Opacity measurements on LULI facility and theroterical interpretation

Fig. 1. Example of sample, hohlraum, and backlighter foil (in the back) use for photo-absorption experiments

Collaborations : CEA/DAM/DIF , LULI/Ecole Polytechnique, CEA/DSM/IRFU/SAp

The absorption experiment is considered in the dense plasma community as the best tool to investigate atomic physics of laboratory plasmas in conditions close to the Local Thermal Equilibrium (LTE). This is especially true in case of the absorption in the X-ray range (of the order of 1000 eV and higher). First reason for is that in the absorption processes the initial states involve deeply bound electrons. The occupations of such levels are well described by Boltzmann statistics when plasma is close to the LTE. Second, the absorption experiments in radiatively heated nearly homogeneous targets can be directly interpreted by atomic physics code with a minimal use of complicated plasma hydrodynamic code. Such experiments have a large impact on plasma atomic physics codes and now allow us to choose the appropriate approximations in the much needed statistical methods in this field.

Experiments principle

Typically, to achieve the required conditions, we use a two lasers configuration. The first laser is used for the heating of the sample. It should let enough energy in the sample so that the plasma thermodynamical conditions are obtained. This is the reason why one uses a kJ visible laser with duration of the order of the nanosecond. The principle of the heating is based on the indirect drive scheme promoted for inertial confinement fusion. The laser is focused in a hohlraum on which the sample is glued on one of its holes, during the interaction an equilibrium X-ray radiation, quasi planckian, will be emitted allowing for an almost uniform heating. A second laser is thereafter used to perform a point projection spectroscopy, it impacts an additional material to produce an X-ray backlighter aimed to radiograph the sample. Preferentially, it is a much shorter laser (e.g. picosecond) since the plasmas conditions should not vary too much during the measurement. With a spectrometer and a sufficient resolution at the end of the measurement chain, we thus obtain the searched measure of the spectral photo-absorption.

Fig. 2. Typical scheme of absorption experimental setups

Fig. 3. Streak camera image in the ZnS experiment with spectral and time resolutions

Absorption measurements in Al and ZnS in EUV range

This work has been performed at the LULI 2000 facility during the PhD thesis work of N. Kontogiannopoulos [Ecole Polytechnique, Dec. 6, 2007]. Absorption of targets is measured using gold cavities. One cavity wall is heated by the main laser beam creating a holhraum inside the cavity; in turn, holhraum heats radiatively the main target in form of a thin film put on a hole in the cavity. The probe radiation from another laser-produced plasma crosses the main target during its hydrodynamic explosion. In this experiment the objective was to study the absorption properties of a plasma mixture composed of a medium (30) and a small (16) Z element. Aluminum targets have been also investigated for comparison and for checking the cavity physics. A typical time-frequency mapping of absorption in ZnS is shown in Fig 3.

We have developed a theoretical interpretation using the superconfiguration code SCO and/or detailed term accounting code HULLAC. This allows one to identify all the main structures appearing in the ZnS spectrum. The computation of relevant complex structures such as 3d-4f in Zn⁶⁺ to Zn⁸⁺ is a serious challenge.

2p-3d absorption structures in neighboring metal plasmas: iron (Z=26), nickel (Z=28), copper (Z=29), zinc (Z=30) and germanium (Z=32)

The experiment has been performed on the LULI 2000 laser. The main new idea of the target and its atomic physics has been to use plasmas of neighbouring Z elements in order to investigate the thermal (configurational) broadening of 2p-3d transition structures. Plasma temperature has been of the order of 20 eV and density of the order of 0.01 g/cm ³. Theoretical analysis indicates that the 2p-3d spin-orbit gap should progressively show up in these spectra with increasing Z overcoming thermal broadening. We have focused our attention on the variation of the 2p-3d transition arrays with the atomic number Z in a limited range of values (Z=26, 28, 29, 30 and 32); we have then compared this Z-dependence with the influence of the plasma temperature that induces a shift and a superposition of structures, due to the thermal-configurational broadening. These studies contribute to the modelling of spin-orbit splitting and related effects such as the interaction between relativistic subconfigurations. Thus the use of different Z elements helps us to decouple inherent atomic physics of highly charged ions from thermal effects. From analysis in similar plasma conditions of plasmas with different Z, we can get information on the competition between jj and intermediate couplings. Moreover the spectral position of the 2p-3d transitions can be used as a "thermometer" since it depends on the temperature. This dependence may also reveal the presence of temperature gradients.

Fig. 4 : Predicted transmission in iron, nickel, copper, zinc and germanium allowing, or not(thin lines), for temperature gradients, at a level already observed in preceding experimental campaign

Fig.5. Measured transmission (black) of a copper plasma compared with two theoretical transmission curves from the statistical SCO code (red) and the detailled HULLAC code (blue) with 0.005 g/cm3 density and 15 eV temperature

On Fig. 5 we present as an example the measured transmission of a radiatively heated copper plasma of 40 μg/cm² areal density and two transmission curves calculated with the SCO code and the HULLAC code for comparison. The absorption structures near the 13 Å wavelength correspond to 2p-3d transitions. The positions of the two spin-orbit-split structures depend on temperature - the theoretical curve at the temperature 15 eV shows the best agreement with respect to the 2p-3d structures in the experimental transmission.

[1] Mesure de coefficients d'absorption de plasmas créés par laser nanoseconde
Thais F., Chenais-Popovics C., Eidmann K., Bastiani S., Blenski T., Gilleron F.
Journal de Physique IV 127, 119 (2005)
[2] Measurement of XUV-absorption spectra of ZnS radiatively heated foils
Kontogiannopoulos N., Bastiani-Ceccotti S., Thais F., Sauvan P., Schott R., Fölsner W., Arnault P., Poirier M., Blenski T.
High Energy Density Physics 3, 149 (2007)
[3] Interpretation of some X-ray and XUV absorption experiments using SCO,
Arnault P., Blenski T., Dejonghe G.
High Energy Density Physics 3, 1 (2007)

#1115 - Màj : 11/10/2018