Bubble preforms for producing glass macroshells for inertial confinement fusion targets

This paper describes a technique for producing spherical gas bubbles in glass that can be used to make inertial confinement fusion (ICF) targets. A glass rod containing an irregularly shaped hole is heated to a temperature where the glass viscosity is low enough so that surface tension forms a bubble from the hole. Buoyancy forces drive the bubble upward in the glass rod as it becomes increasingly spherical. At the proper time, the rising bubble is decelerated and brought to a gradual stop by increasing the glass viscosity by slowly reducing the temperature. With the present technique, 3- to 6-mm-diam spherical bubbles with a distortion of 0.3% have been produced in Corning 7740 and Schott BK-7 glasses. Glass macroshells can be formed from the bubbles trapped in the glass by grinding the outside surface concentric with the highly spherical inner surface. These glass shells, which possess a high degree of geometrical perfection, should be adequate for ICF targets