Benchmark Experiment for Photoionized Plasma Emission from Accretion-Powered X-Ray Sources

A generalized approach to x-ray data modeling for high-energy-density plasma experiments

Accurate understanding of x-ray diagnostics is crucial for both interpreting high-energy-density experiments and testing simulations through quantitative comparisons. X-ray diagnostic models are complex. Past treatments of individual x-ray diagnostics on a case-by-case basis have hindered universal diagnostic understanding. Here, we derive a general formula for modeling the absolute response of non-focusing x-ray diagnostics, such as x-ray imagers, one-dimensional space-resolved spectrometers, and x-ray power diagnostics. The present model is useful for both data modeling and data processing. It naturally accounts for the x-ray crystal broadening. The new model verifies that standard approaches for a crystal response can be good approximations, but they can underestimate the total reflectivity and overestimate spectral resolving power by more than a factor of 2 in some cases near reflectivity edge features. We also find that a frequently used, simplified-crystal-response approximation for processing spectral data can introduce an absolute error of more than an order of magnitude and the relative spectral radiance error of a factor of 3. The present model is derived with straightforward geometric arguments. It is more general and is recommended for developing a unified picture and providing consistent treatment over multiple x-ray diagnostics. Such consistency is crucial for reliable multi-objective data analyses.

Development and integration of photonic Doppler velocimetry as a diagnostic for radiation driven experiments on the Z-machine

Plasma density measurements are key to a wide variety of high-energy-density (HED) and laboratory astrophysics experiments. We present a creative application of photonic Doppler velocimetry (PDV) from which time- and spatially resolved electron density measurements can be made. PDV has been implemented for the first time in close proximity, ∼6 cm, to the high-intensity radiation flux produced by a z-pinch dynamic hohlraum on the Z-machine. Multiple PDV probes were incorporated into the photoionized gas cell platform. Two probes, spaced 4 mm apart, were used to assess plasma density and uniformity in the central region of the gas cell during the formation of the plasma. Electron density time histories with subnanosecond resolution were extracted from PDV measurements taken from the gas cells fielded with neon at 15 Torr. As well, a null shot with no gas fill in the cell was fielded. A major achievement was the low noise high-quality measurements made in the harsh environment produced by the mega-joules of x-ray energy emitted at the collapse of the z-pinch implosion. To evaluate time dependent radiation induced effects in the fiber optic system, two PDV noise probes were included on either side of the gas cell. The success of this alternative use of PDV demonstrates that it is a reliable, precise, and affordable new electron density diagnostic for radiation driven experiments and more generally HED experiments.

Observation of ionization trends in a laboratory photoionized plasma experiment at Z

We report experimental and modeling results for the charge state distribution of laboratory photoionized neon plasmas in the first systematic study over nearly an order of magnitude range of ionization parameter ξ∝F/N_{e}. The range of ξ is achieved by flexibility in the experimental platform to adjust either the x-ray drive flux F at the sample or the electron number density N_{e} or both. Experimental measurements of photoionized plasma conditions over such a range of parameters enable a stringent test of atomic kinetics models used within codes that are applied to photoionized plasmas in the laboratory and astrophysics. From experimental transmission data, ion areal densities are extracted by spectroscopic analysis that is independent of atomic kinetics modeling. The measurements reveal the net result of the competition between photon-driven ionization and electron-driven recombination atomic processes as a function of ξ as it affects the charge state distribution. Results from radiation-hydrodynamics modeling calculations with detailed inline atomic kinetics modeling are compared with the experimental results. There is good agreement in the mean charge and overall qualitative similarities in the trends observed with ξ but significant quantitative differences in the fractional populations of individual ions.

X-ray heating and electron temperature of laboratory photoionized plasmas

We discuss the experimental and modeling results for the x-ray heating and temperature of laboratory photoionized plasmas. A method is used to extract the electron temperature based on the analysis of transmission spectroscopy data that is independent of atomic kinetics modeling. The results emphasized the critical role of x-ray heating and radiation cooling in determining the energy balance of the plasma. They also demonstrated the dramatic impact of photoexcitation on excited-state populations, line emissivity, and radiation cooling. Modeling calculations performed with astrophysical codes significantly overestimated the measured temperature.

GLIDER-A pulsed-current generator for laboratory astrophysics x-ray absorption experiments

In the field of pulse-power, there has always been an interest on small and medium size pulsed-current generators (≤2 MA) which are affordable and of low maintenance. We developed the GLIDER, a compact and modular generator, that drives a gas-puff z-pinch load as a soft x-ray source (0.1-1 keV) for laboratory astrophysics absorption experiments. It comprises 48 bricks, tightly packed in a 1.7 m × 3.5 m × 0.8 m transformer oil container. Its compactness and reliability was enabled owing to unique multilayered oil-soaked insulators, and more than 100 post-hole convolutes. Its stripline includes interchangeable tiles for ease of construction and maintenance. Six triggering units enable current pulse shaping. The GLIDER was tested up to ±60 kV (34 kJ) and produced 2 MA in 450 ns rise time on a 5 nH load. We present grating spectra of K-shell absorption of neutral O and N proving the experimental concept and demonstrating column density and ionization measurements.

A compact multi-plane broadband (0.5-17 keV) spectrometer using a single acid phthalate crystal

Acid phthalate crystals such as KAP crystals are a method of choice to record x-ray spectra in the soft x-ray regime (E ∼ 1 keV) using the large (001) 2d = 26.63 Å spacing. Reflection from many other planes is possible, and knowledge of the 2d spacing, reflectivity, and resolution for these reflections is necessary to evaluate whether they hinder or help the measurements. Burkhalter et al. [J. Appl. Phys., 52, 4379 (1981)] showed that the (013) reflection has efficiency comparable to the 2nd order reflection (002), and it can overlap the main first order reflection when the crystal bending axis ( b -axis) is contained in the dispersion plane, thus contaminating the main (001) measurement in a convex crystal geometry. We present a novel spectrograph concept that makes these asymmetric reflections helpful by setting the crystal b -axis perpendicular to the dispersion plane. In such a case, asymmetric reflections do not overlap with the main (001) reflection and each reflection can be used as an independent spectrograph. Here we demonstrate an achieved spectral range of 0.8-13 keV with a prototype setup. The detector measurements were reproduced with a 3D ray-tracing code.

A time-gated multi-channel x-ray crystal spectrometer on the Shenguang-III laser facility

An eight-channel x-ray flat crystal spectrometer was developed for high energy density physics research at the Shenguang-III (SG-III) laser facility. The spectrometer uses trihydroxymethylaminomethane crystals (2d = 8.78 Å) to record Ti K-shell emission in the photon energy range of 4.65-5.05 keV. The spectrometer couples to an x-ray framing camera to achieve time-resolution. This has four microstrips, and each strip records two snapshots of the emission image. Based on the intersection positioning system with a dual-charge coupled device, the alignment system is easily operated and efficient. The instrument was tested and used for Au hohlraum plasma diagnosis experiments on SG-III. The He-α line and its Li-like satellites and the Ly-α line of a Ti tracer were detected, from which the spectral resolution of the instrument was analyzed. The spectral resolution E/ΔE at the Ti He-α line ranges from about 500 to 880 and mainly limited by the x-ray source size.

Production of photoionized plasmas in the laboratory with x-ray line radiation

In this paper we report the experimental implementation of a theoretically proposed technique for creating a photoionized plasma in the laboratory using x-ray line radiation. Using a Sn laser plasma to irradiate an Ar gas target, the photoionization parameter, ξ=4πF/N_{e}, reached values of order 50ergcms^{-1}, where F is the radiation flux in ergcm^{-2}s^{-1}. The significance of this is that this technique allows us to mimic effective spectral radiation temperatures in excess of 1 keV. We show that our plasma starts to be collisionally dominated before the peak of the x-ray drive. However, the technique is extendable to higher-energy laser systems to create plasmas with parameters relevant to benchmarking codes used to model astrophysical objects.