Measurements of the K-Shell Opacity of a Solid-Density Magnesium Plasma Heated by an X-Ray Free-Electron Laser

The generation of uniformly dense and uniformly heated high-density plasmas created sufficiently rapidly so as to prevent significant hydrodynamic motion during their diagnosis has long been a goal in the field of laser-matter interactions, as such plasmas would provide an environment to make benchmark tests of several fundamental optical, thermodynamic, and transport properties of strongly coupled matter which are important for furthering our understanding of systems relevant to astrophysics , planetary science , and inertial confinement fusion. While heating matter with electrons or ions driven by short-pulse optical light has afforded significant progress in this area, it is with the advent of free-electron lasers (FELs) operating in the x-ray regime, such as the Linac Coherent Light Source (LCLS), that the capability to create hot plasmas at exactly solid densities has been conclusively demonstrated, with the relatively long absorption depths of x rays and absence of prepulse allowing micron-scale matter to be heated on femtosecond time scales with negligible longitudinal temperature variation

Experimental x-ray emission spectra for Mg targets of four different thicknesses, heated by the LCLS x-ray pulse. Each spectrum is the average of spectra from multiple shots, normalised by the target thickness. The lines are labeled according to the charge state of the emitting ion. Also shown is the emissivity extracted by fitting Eq. (1) to the four experimental spectra. The colored region around the fitted emissivity represents its error bar.

¹Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
²Atomic and Molecular Data Unit, Nuclear Data Section, IAEA, P.O. Box 100, A-1400 Vienna, Austria
³SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
⁴Institute of Physics ASCR, Na Slovance 2, 18221 Prague 8, Czech Republic
⁵European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany
⁶Department of Physics, University of California, Berkeley, California 94720, USA