X-ray spectroscopic diagnostics of mix in high growth factor spherical implosions

Abstract

Rayleigh-Taylor (RT) instability of the pusher-fuel interface occurring upon acceleration and deceleration of the pusher is of major concern for current and future ICF experiments. One common diagnostic technique for measuring pusher-fuel mix in spherical implosion experiments involves placing spectroscopic dopants both in the capsule fuel region and the innermost region of the capsule wall adjacent to the fuel. As the degree of pusher-fuel mix is increased the pusher dopant x-ray emission increases relative to that of the fuel dopant. Spherical implosion experiments of this type using Ar and Ti dopants in the fuel and pusher, respectively, are being carried out on Nova. We first show that the Ti He-α Ar He-β line ratio shows promise as a mix diagnostic for high growth factor targets. We then discuss some of the important physical processes underlying Ar and Ti spectral line formation in these targets and discuss how these processes affect the calculation of simulated spectra. The importance of radiative transfer as well as high-density plasma phenomena such as continuum lowering and Stark broadening is demonstrated. The simulated spectra are also observed to be sensitive to assumptions regarding the treatment of electron thermal conduction in the mix region. Spectral postprocessing of 2-D hydrodynamic simulations using detailed line transfer methods has been carried out and implies that simple escape factor treatments must be tested carefully before they can be relied upon. Preliminary comparisons of experimental data with simulation are presented. It is shown that the computed spectra is sensitive to the laser energy and pusher temperature. These comparisons to data also imply that the inclusion of convective effects in computing the electron temperature profile through the mix region is necessary in order to satisfactorily model experimental spectra.