Ultrafine Hydrogen Layer Formation in a Cryogenic Target under External Vibrational Influence

Abstract

A considerable attention has recently been focused on the issue of large target fabrication for inertial confinement fusion (ICF) research on MJ-class laser facilities. The targets require a condensed uniform layer of hydrogen fuel on the inside of a spherical shell. The fusion fuel inside the targets must have such a structure, which supports the fuel layer survivability under target injection and transport through the reactor chamber. Over the last two decades, the Lebedev Physical Institute (LPI), has been devising the structuresensitive methods of forming high-quality hydrogen fuel with an isotropic structure (ultra-fine type of solid layers) to meet the requirements of implosion physics. A considerable anisotropy of HCP-phases of H₂ and D₂ (single crystal or coarse-grained crystalline layers) results in the layer degrading due to roughening of the layer surface before the target reaches the chamber center, or can result in the spherical-shock velocity dependence on the grain orientation. On the contrary, the ultra-fine solid layers have enhanced mechanical strength and thermal stability which is of critical importance for target fabrication, acceleration and injection. To meet the goal of ultra-fine solid layers formation, the LPI has constructed a piezo-vibration module, in which the couple “membrane & target” is driven by an input signal generated due to inverse piezoelectric effect during fuel cooling via the heat conductivity within vibrating targets. It allows one to modify the key experimental parameters (mechanical and thermal) for influencing the fuel microstructure and intensifying the creation of ultimate disordered structures with a large defect density, i.e., isotropic medium. In this report, the modeling results of the processes of cryogenic layer fabrication in the conditions of high-frequency mechanical influence are presented. The investigation is carried out to gain insight into the relation between the microstructure and bulk properties of the fusion fuel, and to fabricate this fuel with a given microstructure within ICF targets.