Using a 2-shock 1D platform at NIF to measure the effect of convergence on mix and symmetry

Abstract

We describe the use of a robust new 1-D like implosion platform at the National Ignition Facility [G. H. Miller et al., Opt. Eng. 43, 2841 (2004)] to study the effect of convergence on mix and shape. Previous experiments suggest that nuclear yields and ion temperature degrade with increased convergence [M. D. Cable et al., Phys. Rev. Lett. 73, 2316 (1994)] due to enhanced perturbation growth and mix, but little has been reported on the distortion of the shape with time. The 2-shock platform was developed [S. F. Khan et al., Phys. Plasmas 23, 042708 (2016)] to maintain a high degree of sphericity during the whole implosion phase and has a thick, uniformly doped (1% Si) plastic CH shell to minimize the effect of mixing due to hydrodynamic feed-through from the outer ablator surface. An inner layer of deuterated plastic (CD) and hydrogen-tritium (DT) gas fill allows for the measurement of DT neutrons produced by the mix between the gas and ablator. DD neutrons provide information about the hot, unmixed CD region. By changing the fill gas density while keeping the capsule diameter, ablator thickness, and Au hohlraum conditions fixed, the x-ray hot spot convergence ratio was varied from 14 to 22. We find that the atomic mix (DT yield) grows linearly as a function of convergence, but since Tion changes as well, it does not necessarily mean that the amount or extent of mix grows linearly as well. We also find the DD yield, which is a measurement of the shell heating, saturates above a certain convergence.