We describe laboratory experiments to generate x-ray photoionized plasmas of relevance to accretion-powered x-ray sources such as neutron star binaries and quasars, with significant improvements over previous work. A key quantity is referenced, namely the photoionization parameter, defined as ξ=4πF/ne where F is the x-ray flux and ne the electron density. This is normally meaningful in an astrophysical steady-state context, but is also commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, time-dependent. 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 astrophysical scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However useful checks on performance can often be made for a limited range of parameters. For example, we show that our 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 a blackbody source with ∼keV spectral temperature. We compare calculations from our in-house plasma modelling code with those from Cloudy and find moderately good agreement for the time evolution of both electron temperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrum with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised argon 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.

Numerical simulation of Rayleigh–Taylor instabilities involving solids has been carried out in a spherically converging geometry. A solid shell is driven toward the center by high-pressure gas outside. Low-pressure gas inside the shell is then compressed, causing the solid shell to slow down and eventually bounce back. Simulation results show the absence of the onset condition unlike in the planar geometry equivalent. Results also show that disturbance growth seems to be strongly associated with the deformation direction instead of the acceleration direction. For example, when the interface is compressed in the cross-stream direction, the disturbance at the gas–solid interface grows even under the Rayleigh–Taylor stable situation.

We study the evolution of the single-mode Richtmyer–Meshkov instability for a wide range of Atwood numbers, shock strengths and perturbation amplitudes using Youngs’ hydrodynamical simulation code TURMOIL. We compare our results to previously published analytic models for the impulsively-driven growth rate, and propose a modification to them to treat the reduction of growth found at high initial perturbation amplitudes and high Mach numbers. It is known that the overall asymmetry between bubbles and spikes and their eventual deceleration can be interpreted as the result of nonlinear coupling to higher harmonic modes. However, we find that for light-to-heavy interfaces at moderate to high impinging shock Mach numbers, the shape of the growing bubbles varies in time, with the initial curved bubble surfaces flattening and inverting to generate a second low velocity jet. For high shock Mach numbers and low initial surface amplitudes, the process of inversion can recur on numerous occasions. We interpret this as being the result of vorticity deposited by the transmitted shock in the bulk of the heavy material, away from the initial interface.

Precise spectral diagnostic modelling of H~{\sc i} and He~{\sc ii} recombination spectra can constrain theoretical models which describe many astrophysical environments. Simple analytic expressions are of interest for collisional $l$-changing rate coefficients that are used by large-scale population modelling codes. We review, clarify and improve-upon the modified Pengelly \&Seaton formulae of Guzm\'an \etal We show that the recent poor results for it shown by Vrinceanu \etal are due to their misinterpretation of its usage. We also detail efficient numerical algorithms which should enable the full quantum mechanical expression for such rate coefficients to be used much more routinely by modelling codes. We illustrate with some collisional-radiative population modelling for hydrogen.

Hydrodynamic instabilities such as Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities usually appear in conjunction with the Kelvin–Helmholtz (KH) instability and are found in many natural phenomena and engineering applications. They frequently result in turbulent mixing, which has a major impact on the overall flow development and other effective material properties. This can either be a desired outcome, an unwelcome side effect, or just an unavoidable consequence, but must in all cases be characterized in any model. The RT instability occurs at an interface between different fluids, when the light fluid is accelerated into the heavy. The RM instability may be considered a special case of the RT instability, when the acceleration provided is impulsive in nature such as that resulting from a shock wave. In this pedagogical review, we provide an extensive survey of the applications and examples where such instabilities play a central role. First, fundamental aspects of the instabilities are reviewed including the underlying flow physics at different stages of development, followed by an overview of analytical models describing the linear, nonlinear and fully turbulent stages. RT and RM instabilities pose special challenges to numerical modeling, due to the requirement that the sharp interface separating the fluids be captured with fidelity. These challenges are discussed at length here, followed by a summary of the significant progress in recent years in addressing them. Examples of the pivotal roles played by the instabilities in applications are given in the context of solar prominences, ionospheric flows in space, supernovae, inertial fusion and pulsed-power experiments, pulsed detonation engines and Scramjets. Progress in our understanding of special cases of RT/RM instabilities is reviewed, including the effects of material strength, chemical reactions, magnetic fields, as well as the roles the instabilities play in ejecta formation and transport, and explosively expanding flows. The article is addressed to a broad audience, but with particular attention to graduate students and researchers who are interested in the state-of-the-art in our understanding of the instabilities and the unique issues they present in the applications in which they are prominent.

We compare the results of two models for the breakup of ejecta particles for a simple test problem. We conclude that while they operate in significantly different manners, their overall results are quite comparable. We note that the chi(2) distribution function widely used in particle breakup modelling is not in fact consistent with the distribution proposed by Hiroyasu & Kadota based on their original experiments, which may resolve the disagreement in particle radius distributions observed in subsequent work. This paper is UK Ministry of Defence (C) Crown Owned Copyright 2020/AWE.

The mean momentum and heavy mass fraction, turbulent kinetic energy, and heavy mass fraction variance fields, as well as the budgets of their transport equations are examined several times during the evolution of a narrowband Richtmyer-Meshkov instability initiated by a Mach 1.84 shock traversing a perturbed interface separating gases with a density ratio of 3. The results are computed using the “quarter scale” data from four algorithms presented in the θ-group study of Thornber et al. [“Late-time growth rate, mixing, and anisotropy in the multimode narrowband Richtmyer-Meshkov instability: The θ-group collaboration,” Phys. Fluids 29, 105107 (2017)]. The present study is inspired by a previous similar study of Rayleigh-Taylor instability and mixing using direct numerical simulation data by Schilling and Mueschke [“Analysis of turbulent transport and mixing in transitional Rayleigh-Taylor unstable flow using direct numerical simulation data,” Phys. Fluids 22, 105102 (2010)]. In addition to comparing the predictions of the data from four implicit large-eddy simulation codes, the budgets are used to quantify the relative importance of the terms in the transport equations, and the balance of the terms is employed to infer the numerical dissipation. Terms arising from the compressibility of the flow are examined, in particular the pressure-dilatation. The results are useful for validation of large-eddy simulation and Reynolds-averaged modeling of Richtmyer-Meshkov instability.

We present a number of fully-conservative, four-equation, multicomponent reconstruction schemes for the Euler equations, which have been designed to maintain both constant pressure and temperature at isothermal contact discontinuities, as is required by thermodynamic consistency. The schemes we discuss have been implemented in AWE's staggered-mesh detailed turbulence modelling code TURMOIL and also in a finite-volume Riemann-solver based code. We show results for a problem where material properties with a high contrast in adiabatic index and material specific heat lead to substantial pressure and temperature perturbations for a simple mass fraction model. The improved schemes maintain the isobaric and isothermal nature of the initial condition to numerical precision. (C) 2019 Published by Elsevier Inc.

The Space Telescope and Optical Reverberation Mapping Project (AGN STORM) on NGC 5548 in 2014 is one of the most intensive multiwavelength AGN monitoring campaigns ever. For most of the campaign, the emission-line variations followed changes in the continuum with a time lag, as expected. However, the lines varied independently of the observed UV-optical continuum during a 60-70 day "holiday," suggesting that unobserved changes to the ionizing continuum were present. To understand this remarkable phenomenon and to obtain an independent assessment of the ionizing continuum variations, we study the intrinsic absorption lines present in NGC 5548. We identify a novel cycle that reproduces the absorption line variability and thus identify the physics that allows the holiday to occur. In this cycle, variations in this obscurer's line-of-sight covering factor modify the soft X-ray continuum, changing the ionization of helium Ionizing radiation produced by recombining helium then affects the level of ionization of some ions seen by the Hubble Space Telescope. In particular, high-ionization species are affected by changes in the obscurer covering factor, which does not affect the optical or UV continuum, and thus appear as uncorrelated changes, a "holiday." It is likely that any other model that selectively changes the soft X-ray part of the continuum during the holiday can also explain the anomalous emission-line behavior observed.

Recent numerical and analytic work has highlighted some shortcomings in our understanding of the dynamics of H II region expansion, especially at late times, when the H II region approaches pressure equilibrium with the ambient medium. Here we reconsider the idealized case of a constant radiation source in a uniform and spherically symmetric ambient medium, with an isothermal equation of state. A thick-shell solution is developed that captures the stalling of the ionization front and the decay of the leading shock to a weak compression wave as it escapes to large radii. An acoustic approximation is introduced to capture the late-time damped oscillations of the H II region about the stagnation radius. Putting these together, a matched asymptotic equation is derived for the radius of the ionization front which accounts for both the inertia of the expanding shell and the finite temperature of the ambient medium. The solution to this equation is shown to agree very well with the numerical solution at all times, and is superior to all previously published solutions. The matched asymptotic solution can also accurately model the variation of H II region radius for a time-varying radiation source.

We discuss the processes controlling ejecta particle production and breakup, as a result of shock wave compression of condensed matter and in broader contexts. We present analytic models for the mass-velocity distribution, particle size and total mass saturation of fluid jets from regular surface perturbations. Numerical calculations illustrate differences in behaviour for isolated features rather than regular periodic surfaces.

We analyze the properties of a number of forms of artificial viscosity, when applied to the Lagrangian phase of a staggered grid Lagrange-remap hydrodynamics code, using a full von Neumann stability analysis. This allows us to derive the numerical dispersion relation for a single step of the scheme. We study the development of shocks within a weakly nonlinear acoustic wave in detail. The second-order von Neumann-Richtmyer artificial viscosity leads to significant post-shock ringing, while adding linear artificial viscosity reduces the accuracy of the scheme to first order and introduces a dependence on flow Mach number. The form of the numerical dispersion relation suggests that to control the post-shock ringing without reducing the accuracy of the solution, a higher-order artificial viscosity is required which will preferentially damp modes close to the mesh scale which are subject to significant phase error. The application of ideas based on the Large Eddy Simulation technique for the modelling of turbulent flows leads to a form of artificial viscous stress similar to that described by Schulz (1964). This artificial stress has several features which are beneficial for accurate modelling of compressible turbulent mixing processes, as demonstrated by a range of other test problems. We show that, in the limit of smooth flows, Christensen's approach of applying limiters to the components of a low-order artificial viscosity is analytically equivalent to a high-order scheme. However, the directionally-split implementation suggested in the original publication leads to damping of low Mach number Kelvin-Helmholtz roll up problems similar to that previously observed for finite-volume Godunov schemes. (C) 2018 Published by Elsevier Inc.

We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination rate to all levels is preserved. At intermediate densities, on the other hand, collisions can lead to ionization of higher-lying excited states, thereby suppressing the dielectronic recombination rate. The improved suppression factors presented here, although highly approximate, allow summed recombination rate coefficients to be used to intermediate densities. There have been several technical improvements to our previously presented fits. For H-through B-like ions the activation log densities have been adjusted to better reproduce existing data. For B-, C-, Al-, and Si-like ions secondary autoionization is now included. The treatment of density discontinuity in electron excitations out of ground state H-, He-, and Ne-like ions has been improved. These refined dielectronic recombination suppression factors are used in the most recent version of the plasma simulation code Cloudy. We show how the ionization and emission spectrum change when this physics is included. Although these suppression factors improve the treatment of intermediate densities, they are highly approximate and are not a substitution for a complete collisional-radiative model of the ionization balance.

We have performed numerical calculations of fluid mixing driven by Rayleigh–Taylor instability for density profiles based on the stratified density experiments of Lawrie & Dalziel (J. Fluid Mech., vol. 688, 2011, pp. 507–527) and Davies Wykes & Dalziel (J. Fluid Mech., vol. 756, 2014, pp. 1027–1057). We find that the late-time mixing profiles are similar to their experimental results for similar initial conditions; we consider a range of additional initial conditions to investigate the robustness of the results. A model for the late-time structure of the mixing layer, based on the maximization of configurational entropy, is compared with the results of the numerical calculations, and shows good agreement.

Accurate rates for energy-degenerate l-changing collisions are needed to determine cosmological abundances and recombination. There are now several competing theories for the treatment of this process, and it is not possible to test these experimentally. We show that the HI two-photon continuum produced by astrophysical nebulae is strongly affected by lchanging collisions. We perform an analysis of the different underlying atomic processes and simulate the recombination and two-photon spectrum of a nebula containing H and He. We provide an extended set of effective recombination coefficients and updated l-changing 2s-2p transition rates using several competing theories. In principle, accurate astronomical observations could determine which theory is correct.

We have performed continuum simulations of ejecta production from multiply-shocked metal surfaces. We describe the underlying computational capability, and discuss the results of our calculations. While still at a developmental stage, these results demonstrate the fundamental linkage between the production of ejecta and sub-surface material failure, explaining qualitative features seen in previous experimental results.

We describe the 2017 release of the spectral synthesis code Cloudy. A major development since the previous release has been exporting the atomic data into external data files. This greatly simplifies updates and maintenance of the data. Many large datasets have been incorporated with the result that we can now predict well over an order of magnitude more emission lines when all databases are fully used. The use of such large datasets is not realistic for most calculations due to the time and memory needs, and we describe the limited subset of data we use by default. Despite the fact that we now predict significantly more lines than the previous Cloudy release, this version is faster because of optimization of memory access patterns and other tuning. The size and use of the databases can easily be adjusted in the command-line interface. We give examples of the accuracy limits using small models, and the performance requirements of large complete models. We summarize several advances in the H- and He-like iso-electronic sequences. We use our complete collisional-radiative models of the ionization of these one and two-electron ions to establish the highest density for which the coronal or interstellar medium (ISM) approximation works, and the lowest density where Saha or local thermodynamic equilibrium can be assumed. The coronal approximation fails at surprisingly low densities for collisional ionization equilibrium but is valid to higher densities for photoionized gas clouds. Many other improvements to the physics have been made and are described. These include the treatment of isotropic continuum sources such as the cosmic microwave background (CMB) in the reported output, and the ability to follow the evolution of cooling non-equilibrium clouds.

We compare the results of the semi-classical (SC) and quantum-mechanical (QM) formalisms for angular-momentum changing transitions in Rydberg atom collisions given in a series of papers by Vrinceanu et al, most recently Vrinceanu et al (2012 Astrophys. J. 747 56), with those of the SC formalism using a modified Monte Carlo realization. We find that this revised SC formalism agrees well with the QM results. This provides further evidence that the rates derived from the QM treatment are appropriate to be used when modeling recombination through Rydberg cascades, an important process in understanding the state of material in the early universe. The rates for Delta l = +/- 1 derived from the QM formalism diverge when integrated to sufficiently large impact parameter, b. Further to the empirical limits to the b integration suggested by Pengelly and Seaton (1964 Mon. Not. R. Astron. Soc. 127 165), we suggest that the fundamental issue causing this divergence in the theory is that it does not fully cater for the finite time taken for such distant collisions to complete.

Cosmological models can be constrained by determining primordial abundances. Accurate predictions of the He I spectrum are needed to determine the primordial helium abundance to a precision of < 1 per cent in order to constrain big bang nucleosynthesis models. Theoretical line emissivities at least this accurate are needed if this precision is to be achieved. In the first paper of this series, which focused on H I, we showed that differences in l-changing collisional rate coefficients predicted by three different theories can translate into 10 per cent changes in predictions for H I spectra. Here, we consider the more complicated case of He atoms, where low-l subshells are not energy degenerate. A criterion for deciding when the energy separation between l subshells is small enough to apply energy-degenerate collisional theories is given. Moreover, for certain conditions, the Bethe approximation originally proposed by Pengelly & Seaton is not sufficiently accurate. We introduce a simple modification of this theory which leads to rate coefficients which agree well with those obtained from pure quantal calculations using the approach of Vrinceanu et al. We show that the l-changing rate coefficients from the different theoretical approaches lead to differences of similar to 10 per cent in He I emissivities in simulations of H II regions using spectral code CLOUDY.

Turbulent Richtmyer-Meshkov instability (RMI) is investigated through a series of high resolution three-dimensional simulations of two initial conditions with eight independent codes. The simulations are initialised with a narrowband perturbation such that instability growth is due to non-linear coupling/backscatter from the energetic modes, thus generating the lowest expected growth rate from a pure RMI. By independently assessing the results from each algorithm and computing ensemble averages of multiple algorithms, the results allow a quantification of key flow properties as well as the uncertainty due to differing numerical approaches. A new analytical model predicting the initial layer growth for a multimode narrowband perturbation is presented, along with two models for the linear and non-linear regimes combined. Overall, the growth rate exponent is determined as theta = 0.292 +/- 0.009, in good agreement with prior studies; however, the exponent is decaying slowly in time. Also, theta is shown to be relatively insensitive to the choice of mixing layer width measurements. The asymptotic integral molecular mixing measures Theta = 0.792 +/- 0.014, Xi = 0.800 +/- 0.014, and psi = 0.782 +/- 0.013 are lower than some experimental measurements but within the range of prior numerical studies. The flow field is shown to be persistently anisotropic for all algorithms, at the latest time having between 49% and 66% higher kinetic energy in the shock parallel direction compared to perpendicular and does not show any return to isotropy. The plane averaged volume fraction profiles at different time instants collapse reasonably well when scaled by the integral width, implying that the layer can be described by a single length scale and thus a single theta. Quantitative data given for both ensemble averages and individual algorithms provide useful benchmark results for future research. Published by AIP Publishing.