Session GO08: HED: Pulsed Power Laboratory Astrophysics

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The High Amperage Driver for Extreme States, HADES, is a multi-cavity pulsed-power driver based on linear transformer driver technology. Designed as a compact machine, HADES uses current-adding transmission lines to combine 2x600kA in less than 250 ns via a post-hole assembly. We are presenting here the engineering validation of current adding technology and how it relates to machine triggering and load inductance. Key design features such as parallel cavity charging, and independent brick triggering will be discussed.   

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We present results from a laboratory platform capable of producing differentially rotating plasmas aiming to study angular momentum transport in conditions relevant to those astrophysical disks and jets. A 1.4 MA electrical current pulse drives 8 radial plasma jets carrying angular momentum relative to the setup's common axis. When the jets merge around the common axis of the experiment, a hollow rotating plasma column is formed and a hollow, highly collimated jet is ejected along the axis of the system. Laser interferometry used to determine the radial density profile provides measurements of the density depletion at the jet axis. Optical Thomson scattering measurements reveal the jet spins at \textasciitilde 10 km/s with Ti \textasciitilde 70 eV, Te \textasciitilde 30 eV, resulting in a subsonic M \textasciitilde 0.6 rotational motion, whereas estimates from XUV images indicate a supersonic axial flow M \textasciitilde 3. Furthermore, Re \textasciitilde 10$^{\mathrm{5}}$, Rm \textasciitilde 10$^{\mathrm{2}}$, hence Pm \textless \textless 1 putting our experiment in the regime relevant to young stellar object disks and jets. Supported by US Department of Energy Awards DE-F03-02NA00057 {\&} DE-SC-0001063 and the Royal Astronomical Society. V. Valenzuela-Villaseca is funded by the Imperial College President's PhD Scholarships.   

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We will explore the physical and astrophysical motivations for the current experiments conducted by the Wootton Center for Astrophysical Plasma Properties (WCAPP) on the Z-machine at Sandia National Laboratories (SNL). This work informs our understanding of the Sun and Sun-like stars, radiation dominated plasma in accretion disks around compact objects---including supermassive black holes at the centers of galaxies, and our understanding of the compact endpoint of almost all stars, the white dwarf stars. The solution to significant puzzles surrounding these objects will come from reproducing, studying and benchmarking the underlying physics of these plasmas in the laboratory. The talks that follow in this section will detail the progress we have made in each of these areas.   

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White dwarf stars with carbon-dominated atmospheres (the hot DQs) likely form from the merger of two white dwarfs. In some cases, such mergers end explosively as type Ia supernovae, which are important as cosmological distance probes, but the conditions for detonation are unclear. A better understanding of the hot DQs, therefore, has potential to clarify a key process relevant to cosmology. In particular, existing astronomical spectra of the hot DQ stars could be used to derive a mass distribution, which might reveal a cutoff mass beyond which mergers explode. These spectra could also enable mass-radius determinations, which might indicate core compositions resulting from nuclear burning in a merger. These goals hinge on the ability of spectral models to deliver accurate surface gravities. However, these models have not been vetted against laboratory data, and the input C\,{\sc ii} line broadening remains uncertain. We have, therefore, begun a series of experiments on the Z machine at Sandia National Laboratories to test these models and have successfully measured carbon plasma in emission and absorption at the temperatures and densities of hot DQ photospheres. We will present the early results of this work and discuss future directions.   

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Whether the context is astrophysical or laboratory plasmas, fitting the lines in observed spectra with theoretical models is often the most easily-applied technique for constraining plasma conditions. To obtain reliable results, the important physical effects must be properly taken into account. One important effect that is relatively unconstrained experimentally is the continuum lowering/occupation probability of individual states. In this talk we show how fits to astrophysical and laboratory data are affected by different formalisms for the occupation probability. Using data from the White Dwarf Photosphere Experiment (WDPE) fielded at Sandia's Z-machine, we examine which configurations of our hydrogen gas cell are most effective for measuring this critical aspect of theory.   

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We discuss an experimental effort to create and study laboratory photoionized plasmas relevant to the extreme conditions in x-ray binaries and active galactic nuclei. The physics models astronomers rely on to study such objects have had little laboratory testing due to the difficulty of accessing this plasma regime. Using the Z-Machine at Sandia National Labs, the experiment uses the intense broadband x-ray flux from a Z-pinch to drive and backlight a neon photoionized plasma contained in a cm-scale gas cell with atom number densities of 10$^{17}$ to 10$^{18}$ cm$^{-3}$. At the available gas cell positions, the x-ray flux reaches a peak of order 10$^{12}$ W/cm$^{2[1]}$. Combinations of these parameters span an order of magnitude in ionization parameter value allowing the study of trends in astrophysically relevant photoionized plasmas. This differs from previous experiments characteristic of single values of ionization parameter. With K-shell line absorption spectroscopy, the resulting plasma conditions (e.g. ion areal densities and charge state distribution) are determined, which can be compared with simulation results to test atomic kinetics models for photoionized plasmas. $^{[1]}$R.C. Mancini et al, Phys. Rev. E 101, 051201(R) (2020).   

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Photoionized silicon experiments were performed using the Z machine at Sandia National Laboratories. These data represent the first benchmark emission spectra suitable to test the theoretical assumptions in astrophysical models of accretion-powered photoionized plasmas. Additionally, a high spectral resolution ($\lambda $/$\delta \lambda $\textasciitilde 9200) spectrometer was conceived to record that emission. This instrument yielded unprecedented resolution for plasma emission with detections of spectral lines unobserved previously. The combination of a low-density plasma, the highly resolving quartz crystal, the minimum source size effect in the spherical geometry and the highly resolving x-ray film, all made these high-spectrally-resolved observations possible. These data allow for measurements of relative wavelengths for these lines which can be used to test model predictions for multiple silicon charge (He-like to B-like) and level states within charge states. We discuss how the results could be used to expand line databases with constrained uncertainties.   

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Iron opacities measured at stellar interior conditions were significantly higher than calculated [Bailey, Nature 517, 56 (2015)]. While this helps resolve a decade old solar problem, the question remains: What is responsible for the model-data discrepancy? This question is difficult to answer due to complex nature of the discrepancies; The calculated iron opacities have (1) narrower spectral lines, (2) deeper opacity windows, and (3) weaker quasi-continuum at short wavelength than the measurements. Recent systematic study of chromium, iron, and nickel opacities [Nagayama, PRL 122, 235001 (2019)] suggested possible model refinements for the two of the discrepancies but deepened the mystery on the quasi-continuum discrepancy; significant quasi-continuum discrepancy was observed only from iron and only at the higher temperatures and densities. For the last three years, we have been revisiting the iron results by performing more experiments and refining the data-analysis method. We will summarize the new iron opacity results and discuss its implication for the solar problem. Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DOE's NNSA under contract DE-NA0003525.   

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No convincing model revisions nor systematic experimental errors have yet resolved the model/data discrepancy in Fe opacity measurements at high temperatures \textgreater 180 eV and high electron densities \textgreater 3x10$^{\mathrm{22}}$ cm$^{\mathrm{-3}}$ [Bailey et al, Nature (2015), Nagayama et al. PRL (2019)]. This injects uncertainty into stellar interior models. Systematic errors from unresolved temporal gradients are one possible hypothesis, despite evidence that such errors are unimportant. Data recorded on x-ray film provide measurements over a time determined by the backlighter duration, but direct time-resolved measurements didn't exist until now. The novel hCMOS Ultra-fast X-ray Imager technology developed at Sandia National Laboratories and implemented in the opacity spectrometers allows such tests for the first time. Mg K-shell absorption were recorded to measure the opacity sample evolution across low and high temperature and density conditions. These measurements enable further evaluation of possible temporal gradient effects, test simulation predictions, and can optimize future opacity experiment designs.   

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In the early days of quantum electrodynamics, Maria Goeppert-Mayer predicted quantum two-photon processes. Two-photon emission was observed in accelerator beam-foil spectroscopy and both two-photon absorption and Raman scattering of visible light are well-known in nonlinear optics. Recent opacity measurements at the Sandia Z-machine found opacities larger than code predictions for Fe foils at temperatures \textasciitilde 200 eV. We investigate the question whether two-photon effects can increase the opacity of hot plasmas containing many-electron ions. Our calculations use second-order perturbation theory. Dipole transition matrix-elements involving continuum states are obtained by analytic continuation of bound-bound matrix elements, a method recently shown to be very accurate. We present results for various plasma conditions and some cases find large extra opacity. The theory can be tested by opacity measurements and also by experiments with X-ray free-electron lasers. If the results are substantiated, two-photon opacity is likely to be important for stellar interiors. Sandia National Laboratories is a multi-mission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. DOE's NNSA under contract DE-NA0003525.   

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Laboratory experiments typically test opacity models by volumetrically heating a sample with x-rays and measuring spectrally-resolved transmission by viewing a bright backlighter through the sample with a spectrometer. A potential problem is that any background signal contaminating the spectrum will cause the inferred opacity to be too low. Backgrounds can arise from many sources, including sample self-emission, emission from nearby unresolved plasmas, high-order crystal reflections, high energy x-rays, or instrument fluorescence. Methods developed to measure background signals in opacity experiments a the the Sandia Z facility are discussed.   

Live

Iron opacity at electron densities and temperatures similar to solar interior conditions was obtained using the Z machine at Sandia National Laboratories. It was found to be 30-400{\%} higher than what is used in standard solar models. In contrast, it is expected that opacity near solar conditions should be lower than the mass attenuation coefficients of x-ray radiation at room temperature (cold opacity). The caveat is that experimental values for opacity at room temperature are reported to within 10{\%} error at best. The present project attempts to reduce these errors. Cold opacity is determined here using transmission measurements of an iron foil at three different characteristic line energies in the soft x-ray 6-13 {\AA} range. The required areal density is independently measured using Rutherford Backscattering Spectroscopy with evaluated accuracy using an ion beamline at Sandia. Initial transmission measurements have shown that a few percent error on transmission could be achieved.   

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Testing oxygen opacity calculations is important for understanding the Sun and white dwarf stars. Near the solar convection zone base, T$_{\mathrm{e}}$\textasciitilde 180 eV, n$_{\mathrm{e}}$ \textasciitilde 9e22 electrons/cc, and oxygen is mostly H-like or fully-stripped. Highly-ionized oxygen produces a relatively simple opacity spectrum, but its calculation relies on untested approximations for continuum lowering and line broadening. We initiated stellar oxygen opacity measurements on Z with progress in two key areas: i) target fabrication and ii) extending the measurement spectral range. We created SiO$_{\mathrm{2}}$ half-moon samples with oxygen areal density greater than 1e19 atoms/cm\textasciicircum 2 and successfully recorded transmission through the heated sample on Z. The 6-18.5 Angstrom spectral range includes the critical oxygen bound-free absorption, the Ly beta transition, and the opacity window region on the short wavelength side of the Ly alpha line. The Si K-shell spectrum is less perturbed by density effects and provides plasma diagnostic information. Experimental results and prospects for refining stellar interior calculations will be discussed. Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DOE's NNSA under contract DE- NA0003525.   

On the relative importance of the different initial conditions that seed the electrothermal instability

The electrothermal instability (ETI) plays an important role in the thermal and hydrodynamic evolution of dense metallic systems driven with extreme electrical pulses. The instability grows from gradients in the electrical resistivity, and is responsible for hampering numerous applications of pulsed-power technology. For the first time, metal surfaces have been tracked with approximately 20 um accuracy throughout an experiment. This tracking reveals no clear correlation between target defects and non-uniform thermal emissions indicative of the ETI. Additionally, the relative influence of surface topography and purity of metal composition will be compared. Data indicate enhanced stability to ETI may be found by employing ultra-pure materials.