293 results found
Paddock 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: 0741-3335
Low 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.
Hoarty 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: 1070-664X
Recent 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.
Crilly 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: 1574-1818
SpK is part of the numerical codebase at Imperial College London used to model high energy density physics (HEDP) experiments. SpK is an efficient atomic and microphysics code used to perform detailed configuration accounting calculations of electronic and ionic stage populations, opacities and emissivities for use in post-processing and radiation hydrodynamics simulations. This is done using screened hydrogenic atomic data supplemented by the NIST energy level database. An extended Saha model solves for chemical equilibrium with extensions for non-ideal physics, such as ionisation potential depression, and non thermal equilibrium corrections. A tree-heap (treap) data structure is used to store spectral data, such as opacity, which is dynamic thus allowing easy insertion of points around spectral lines without a-priori knowledge of the ion stage populations. Results from SpK are compared to other codes and descriptions of radiation transport solutions which use SpK data are given. The treap data structure and SpK’s computational efficiency allows inline post-processing of 3D hydrodynamics simulations with a dynamically evolving spectrum stored in a treap.
Watt 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: 2469-9888
A 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.
Breach 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: 1574-1818
Inertial 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.
Beesley 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: 2211-3797
This 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.
Singh 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: 0263-0346
We 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.
Halliday JWD, Crilly A, Chittenden J, et al., 2022, Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator, Physics of Plasmas, Vol: 29, Pages: 1-13, ISSN: 1070-664X
We present first results from a novel experimental platform which is able toaccess physics relevant to topics including indirect-drive magnetised ICF;laser energy deposition; various topics in atomic physics; and laboratoryastrophysics (for example the penetration of B-fields into HED plasmas). Thisplatform uses the X-Rays from a wire array Z-Pinch to irradiate a silicontarget, producing an outflow of ablated plasma. The ablated plasma expands intoambient, dynamically significant B-fields (~5 T) which are supported by thecurrent flowing through the Z-Pinch. The outflows have a well-defined(quasi-1D) morphology, enabling the study of fundamental processes typicallyonly available in more complex, integrated schemes. Experiments were fielded onthe MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). On this machine awire array Z-Pinch produces an X-Ray pulse carrying a total energy of ~15 kJover ~30 ns. This equates to an average brightness temperature of around 10 eVon-target.
McLean KW, Rose SJ, 2022, Multi-group radiation diffusion convergence in low-density foam experiments, Journal of Quantitative Spectroscopy and Radiative Transfer, Vol: 280, Pages: 1-13, ISSN: 0022-4073
We present an in-depth analysis of a Marshak radiation wave moving through an iron-oxide (Fe2O3) foamusing a 1D multigroup diffusive radiation transport model, MDART (Multigroup Diffusion Algorithm forRadiation Transport). We consider the consequences of under-resolving the group structure and addresshow this could lead to incorrect conclusions in the analysis of general supersonic radiation wave experiments. We also provide an analysis of the types of experimental outcome one may incorrectly link tophysical effects but are in fact due to poor simulation practice.
Halliday JWD, Crilly A, Chittenden J, et al., 2022, An Experimental Study of Magnetic Flux Penetration in Radiatively Driven Plasma Flows, ISSN: 0730-9244
In this talk we present measurements from a novel platform in which the X-Rays from a wire-array Z-Pinch irradiate a silicon target, producing an outflow of ablated silicon plasma. This ablated plasma expands into ambient, dynamically significant magnetic fields (B ∼ 5 T) which are supported by the current flowing through the Z-Pinch.
Kettle B, Hollatz D, Gerstmayr E, et al., 2021, A laser-plasma platform for photon-photon physics: the two photon Breit-Wheeler process, New Journal of Physics, Vol: 23, ISSN: 1367-2630
We describe a laser–plasma platform for photon–photon collision experiments to measure fundamental quantum electrodynamic processes. As an example we describe using this platform to attempt to observe the linear Breit–Wheeler process. The platform has been developed using the Gemini laser facility at the Rutherford Appleton Laboratory. A laser Wakefield accelerator and a bremsstrahlung convertor are used to generate a collimated beam of photons with energies of hundreds of MeV, that collide with keV x-ray photons generated by a laser heated plasma target. To detect the pairs generated by the photon–photon collisions, a magnetic transport system has been developed which directs the pairs onto scintillation-based and hybrid silicon pixel single particle detectors (SPDs). We present commissioning results from an experimental campaign using this laser–plasma platform for photon–photon physics, demonstrating successful generation of both photon sources, characterisation of the magnetic transport system and calibration of the SPDs, and discuss the feasibility of this platform for the observation of the Breit–Wheeler process. The design of the platform will also serve as the basis for the investigation of strong-field quantum electrodynamic processes such as the nonlinear Breit–Wheeler and the Trident process, or eventually, photon–photon scattering.
Baggott RA, Rose SJ, Mangles SPD, 2021, Temperature Equilibration Due to Charge State Fluctuations in Dense Plasmas, ISSN: 0730-9244
The charge states of ions in dense plasmas fluctuate due to collisional ionization and recombination. In this work we show how, by modifying the ion interaction potential, these fluctuations can mediate energy exchange between the plasma electrons and ions. We also develop a theoretical framework for this novel electron-ion energy transfer mechanism.
Rose SJ, Hatfield PW, 2021, Astronomy Domine: advancing science with a burning plasma, Contemporary Physics, Vol: 62, Pages: 14-23, ISSN: 0010-7514
Inertial Confinement Fusion (ICF) is a subject that has been studied for decades, because of its potential for clean energy generation. Although thermonuclear fusion has been achieved, the energy out has always been considerably less than the energy in, so high energy gain with a burning thermonuclear plasma is still some way off. A multitude of new science has come from the ICF programme that is relevant outside the field (typically in astrophysics). What we look at in this text is what new science can come from the much more extreme conditions that would be created in the laboratory if a burning ICF plasma could be created -- in terms of energy density the most extreme macroscopic environment ever created. We show that this could impact science from particle physics through astrophysics and on to cosmology. We also believe that the experiments that we propose here are only a small part of the science that will be opened up when a burning thermonuclear plasma is created in the laboratory.
Baggott RA, Rose S, Mangles SPD, 2021, Temperature equilibration due to charge state fluctuations in dense plasmas, Physical Review Letters, Vol: 27, ISSN: 0031-9007
The charge states of ions in dense plasmas fluctuate due to collisionalionization and recombination. Here we show how, by modifying the ioninteraction potential, these fluctuations can mediate energy exchange betweenthe plasma electrons and ions. Moreover, we develop a theory for this novelelectron-ion energy transfer mechanism. Calculations using a random walkapproach for the fluctuations suggest that the energy exchange rate from chargestate fluctuations could be comparable to direct electron-ion collisions. Thismechanism is, however, predicted to exhibit a complex dependence on thetemperature and ionization state of the plasma, which could contribute to ourunderstanding of significant variation in experimental measurements ofequilibration times.
Spencer Kelly R, Hart LJF, Rose SJ, 2021, An investigation of efficient muon production for use in muon catalyzed fusion, Journal of Physics: Energy, Vol: 3, Pages: 1-7, ISSN: 2515-7655
We model the energy cost of producing muons for use in muon catalyzed fusion and show that by careful design the cost can be reduced by a factor of 2.5 below current values. This is done by recapturing the kinetic energy of waste particles and generating heat through tritium breeding. When put together with the modeling of muon catalyzed fusion we estimate that electrical output/electrical input of 14% can be achieved currently.
Hatfield PW, Gaffney JA, Anderson GJ, et al., 2021, The data-driven future of high-energy-density physics, Nature, Vol: 593, Pages: 351-361, ISSN: 0028-0836
High-energy-density physics is the field of physics concerned with studying matter at extremely high temperatures and densities. Such conditions produce highly nonlinear plasmas, in which several phenomena that can normally be treated independently of one another become strongly coupled. The study of these plasmas is important for our understanding of astrophysics, nuclear fusion and fundamental physics—however, the nonlinearities and strong couplings present in these extreme physical systems makes them very difficult to understand theoretically or to optimize experimentally. Here we argue that machine learning models and data-driven methods are in the process of reshaping our exploration of these extreme systems that have hitherto proved far too nonlinear for human researchers. From a fundamental perspective, our understanding can be improved by the way in which machine learning models can rapidly discover complex interactions in large datasets. From a practical point of view, the newest generation of extreme physics facilities can perform experiments multiple times a second (as opposed to approximately daily), thus moving away from human-based control towards automatic control based on real-time interpretation of diagnostic data and updates of the physics model. To make the most of these emerging opportunities, we suggest proposals for the community in terms of research design, training, best practice and support for synthetic diagnostics and data analysis.
Perez-Callejo G, Marley E, Liedahl DA, et al., 2021, Demonstration of geometric effects and resonant scattering in the X-Ray spectra of high-energy-density plasmas, Physical Review Letters, Vol: 126, Pages: 1-7, ISSN: 0031-9007
In a plasma of sufficient size and density, photons emitted within the system have a probability of being reabsorbed and reemitted multiple times—a phenomenon known in astrophysics as resonant scattering. This effect alters the ratio of optically thick to optically thin lines, depending on the plasma geometry and viewing angle, and has significant implications for the spectra observed in a number of astrophysical scenarios, but has not previously been studied in a controlled laboratory plasma. We demonstrate the effect in the x-ray spectra emitted by cylindrical plasmas generated by high power laser irradiation, and the results confirm the geometrical interpretation of resonant scattering.
Rose S, Hatfield P, Scott R, 2020, Modelling burning thermonuclear plasma, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 378, Pages: 1-8, ISSN: 1364-503X
Considerable progress towards the achievement ofthermonuclear burn using inertial confinement fusion has beenachieved at the National Ignition Facility (NIF) in the USA inthe last few years. Other drivers, such as the Z-machine atSandia, are also making progress towards this goal. A burningthermonuclear plasma would provide a unique and extremeplasma environment; in this paper we discuss a) differenttheoretical challenges involved in modelling burning plasmasnot currently considered, b) the use of novel machine learningbased methods that might help large facilities reach ignition,and c) the connections that a burning plasma might have tofundamental physics, including QED studies, and the replicationand exploration of conditions that last occurred in the first fewminutes after the Big Bang.
Pérez-Callejo G, Jarrott LC, Liedahl DA, et al., 2020, Measuring the oscillator strength of intercombination lines of helium-like V ions in a laser-produced-plasma, Journal of Quantitative Spectroscopy and Radiative Transfer, Vol: 256, ISSN: 0022-4073
We present results of measurements of the oscillator strength of intercombination lines of He-like Vanadium ions in high energy density (HED) laser-produced-plasmas and compare them with the simulations from commonly used codes and data from the NIST database. Whilst not yet sufficiently accurate to constrain different trusted atomic-physics models for the particular system studied, our results are in agreement with the available data within experimental error bars, yet differ from cruder approximations of the oscillator strength used in certain atomic-kinetics packages, suggesting that this general method could be further extended to be used as a measurement of the oscillator strength of additional atomic transitions under the extreme conditions that are achieved in HED experiments.
Pérez-Callejo G, Barrios MA, Liedahl DA, et al., 2020, A novel method to measure ion density in ICF experiments using x-ray spectroscopy of cylindrical tracers, Physics of Plasmas, Vol: 27, Pages: 112714-1-112714-11, ISSN: 1070-664X
The indirect drive approach to inertial confinement fusion has undergone important advances in the past few years. Improvements in temperature and density diagnostic methods are leading to more accurate measurements of the plasma conditions inside the Hohlraum and therefore to more efficient experimental designs. The implementation of dot spectroscopy has proven to be a versatile approach to extracting space- and time-dependent electron temperatures. In this method, a microdot of a mid-Z material is placed inside the Hohlraum and its K-shell emission spectrum is used to determine the plasma temperature. However, radiation transport of optically thick lines acting within the cylindrical dot geometry influences the outgoing spectral distribution in a manner that depends on the viewing angle. This angular dependence has recently been studied in the high energy density regime at the OMEGA laser facility, which allowed us to design and benchmark appropriate radiative transfer models that can replicate these geometric effects. By combining these models with the measurements from the dot spectroscopy experiments at the National Ignition Facility, we demonstrate here a novel technique that exploits the transport effects to obtain time-resolved measurements of the ion density of the tracer dots, without the need for additional diagnostics. We find excellent agreement between experiment and simulation, opening the possibility of using these geometric effects as a density diagnostic in future experiments.
White S, Irwin R, Warwick R, et al., 2020, Generation of photoionized plasmas in the laboratory: Analogues to astrophysical sources, Laboratory Astrophysics fFrom Observation to Interpretation, Publisher: Cambridge University Press, Pages: 321-325, ISSN: 1743-9213
Implementation of a novel experimental approach using a bright source of narrowband x-ray emission has enabled the production of a photoionized argon plasma of relevance to astrophysical modelling codes such as Cloudy. We present results showing that the photoionization parameter ζ = 4πF/ne generated using the VULCAN laser was ≈ 50 erg cm s−1, higher than those obtained previously with more powerful facilities. Comparison of our argon emission-line spectra in the 4.15 - 4.25 Å range at varying initial gas pressures with predictions from the Cloudy code and a simple time-dependent code are also presented. Finally we briefly discuss how this proof-of-principle experiment may be scaled to larger facilities such as ORION to produce the closest laboratory analogue to a photoionized plasma.
Baggott R, Rose S, Mangles S, 2020, Calculating opacity in hot, dense matter using second-order electron-photon and two-photon transitions to approximate line broadening, Physical Review Letters, Vol: 125, Pages: 145002 – 1-145002 – 5, ISSN: 0031-9007
Calculations of the opacity of hot, dense matter require models for plasma line broadening. How-ever, the most general theories are too complex to calculate directly and some approximation is inevitably required. The most widely-used approaches focus on the line centre, where a Lorentzian shape is obtained. Here, we demonstrate that in the opposite limit, far from the line centre, the opacity can be expressed in terms of second-order transitions, such as electron-photon and two-photon processes. We suggest that this insight could form the basis for a new approach to improve calculations of opacity in hot, dense matter. Preliminary calculations suggest that this approach could yield increased opacity away from absorption lines.
Behm K, Hussein AE, Zhao TZ, et al., 2020, Demonstration of femtosecond broadband X-rays from laser wakefield acceleration as a source for pump-probe X-ray absorption studies, High Energy Density Physics, Vol: 35, Pages: 1-5, ISSN: 1574-1818
We present X-ray absorption measurements near the K-edge of laser heated aluminum in a pump-probe configuration using X-rays generated in a laser wakefield accelerator. A 30 fs duration laser pulse from the Herculeslaser system was split into two beamlines, with one used to heat a 4 µm thickness Al foil and the second to drive a laser wakefield accelerator. The laser-heated plasma was probed at various pump-probe delays using the femtosecond duration X-rays generated by betatron oscillations of the electrons in the wakefield. We observe an apparent blue-shift of the K-edge occurring on a sub-picosecond timescale in the transmission spectra.
McLean KW, Rose SJ, 2020, Corrections to weighted opacities and energy exchange rate in 3-T radiation-hydrodynamics, High Energy Density Physics, Vol: 35, Pages: 1-5, ISSN: 1574-1818
It is often the case that high energy density systems can be well described and simulated in the 3T approximation, where electrons, ions and the radiation field are defined at unique temperatures given by Te, Ti, Tr. The difference in temperature between the electrons and radiation field is important when calculating weighted opacities and electron-radiation energy exchange rates. Often, it is assumed that Tr ≈ Te, meaning the quantities can be calculated as functions of Te only. This paper explores the consequences that arise when one uses this assumption in regions where Tr ≠ Te. Mutliplicative correction factors are derived for the Rosseland and Planckian mean opacities (κR and κP) and for the electron-radiation energy exchange rate. We find that there exists a very small region of parameter space where the corrections are negligible. However, for the majority of parameter space explored, numerical corrections vary from factors of 2 to multiple orders of magnitude.
Hobbs LMR, Burridge D, Hill MP, et al., 2020, X-ray-line coincidence photopumping in a potassium-chlorine mixed plasma, Physical Review A, Vol: 101, Pages: 053431-1-053431-8, ISSN: 2469-9926
Exploiting the multiple long pulse capability and suite of x-ray diagnostics of the Orion laser, we have set out to explore line coincidence photopuming—the enhancement in population of an atomic level brought on by resonant absorption of x rays from a different emitting ion. Unlike previous work, the two ions are in the same plasma and so the experiment is an x-ray analog of the well-known Bowen resonance fluorescence mechanism that operates in astrophysical situations in the optical region. Our measurements have shown enhanced fluorescence in a chlorine plasma, attributable to line coincident photopumping from co-mixed potassium ions. To detect this relatively low signal-to-noise phenomenon, the data from multiple shots are combined, and the statistical method of bootstrapping is used to assign a confidence value to the measured enhancement, resulting in an estimate of the enhancement of 39±1618% compared to the null case, where no pumping occurs. The experimental results have been compared to coupled radiation-transport and radiation hydrodynamics simulations using the cretin code together with the nym radiation hydrodynamics model and agreement has been found, with the simulations also predicting modest enhancement.
Nilsen J, Burridge D, Hobbs LMR, et al., 2020, Enhanced fluorescence from X-Ray line coincidence pumping, 16th International Conference on X-ray Lasers, Publisher: Springer International Publishing, Pages: 29-35, ISSN: 0930-8989
Many resonant photo-pumped X-ray laser schemes that use a strong pump line such as Ly-α or He-α to populate the upper laser state of a separate lasing material have been proposed over the last four decades but none have been demonstrated. As a first step to creating a photo-pumped X-ray laser we have decided to reinvestigate some of these schemes at the Orion laser facility with the goal to show enhanced fluorescence. In particular we look at using the Ly-α or He-α K lines to pump the 1s–3p and 4p transitions in H-like Cl and see fluorescence on the 4f–3d line at 65 Å and the 3d–2p line at 23 Å. Preliminary experiments are presented that show a modest enhancement. As an alternative we also look at enhancing the 2p–2s line in Ne-like Ge at 65 Å using the Ly-α Mg line to photo-pump the 2s–3p line of Ne-like Ge. Calculations are presented that suggest modest enhancements of 2.5.
Bailie D, Hyland C, Singh R, et al., 2020, An investigation of the L-shell X-ray conversion efficiency for laser-irradiated tin foils, Plasma Science and Technology, Vol: 22, Pages: 1-7, ISSN: 1009-0630
We have used the Shenguang II laser in third harmonic (351 nm) to investigate the emission ofL-shell radiation in the 3.3 to 4.4 keV range generated using thin foils of Sn coated onto a parylenesubstrate with irradiation of order 1015 Wcm−2 and nanosecond pulse duration. In our experiment,we have concentrated on assessing the emission on the non-laser irradiated side as this allows anexperimental geometry relevant to experiments on photo-ionised plasmas where a secondary targetmust be placed close to the source, to achieve X-ray fluxes appropriate to astrophysical objects.Overall L-shell conversion efficiencies are estimated to be of order 1%, with little dependence onSn thickness between 400 and 800 nm.
Hatfield P, Rose S, Scott R, et al., 2020, Using sparse Gaussian processes for predicting robust inertial confinement fusion implosion yields, IEEE Transactions on Plasma Science, Vol: 48, Pages: 14-21, ISSN: 0093-3813
Here, we present the application of an advanced sparse Gaussian process-based machine learning algorithm to the challenge of predicting the yields of inertial confinement fusion (ICF) experiments. The algorithm is used to investigate the parameter space of an extremely robust ICF design for the National Ignition Facility, the ``Simplest Design''; deuterium-tritium gas in a plastic ablator with a Gaussian, Planckian drive. In particular, we show that: 1) GPz has the potential to decompose uncertainty on predictions into uncertainty from lack of data and shot-to-shot variation; 2) it permits the incorporation of science-goal-specific cost-sensitive learning (CSL), e.g., focusing on the high-yield parts of parameter space; and 3) it is very fast and effective in high dimensions.
Kettle B, Gerstmayr E, Streeter MJV, et al., 2019, Single-shot multi-keV X-ray absorption spectroscopy using an ultrashort laser-wakefield accelerator source, Physical Review Letters, Vol: 123, Pages: 254801-1-254801-6, ISSN: 0031-9007
Single-shot absorption measurements have been performed using the multi-keV x rays generated by a laser-wakefield accelerator. A 200 TW laser was used to drive a laser-wakefield accelerator in a mode which produced broadband electron beams with a maximum energy above 1 GeV and a broad divergence of ≈15 mrad FWHM. Betatron oscillations of these electrons generated 1.2±0.2×106 photons/eV in the 5 keV region, with a signal-to-noise ratio of approximately 300∶1. This was sufficient to allow high-resolution x-ray absorption near-edge structure measurements at the K edge of a titanium sample in a single shot. We demonstrate that this source is capable of single-shot, simultaneous measurements of both the electron and ion distributions in matter heated to eV temperatures by comparison with density functional theory simulations. The unique combination of a high-flux, large bandwidth, few femtosecond duration x-ray pulse synchronized to a high-power laser will enable key advances in the study of ultrafast energetic processes such as electron-ion equilibration.
Beesley JJ, Rose SJ, 2019, Free electron relativistic correction factors to collisional excitation and ionisation rates in a plasma, High Energy Density Physics, Vol: 33, ISSN: 1574-1818
This paper presents approximate correction factors to collisional excitation and ionisation rates which account for special relativity in the free electron motion as a function of free electron temperature and the threshold energy of the reaction. These are calculated by taking the ratio of collisional rates derived from simple empirical cross sections using relativistic and non-relativistic mechanics. These results are extended to de-excitation and three body recombination rates using detailed balance. It is found that the relativistic correction is significant in regimes potentially important to galactic intracluster media and diagnostic dopants in burning ICF plasmas.
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