Experiments with a thin-walled stainless-steel vacuum chamber

The application of a reasonably thin stainless-steel wall has been recently suggested as an alternative approach in ultra-high-vacuum (UhV) chamber construction, since the hydrogen outgassing rate q(out) can be decreased much easily to the level required to reach UHV and EXV. Two identical stainless-steel chambers (AISI type 304, volume 121) with uniform wall thickness (0.6 mm) were constructed. Each was equipped with a miniature ion-getter pump and spinning rotor gauge (SRG) thimble, They were pumped down and initially degassed by a moderate bakeout (2.5 h, 150 degrees C) using a turbomolecular pump system. The gas accumulation method was applied for the determination of q(out) and the total amount of released gases. A capacitance manometer was used during bakeout but a SRG was applied after the chamber was sealed off. The q(out) at room temperature after initial bakeout was in the order of congruent to 3 x 10(-12) mbar l/(cm(2) s). SRC measurements of q(out) at 50 degrees C over several days showed a tendency of slight decrease, The bakeout of the chambers was being repeated in a duration of 72 h at 200 degrees C. The amount of released hydrogen was much smaller than expected, whereas the decrease of q(out) was noticeable. In fact, it was one tenth of the value before the bakeout. The results show the benefit of using a moderately thin wall and agree with the model of recombination-limited hydrogen outgassing from stainless steel. (C) 2000 American Vacuum Society. [S0734-2101(00)09504-X].