Absorption spectroscopy of hot dense matter through direct-drive implosions with multiple dopants

The need to accurately predict radiation transport in stellar interiors and inertial confinement fusion targets provides a clear motivation for spectral opacity measurements of hot dense plasmas. In principle, laser-driven implosions provide a platform to access extreme plasma conditions (densities ρ>ρ0, electron temperatures Te> 400 eV), hard to reach by other methods. In this scheme, an element embedded in the shell is compressed by capsule convergence and heated by conduction from the central hot spot, which also provides a bright broadband x-ray source for absorption spectroscopy. We present an experimental study of absorption spectroscopy with capsules combining two distinct tracer element layers, respectively, titanium and vanadium, embedded in the plastic shell of direct-drive laser implosions at the OMEGA laser facility. The presented work is one of the scarce studies on absorption spectroscopy in an implosion configuration so far, and the use of two elements simultaneously has the advantage of bringing more stringent constraints on the atomic-physics models underlying our radiation-hydrodynamics simulations. Using two layers at different depths also enables us to characterize and benchmark the simulated density and temperature gradients inside the shell. Comparisons between simulated and measured time-integrated and time-resolved spectra are presented, and a good agreement is found, which validates our radiation-hydrodynamics and atomic physics models. The measurements presented here pave the way toward future opacity studies of elements increasingly relevant for astrophysical applications.