P. Angelo

Access to Spectrally Resolved Ultra‐Dense Hot Low Z Emissivities and Opacities

We present here experimental studies of broadened bound‐bound emissivities and opacities of low Z plasmas that are the best candidates for exhibiting ion and electron correlations. First we report on an emission experiment where a new target design is used to access the highest densities. Such targets irradiated by an intense long laser pulse generate plasmas well adapted to model and extract opacities. The measurements are compared to theoretical results obtained from simulations involving new atomic/molecular physics models that take into account detailed line profiles. We end up with the description of an absorption experiment in progress, this experiment using the same targets.

Low Z opacities at high densities

Abstract We present an experimental study devoted to measuring the opacity of bound–bound transitions in ultra-dense, hot, low Z plasmas, which are at the extreme limit for conditions of both emission spectroscopy and absorption spectroscopy. In this work, we develop an absorption spectroscopy experiment specially adapted to high-density diagnostics, using newly designed structured targets and an ultra-high resolution spectrograph. An aluminum plasma is chosen as the first candidate and the opacity of the He-like 1s 2 –1s2p (He β ) and 1s 2 –1s3p (He γ ) transitions are measured.

Improving the dicenter model for hot dense plasmas: molecular stark effect

Abstract In previous work of the dicenter model, used to take into account the ionic correlation effects in hot dense plasmas, only considered a single perturbing ion (i.e., the nearest neighbor ion) thus limiting its range of applicability. The improvement proposed in the present work includes the effect of all perturbing ions through a quasistatic external microfield acting on a “quasi-molecule”. This enlarges the domain of validity of the dicenter model. For low densities this new alternative model gives line widths in agreement with standard “monocenter” profiles without artificially reducing the electron screening inside the dicenter emitting cell. This reduced screening, which had been employed to treat the repulsive interactions between the quasi-molecule and the other perturbing ions, is removed in the present work.

A comparison of two atomic models for the radiative properties of dense hot low Z plasmas

Abstract In this work, two different atomic models (ANALOP based on parametric potentials and IDEFIX based on the dicenter model) are used to calculate the opacities for bound–bound transitions in hot dense, low Z plasmas, and the results are compared to each other. In addition, the ANALOP code has been used to compute free–bound cross sections for hydrogen-like ions.

Semiclassical analytical approach to the description of quasimolecular optical transitions

A formula was obtained that describes asymptotically forbidden quasimolecular optical transitions in the frame of the semiclassical approach. It is particularly relevant for the weak extrema in the difference between the ground- and excitedstate interaction potentials. When averaged over impact parameters and velocity distribution the formula agreed reasonably well with the recent experimental data for the Ca(4 1 S → 3 1 D) + He transition. This paper is devoted to the problem of analytical descriptions of quasimolecular optical transitions which are asymptotically forbidden at large interatomic distances R. It is well known that spectral line shapes (SLS) produced by the allowed atomic transitions show mainly Lorentzian distributions with far wings attributed to collisions with buffer gas atoms (Sobelman 1979). In contrast, little is known about the SLS for transitions caused by a pair of colliding atoms and which are forbidden at large R values (asymptotically forbidden transitions). A typical example for such transitions, which can be referred to as quasimolecular optical transitions, is optical depopulation of metastable atoms, especially those of the second group and of rare gases. The analytical description of asymptotically forbidden transitions presents great difficulties in view of the considerable difference in the behaviour of the potential energy curves and the optical transition moments, which are required to complete the SLS calculation. For instance, the transition moments in this situation cannot be approximated by constant values as in the case of allowed transitions. However, in many cases the quasimolecular optical transitions result from a change of angular momentum during the collision (Devdariani 1993) due to predominantly short-range repulsion. Here, potential energy curves and optical transition probability (or radiation width �(R) ) can be reasonably approximated by exponential functions. It should be noted that

Alternative treatment of line broadening in dense and hot plasmas

A spectral line shape code using the “quasimolecular model” has been developed for dense and hot plasmas. First this alternative treatment is justified. Then the importance of using a two-center basis and the effects of the plasma screening are discriminated. Specific hydrogen-like and helium-like transitions are studied for the exhibition of dense plasma effects (PPS, asymmetries, satellite features) and for a comparison with experimental results. It is shown that satellites due to extrema are enhanced by the ion dynamics correction.

Advanced Simulations for Signatures of Charge Exchange in Heterogeneous Plasma Emission

We present an advanced theory of x‐dips in spectral lines emitted from laser‐produced plasmas. We compare predictions of this theory with our previous experimental results where, in the process of a laser irradiation of targets made out of aluminum carbide, we observed two dips in the Lyγ aluminum line perturbed by fully stripped carbon. Our theory gives a reasonable agreement with our experimental results. The results are of importance for the diagnostics of fundamental processes as it opens up a way to experimentally produce not‐yet‐available fundamental data on charge exchange between multi‐charged ions, virtually inaccessible by other experimental methods. From the theoretical viewpoint, the x‐dips are the only one signature of charge exchange in profiles of spectral lines emitted by plasmas and they are the only one quasi‐molecular phenomenon that could be observed at relatively “low” densities of laser‐produced plasmas, all those aspects emphasize the interest for studying heterogeneous plasma emission.

Ionic correlation effects on opacity and emissivity of ultra-dense hot low Z plasmas

The present work is devoted to the study of opacities for ultra-dense, hot, low Z plasmas. For those highly correlated plasmas, similar to some astrophysical situations and to implosion experiments, ionic and electronic correlation effects modify drastically the atomic and radiative properties. Up to now the quasi-molecular model has been shown to be efficient for an exact treatment of the spectral line shapes. The aim of this work is to use the same approach for the computation of the photo-excitation cross sections, the opacity and the emissivity characterizing ultra-dense and hot plasmas.

Time resolved FLy β spectroscopic measurements in colliding foil experiments

The experimental results we present here concern the time evolution of the main plasma parameters in the region between two thin laser accelerated Teflon foils as deduced from fluorine H-like and recombination continuum emission, as well as the time evolution of Fluorine Lyβ line profile. Two X-ray streak cameras with a time resolution of 10 ps have then been used, recording the emission of a 25 μm sized plasma slice between the foils, to be compared to the initial distance (≈100 μm). To obtain a well resolved spectrum despite of the low streak camera photocathode spatial resolution, a high dispersive TlAP spectrometer has been used to record the Lyβ line profile. In this way we can associate any particular line profile we record to the plasma conditions producing it. In conclusion this experiment allows quantitative comparison with hydrodynamics code results, post-processed by atomic physics and spectra codes.

Charge exchange between two nearest neighbour ions immersed in a dense plasma

In dense plasmas the quasimolecular model is relevant to describe the radiative properties: two nearest neighbor ions remain close to each other during a time scale of the order of the emission time. Within the frame of a quasistatic approach it has been shown that hydrogen-like spectral line shapes can exhibit satellite-like features. In this work we present the effect on the line shapes of the dynamical collision between the two ions exchanging transiently their bound electron. This model is suitable for the description of the core, the wings and the red satellite-like features. It is post-processed to the self consistent code (IDEFIX) giving the adiabatic transition energies and the oscillator strengths for the transient molecule immersed in a dense free electron bath. It is shown that the positions of the satellites are insensitive to the dynamics of the ion-ion collision. Results for fluorine Lyβ are presented.