X-ray Thomson Scattering Measurements From Hohlraum Targets on the Omega Laser

Alison Saunders, University of California, Berkeley

X-ray Thomson scattering (XRTS) is a powerful diagnostic for probing warm and hot dense matter. The electron temperature and density of a plasma can be inferred from the profile of inelastically scattered X-rays and the ratio of elastic to inelastic scattering provides the ionization[1]. Ionization potential depression (IPD) strongly impacts ionization in dense plasmas. We lack experimental data to benchmark the state of IPD models to date. To that end, we started XRTS measurements from indirectly driven solid CH plastic spheres at the National Ignition Facility (NIF)[2,3,4]. In these experiments, a spherically converging shock wave compressed the CH to about 6 times solid density and about 100 eV temperature. These experiments find a 10 percent higher ionization state in carbon than predicted by the widely used Stewart-Pyatt model[5]. We aim to investigate more systematically the ionization balance in this interesting plasma regime through hohlraum targets on the Omega laser facility.
The Omega setup is a downscaled version of the NIF experiments. It consists of a 1-mm diameter gold hohlraum target with a 500-micron CH sphere in the center. Twenty laser beams create the hohlraum drive and three laser beams heat a zinc backlighter foil to provide Zn He-a X-ray line emission at 9.0 keV for the XRTS measurement. The X-rays are scattered at 110 degrees and collected by the zinc spectrometer, ZSPEC, which uses a highly oriented pyrolytic graphite crystal and records spectral data in the 7.5-11 keV energy range. Data from this experiment will help provide more insight into the ionization state of warm dense plasmas and hence the equation of state of materials important to planetary cores, white dwarfs and inertial confinement fusion.

[1] S.H. Glenzer and R. Redmer, Reviews of Modern Physics 81 (2009).
[2] T. Doeppner et al., Journal of Physics: Conference Series 500, 192019 (2014).
[3] A.L. Kritcher et al., High Energy Density Physics 10 (2014).
[3] A.L. Kritcher et al., High Energy Density Physics 10 (2014).
[4] D. Chapman et al., Phys. Plasmas 21, 082709 (2014).
[5] D. Kraus et al., submitted to Phys. Rev. Lett. (2016).

Abstract Author(s): A. Saunders, A. Jenei, T. Doeppner, D. Kraus, R. Falcone, J. Nilsen, D. Swift