Opacity and radiative power losses calculations

In this contribution we present results on opacity and radiative power losses in laser-produced plasmas. We focus our attention on the inner shell transition array 1s–2p in an aluminum plasma. At high densities, electron, Doppler and ion Stark broadening play a role in line merging. This is why the PPP line-shape code developed at Université de Provence was adapted to calculate opacity and radiative power losses in Al and Ge ions. Atomic physics data required in PPP calculations is provided by an MCDF code. Comparison with experiments is discussed.     

Absorption spectroscopy of mid and neighboring Z plasmas: Iron, nickel,copper and germanium

Opacities of four medium Z element plasmas (iron, nickel, copper and germanium) have been measured at the LULI-2000 facility in similar conditions: temperatures between 15 and 25 eV and densities between 2 and 10 mg/cm³, in a wavelength range (8–18 Å) including the strong 2p–3d structures.Two laser beams from the LULI facility were used in the nanosecond-picosecond configuration. The NANO-2000 beam (at λ = 0.53 μm) heated a gold hohlraum with an energy between 30 and 150 J with a duration of 0.6 ns. Samples covering half a hohlraum hole were thus radiatively heated. The picosecond pulse PICO-2000 beam (at λ = 1.053 μm) has been used to produce a short (about 10 ps) X-ray backlighter in order to reduce time variations of temperatures and densities during the measurement. A crystal high-resolution spectrometer was used as the main diagnostic to record at the same time the non-absorbed and the absorbed backlighter spectra. Radiation temperatures were measured using a broadband spectrometer. 1D and 2D simulations have been performed in order to estimate hydrodynamic plasmas parameters.The measured spectra have been compared with theoretical ones obtained using either the superconfiguration code SCO or the detailed term accounting code HULLAC. These comparisons allow us to check the modeling of the statistical broadening and of the spin-orbit splitting of the 2p–3d transitions and related effects such as the interaction between relativistic subconfigurations belonging to the same non-relativistic configuration.     

K-shell spectroscopy of plasmas created by intense laser irradiation of micron-scale pyramid and sphere targets

K-shell spectra of targets with microstructured features irradiated by an intense femtosecond laser have been studied. Examination of K_α emission from laser irradiated Si targets coated with micron-scale polystyrene spheres indicates that the emission is enhanced by a factor of 3 over emission from planar solids. Sphere-coated targets also emit K-shell He-like Si radiation indicating the presence of a hot dense plasma beneath the microspheres. Furthermore, K_α from Ti foils coupled to micro-tipped reentrant pyramid and wedge shaped targets has been studied, however, no significant enhancement of the K_α yield is observed for these kinds of targets. These studies illustrate that, with correct tailoring of the target surface, field enhancements can be used to increase X-ray emission from intensely irradiated targets.     

Zeeman–Stark line shape codes including ion dynamics

Ion dynamics is revisited in the presence of a magnetic field. This problem is of interest for the characterization of the ITER divertor plasmas, where Zeeman–Stark line shapes are both a diagnostic tool and an ingredient in the transport codes used for radiative transfer simulations. Comparisons between ab initio simulations of Zeeman–Stark profiles and calculations performed using the computationally efficient frequency fluctuation model are presented for the Lyman α and Balmer α lines.     

Aluminium Lyman-beta satellite emission in non-Maxwellian dense laser produced plasmas

High-energy decay channels of the Al Lyman-β satellite have been observed in X-ray emission from highly ionized plasma jets created by intense laser irradiation of aluminium foil targets. Atomic structure calculations show that the Lyman β satellite emission consists from six emission groups close to the He-like Al 1s²-1s4p (He_γ) and 1s²-1s5p (He_δ) resonance lines. This provides new possibilities for space resolved analysis of high density plasmas. Non-Maxwellian simulations of the plasma emission carried out with the MARIA code demonstrate that the intensity ratios of the Lyman-β satellites and the He_γ and He_δ resonance lines are very sensitive to the bulk electron temperature. In contrast to standard diagnostic methods, parameter studies show that this bulk electron diagnostics is practically unaffected by suprathermal electrons having less than 10% of the bulk electron density.     

X-ray line emissions from tamped thin aluminum targets driven by subpicosecond-duration laser pulses

We report on atomic kinetics and X-ray line spectra modeling work of plasmas generated by high-intensity, ultrashort-duration pulsed lasers. Our work is motivated by the need to analyze and interpret experiments with laser-irradiated layered targets performed at the Max-Planck-Institut für Quantenoptik. The focus of this Paper is on the theoretical characterization of the properties of X-ray line emissions as signatures of plasma conditions. Our model considers several spectral features with detailed attention paid to atomic kinetics, intrinsic spectral lineshapes in a high-density plasma environment (in particular Stark broadening and line shift effects), and spectroscopic-quality radiation transport (opacity effects). We apply our model to the analysis of time-integrated K-shell aluminum X-ray line spectra and time-resolved total line intensities obtained from the layered targets. Modeling calculations indicate that red line shifts observed in these experiments cannot be explained by shifts in the centers of gravity of composite spectral features due to blending with enhanced satellite contributions, but are consistent with intrinsic line shift effects in both resonance and satellite lines. We also investigate the sensitivity of our results to the selection of one of three adopted models for laser-energy deposition and transport within the target.     

Modeling of the oscillating field effects on the Heβ line emission in short pulse laser-produced plasmas

Strong oscillating fields may induce strong modifications of the emission spectra of ions. We discuss here the possibility of observing such effects in actual laser experiments where space- and time-integration effects can easily mask their existence. Focusing on the Al He_β transition, we first discuss the calculation of its spectral broadening in the presence of a strong laser field. Then, starting from 1D hydro-simulations of short, intense, laser pulse-produced plasmas that provide the density, temperature and laser intensity profiles as a function of time, full integrated collisional-radiative calculations of the laser field-dependent emissivity of the Al He_β line, are presented.     

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.     

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.     

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.     

Complex hydrodynamics in heated and shocked conditions

In inertial confinement fusion double-shell designs, the inner shell experiences heating that can amplify non-uniformities and consequently enhance mixing, which degrades capsule performance. Recent OMEGA experiments study the time-dependent evolution of mix under heated and shocked conditions. In each experiment, a cylindrical Be tube was filled with a layered system of a BeCu disk and low-density CH foam. The BeCu disks were machined with a multi-mode perturbation representative of the target surface roughness present in ICF capsules. The targets were heated from one end using a hohlraum and subsequently shocked using direct-drive from the opposite end. X-ray radiography was used to quantitatively diagnose the transmission profiles of the disk/foam interface. We focus primarily on an assessment of the applicability of the radiation transport models available in the RAGE (Radiation Adaptive Grid Eulerian) hydrodynamics code. These include grey diffusion, several types of multi-group diffusion, and a new frequency-dependent source capability that addresses the NLTE nature of the laser energy deposition.     

The effect of improved satellite line shapes on the argon Heβ spectral feature

The effect of the interference term in the electron broadening of the Li-like satellites to the Ar Heβ line has been investigated in the standard Stark broadening theory. Although the interference term has negligible or small effect on satellite lines with spectator electrons in n = 2 and n = 3 shells, it shows significant narrowing for lines with n = 4 spectator electrons. Nevertheless, the improved n = 4 satellite line shape makes a small difference in the emergent intensity distribution of the composite spectral feature. Accordingly, this change in line shape does not affect previous diagnosis of plasma conditions in Ar-doped ICF implosion cores.     

Temporal and spectral behavior of sub-picosecond laser-created X-ray sources from low- to moderate-Z elements

We report on a set of experiments in which solid targets of different atomic numbers (Z) were irradiated with laser pulses of time durations ranging from 300 fs to 33 ps, and energies up to 26 J. The time-resolved X-ray emission in the 7.6–8.1 Å spectral range was measured using an ultra-fast X-ray streak camera coupled with a conical Bragg crystal. In this way we were able to follow the dramatic modification of the spectral features as a function of the laser duration. The features evolve from a “ns-type” emission, characterized by narrow and well-defined spectral lines, to very broad spectral features, due not only to the Stark broadening but also to the proliferation of satellites lines. The measured spectra also show strong time dependence, which allows us to follow the time evolution of the hydrodynamic parameters. We then compare the derived parameter with the CHIVAS hydro-radiative simulations. The experimental results are also compared with the AVERROES/TRANSPEC collisional-radiative code, and with precise spectral line shape calculations (PPP and PrismSPECT). The results seem to indicate regimes of interaction where hot electrons play an important role on spectral line formation.     

Robust algorithm for computing quasi-static stark broadening of spectral lines

A method previously developed to solve large-scale linear systems is applied to quasi-static Stark line broadening calculations. The method is formally exact, numerically stable, and allows optimization of the integration over the quasi-static field to assure numerical accuracy. Furthermore, the method is computationally efficient compared to the conventional approach, particularly for complex problems involving large matrices.     

X-ray spectral measurements and collisional-radiative modeling of hot,gold plasmas at the omega laser

M-Band and L-Band Gold spectra between 3 and 5 keV and 8 and 13 keV, respectively, have been recorded by a photometrically calibrated crystal spectrometer. The spectra were emitted from the plasma in the laser deposition region of a ‘hot hohlraum’. This is a reduced-scale hohlraum heated with ≈9 kJ of 351 nm light in a 1 ns square pulse at the OMEGA laser. The space- and time-integrated spectra included L-Band line emission from Co-like to Ne-like gold. The three L-Band line features were identified to be the 3s → 2p, 3d_5/2 → 2p_3/2 and 3d_3/2 → 2p_1/2 transitions at ≈9 keV, ≈10 keV and ≈13 keV, respectively. M-Band 5f → 3d, 4d → 3p, and 4p → 3s transition features from Fe-like to P-like gold were also recorded between 3 and 5 keV. Modeling from the radiation–hydrodynamics code LASNEX, the collisional-radiative codes FLYCHK and SCRAM, and the atomic structure code FAC were used to model the plasma and generate simulated spectra for comparison with the recorded spectra. Through these comparisons, we have determined the average electron temperature of the emitting plasma to be between 6.0 and 6.5 keV. The electron temperatures predicted by LASNEX appear to be too large by a factor of about 1.5.     

Three-dimensional elastic behavior of a nonhomogeneous layer

Summary  This paper deals with the theoretical treatment of a three-dimensional elastic problem governed by a cylindrical coordinate system (r,θ,z) for a medium with nonhomogeneous material property. This property is defined by the relation G(z)=G ₀(1+z/a) ^m where G ₀,a and m are constants, i.e., shear modulus of elasticity G varies arbitrarily with the axial coordinate z by the power product form. We propose a fundamental equation system for such nonhomogeneous medium by using three kinds of displacement functions and, as an illustrative example, we apply them to an nonhomogeneous thick plate (layer) subjected to an arbitrarily distributed load (not necessarily axisymmetric) on its surfaces. Numerical calculations are carried out for several cases, taking into account the variation of the nonhomogeneous parameter m. The numerical results for displacement and stress components are shown graphically. Received 10 May 1999; accepted for publication 15 August 1999     

Measurements of niobium absorption spectra in plasmas with nearly full M-shell configurations

A systematic study has been carried out on the changes in the L-shell absorption structure of niobium as a result of changing the population of the n = 3 shell from full to having vacancies in the 3d level. The niobium spectra were measured in the 2–3 keV frequency range, which spanned the 2p-nd transitions where 3 ≤ n ≤ 11. In addition to the detailed structure in these arrays the data also show 2s-4p and 2p-4s transitions and the bound-free L edge. The frequencies and widths of transition arrays, transmission between arrays, and the absorption due to the bound-free edge, can be seen in the data. The sample conditions were found from a combination of two-dimensional radiation-hydrodynamics calculations using the AWE NYM code and flux measurements using X-ray diodes, measurements of 1s-2p absorption spectra in aluminium and mixed aluminium/niobium samples. The electron temperature error, inferred from the modelling, is ±2 eV, with a density error of 30%. The data were recorded over the temperature range from 28 to 45 eV and show marked changes in the spectra over this range.The data were compared to spectra predicted by the AWE CASSANDRA [B.J.B. Crowley, J.W.O. Harris, J. Quant. Spectrosc. Radiat. Transfer 71 (2000) p. 257] opacity code. The calculated spectra were able to reproduce the measurements reasonably well. However, there are some differences in line positions that cannot be accounted for by gradients and there are differences in the array structure in the prediction and the measurements, with additional structure predicted but not seen in the measurements. There is also lower transmission on the blue side of the 2p-3d transition arrays compared to prediction.