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Journal articleKettle B, Colgan C, Los E, et al., 2024,
Extended X-ray absorption spectroscopy using an ultrashort pulse laboratory-scale laser-plasma accelerator
, Communications Physics, Vol: 7, ISSN: 2399-3650Laser-driven compact particle accelerators can provide ultrashort pulses of broadband X-rays, well suited for undertaking X-ray absorption spectroscopy measurements on a femtosecond timescale. Here the Extended X-ray Absorption Fine Structure (EXAFS) features of the K-edge of a copper sample have been observed over a 250 eV window in a single shot using a laser wakefield accelerator, providing information on both the electronic and ionic structure simultaneously. This unique capability will allow the investigation of ultrafast processes, and in particular, probing high-energy-density matter and physics far-from-equilibrium where the sample refresh rate is slow and shot number is limited. For example, states that replicate the tremendous pressures and temperatures of planetary bodies or the conditions inside nuclear fusion reactions. Using high-power lasers to pump these samples also has the advantage of being inherently synchronised to the laser-driven X-ray probe. A perspective on the additional strengths of a laboratory-based ultrafast X-ray absorption source is presented.
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Journal articleRiley D, Singh RL, White S, et al., 2024,
Generation of photoionized plasmas in the laboratory of relevance to accretion-powered x-ray sources using keV line radiation
, High Energy Density Physics, Vol: 51, ISSN: 1574-1818We describe laboratory experiments to generate x-ray photoionized plasmas of relevance to accretion-powered xray sources such as neutron star binaries and quasars, with significant improvements over previous work. A keyquantity is referenced, namely the photoionization parameter, defined as ξ = 4πF/ne where F is the x-ray flux andne the electron density. This is normally meaningful in an astrophysical steady-state context, but is alsocommonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, timedependent. We demonstrate emission-weighted values of ξ > 50 erg-cm s− 1 using laser-plasma x-ray sources,with higher results at the centre of the plasma which are in the regime of interest for several astrophysicalscenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by themany orders of magnitude differences in time and spatial scales, but also other plasma parameters. Howeveruseful checks on performance can often be made for a limited range of parameters. For example, we show thatour use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from ablackbody source with ~keV spectral temperature. We compare calculations from our in-house plasma modellingcode with those from Cloudy and find moderately good agreement for the time evolution of both electrontemperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrumwith experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionisedargon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.
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Journal articleXu L, Yu X, Favier CD, et al., 2024,
Development of an experimental method for well-controlled blast induced traumatic limb fracture in rats
, Defence Technology, Vol: 34, Pages: 168-176, ISSN: 2214-9147Heterotopic ossification (HO) is a consequence of traumatic bone and tissue damage, which occurs in 65% of military casualties with blast-associated amputations. However, the mechanisms behind blast-induced HO remain unclear. Animal models are used to study blast-induced HO, but developing such models is challenging, particularly in how to use a pure blast wave (primary blast) to induce limb fracture that then requires an amputation. Several studies, including our recent study, have developed platforms to induce limb fractures in rats with blast loading or a mixture of blast and impact loading. However, these models are limited by the survivability of the animal and repeatability of the model. In this study, we developed an improved platform, aiming to improve the animal's survivability and injury repeatability as well as focusing on primary blast only. The platform exposed only one limb of the rat to a blast wave while providing proper protection to the rest of the rat's body. We obtained very consistent fracture outcome in the tibia (location and pattern) in cadaveric rats with a large range of size and weight. Importantly, the rats did not obviously move during the test, where movement is a potential cause of uncontrolled injury. We further conducted parametric studies by varying the features of the design of the platform. These factors, such as how the limb is fixed and how the cavity through which the limb is placed is sealed, significantly affect the resulting injury. This platform and test setups enable well-controlled limb fracture induced directly by pure blast wave, which is the fundamental step towards a complete in vivo animal model for blast-induced HO induced by primary blast alone, excluding secondary and tertiary blast injury. In addition, the platform design and the findings presented here, particularly regarding the proper protection of the animal, have implications for future studies investigating localized blast injuries, such as blast induced br
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Journal articleDowhan GV, Shah AP, Sporer BJ, et al., 2024,
High-magnification Faraday rotation imaging and analysis of X-pinch implosion dynamics
, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 95, ISSN: 0034-6748 -
Journal articlePérez-Callejo G, Gawne T, Preston TR, et al., 2024,
Dielectronic satellite emission from a solid-density Mg plasma: relationship to models of ionization potential depression
, Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, Vol: 109, ISSN: 1539-3755We report on experiments where solid-density Mg plasmas are created by heating with the focused output of the Linac Coherent Light Source x-ray free-electron laser. We study the K-shell emission from the helium- and lithium-like ions using Bragg crystal spectroscopy. Observation of the dielectronic satellites in lithium-like ions confirms that the M-shell electrons appear bound for these high charge states. An analysis of the intensity of these satellites indicates that when modeled with an atomic-kinetics code, the ionization potential depression model employed needs to produce depressions for these ions which lie between those predicted by the well known Stewart-Pyatt and Ecker-Kroll models. These results are largely consistent with recent density functional theory calculations.
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Journal articleRead J, Burdiak G, Bland SN, et al., 2024,
Point projection radiography of electromagnetically accelerated flyer plates with an external X-pinch driver (vol 95, 023508, 2024)
, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 95, ISSN: 0034-6748 -
Journal articleRead J, Burdiak G, Bland SN, et al., 2024,
Point projection radiography of electromagnetically accelerated flyer plates with an external X-pinch driver
, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 95, ISSN: 0034-6748 -
Journal articlePaddock RW, Li TS, Kim E, et al., 2024,
Energy gain of wetted-foam implosions with auxiliary heating for inertial fusion studies
, Plasma Physics and Controlled Fusion, Vol: 66, ISSN: 0741-3335Low convergence ratio implosions (where wetted-foam layers are used to limit capsule convergence, achieving improved robustness to instability growth) and auxiliary heating (where electron beams are used to provide collisionless heating of a hotspot) are two promising techniques that are being explored for inertial fusion energy applications. In this paper, a new analytic study is presented to understand and predict the performance of these implosions. Firstly, conventional gain models are adapted to produce gain curves for fixed convergence ratios, which are shown to well-describe previously simulated results. Secondly, auxiliary heating is demonstrated to be well understood and interpreted through the burn-up fraction of the deuterium-tritium fuel, with the gradient of burn-up with respect to burn-averaged temperature shown to provide good qualitative predictions of the effectiveness of this technique for a given implosion. Simulations of auxiliary heating for a range of implosions are presented in support of this and demonstrate that this heating can have significant benefit for high gain implosions, being most effective when the burn-averaged temperature is between 5 and 20 keV.
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Journal articleMaler D, Belozerov O, Godinger A, et al., 2024,
Multi frame radiography of supersonic water jets interacting with a foil target
, Journal of Applied Physics, Vol: 135, ISSN: 0021-8979Pulsed-power-driven underwater electrical explosion of cylindrical or conical wire arrays produces supersonic water jets that emerge from a bath, propagating through the air above it. Interaction of these jets with solid targets may represent a new platform for attaining materials at high pressure (>1010 Pa) conditions in a university-scale laboratory. However, measurements of the internal structure of such jets and how they interact with targets are difficult optically due to large densities and density contrasts involved. We utilized multi-frame x-ray radiographic imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility to explore the water jet and its interaction with a 50 μm thick copper foil placed a few mm from the surface of water. The jet was generated with a ∼130 kA-amplitude current pulse of ∼450 ns rise time applied to a conical wire array. X-ray imaging revealed a droplet-type structure of the jet with an average density of <400 kg/m3 propagating with a velocity of ∼1400 m/s. Measurements of deformation and subsequent perforation of the target by the jet suggested pressures at the jet–target interface of ∼5 × 109 Pa. The results were compared to hydrodynamic simulations for better understanding of the jet parameters and their interaction with the foil target. These results can be used in future research to optimize the platform, and extend it to larger jet velocities in the case of higher driving currents supplied to the wire array.
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Journal articleRack A, Lukić B, Chapman DJ, et al., 2024,
Dynamic loading platforms coupled to ultra-high speed X-ray imaging at beamline ID19 of the European Synchrotron ESRF
, High Pressure Research, ISSN: 0895-7959The intersection of dynamic compression, high-rate material response and X-ray science has seen rapid growth, leading to the establishment of specialized end-stations at international facilities such as Linac Coherent Light Source LCLS (Matter at Extreme Conditions–MEC) and Advanced Photon Source APS (Dynamic Compression Sector–DCS), both USA. Although these facilities excel in working with X-rays tailored for small material volumes (i.e. (Formula presented.)), it needs a different approach to delve into subsequent processes. This is particularly the case in the transition from the micro- to mesoscale: here the ESRF distinguishes itself. The large beam size (several cm2) of the ID19 beamline, in conjunction with a strong high energy component, source flux density, and outstanding imaging sensitivity, enables sub-surface visualization of engineering-scale structures as well as natural systems in representative volume, under high rate and shock. This is particularly valuable when studying materials with complex mesostructures and heterogeneities on relevant volumetric scales, which often dominate the dynamic material response. The study of the behavior of materials under dynamic loading presents a unique challenge due to inherently spanning over multiple lengths- and timescales. The evolution of sudden (thermo)mechanical excitation, starting from the lattice scale and progressing through grains, phase domains, and ultimately to structures, exhibits a spectrum of responses spanning from the microscopic to bulk length scales. Consequently, a diverse range of diagnostics as well as driver instrumentation is required to identify, study, and characterize this material response spectrum. This article shall introduce platforms available at beamline ID19 and underline their potential by selected showcase applications. Community access proposals such as the beamtime Block Allocation Group (BAG) allow for access in a routine manner.
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Journal articlePachman J, Chapman DJ, Foglar M, et al., 2024,
Shock response of concrete, fibre concrete and ultrahigh performance concrete
, INTERNATIONAL JOURNAL OF IMPACT ENGINEERING, Vol: 183, ISSN: 0734-743X -
Journal articleGusev AI, Lavrinovich I, Bland S, et al., 2023,
New SOS diode pumping circuit based on an all-solid-state spiral generator for high-voltage nanosecond applications
, IEEE Transactions on Plasma Science, Vol: 51, Pages: 2858-2856, ISSN: 0093-3813Semiconductor opening switch (SOS) diodes are capable to switch currents with a density of more than 1 kA/cm 2 and withstand nanosecond pulses with an amplitude of up to 1 MV. SOS diodes, however, require a specific pumping circuit that must simultaneously provide forward and reverse pumping currents with a time of ∼ 500 and ∼ 100 ns, respectively. Such a pumping circuit with energies > 1 J typically requires a gas-discharge switch or a low-efficient solid-state solution. This study proposes a novel approach to pumping SOS diodes based on a spiral generator (SG) (also known as a vector inversion generator). Due to its wave characteristics, the SG produces a bipolar current discharge that meets the time duration and current amplitude required to pump an SOS diode. Moreover, the initial pulse from the spiral typically has a relatively low current amplitude compared to the opposite polarity secondary pulse, so the SOS diode can operate at very high efficiencies. This idea has been tested using an all-solid-state SG coupled with large-area SOS diodes (1 cm 2 ). With this combination, a voltage pulse of 62 kV having a rise time of only 11 ns was obtained on an open circuit load (3 pF, 1 M Ω ). The experiments were highly repeatable, with no damage to the components despite multiple tests. There is significant scope to further improve the results, with simple alterations to the SG.
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Journal articleBailie D, White S, Irwin R, et al., 2023,
K-Edge Structure in Shock-Compressed Chlorinated Parylene
, ATOMS, Vol: 11 -
Journal articleDatta R, Angel J, Greenly JB, et al., 2023,
Plasma flows during the ablation stage of an over-massed pulsed-power-driven exploding planar wire array
, PHYSICS OF PLASMAS, Vol: 30, ISSN: 1070-664X -
Journal articleLee JJ, Mohammed AA, Pullen A, et al., 2023,
Mechanical characterisation of 3D printed lightweight lattice structures with varying internal design alterations
, MATERIALS TODAY COMMUNICATIONS, Vol: 36 -
Journal articleHoarty DJ, Morton J, Rougier JC, et al., 2023,
Radiation burnthrough measurements to infer opacity at conditions close to the solar radiative zone–convective zone boundary
, Physics of Plasmas, Vol: 30, Pages: 1-15, ISSN: 1070-664XRecent measurements at the Sandia National Laboratory of the x-ray transmission of iron plasma have inferred opacities much higher than predicted by theory, which casts doubt on modeling of iron x-ray radiative opacity at conditions close to the solar convective zone-radiative zone boundary. An increased radiative opacity of the solar mixture, in particular iron, is a possible explanation for the disagreement in the position of the solar convection zone-radiative zone boundary as measured by helioseismology and predicted by modeling using the most recent photosphere analysis of the elemental composition. Here, we present data from radiation burnthrough experiments, which do not support a large increase in the opacity of iron at conditions close to the base of the solar convection zone and provide a constraint on the possible values of both the mean opacity and the opacity in the x-ray range of the Sandia experiments. The data agree with opacity values from current state-of-the-art opacity modeling using the CASSANDRA opacity code.
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Journal articleCrilly AJ, Niasse NPL, Fraser AR, et al., 2023,
SpK: A fast atomic and microphysics code for the high-energy-density regime
, High Energy Density Physics, Vol: 48, Pages: 1-12, ISSN: 1574-1818SpK is part of the numerical codebase at Imperial College London used to model high energy density physics (HEDP) experiments. SpK is an efficient atomic and microphysics code used to perform detailed configuration accounting calculations of electronic and ionic stage populations, opacities and emissivities for use in post-processing and radiation hydrodynamics simulations. This is done using screened hydrogenic atomic data supplemented by the NIST energy level database. An extended Saha model solves for chemical equilibrium with extensions for non-ideal physics, such as ionisation potential depression, and non thermal equilibrium corrections. A tree-heap (treap) data structure is used to store spectral data, such as opacity, which is dynamic thus allowing easy insertion of points around spectral lines without a-priori knowledge of the ion stage populations. Results from SpK are compared to other codes and descriptions of radiation transport solutions which use SpK data are given. The treap data structure and SpK’s computational efficiency allows inline post-processing of 3D hydrodynamics simulations with a dynamically evolving spectrum stored in a treap.
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Journal articleWahler S, Klapoetke TM, Proud WG, 2023,
Testing open-source tools for optical chemical structure recognition on novel nitrogen-rich energetic materials
, JOURNAL OF ENERGETIC MATERIALS, ISSN: 0737-0652 -
Journal articleWatt RA, Rose SJ, Kettle B, et al., 2023,
Monte Carlo modeling of the linear Breit-Wheeler process within the geant4 framework
, Physical Review Accelerators and Beams, Vol: 26, Pages: 1-7, ISSN: 2469-9888A linear Breit-Wheeler module for the code geant4 has been developed. This allows signal-to-noise ratio calculations of linear Breit-Wheeler detection experiments to be performed within a single framework. The interaction between two photon sources is modeled by treating one as a static field, then photons from the second source are sampled and tracked through the field. To increase the efficiency of the module, we have used a Gaussian process regression, which can lead to an increase in the calculation rate by a factor of up to 1000. To demonstrate the capabilities of this module, we use it to perform a parameter scan, modeling an experiment based on that recently reported by Kettle et al. [New J. Phys. 23, 115006 (2021)]. We show that colliding 50-fs duration γ rays, produced through bremsstrahlung emission of a 100 pC, 2-GeV laser wakefield accelerator beam, with a 50-ps x-ray field, generated by a germanium burn-through foil heated to temperatures >150 eV, this experiment is capable of producing >1 Breit-Wheeler pair per shot.
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Journal articleTsukada H, Nguyen T-TN, Breeze J, et al., 2023,
The risk of fragment penetrating injury to the heart
, Journal of The Mechanical Behavior of Biomedical Materials, Vol: 141, Pages: 1-6, ISSN: 1751-6161Injury due to the penetration of fragments into parts of the body has been the major cause of morbidity and mortality after an explosion. Penetrating injuries into the heart present very high mortality, yet the risk associated with such injuries has not been quantified. Quantifying this risk is key in the design of personal protection and the design of infrastructure.This study is the first quantitative assessment of cardiac penetrating injuries from energised fragments. Typical fragments (5-mm sphere, 0.78-g right-circular cylinder and 1.1-g chisel-nosed cylinder) were accelerated to a range of target striking velocities using a bespoke gas-gun system and impacted ventricular and atrial walls of lamb hearts. The severity of injury was shown to not depend on location (ventricular or atrial wall). The striking velocity with 50% probability of critical injury (Abbreviated Injury Scale (AIS) 5 score) ranged between 31 and 36 m/s across all 3 fragments used. These findings can help directly in reducing morbidity and mortality from explosive events as they can be implemented readily into models that aim to predict casualties in an explosive event, inform protocols for first responders, and improve design of infrastructure and personal protective equipment.
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Journal articleStrucka J, Lukic B, Koerner M, et al., 2023,
Synchrotron radiography of Richtmyer–Meshkov instability driven by exploding wire arrays
, Physics of Fluids, Vol: 35, Pages: 1-11, ISSN: 1070-6631We present a new technique for the investigation of shock-driven hydrodynamic phenomena in gases, liquids, and solids in arbitrary geometries. The technique consists of a pulsed power-driven resistive wire array explosion in combination with multi-MHz synchrotron radiography. Compared to commonly used techniques, it offers multiple advantages: (1) the shockwave geometry can be shaped to the requirements of the experiment, (2) the pressure (P > 300 MPa) generated by the exploding wires enables the use of liquid and solid hydrodynamic targets with well-characterized initial conditions (ICs), (3) the multi-MHz radiography enables data acquisition to occur within a single experiment, eliminating uncertainties regarding repeatability of the ICs and subsequent dynamics, and (4) the radiographic measurements enable estimation of compression ratios from the x-ray attenuation. In addition, the use of a synchrotron x-ray source allows the hydrodynamic samples to be volumetrically characterized at a high spatial resolution with synchrotron-based microtomography. This experimental technique is demonstrated by performing a planar Richtmyer–Meshkov instability (RMI) experiment on an aerogel–water interface characterized by Atwood number A 0 ∼ − 0.8 and Mach number M ∼ 1.5. The qualitative and quantitative features of the experiment are discussed, including the energy deposition into the exploding wires, shockwave generation, compression of the interface, startup phase of the instability, and asymptotic growth consistent with Richtmyer's impulsive theory. Additional effects unique to liquids and solids—such as cavitation bubbles caused by rarefaction flows or initial jetting due to small perturbations—are observed. It is also demonstrated that the technique is not shape dependent by driving a cylindrically convergent RMI experiment.
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Conference paperMundy T, Bland S, Lebedev S, et al., 2023,
Novel Experiment for Scaled Power Flow Studies Towards Next-Generation Pulsed Power
, ISSN: 2158-4915In order to develop a better understanding of current losses in the magnetically insulated region of high-power pulsed power machines, it is crucial to be able to conduct experiments at scale in smaller facilities. Here, we present a novel experiment that has been tested on the MAGPIE driver at Imperial College. The targets are inexpensive and easy to customize for experiments ranging from power flow to warm dense matter. Simulations in COMSOL indicated electric fields of up to 600 MV/m and magnetic fields of up to 300 T could be produced on MAGPIE. In initial testing, Electric fields exceeding 100 MV/m and magnetic fields exceeding 50 T were generated, and both magnetically insulated transmission and plasma-shorted transmission were demonstrated.
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Conference paperMarrow K, Mundy T, Halliday J, et al., 2023,
Radiative Instabilities in the Stagnation Layer of Colliding, X-Ray Driven Plasma Flows
, ISSN: 0730-9244We summarise existing results and future avenues of research from a novel experimental platform [1] fielded on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). This platform uses the x-ray pulse emitted from a wire array z-pinch to drive plasma ablation from a target. The radiatively driven outflow has a uniform (quasi-1D) structure and expands into the ambient magnetic field produced by the z-pinch.
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Journal articleBreach O, Hatfield P, Rose S, 2022,
Optimising point source irradiation of a capsule for maximum uniformity
, High Energy Density Physics, Vol: 45, Pages: 1-7, ISSN: 1574-1818Inertial Confinement Fusion involves the implosion of a spherical capsule con-taining thermonuclear fuel. The implosion is driven by irradiating the outsideof the capsule by X-rays or by optical laser irradiation, where in each casethe highest uniformity of irradiation is sought. In this paper we consider thetheoretical problem of irradiation of a capsule by point sources of X-rays, andconfigurations which maximize uniformity are sought. By studying the root-mean-square deviation in terms of different order harmonic modes, we ratio-nalise the dependence of uniformity on distance d of the point sources fromthe centre of a capsule. After investigating simple configurations based onthe Platonic solids, we use a global optimisation algorithm (basin-hopping)to seek better arrangements. The optimum configurations are found to de-pend strongly on d; at certain values which minimise nonuniformity, theseinvolve grouping of sources on the vertices of octahedra or icosahedra, whichwe explain using a modal decomposition. The effect of uncertainties in bothposition and intensity is studied, and lastly we investigate the illuminationof a capsule whose radius is changing with time.
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Journal articleRankin IA, Nguyen T-TN, McMenemy L, et al., 2022,
Protective clothing reduces lower limb injury severity against propelled sand debris in a laboratory setting
, Human Factors and Mechanical Engineering for Defense and Safety, Vol: 6, Pages: 1-7, ISSN: 2509-8004The contribution of energised environmental debris to injury patterns of the blast casualty is not known. The extent to which personal protective equipment (PPE) limits the injuries sustained by energised environmental debris following an explosive event is also not known. In this study, a cadaveric model exposed to a gas-gun mediated sand blast was utilised which reproduced soft-tissue injuries representative of those seen clinically following blast. Mean sand velocity across experiments was 506 ± 80 ms−1. Cadaveric samples wearing standard-issue PPE were shown to have a reduced injury severity to sand blast compared to control: a statistically significant reduction was seen in the total surface area (143 mm2 vs. 658 mm2, p = 0.004) and depth of injuries (0 vs. 23 deep injuries, odds ratio = 0.0074, 95% confidence intervals 0.0004–0.1379). This study is the first to recreate wounds from propelled sand in a human cadaveric model. These findings implicate environmental debris, such as sand ejected from a blast event, as a critical mechanism of injury in the blast casualty. Tier 1 pelvic PPE was shown to reduce markedly the severity of injury. This injury mechanism should be a key focus of future research and mitigation strategies.
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Journal articleNguyen T-TN, Tsukada H, Breeze J, et al., 2022,
The critical role of a backing material in assessing the performance of soft ballistic protection
, Human Factors and Mechanical Engineering for Defense and Safety, Vol: 6, Pages: 1-11, ISSN: 2509-8004Penetrating trauma by energised fragments is the most common injury from an explosive event. Fragment penetrations to the truncal region can result in lethal haemorrhage. Personal armour is used to mitigate ballistic threats; it comprises hard armour to protect from high-velocity bullets and soft armour to protect against energised fragments and other ballistic threats (such as from a hand gun) with low impact velocities. Current testing standards for soft armour do not focus on realistic boundary conditions, and a backing material is not always recommended. This study provides a comprehensive set of evidence to support the inclusion of a backing used in testing of soft body armour. Experiments were performed with a gas-gun system using fragment-simulating projectiles (FSPs) of different shapes and sizes to impact on a woven aramid and a knitted high-performance polyethylene ballistic fabric, with and without the ballistic gelatine soft tissue simulant as the backing material. The results showed statistically significant differences in the impact velocities at 50% risk (V50) of fabric perforation across all test configurations when the gelatine backing was used. Furthermore, the backing material enabled the collection of injury-related metrics such as V50 of tissue-simulant penetrations as well as depth of penetration against impact velocity. The normalised energy absorbed by the fabric could also be calculated when the backing material was present. This study confirms that a backing material is essential, particularly when assessing the performance of single layer fabrics against FSPs of low mass. It also demonstrates the additional benefits provided by the backing for predicting injury outcomes.
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Journal articleBeesley JJ, Rose SJ, 2022,
High-temperature limit of Breit-Wheeler pair production in a black-body field
, Results in Physics, Vol: 41, Pages: 1-3, ISSN: 2211-3797This paper presents an analytic expression for the high-temperature limit of Breit-Wheeler pair production in a black-body field to lowest order in perturbation theory, of interest in early-universe cosmology. The limit is found to be a good approximation for temperatures above about three times the electron rest energy. It is also found that coupling to low-energy processes remains important at arbitrarily high temperatures, due to the exchange of a low-energy virtual fermion near the mass shell. This appears mathematically in the rate as a logarithmic factor of the photon temperature divided by the electron rest mass.
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Journal articleLavrinovich I, Gusev AI, Bland S, et al., 2022,
2-kV thyristor triggered in impact-ionization wave mode by a solid-state spiral generator
, IEEE Transactions on Plasma Science, Vol: 50, Pages: 3443-3451, ISSN: 0093-3813Impact-ionization wave triggering of a thyristor enables it to switch significantly higher currents with much faster rise times ( dI/dt ) than through conventional triggering; indeed tests on commercial components demonstrate that both current and dI/dt can be increased an order of magnitude over their specified datasheet values by utilizing impact ionization. However, creating an impact ionization wave places stringent requirements on the generator used to trigger the thyristor—particularly the trigger pulse must have a voltage rise rate ( dV/dt ) of more than 1 kV/ns and an amplitude over twice the thyristors static breakdown voltage. Given the capacitance of a thyristor is relatively large, often hundreds of pF, this is difficult to achieve with many common triggering methods. In this study, we present a bespoke, cost-effective, trigger generator that has been developed based on spiral/vector inversion techniques coupled to an optimized sharpening circuit. Using this generator, both a 2-kV single thyristor and a 4-kV stack of two thyristors in series were triggered in the impact-ionization mode. The thyristors had a wafer diameter of 32 mm and capacitances of 370 pF. With a single thyristor 100 shots were performed with it switching a peak current of 1.25 kA and an associated dI/dt of 12 kA/ μs . With two thyristors, peak currents of 2.6 kA and with dI/dt of 25 kA/ μs were achieved. In all experiments no degradation of the semiconductor structure was observed. The work opens the way for developing very powerful, but still compact, solid-state trigger generators and larger pulsers for a wide range of pulsed power applications.
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Journal articleYu X, Nguyen T, Wu T, et al., 2022,
Non-lethal blasts can generate cavitation in cerebrospinal fluid while severe helmeted impacts cannot: a novel mechanism for blast brain injury
, Frontiers in Bioengineering and Biotechnology, Vol: 10, ISSN: 2296-4185Cerebrospinal fluid (CSF) cavitation is a likely physical mechanism for producing traumatic brain injury (TBI) under mechanical loading. In this study, we investigated CSF cavitation under blasts and helmeted impacts which represented loadings in battlefield and road traffic/sports collisions. We first predicted the human head response under the blasts and impacts using computational modelling and found that the blasts can produce much lower negative pressure at the contrecoup CSF region than the impacts. Further analysis showed that the pressure waves transmitting through the skull and soft tissue are responsible for producing the negative pressure at the contrecoup region. Based on this mechanism, we hypothesised that blast, and not impact, can produce CSF cavitation. To test this hypothesis, we developed a one-dimensional simplified surrogate model of the head and exposed it to both blasts and impacts. The test results confirmed the hypothesis and computational modelling of the tests validated the proposed mechanism. These findings have important implications for prevention and diagnosis of blast TBI.
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Journal articleSingh RL, White S, Charlwood M, et al., 2022,
L-shell X-Ray conversion yields for laser-irradiated tin and silver foils
, Laser and Particle Beams, Vol: 2022, Pages: 1-10, ISSN: 0263-0346We have employed the VULCAN laser facility to generate a laser plasma X-ray source for use in photoionization experiments. A nanosecond laser pulse with an intensity of order 1015 Wcm−2 was used to irradiate thin Ag or Sn foil targets coated onto a parylene substrate, and the L-shell emission in the 3.3–4.4 keV range was recorded for both the laser-irradiated and nonirradiated sides. Both the experimental and simulation results show higher laser to X-ray conversion yields for Ag compared with Sn, with our simulations indicating yields approximately a factor of two higher than those found in the experiments. Although detailed angular data were not available experimentally, the simulations indicate that the emission is quite isotropic on the laser-irradiated side but shows close to a cosine variation on the nonirradiated side of the target as seen experimentally in the previous work.
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