Outgassing performances of an industrial prototype tube for the Virgo antenna

The Virgo detector, a gravitational wave interferometric antenna, is described with reference to its vacuum performance specifications, which concern essentially the hydrogen outgassing rate and hydrocarbon contamination. The hydrogen outgassing rate of an industrial scale stainless steel (304 L) vessel (1.2 m diameter, 7.5 m length) was measured after two different annealing processes: 950 °C vacuum firing and 400 °C air firing. The results are compared with the predictions of the classical theory of hydrogen diffusion and solubility in stainless steel. An ultimate outgassing rate as low as 1.1 10 −12 Pa m s −1 at 20 °C was achieved after a standard 7 day- 150 °C bake out, on a 40 m 2 area vacuum vessel. Low hydrocarbon partial pressures were measured.