Permeation of helium and hydrogen from glass-microsphere laser targets

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Abstract
Isothermal outgassing curves of hollow glass microspheres filled with helium, hydrogen, or deuterium gas have been determined. Four million 40–45‐μm soda‐lime glass microspheres, similar to laser‐fusion targets, were filled by gas permeation at 693–763 K and outgassed at 292–573 K. The permeabilities were calculated by an exponential theory, and they agree to an order of magnitude with the literature values. The outgassing curves are not pressure dependent. Two irregularities are apparent. First, the counting of target‐quality individual D‐T–filled microspheres shows a permeability spread of an order of magnitude from one microsphere to the next, which may be caused by variable chemical composition. Second, all the gases show deviations from exponential behavior in the form of tails at long times. Chemical reaction of the hydrogen with the glass, as well as incomplete filling and outgassing, may cause the hydrogen tails; the cause of the helium tails is not known.