Patent Number: 043808557
Section: summary

DESCRIPTION The present invention relates to methods for filling hollow shells with gas and in particular to a method for filling hollow glass shells with gases which do not readily diffuse through the glass walls of the shells. The invention is especially suitable for use in making laser fusion targets. Such targets are mounted in a target chamber and illuminated with a high power laser. Previously, glass microballoon laser fusion targets (see U.S. Pat. No. 4,034,032 issued July 5, 1977) have been filled with gases which can permeate or diffuse through the glass walls of the microballoons. Only light gases such as neon, hydrogen and helium and the isotopes thereof can readily diffuse through the glass walls of the microballoon. Filling with other gases by diffusion is difficult or impossible. It is nevertheless desirable to provide microballoon laser fusion targets filled with gases such as argon, deuterium sulphide and xenon, either alone or in mixtures with the lighter gases mentioned above in order to provide laser fusion targets. Such targets have particular utility in studies of the effects of high energy laser radiation particularly as such effects occur during the laser fusion process. Accordingly, it is an object of the present invention to provide an improved method of filling or charging hollow shells such as glass microballoons with gases and particularly such gases as do not readily diffuse through the shell. It is a further object of the present invention to provide an improved method of filling glass microballoons having diameters in the range of 20 to 400 micrometers (um) which does not rely on the permeability characteristics of the glass walls of such microballoons. It is a further object of the present invention to provide an improved method of filling glass microballoons with minimum perturbation of the surface finish and geometry of the balloons. It is a still further object of the present invention to provide an improved method for filling microballoons which also enables the balloons to be coated on the inside surface thereof. It is a still further object of the present invention to provide an improved method of filling microballoons with gas to pressures above atmospheric pressure. Briefly described, the method of filling a hollow shell for use as a laser fusion target in accordance with the invention is carried out by drilling a hole through the wall of the shell. A plug having a melting temperature lower than that of the shell is then located on the shell over the hole. The plug is very small and is sometimes referred to as a microsphere hereinafter. The shell with the plug located over the hole is then placed in an evacuable pressure vessel which can be heated. In this vessel the shell can be evacuated of atmospheric gases and filled with the fill gas to the desired pressure. The gases enter and leave the shell between the plug and the perimeter of the hole. The vessel containing the shell with the plug located over the hole may then be heated to melt the plug to seal the hole. The vessel is then cooled to resolidify the plug and thus form a barrier against escape of the gas from the shell. The shell may be a glass microballoon which, when filled with the gas, provides the laser fusion target itself or the component of a more complex target configuration such as a multi-coated or multi-shelled target.