Abstract:

Acoustic cavity resonances were used to examine the interior surface geometry and gas fill pressures of various prototype inertial confinement fusion targets. A representative target consists of a millimeter-sized spherical beryllium shell filled to high pressure with deuterium/tritium (DT) gas at room temperature. Below the triple point of DT the gas forms a solid and redistributes itself symmetrically within the shell through a process known as beta-layering. The thickness of the solid DT layer, and the symmetry and smoothness of each surface must adhere to strict specifications for ignition to occur. Sound-speed measurements at room temperature provide the gas fill pressure and thus the solid DT layer thickness upon cooldown. These measurements rely upon an accurate equation of state. Degenerate-mode splitting of the cavity resonances provides interior surface geometry information. This technique was applied to a variety of deuterium and helium filled shells at pressures up to 356 atm. Several of these shells were manufactured with known interior surface perturbations. Measured mode splitting is compared with theory and the utility of the technique for cryogenic targets is examined.