OUTGASSING OF THIN WALL STAINLESS-STEEL CHAMBER

Stainless steel with its unique mechanical properties is the most freq uently used material for manufacturing of vacuum chambers and componen ts. Its disadvantage is hydrogen outgassing which makes obtaining of u ltrahigh vacuum (UHV) troublesome. Vacuum annealing, prolonged in situ bake out, air bake, low-permeable coatings, and other, usually expens ive methods, are applied to reduce the outgassing rate and the concent ration of hydrogen. Recombination limited outgassing was shown to expl ain the required duration and temperature of bake out better than diff usion limited outgassing. In our experiments, thin wall stainless stee l chambers (0.15 mm) were examined with requirement of minimising the outgassing rate. They were cleaned and baked out in situ at 540 K resu lting in several ten diffusion time constants. Pressure rise was defec ted by spinning rotor gauge (SRG) after seal-off. Measurements were pe rformed for thousands of hours to exclude transient phenomena. Pressur e rise slope at room temperature at 10(-5) mbar of H-2 equivalent pres sure gave q(out) congruent to 10(-15) mbar I/(s cm(2)) and another sam ple measured at 10(-6) mbar gave q(out) congruent to 10(-17) mbar I/(s cm(2)). Numerical simulations of bake out with different values of re combination coefficient K, were made for comparison. Calculated and me asured results are within limits showing that in thin wall stainless s teel chambers extremely low outgassing rates can be obtained in modera te bake out procedure. (C) 1998 Elsevier Science Ltd. All rights reser ved.