. These show that sufficient azimuthal magnetic field can penetrate from the liner into the fuel to qualitatively reproduce the observed spectral asymmetry.
Bendixsen LSC, Bott-Suzuki SC, Cordaro SW, et al., 2016, Axial mass fraction measurements in a 300kA dense plasma focus, Physics of Plasmas, Vol: 23, ISSN: 1089-7674
The dynamics and characteristics of the plasma sheath during the axial phase in a ∼300 kA, ∼2 kJ dense plasma focus using a static gas load of Ne at 1–4 Torr are reported. The sheath, which is driven axially at a constant velocity ∼105 m/s by the j × B force, is observed using optical imaging, to form an acute angle between the electrodes. This angle becomes more acute (more parallel to the axis) along the rundown. The average sheath thickness nearer the anode is 0.69 ± 0.02 mm and nearer the cathode is 0.95 ± 0.02 mm. The sheath total mass increases from 1 ± 0.02 μg to 6 ± 0.02 μg over the pressure range of 1–4 Torr. However, the mass fraction (defined as the sheath mass/total mass of cold gas between the electrodes) decreases from 7% to 5%. In addition, the steeper the plasma sheath, the more mass is lost from the sheath, which is consistent with radial and axial motion. Experimental results are compared to the Lee code when 100% of the current drives the axial and radial phase.
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