T. Blenski

Opacity Studies of Iron in the 15-30eV Temperature Range

Absorption of the 2p-3d transitions of iron has been measured using point projection spectroscopy. Thin C tamped Fe foils were heated around 20 eV by X-rays generated in gold spherical hohlraums irradiated by the high-power laser ASTERIX IV. Absorption of Fe V to Fe X has been observed in the spectral vicinity of 730 eV (17 A). The Ag backlighter source and absorbed spectra were recorded on the same shot by a TlAP crystal spectrograph. The experimental spectra have been reproduced by the two superconfiguration local thermodynamic equilibrium codes SCO and STA. Detailed statistical calculations of the different ionic structures have also been performed with the Spin Orbit Split Arrays method, allowing the determination of ion populations. The electron temperature and average ionization obtained by fitting the experiment with the different calculations were compared with radiative hydrodynamic simulations.

A consistent approach for mixed detailed and statistical calculation of opacities in hot plasmas

Abstract Absorption and emission spectra of plasmas with multicharged-ions contain transition arrays with a huge number of coalescent electric-dipole (E1) lines, which are well suited for treatment by the unresolved transition array and derivative methods. But, some transition arrays show detailed features whose description requires diagonalization of the Hamiltonian matrix. We developed a hybrid opacity code, called SCORCG, which combines statistical approaches with fine-structure calculations consistently. Data required for the computation of detailed transition arrays (atomic configurations and atomic radial integrals) are calculated by the superconfiguration code SCO (Super-Configuration Opacity), which provides an accurate description of the plasma screening effects on the wave-functions. Level energies as well as position and strength of spectral lines are computed by an adapted RCG routine of R. D. Cowan. The resulting code provides opacities for hot plasmas and can handle mid-Z elements. The code is also a powerful tool for the interpretation of recent laser and Z-pinch experimental spectra, as well as for validation of statistical methods.

Opacity measurements of tamped NaBr samples heated by z-pinch X-rays

Abstract Laboratory measurements provide benchmark data for wavelength-dependent plasma opacities to assist inertial confinement fusion, astrophysics, and atomic physics research. There are several potential benefits to using z-pinch radiation for opacity measurements, including relatively large cm-scale lateral sample sizes and relatively-long 3– 5 ns experiment durations. These features enhance sample uniformity. The spectrally resolved transmission through a CH-tamped NaBr foil was measured. The z-pinch produced the X-rays for both the heating source and backlight source. The (50±4) eV foil electron temperature and (3±1)×10 21 cm −3 foil electron density were determined by analysis of the Na absorption features. LTE and NLTE opacity model calculations of the n=2 to 3, 4 transitions in bromine ionized into the M-shell are in reasonably good agreement with the data.

Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO2 and Ar clusters with high-intensity femtosecond laser pulses with qlas≃1018 W/cm2 are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is observed to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relative number of excited ions (≃10−2–10−3) with energies of 0.1–1 MeV in such a plasma. Possible mechanisms underlying the acceleration of ions to high energies are discussed. It is shown that the experimental results can be attributed to the influence of ponderomotive forces in standing waves generated by the reflection of laser radiation from the clusters.

Radiative properties of stellar plasmas and open challenges

The lifetime of solar-like stars, the envelope structure of more massive stars, and stellar acoustic frequencies largely depend on the radiative properties of the stellar plasma. Up to now, these complex quantities have been estimated only theoretically. The development of the powerful tools of helio- and astero- seismology has made it possible to gain insights on the interiors of stars. Consequently, increased emphasis is now placed on knowledge of the monochromatic opacity coefficients. Here we review how these radiative properties play a role, and where they are most important. We then concentrate specifically on the envelopes of β Cephei variable stars. We discuss the dispersion of eight different theoretical estimates of the monochromatic opacity spectrum and the challenges we need to face to check these calculations experimentally.

Radiative heating of B, Al and Ni thin foils at 15–25 eV temperatures

Several thin foils have been heated by X-ray radiation emitted by a small gold cavity irradiated with the LULI facility Nd glass laser. The foils were heated to moderate temperatures (10–25 eV) in conditions near local thermodynamic equilibrium. Measurements of the absorption of Kα transitions of aluminum, of the K-shell transitions of boron, and of the n=2–3 and 4 transitions of nickel are reported. A 5000 l/mm transmission grating spectrograph recorded the boron data, and a flat thallium hydrogen phthalate (TlAP) crystal was used for aluminum and nickel absorption measurements. The nickel and aluminum spectra indicate a higher temperature (22–25 eV) than the boron case (14 eV), explained by a difference in the absorption spectra of these elements. In the Ni case the measured 2p-3d spin–orbit–split absorption structures compared well with theoretical spectra obtained with the SCO superconfiguration code using the spin orbit split transition arrays formalism (SOSA). The heating of the foils as calculated with radiative hydrodynamic simulations is very similar to the experimental data.

Self-consistent approach for the thermodynamics of ions in dense plasmas in the superconfiguration approximation

Abstract We propose a new thermodynamic approach to ions in dense plasmas taking into account the screening by free electrons. The ions in fundamental and excited states are represented by a group of superconfigurations. Each superconfiguration containing an integer number of bound electrons is totally screened by free electrons in a Wigner–Seitz (WS) sphere. The minimisation of the plasma free energy with respect to the WS radii of the ions, under the condition that the specific mass of the plasma is preserved, leads to the equality of the electronic pressure for all ions. This pressure, as well as the WS radii and the distribution of ionic probabilities, are calculated by iteration. In this way, our approach not only gives the charge neutrality of the plasma, but also assures that the plasma environment is the same for each ion. In principle, the proposed approach allows one to apply the superconfiguration method to calculate radiative properties of plasmas for higher densities than those encountered in transmission experiments. Numerical examples for nickel, aluminium and samarium plasmas will be given. Comparisons with results from the previous statistical approach using the same WS radius for each ion charge state will also be discussed.

Quantum mechanical model for the study of pressure ionization in the superconfiguration approach

The knowledge of plasma equation of state and photoabsorption requires suitable and realistic models for the description of ions. The number of relevant electronic configurations of ions in hot dense plasmas can be immense (increasing with atomic number Z). In such cases, calculations relying on the superconfiguration approximation appear to be among the best statistical approaches to photoabsorption in plasmas. The superconfiguration approximation enables one to perform rapid calculation of averages over all possible configurations representing excited states of bound electrons. We present a thermodynamically consistent model involving detailed screened ions (described by superconfigurations) in plasmas. The density effects are introduced via the ion-sphere model. In the usual approaches, bound electrons are treated quantum mechanically while free electrons are described within the framework of semi-classical Thomas–Fermi theory. Such a hybrid treatment can lead to discontinuities in the thermodynamic quantities when pressure ionization occurs. We propose a model in which all electrons (bound and free) are treated quantum mechanically. Furthermore, resonances are carefully taken into account in the self-consistent calculation of the electronic structure of each superconfiguration. The model provides the contribution of electrons to the main thermodynamic quantities, together with a treatment of pressure ionization, and gives a better insight into the electronic properties of hot dense plasmas.

Absorption measurements of radiatively heated multi-layered Al/Ni foils

Abstract Mixtures of light and mid- Z elements have been used to measure the absorption of the mid- Z element Ni, the temperature is inferred from the K-shell absorption of the light element Al. Here we test this method by comparing the temperatures deduced from Al K α transitions and nickel L-shell absorption spectra in Al/Ni multilayers and bilayers. The ionisation state is obtained by comparison of the Al and Ni spectra with calculations assuming local thermodynamic equilibrium. The temperatures obtained from the experiment are compared with hydrodynamic simulations predictions. Simulation code results show that the density differs by a factor of 2 in the two elements. This has to be taken into account in the determination of the temperature.

Absorption of local thermodynamic equilibrium aluminum at different densities

Abstract Increasing the range of plasma parameters accessible for laboratory absorption coefficients measurements is of interest for astrophysical applications. Aluminum is of special interest as its 1s–2p inner shell absorption transitions permit one to precisely determine the ionization balance that is strongly dependent on the electron temperature. A method to increase the density of the probed sample was tested on aluminum confined by carbon tampers of different thickness (8– 70 μg / cm 2 ). This created a density increase in the aluminum of a factor of ∼10. Measurements showed that the aluminum ionization decreases substantially with increasing carbon thickness. Radiative hydrodynamic simulations showed that density and temperature gradients could not be neglected and had to be taken into account in calculating the absorption structures with the atomic physics code HULLAC. Very good agreement between theory and experiment was obtained by coupling HULLAC with hydrodynamic simulations.