A novel method to measure ion density in ICF experiments using x-ray spectroscopy of cylindrical tracers

Abstract

The indirect drive approach to inertial confinement fusion (ICF) has undergone important advances in the past years. The improvements in temperature and density diagnostic methods are leading to more accurate measurements of the plasma conditions inside the hohlraum and therefore to more efficient experimental designs. The implementation of dot spectroscopy has proven to be a versatile approach to extracting spaceand time-dependent electron temperatures. In this method a microdot of a mid-Z material is placed inside the hohlraum and its K-shell emission spectrum is used to determine the plasma temperature. However, radiation transport of optically thick lines acting within the cylindrical dot geometry influences the outgoing spectral distribution in a manner that depends on the viewing angle. This angular dependence has recently been studied in the high energy density (HED) regime at the OMEGA laser facility, which allowed us to design and benchmark appropriate radiative transfer models that can replicate these geometric effects. By combining these models with the measurements from the dot spectroscopy experiments at the National Ignition Facility (NIF), we demonstrate here a novel technique that exploits the transport effects to obtain time-resolved measurements of the ion density of the tracer dots, without the need for additional diagnostics. We find excellent agreement between experiment and simulation, opening the possibility of using these geometric effects as a density diagnostic in future experiments.

Demonstration of Geometric Effects and Resonant Scattering in the X-Ray Spectra of High-Energy-Density Plasmas

“…This led to the conclusion that the O VI emission in certain solar spectra originated from a slab-like geometry in the solar atmosphere rather than from cylindrical structures [41]. The same effect also plays a role in the spectra emitted from plasmas created in the laboratory, and recently a geometric analysis of dot-spectroscopy experiments at the National Ignition Facility [42] (NIF) has yielded the first time-resolved measurements of coupled temperature and density inside an inertial confinement fusion (ICF) hohlraum [43].…”

“…The density of the plasma was measured from the XRPINH data, by imposing a condition of conservation of particles. The temperature distribution within the plasma was extracted by fitting the Sc and V K-shell spectra to a combination of synthetic spectra produced by the code Cretin [39] using a genetic algorithm as described in [43,54]. These are shown in Figure 3, with the obtained temperatures on the left and the density of scandium ions on the right.…”