Publications
67 results found
Power D, Mijin S, Wigram M, et al., 2023, Scaling laws for electron kinetic effects in tokamak scrape-off layer plasmas, Nuclear Fusion, Vol: 63, Pages: 1-16, ISSN: 0029-5515
Tokamak edge (scrape-off layer (SOL)) plasmas can exhibit non-local transport in the direction parallel to the magnetic field due to steep temperature gradients. This effect along with its consequences has been explored at equilibrium for a range of conditions, from sheath-limited to detached, using the 1D kinetic electron code SOL-KiT, where the electrons are treated kinetically and compared to a self-consistent fluid model. Line-averaged suppression of the kinetic heat flux (compared to Spitzer-Härm) of up to 50% is observed, contrasting with up to 98% enhancement of the sheath heat transmission coefficient, γe. Simple scaling laws in terms of basic SOL parameters for both effects are presented. By implementing these scalings as corrections to the fluid model, we find good agreement with the kinetic model for target electron temperatures. It is found that the strongest kinetic effects in γe are observed at low-intermediate collisionalities, and tend to increase (keeping upstream collisionality fixed) at increasing upstream densities and temperatures. On the other hand, the heat flux suppression is found to increase monotonically as upstream collisionality decreases. The conditions simulated encompass collisionalities relevant to current and future tokamaks.
Arran C, Bradford P, Dearling A, et al., 2023, Measurement of Magnetic Cavitation Driven by Heat Flow in a Plasma., Phys Rev Lett, Vol: 131
We describe the direct measurement of the expulsion of a magnetic field from a plasma driven by heat flow. Using a laser to heat a column of gas within an applied magnetic field, we isolate Nernst advection and show how it changes the field over a nanosecond timescale. Reconstruction of the magnetic field map from proton radiographs demonstrates that the field is advected by heat flow in advance of the plasma expansion with a velocity v_{N}=(6±2)×10^{5} m/s. Kinetic and extended magnetohydrodynamic simulations agree well in this regime due to the buildup of a magnetic transport barrier.
Power D, Mijin S, Militello F, et al., 2021, Ion-electron energy transfer in kinetic and fluid modelling of the tokamak scrape-off layer, EUROPEAN PHYSICAL JOURNAL PLUS, Vol: 136, ISSN: 2190-5444
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Hill DW, Ridgers CP, Kingham RJ, et al., 2021, Vlasov-Fokker-Planck simulations of pre-magnetized ablating planar targets, PHYSICS OF PLASMAS, Vol: 28, ISSN: 1070-664X
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Ridgers CP, Arran C, Bissell JJ, et al., 2021, The inadequacy of a magnetohydrodynamic approach to the Biermann battery, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 379, Pages: 1-13, ISSN: 1364-503X
Magnetic fields can be generated in plasmas by the Biermann battery when the electric field produced by the electron pressure gradient has a curl. The commonly employed magnetohydrodynamic (MHD) model of the Biermann battery breaks down when the electron distribution function is distorted away from Maxwellian. Using both MHD and kinetic simulations of a laser-plasma interaction relevant to inertial confinement fusion we have shown that this distortion can reduce the Biermann-producing electric field by around 50%. More importantly, the use of a flux limiter in an MHD treatment to deal with the effect of the non-Maxwellian electron distribution on electron thermal transport leads to a completely unphysical prediction of the Biermann-producing electric field and so results in erroneous predictions for the generated magnetic field.
Tubman ER, Joglekar AS, Bott AFA, et al., 2021, Observations of pressure anisotropy effects within semi-collisional magnetized plasma bubbles, Nature Communications, Vol: 12, Pages: 334-334, ISSN: 2041-1723
Magnetized plasma interactions are ubiquitous in astrophysical and laboratory plasmas. Various physical effects have been shown to be important within colliding plasma flows influenced by opposing magnetic fields, however, experimental verification of the mechanisms within the interaction region has remained elusive. Here we discuss a laser-plasma experiment whereby experimental results verify that Biermann battery generated magnetic fields are advected by Nernst flows and anisotropic pressure effects dominate these flows in a reconnection region. These fields are mapped using time-resolved proton probing in multiple directions. Various experimental, modelling and analytical techniques demonstrate the importance of anisotropic pressure in semi-collisional, high-β plasmas, causing a reduction in the magnitude of the reconnecting fields when compared to resistive processes. Anisotropic pressure dynamics are crucial in collisionless plasmas, but are often neglected in collisional plasmas. We show pressure anisotropy to be essential in maintaining the interaction layer, redistributing magnetic fields even for semi-collisional, high energy density physics (HEDP) regimes.
Mijin S, Militello F, Newton S, et al., 2020, Kinetic effects in parallel electron energy transport channels in the scrape-off layer, Plasma Physics and Controlled Fusion, Vol: 62, Pages: 125009-125009, ISSN: 0741-3335
We present an analysis of parallel electron energy transport in the scrape-off layer (SOL), considering the convective and conductive channels, as well as the radiation and neutral inelastic energy transfer channels involving atomic deuterium. Kinetic effects in both equilibria and conductive transients are explored by utilizing the capability of the SOL-KiT code to treat electrons as either a fluid or kinetically. We find kinetic effects in multiple channels, with an emphasis on those occurring during the investigated conductive transients. Energetic electron effects in the heat flux, as well as a modification of ionization rates of up to 40% compared to Maxwellian rates during perturbations in detached conditions, are reported and discussed.
Mijin S, Militello F, Newton S, et al., 2020, Kinetic and fluid simulations of parallel electron transport during equilibria and transients in the scrape-off layer, Plasma Physics and Controlled Fusion, Vol: 62, ISSN: 0741-3335
We present the first parallel electron transport results obtained using the newly developed 1D transport code SOL-KiT. With the capability to switch between consistent kinetic and fluid models for the electrons, we explore and report the differences in both equilibrium and transient simulations. Significant kinetic effects are found during transients, especially in the behaviour of the electron sheath heat transmission coefficient, which shows up to an eightfold increase. Equilibria are obtained for an input power scan with parameters relevant to medium size tokamaks. Detached equilibria are found to persist to higher input powers when electrons are treated kinetically. Furthermore, non-monotonic behaviour of the electron sheath heat transmission coefficient is observed in the power scan, with values being up to 40% above the classical value. We discuss the implications of the pr
Bell AR, Kingham RJ, Watkins H, et al., 2020, Instability in a magnetised collisional plasma driven by a heat flow or a current, Plasma Physics and Controlled Fusion, Vol: 62, ISSN: 0741-3335
We solve the linearised Vlasov-Fokker-Planck (VFP) equation to show that heat flow or an electrical current in a magnetized collisional plasma is unstable to the growth of a circularly polarised transverse perturbation to a zeroth order uniform magnetic field. The Braginskii (1965) transport equations exhibit the same instability in the appropriate limit. This is relevant to laser-produced plasmas, inertial fusion energy (IFE) and to dense cold interstellar plasmas.
Shi Y, Weichman K, Kingham RJ, et al., 2020, Magnetic field generation in a laser-irradiated thin collisionless plasma target by return current electrons carrying orbital angular momentum, NEW JOURNAL OF PHYSICS, Vol: 22, Pages: 1-11, ISSN: 1367-2630
Magnetized high energy density physics offers new opportunities for observing magnetic field-related physics for the first time in the laser–plasma context. We focus on one such phenomenon, which is the ability of a laser-irradiated magnetized plasma to amplify a seed magnetic field. We performed a series of fully kinetic 3D simulations of magnetic field amplification by a picosecond-scale relativistic laser pulse of intensity 4.2 × 1018 W cm−2 incident on a thin overdense target. We observe axial magnetic field amplification from an initial 0.1 kT seed to 1.5 kT over a volume of several cubic microns, persisting hundreds of femtoseconds longer than the laser pulse duration. The magnetic field amplification is driven by electrons in the return current gaining favorable orbital angular momentum from the seed magnetic field. This mechanism is robust to laser polarization and delivers order-of-magnitude amplification over a range of simulation parameters.
Mijin S, Antony A, Militello F, et al., 2020, SOL-KiT -- fully implicit code for kinetic simulation of parallel electron transport in the tokamak Scrape-Off Layer, Publisher: arXiv
Here we present a new code for modelling electron kinetics in the tokamakScrape-Off Layer (SOL). SOL-KiT (Scrape-Off Layer Kinetic Transport) is a fullyimplicit 1D code with kinetic (or fluid) electrons, fluid (or stationary) ions,and diffusive neutrals. The code is designed for fundamental exploration ofnon-local physics in the SOL and utilizes an arbitrary degree Legendrepolynomial decomposition of the electron distribution function, treating bothelectron-ion and electron-atom collisions. We present a novel method forensuring particle and energy conservation in inelastic and superelasticcollisions, as well as the first full treatment of the logical boundarycondition in the Legendre polynomial formalism. To our knowledge, SOL-KiT isthe first fully implicit arbitrary degree harmonic kinetic code, offering aconservative and self-consistent approach to fluid-kinetic comparison with itsintegrated fluid electron mode. In this paper we give the model equations andtheir discretizations, as well as showing the results of a number ofverification/benchmarking simulations.
Watkins HC, Kingham RJ, 2019, Non-local Corrections to Collisional Transport in Magnetised Plasmas
In modern inertial fusion experiments there is a complex interplay betweennon-locality and magnetisation that can greatly influence transport. In thiswork we use a matrix recursion method to include higher-order correctionsbeyond the diffusion approximation usually used for magnetised plasmas. Workingin the linear regime, we show this can account for arbitrary orders of thedistribution function expansion in Knudsen non-locality parameter$k\lambda_{ei}$. Transport coefficients, such as thermal conductivity, deviatesignificantly from the magnetised diffusive approximation. In particular weshow how higher orders of the expansion contribute to transport asynchronouslyparallel, perpendicular and cross-perpendicular to a uniform magnetic fieldperpendicular to plasma perturbation.
Shi Y, Vieira J, Trines RMGM, et al., 2018, Magnetic field generation in plasma waves driven by copropagating intense twisted lasers, Physical Review Letters, Vol: 121, ISSN: 0031-9007
We present a new magnetic field generation mechanism in underdense plasmas driven by the beating of two, copropagating, Laguerre-Gaussian orbital angular momentum laser pulses with different frequencies and also different twist indices. The resulting twisted ponderomotive force drives up an electron plasma wave with a helical rotating structure. To second order, there is a nonlinear rotating current leading to the onset of an intense, static axial magnetic field, which persists over a long time in the plasma (ps scale) after the laser pulses have passed by. The results are confirmed in three-dimensional particle-in-cell simulations and also theoretical analysis. For the case of 300 fs duration, 3.8×1017 W/cm2 peak laser intensity we observe magnetic field of up to 0.4 MG. This new method of magnetic field creation may find applications in charged beam collimation and microscale pinch.
Watkins HC, Kingham RJ, 2018, Magnetised thermal self-focusing and filamentation of long-pulse lasers in plasmas relevant to magnetised ICF experiments, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
In this paper we study the influence of the magnetised thermal conductivityon the propagation of a nanosecond $10^{14} \mathrm{Wcm}^{-2}$ laser in anunderdense plasma by performing simulations of a paraxial model laser in aplasma with the full Braginskii magnetised transport coefficients. Analytictheory and simulations show the shortening of the self-focal length of a laserbeam in a plasma as a result of the reduction of the plasma thermalconductivity in a magnetic field. Furthermore the filamentation of a laser viathe thermal mechanism is found to have an increased spatial growth rate in amagnetised plasma. We discuss the effect of these results on recent magnetisedinertial fusion experiments where filamentation can be detrimental to laserpropagation and uniform laser heating. We conclude the application of externalmagnetic fields to laser-plasma experiments requires the inclusion of theextended electron transport terms in simulations of laser propagation.
Hill D, Kingham RJ, 2018, Enhancement of pressure perturbations in ablation due to kinetic magnetized transport effects under direct-drive inertial confinement fusion relevant conditions, Physical Review E, Vol: 98, ISSN: 1539-3755
We present kinetic two-dimensional Vlasov-Fokker-Planck simulations, including both self-consistent magnetic fields and ablating ion outflow, of a planar ablating foil subject to nonuniform laser irradiation. Even for small Hall parameters (ωτei≲0.05) self-generated magnetic fields are sufficient to invert and enhance pressure perturbations. The mode inversion is caused by a combination of the Nernst advection of the magnetic field and the Righi-Leduc heat flux. Nonlocal effects modify these processes. The mechanism is robust under plasma conditions tested; it is amplitude independent and occurs for a broad spectrum of perturbation wavelengths, λp=10–100μm. The ablating plasma response to a dynamically evolving speckle pattern perturbation, analogous to an optically smoothed beam, is also simulated. Similar to the single-mode case, self-generated magnetic fields increase the degree of nonuniformity at the ablation surface by up to an order of magnitude and are found to preferentially enhance lower modes due to the resistive damping of high mode number magnetic fields.
Brodrick JP, Sherlock M, Farmer WA, et al., 2018, Incorporating kinetic effects on Nernst advection in inertial fusion simulations, Plasma Physics and Controlled Fusion, Vol: 60, ISSN: 0741-3335
We present a simple method to incorporate nonlocal effects on the Nernst advection of magnetic fields down steep temperature gradients, and demonstrate its effectiveness in a number of inertial fusion scenarios. This is based on assuming that the relationship between the Nernst velocity and the heat flow velocity is unaffected by nonlocality. The validity of this assumption is confirmed over a wide range of plasma conditions by comparing Vlasov–Fokker–Planck and flux-limited classical transport simulations. Additionally, we observe that the Righi–Leduc heat flow is more severely affected by nonlocality due to its dependence on high velocity moments of the electron distribution function, but are unable to suggest a reliable method of accounting for this in fluid simulations.
Brodrick JP, Kingham RJ, Marinak MM, et al., 2017, Testing nonlocal models of electron thermal conduction for magnetic and inertial confinement fusion applications, PHYSICS OF PLASMAS, Vol: 24, ISSN: 1070-664X
Three models for nonlocal electron thermal transport are here compared against Vlasov-Fokker-Planck (VFP) codes to assess their accuracy in situations relevant to both inertial fusion hohlraums and tokamak scrape-off layers. The models tested are (i) a moment-based approach using an eigenvector integral closure (EIC) originally developed by Ji, Held, and Sovinec [Phys. Plasmas 16, 022312 (2009)]; (ii) the non-Fourier Landau-fluid (NFLF) model of Dimits, Joseph, and Umansky [Phys. Plasmas 21, 055907 (2014)]; and (iii) Schurtz, Nicolaï, and Busquet’s [Phys. Plasmas 7, 4238 (2000)] multigroup diffusion model (SNB). We find that while the EIC and NFLF models accurately predict the damping rate of a small-amplitude temperature perturbation (within 10% at moderate collisionalities), they overestimate the peak heat flow by as much as 35% and do not predict preheat in the more relevant case where there is a large temperature difference. The SNB model, however, agrees better with VFP results for the latter problem if care is taken with the definition of the mean free path. Additionally, we present for the first time a comparison of the SNB model against a VFP code for a hohlraum-relevant problem with inhomogeneous ionisation and show that the model overestimates the heat flow in the helium gas-fill by a factor of ∼2 despite predicting the peak heat flux to within 16%.
Read MP, Kingham RJ, Bissell JJ, 2016, The influence of magnetised electron transport on thermal self-focusing and channelling of nanosecond laser beams, 9th International Conference on Inertial Fusion Sciences and Applications (IFSA 2015), Publisher: IOP Publishing, ISSN: 1742-6588
The propagation of a nanosecond IR laser pulse through an under-dense (0.01 — 0.1ncr) magnetised laser-plasma is considered. The interplay between magnetised transport, B-field evolution and plasma hydrodynamics in the presence of a dynamically evolving beam are investigated by means of a paraxial wave solving module coupled to CTC, a 2D MHD code including Braginskii electron transport and IMPACT, a 2D implicit Vlasov-Fokker-Planck (VFP) code with magnetic fields. Magnetic fields have previously been shown to improve density channel formation for plasma waveguides however fluid simulations presented here indicate that Nernst advection can result in the rapid cavitation of magnetic field in the laser-heated region resulting in beam defocusing. Kinetic simulations indicate that strong non-local transport is present leading to the fluid code overestimating heat-flow and magnetic field advection and resulting in the recovery of beam channelling for the conditions considered.
Joglekar AS, Ridgers CP, Kingham RJ, et al., 2016, Kinetic modeling of Nernst effect in magnetized hohlraums, Physical Review E, Vol: 93, ISSN: 1539-3755
We present nanosecond time-scale Vlasov-Fokker-Planck-Maxwell modeling of magnetized plasma transport and dynamics in a hohlraum with an applied external magnetic field, under conditions similar to recent experiments. Self-consistent modeling of the kinetic electron momentum equation allows for a complete treatment of the heat flow equation and Ohm's law, including Nernst advection of magnetic fields. In addition to showing the prevalence of nonlocal behavior, we demonstrate that effects such as anomalous heat flow are induced by inverse bremsstrahlung heating. We show magnetic field amplification up to a factor of 3 from Nernst compression into the hohlraum wall. The magnetic field is also expelled towards the hohlraum axis due to Nernst advection faster than frozen-in flux would suggest. Nonlocality contributes to the heat flow towards the hohlraum axis and results in an augmented Nernst advection mechanism that is included self-consistently through kinetic modeling.
Pasley J, Bush IA, Robinson APL, et al., 2015, Generation of shock waves in dense plasmas by high-intensity laser pulses, Nukleonika, Vol: 60, Pages: 193-198, ISSN: 1508-5791
When intense short-pulse laser beams (I > 1022 W/m2, τ < 20 ps) interact with high density plasmas, strong shock waves are launched. These shock waves may be generated by a range of processes, and the relative significance of the various mechanisms driving the formation of these shock waves is not well understood. It is challenging to obtain experimental data on shock waves near the focus of such intense laser–plasma interactions. The hydrodynamics of such interactions is, however, of great importance to fast ignition based inertial confinement fusion schemes as it places limits upon the time available for depositing energy in the compressed fuel, and thereby directly affects the laser requirements. In this manuscript we present the results of magnetohydrodynamic simulations showing the formation of shock waves under such conditions, driven by the j × B force and the thermal pressure gradient (where j is the current density and B the magnetic field strength). The time it takes for shock waves to form is evaluated over a wide range of material and current densities. It is shown that the formation of intense relativistic electron current driven shock waves and other related hydrodynamic phenomena may be expected over time scales of relevance to intense laser–plasma experiments and the fast ignition approach to inertial confinement fusion. A newly emerging technique for studying such interactions is also discussed. This approach is based upon Doppler spectroscopy and offers promise for investigating early time shock wave hydrodynamics launched by intense laser pulses.
Robinson APL, Strozzi DJ, Davies JR, et al., 2014, Theory of fast electron transport for fast ignition, Nuclear Fusion, Vol: 54, ISSN: 1741-4326
Fast ignition (FI) inertial confinement fusion is a variant of inertial fusion in which DT fuel is first compressed to high densityand then ignited by a relativistic electron beam generated by a fast (<20 ps) ultra-intense laser pulse, which is usually broughtin to the dense plasma via the inclusion of a re-entrant cone. The transport of this beam from the cone apex into the dense fuelis a critical part of this scheme, as it can strongly influence the overall energetics. Here we review progress in the theory andnumerical simulation of fast electron transport in the context of FI. Important aspects of the basic plasma physics, descriptionsof the numerical methods used, a review of ignition-scale simulations, and a survey of schemes for controlling the propagationof fast electrons are included. Considerable progress has taken place in this area, but the development of a robust, high-gain FI‘point design’ is still an ongoing challenge.
Williams BER, Kingham RJ, 2013, Hybrid simulations of fast electron propagation including magnetized transport and non-local effects in the background plasma, 40th Conference of the European-Physical-Society on Plasma Physics, Publisher: IOP PUBLISHING LTD, ISSN: 0741-3335
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Williams BER, Kingham RJ, Bissell JJ, 2013, Heat flux effects on magnetic field dynamics in solid density plasmas traversed by relativistic electron beams, PLASMA PHYSICS AND CONTROLLED FUSION, Vol: 55, ISSN: 0741-3335
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Bissell JJ, Ridgers CP, Kingham RJ, 2013, Super-Gaussian transport theory and the field-generating thermal instability in laser–plasmas, New Journal of Physics, Vol: 15, Pages: 025017-025017
Bissell JJ, Kingham RJ, Ridgers CP, 2012, Magnetothermal instability in laser plasmas including hydrodynamic effects, PHYSICS OF PLASMAS, Vol: 19, ISSN: 1070-664X
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Thomas AGR, Tzoufras M, Robinson APL, et al., 2012, A review of Vlasov-Fokker-Planck numerical modeling of inertial confinement fusion plasma, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 231, Pages: 1051-1079, ISSN: 0021-9991
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- Citations: 52
Willingale L, Thomas AGR, Nilson PM, et al., 2011, Proton probe measurement of fast advection of magnetic fields by hot electrons, PLASMA PHYSICS AND CONTROLLED FUSION, Vol: 53, ISSN: 0741-3335
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Ridgers CP, Sherlock M, Evans RG, et al., 2011, Superluminal sheath-field expansion and fast-electron-beam divergence measurements in laser-solid interactions, PHYSICAL REVIEW E, Vol: 83, ISSN: 1539-3755
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- Citations: 13
Bush IA, Robinson APL, Kingham R, et al., 2010, Cavitation and shock wave formation in dense plasmas by relativistic electron beams, PLASMA PHYS CONTR F, Vol: 52, ISSN: 0741-3335
The propagation of a high current relativistic electron beam through a dense plasma, for example, in fast-ignition inertial confinement fusion, produces strong heating and magnetic field generation. The j x B force and thermal pressure gradient that the return current creates may in fact cavitate and cause shock waves in the plasma around the electron beam.Here we investigate this effect in different regimes of plasma density and hot electron current. An analytic model has been developed that gives good estimates of the density, pressure, magnetic field and velocity obtained in the plasma. This model is compared against the results from an MHD code that includes the effects of resistive field growth, Ohmic heating and the j x B force. The strength of the cavitation is found to be dependent upon the ratio between j(2) and the initial mass density. It is shown that cavitation is indeed relevant to fast-ignition, and is strong enough to launch shocks in certain circumstances.
Bissell JJ, Ridgers CP, Kingham RJ, 2010, Field Compressing Magnetothermal Instability in Laser Plasmas, PHYSICAL REVIEW LETTERS, Vol: 105, ISSN: 0031-9007
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- Citations: 21
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