Modeling of K-shell Al and Mg radiation from compact single, double planar and cylindrical alloyed Al wire array plasmas produced on the 1 MA Zebra generator at UNR

Radiative emission from alloyed Al single, double and compact cylindrical wire arrays have been studied using the 1 MA Zebra UNR generator. Single planar wire arrays using ten wires and double planar wire arrays and compact cylindrical wire arrays (CCWA) that both had sixteen wires were utilized. The wire composition is Al-5056 (95% of Al and 5% of Mg). We have observed that implosion of these alloyed Al wire loads generated optically thick Al plasmas that can be diagnosed using K-shell Mg lines. In particular, among the considered loads, the K-shell lines of Al from implosions of the double planar wire arrays have the highest optical depth for He-like Al resonance transitions, which occurred near the stagnation phase. X-ray time-gated and time-integrated spectra and pinhole images as well as photoconductive detectors signals were analyzed to provide information on the plasma parameters; electron temperatures and densities, implosion dynamics features and power and yields of the X-ray radiation. Previously developed non-LTE models were applied to model axially-resolved time-integrated, as well as time-gated spatially-integrated, K-shell spectra from Al and Mg. The derived time-dependent electron temperature, density and axial opacity were studied and compared. In addition, the wire ablation dynamics model (WADM) was used to calculate the kinetic energy of the plasma, which with the aid of a Local Thermal Equilibrium (LTE) magneto-hydrodynamics (MHD) simulation, allowed to estimate the precursor and stagnated z-pinch plasma electron temperatures from implosions of wire array loads.     

Spectroscopic analysis of Cu wire array implosions on the refurbished Z generator

Experimental investigations of pinches on the refurbished Z (ZR) generator using Cu arrays have been initiated and more are planned for the near future. Significant X-ray emissions in the K-shell from moderately high atomic number plasmas such as Cu generate extreme interest. However, the production of these hard photons from high Z materials comes with a price. There is substantial loss of radiative yield due to stripping through many electrons present in high Z materials to reach to the H- or He-like ionization stages. Production of hard X-rays for materials with atomic number higher than Cu such as Kr is very difficult and theoretical predictions are even more uncertain. Previous experimental efforts using Cu as a plasma pinch load are encouraging and promote further investigations of this element on the refurbished Z machine for achieving photon energies higher than 5 keV and obtaining sufficient radiative yield. We will analyze the ionization dynamics and generate Cu spectrum using the temperature and density conditions obtained from 1-D non-LTE radiation hydrodynamics simulations of Cu wire array implosions on ZR. These results will be compared with K- and L-shell experimental spectrum of shot Z 1975. Theoretical K- and L-shell spectroscopy provides validation of atomic and plasma modeling when compared to available experimental data and also provides useful diagnostics for the plasma parameters. Our self-consistently generated non-LTE collisional-radiative model employs an extensive atomic level structure and data for all dominant atomic processes that are necessary to model accurately the pinch dynamics and the spectroscopic details of the emitted radiation.     

Searching for efficient X-ray radiators for wire array Z-pinch plasmas using mid-atomic-number single planar wire arrays on Zebra at UNR

We continue to search for more efficient X-ray radiators from wire array Z-pinch plasmas. The results of recent experiments with single planar wire array (SPWA) loads made from mid-atomic-number material wires such as Alumel, Cu, Mo, and Ag are presented and compared. In particular, two new efficient X-ray radiators, Alumel (95% Ni, 2% Al, and 2% Si) and Ag, are introduced, and their radiative properties are discussed in detail. The experiments were performed on the 1 MA Zebra generator at UNR. The X-ray yields from such mid-atomic-number SPWAs exceed twice those from low-atomic-number SPWAs, such as Al, and increase with the atomic number to reach more than 27–29 kJ for Ag. To consider the main contributions to the total radiation, we divided the time interval of the Z-pinch dynamic where wire ablation and implosion, stagnation, and plasma expansion occur in corresponding phases and studied the radiative and implosion characteristics within them. Theoretical tools such as non-LTE kinetics and wire ablation dynamic models were applied in the data analysis. These results and the models developed have much broader applications, not only for SPWAs on Zebra, but for other HED plasmas with mid-atomic-number ions.     

K-α emission spectroscopic analysis from a Cu Z-pinch

Advances in diagnostic techniques at the Sandia Z-facility have facilitated the production of very detailed spectral data. In particular, data from the copper nested wire-array shot Z1975 provides a wealth of information about the implosion dynamics and ionization history of the pinch. Besides the dominant valence K- and L-shell lines in Z1975 spectra, K-α lines from various ionization stages were also observed. K-shell vacancies can be created from inner-shell excitation and ionization by hot electrons and from photo-ionization by high-energy photons; these vacancies are subsequently filled by Auger decay or resonance fluorescence. The latter process produces the K-α emission. For plasmas in collisional equilibrium, K-α emission usually occurs from highly charged ions due to the high electron temperatures required for appreciable excitation of the K-α transitions. Our simulation of Z1975 was carried out with the NRL 1-D DZAPP non-LTE radiation-hydrodynamics model, and the resulting K- and L-shell synthetic spectra are compared with measured radiation data. Our investigation will focus on K-α generation by both impacting electrons and photons. Synthetic K-α spectra will be generated either by self-consistently calculating the K-shell vacancy production in a full Z-pinch simulation, or by post-processing data from a simulation. The analysis of these K-α lines as well as K- and L-shell emission from valence electrons should provide quantitative information about the dynamics of the pinch plasma.     

Signature of externally introduced laser fields in X-ray emission of multicharged ions

Theory predicts that the presence of strong single-frequency electric fields results in appearance of satellite or dip structures in X-ray spectral lines emitted from hot dense plasmas. Emission from multicharged ions is measured to determine the effects of laser field. A ps-laser beam was split into two parts: the first created an expanding plasma, while the second, which was temporally synchronized, irradiated the plasma at a varying distances in a direction perpendicular to the target normal. The field introduced by the second beam perturbed the plasma environment in the vicinity of radiators. The spatially resolved X-ray spectra were recorded using the high-resolution toroidally bent crystal spectrometer combined with a CCD detector. Spectrally resolved features are observed in broadened Al Heβ line profiles that are consistent with predicted spectra. The predicted spectra are derived from a combination of hydrodynamic plasma modeling post-processed by theoretical models that include the effect of externally introduced laser fields. The possible mixing of higher-intensity fields is qualitatively explained by a combination of fluid and one-dimensional PIC simulations that indicate redistribution of the fields and density fluctuations due to a presence of parametric instabilities.     

A computational study of x-ray emission from high-Z x-ray sources on the National Ignition Facility laser

We have begun to use 350–500 kJ of 1/3-micron laser light from the National Ignition Facility (NIF) laser to create millimeter-scale, bright multi-keV x-ray sources. In the first set of shots we achieved 15%–18% x-ray conversion efficiency into Xe M-shell (∼1.5–2.5 keV), Ar K-shell (∼3 keV) and Xe L-shell (∼4–5.5 keV) emission (Fournier et al., Phys. Plasmas 17, 082701, 2010), in good agreement with the emission modeled using a 2D radiation-hydrodynamics code incorporating a modern Detailed Configuration Accounting atomic model in non-LTE (Colvin et al., Phys. Plasmas, 17, 073111, 2010). In this paper we first briefly review details of the computational model and comparisons of the simulations with the Ar/Xe NIF data. We then discuss a computational study showing sensitivity of the x-ray emission to various beam illumination details (beam configuration, pointing, peak power, pulse shape, etc.) and target parameters (size, initial density, etc.), and finally make some predictions of how the x-ray conversion efficiency expected from NIF shots scales with atomic number of the emitting plasma.     

Plasma jets produced by low energy laser pulse interaction with planar and cratered targets

Laser experiments of the plasma jet formation using nanosecond laser pulses with low energy, i.e., <20 J, are presented. Planar and cratered gadolinium and aluminum targets are irradiated with laser intensities of several 10¹⁴ W/cm². Spatially-resolved time-integrated X-ray spectra were recorded in the spectral range from 7 to 10 Å. A jet-like structure is obtained from aluminum targets with a preformed crater, which is not seen in planar target irradiation. For gadolinium, a jet is observed from both planar and preformed cratered targets, suggesting that the collimation is dominated by radiative cooling. A radiation-hydrodynamics code coupled to a non-LTE ionization code was used to model the plasma. The calculated plasma emission was found to be consistent with the experimental results.     

Collisional–radiative studies of carbon plasmas

A detailed plasma kinetics model for carbon is presented which has been calculated using the Los Alamos suite of atomic kinetics codes. Models have been generated in the configuration-average approximation and the fine-structure approximation, which includes configuration-interaction and intermediate-coupling effects. These models have been used to generate relevant plasma quantities, ranging from the average ionization stage to detailed emission spectra. For many of the quantities examined, and for certain plasma conditions, significant differences exist between the configuration-average model and the detailed fine-structure model, indicating that such a detailed fine-structure model is necessary to accurately model atomic properties in such plasmas.     

Time- and space-resolved spectroscopy of dynamic hohlraum interiors

A dynamic hohlraum is created when an annular z-pinch plasma implodes onto a cylindrical 0.014 g/cc 6-mm-diameter CH₂ foam. The impact launches a radiating shock that propagates toward the axis at 350 μm/ns. The radiation trapped by the tungsten z-pinch plasma forms a 200 eV hohlraum that provides X-rays for indirect drive inertial confinement fusion capsule implosion experiments. We are developing the ability to diagnose the hohlraum interior using emission and absorption spectroscopy of Si atoms added as a tracer to the central portion of the foam. Time- and space-resolved Si spectra are recorded with an elliptical crystal spectrometer viewing the cylindrical hohlraum end-on. A rectangular aperture at the end of the hohlraum restricts the field of view so that the 1D spectrometer resolution corresponds approximately to the hohlraum radial direction. This enables distinguishing between spectra from the unshocked radiation-heated foam and from the shocked foam. Typical spectral lines observed include the Si Lyα with its He-like satellites and the He-like resonance sequence including Heα, Heβ, and Heγ, along with some of their associated Li-like satellites. Work is in progress to infer the hohlraum conditions using collisional–radiative modeling that accounts for the radiation environment and includes both opacity effects and detailed Stark broadening calculations. These 6-mm-scale radiation-heated plasmas might eventually also prove suitable for testing Stark broadening line profile calculations or for opacity measurements.     

Unlocking the secrets of supernovae through their light curves,spectra & polarization

Utilizing the radiative transfer code cmfgen, we have undertaken time-dependent radiative transfer calculations that compute the light curve and spectra for Type Ia, Ib, Ic, and II supernovae (SNe) through the photospheric and nebular phases. The non-LTE calculations allow for a multitude of atomic processes (bound–bound, bound–free, free–free, collisional, charge exchange, and Penning ionization) and for non-thermal excitation and ionization from non-thermal electrons created by the degradation in energy of high-energy (～1 MeV) gamma-rays. The proper inclusion of all these processes requires a vast amount of atomic data. Not all the atomic data is available, and the quality of the available atomic data varies considerably. We have confirmed the results of Utrobin and Chugai (2005) that time dependent terms must be included in the statistical equilibrium equations in order to model the Hα evolution of SN 1987A, shown that time dependent terms influence other spectral features, and shown that these conclusions also apply to the modeling of Type II SNe in general. The inclusion of non-thermal processes has allowed us to model Hα and He i emission in Type II SNe into the nebular phase, and to model the He i emission in Type Ib and Ic SNe. Our calculations show that the He deficiency in Ic SNe is unlikely to be real – instead, the absence of He i on SNe Ic spectra is more likely related to inefficient excitation of He iions. Simply by varying the amount of mixing we are able to create spectra of Type Ib and Ic SNe using the SAME progenitor model. Based on a new grid of SNe Ib/c models, we confirm previous findings that the typically fast-rising narrow-peak fast-declining SNe Ib/c light curves imply ejecta masses $\mathrm{?}5{\text{M}}_{\odot }$, favoring intermediate-mass massive stars in interacting binaries. We are successfully applying cmfgen to model Type Ia SNe, and are currently exploring the role of mixing and non-thermal processes in these SNe. We highlight the differences between the various classes of SNe.     

Investigation of characteristics of hard x-rays produced during implosions of wire array loads on 1.6 MA Zebra generator

Experimental study of the characteristics of hard x-ray (HXR) emission from multi-planar wire arrays and compact-cylindrical wire arrays (CCWA) plasmas on the 1.6 MA Zebra generator at UNR has been carried out. The characteristics of HXR produced by multi-planar wire arrays such as single, double, and triple planar as well as compact-cylindrical wire arrays made from Al, Cu, brass, Mo, and W were analyzed. Data from spatially resolved time-integrated and spatially integrated time-gated x-ray spectra recorded by LiF spectrometers, x-ray pinhole images, and signals from fast x-ray detectors have been used to study spatial distribution and time history of HXR emission with different loads. The dependence of the HXR yield and power on the wire material, geometry of the load and load mass is observed. Both HXR yield and power are minimum for Al and maximum for W loads. The HXR yield increases with the rise of the atomic number of the material for all loads. The presence of aluminum wires in the load with the main material such as Cu, Mo, or W in combined wire arrays decreases HXR yield. For W plasma, the intensity of cold L-shell spectral lines (1–1.5 Å) correlates with corresponding amplitude of HXR signals which may suggest the evidence of generation of electron beams in plasma. It is found that HXRs are generated from different plasma regions by the interaction of electron-beam with the plasma trailing mass, with the material of anode and due to thermal radiation from plasma bright spots. The theoretical assumption of thermal mechanism of HXR emission predicts the different trends for dependency of HXR power on atomic number and load mass.     

X-ray absorption spectroscopy for wire-array Z-pinches at the non-radiative stage

Absorption spectroscopy was applied to wire-array Z-pinches on the 1 MA pulsed-power Zebra generator at the Nevada Terawatt Facility (NTF). The 50 TW Leopard laser was coupled with the Zebra generator for X-ray backlighting of wire arrays at the ablation stage. Broadband X-ray emission from a laser-produced Sm plasma was used to backlight Al star wire arrays in the range of 7–9 Å. Two time-integrated X-ray conical spectrometers recorded reference and absorption spectra. The spectrometers were shielded from the bright Z-pinch X-ray burst by collimators. The comparison of plasma-transmitted spectra with reference spectra indicates absorption lines in the range of 8.1–8.4 Å. Analysis of Al K-shell absorption spectra with detailed atomic kinetics models shows a distribution of electron temperature in the range of 10–30 eV that was fitted with an effective two-temperature model. Temperature and density distributions in wire-array plasma were simulated with a three-dimension magneto-hydrodynamic code. Post-processing of this code’s output yields synthetic transmission spectrum which is in general agreement with the data.     

Versatile code DLAYZ for investigating population kinetics and radiative properties of plasmas in non-local thermodynamic equilibrium

A versatile code DLAYZ based on collisional-radiative model is developed for investigating the population kinetics and radiative properties of plasmas in non-local thermodynamic equilibrium. DLAYZ is implemented on the detailed level accounting (DLA) approach and can be extended to detailed configuration accounting (DCA) and hybrid DLA/DCA approaches. The code can treat both steady state and time-dependent problems. The implementation of the main modules of DLAYZ is discussed in detail including atomic data, rates, population distributions and radiative properties modules. The complete set of basic atomic data is obtained using relativistic quantum mechanics. For dense plasmas, the basic atomic data with plasma screening effects can be obtained. The populations are obtained by solving the coupled rate equations, which are used to calculate the radiative properties. A parallelized version is implemented in the code to treat the large-scale rate equations. Two illustrative examples of a steady state case for carbon plasmas and a time-dependent case for the relaxation of a K-shell excited argon are employed to show the main features of the present code.     

An experimental platform for creating white dwarf photospheres in the laboratory

We present an experimental platform for measuring hydrogen Balmer emission and absorption line profiles for plasmas with white dwarf (WD) photospheric conditions (T_e ～1 eV, n_e ～10¹⁷ cm^?3). These profiles will be used to benchmark WD atmosphere models, which, used with the spectroscopic method, are responsible for determining fundamental parameters (e.g., effective temperature, mass) for tens of thousands of WDs. Our experiment, performed at the Z Pulsed Power Facility at Sandia National Laboratories, uses the large amount of X-rays generated from a z-pinch dynamic hohlraum to drive plasma formation in a gas cell. The platform is unique compared to past hydrogen line profile experiments in that the plasma is radiation-driven. This decouples the heating source from the plasma to be studied in the sense that the radiation temperature causing the photoionization is independent of the initial conditions of the gas. For the first time we measure hydrogen Balmer lines in absorption at these conditions in the laboratory for the purpose of benchmarking Stark-broadened line shapes. The platform can be used to study other plasma species and to explore non-LTE, time-dependent collisional-radiative atomic kinetics.     

Role of dielectronic recombination and autoionizing states in the dynamic equilibrium of non-LTE plasmas

The dielectronic-recombination phenomenon is an essential component of the dynamical-equilibrium regime of non-LTE plasmas. This phenomenon is a sequence of two atomic processes, namely, resonant capture followed by spontaneous emission or collisional de-excitation. It relies on the existence of singly- and doubly-excited autoionizing states, which are blends of quasi-bound and continuum states. In most cases, a very large number of such states ought to be accounted for in the calculations of collisional-radiative models; however, for computational reasons it is necessary to consider level ensembles, namely electronic configurations, or superconfigurations. Formulas for the rates of resonant capture and autoionization are given for transitions between levels, between configurations, and between superconfigurations. Results of collisional-radiative calculations and comparisons between the effects of the different atomic processes are presented.