The experiments described here were carried out on the LULI 2000 laser installation. They consist in creating a plasma by indirect heating on a target (usually metallic) and probing it in the X or extreme-UV (XUV) domain by a broadband beam named “backlighter”. The target transmission is measured either in X-ray range, namely from 700 eV to 1800 eV, or in XUV range, from 50 eV to 250 eV approximately. The heating conditions and the electronic density of the plasma are such that local thermodynamic equilibrium assumption is reasonable.
About ten opacity measurement campaigns have been carried out by our team over the last decades using various indirect heating schemes. The first experiments were done with the sample glued to the aperture of the gold sphere used for heating [Thais2003, Loisel 2009]. Then we have used a geometry where a second gold sphere was added symmetrically to the first one with respect to the sample. The detection axis coincided with the line passing through the center of the spheres. As a result the “parasitic” self-emission of the cavities was detected simultaneously with the radiation transmitted by the sample. To prevent this, a third geometry has been elaborated. During recent campaigns the detection has been done either in the X-ray range, or simultaneously in the X-ray and XUV range.
This campaign was aimed at studying the absorption of nickel, copper and aluminum in the X-ray and XUV domains. Only the results in X-ray range have proved to be usable and are presented here. A typical transmission measurement scheme is shown in Fig. 1.
The sample to be probed is placed between two gold spheres or “hohlraum” heated by laser beams of about 1.5 ns duration, with a wavelength of 0.526 μm (2 omega) and energy from 100 to 600 J. The presence of two cavities ensures some symmetry in the heating of the sample. One of the novelties of this campaign lies in the geometry adopted: the axis joining the centers of the spheres is perpendicular to that of the radiography in order to avoid as much as possible that the radiation of the cavities reaches the detector. The sample is oriented at 45° with respect to the X-ray axis and the spheres axis (Figure 1). The backlighter used during the last campaigns is characterized by a duration of about 1 ns, a wavelength of 0.526 µm, and an energy of about 10 J.
The spectra measured for four separate shots are shown in Fig. 2 [Dozieres2015]. They are compared to calculations made using the SCO-RCG hybrid code [Porcherot2014]. These calculations take into account the presence of carbon tampers assuming their temperature is 20 eV. One observes good or very good agreement on the positions and depths of the 2p-3d structures of nickel and copper, and 1s-2p of aluminum. There is also a good agreement on 2p-4d structures around 1000 eV for nickel and 1100 eV for copper. Different components 2p-4d are visible, because of the fine structure 2p and the presence of several states of charge. The measurement of these components is valuable for the theoretical interpretation because their relative depth is highly dependent on temperature.
During this campaign for which the detection was not resolved in time the energy of the target heating shots was deliberately reduced to about 100 J to prevent the radiation of the cavities from reaching the detector. Several shots could be exploited in the domain X [Dozieres2018]. For some shots heating was done with gold leaves and not cavities. This makes it possible to obtain higher sample temperatures to the detriment of the homogeneity of the heating. A first spectrum has been obtained in the XUV domain. Due to the lack of detection resolved in time during this campaign, this spectrum is strongly affected by the own emission of the cavities.
During this campaign for which XUV measurements were planned while no time-resolved detection was available, the energy of the target-heating shots was deliberately reduced to about 100 J to prevent the radiation of the cavities from reaching the detector. Several shots could be exploited in the X-ray range [Dozieres2018]. For some shots heating was done with gold foils instead of cavities. This makes it possible to obtain higher sample temperatures to the detriment of the homogeneity of the heating. A first spectrum has been obtained in the XUV domain. Due to the lack of detection resolved in time during this campaign, this spectrum is strongly affected by the own emission of the cavities.
An example of spectrum measured in the X-ray domain is shown in Fig. 3a and 3b for shot 75, compared with the SCO-RCG and FAC codes, respectively. These calculations take into account the presence of carbon buffers. There is a fairly good agreement concerning the transitions 2p-3d (energy around 860–890 eV) and 2p-4d (E from 920 to 940 eV). The maximum transmission between the two peaks at 0.35 is imperfectly reproduced by the calculations, probably because of the presence of temperature and density gradients. More details are available in the manuscript [Dozieres2018].