Hydrodynamic growth and mix experiments at National Ignition Facility

V A Smalyuk,C Cerjan,G Grim,H F Robey,D Rowley,T Parham,M Mintz,H.-S Park,B A Remington,O A Hurricane ,John Edwards ,J L Kline ,B A Hammel,J P Knauer ,J D Kilkenny ,J M Mcnaney ,W W Hsing ,A Nikroo,S V Weber ,Daniel Clark ,Kumar Raman ,Robert Tipton ,D T Casey ,O L Landen ,S W Haan,J A Caggiano ,Jesse Pino ,A V Hamza ,C B Yeamans

doi:10.1088/1742-6596/688/1/012113

Abstract

Hydrodynamic growth and its effects on implosion performance and mix were studied at the National Ignition Facility (NIF). Spherical shells with pre-imposed 2D modulations were used to measure Rayleigh-Taylor (RT) instability growth in the acceleration phase of implosions using in-flight x-ray radiography. In addition, implosion performance and mix have been studied at peak compression using plastic shells filled with tritium gas and imbedding localized CD diagnostic layer in various locations in the ablator. Neutron yield and ion temperature of the DT fusion reactions were used as a measure of shell-gas mix, while neutron yield of the TT fusion reaction was used as a measure of implosion performance. The results have indicated that the low-mode hydrodynamic instabilities due to surface roughness were the primary culprits to yield degradation, with atomic ablator-gas mix playing a secondary role.

Highlights

Hydrodynamic instabilities and mix play a central role in the performance degradation; any drive asymmetries and surface imperfections are amplified by the hydrodynamic instabilities during implosion resulting in a distorted shell with reduced hot-spot temperature, volume, and pressure [1]
Since the discrepancies were especially pronounced in implosions with significant inferred ablator-fuel mix [2], it suggested another hypothesis that atomic mix was a major contributor to yield degradation because there were no models of atomic mix used in 2D simulations
While the measured growth factor at mode 30 was close to that predicted, the measured growth factors at modes 60 and 90 were larger than predicted by factors of ~1.3 and ~2.5, respectively. These results demonstrated that ablation-front growth factors were under-predicted in the simulations used to model the performance of layered DT implosions

Summary

Introduction

Hydrodynamic instabilities and mix play a central role in the performance degradation; any drive asymmetries and surface imperfections are amplified by the hydrodynamic instabilities during implosion resulting in a distorted shell with reduced hot-spot temperature, volume, and pressure [1]. Layered DT implosions have previously been modeled using 2D simulations intended to capture performance degradation due to instabilities and drive asymmetries [3], but lacking a model to predict atomic ablator-fuel mix [4]. These simulations over-predicted the yields by a factors from ~5 to ~30 for high-compression implosions. As a way to explain the measured performance, 2D simulations used large, un-physical multipliers (up to 3-5x) on the capsule surface roughness to bring simulated yields down to the measured levels [3] This prompted a hypothesis that the instability growth factors were larger than in 2D simulations.

Ablation-front Rayleigh-Taylor instability experiments

Atomic-mix experiments in the deceleration phase

Conclusions