Quantitative measurement of outgassing and degassing rate for various materials used in cryogenic pumps

Abstract

Outgassing from materials is a principal impediment for creating and maintaining clean and ultrahigh-vacuum environments. Data about out-gassing rates of different materials is very important for designing any vacuum system operating in ultra- high vacuum regime. Gas load due to outgassing from different materials determines the size (capacity) of pumps and order of vacuum that can be achieved in the system. Data on outgassing rates of materials are available in literature but not for all materials especially cryogenic pump materials at the required temperature of operation. To know the outgassing and degassing rate of such materials an experimental system was set up. The facility is having base outgassing rate of ∼ 5x10-12 mabr-l/s-cm2 measured through American Vacuum Society (AVS) recommended standard practices. The ultimate vacuum achieved was ∼5 x10-9 mbar having a sample chamber volume of 8 liters. Development of cryogenic pump involves selection of many materials like sorbents, black coatings, adhesive, sealant and support materials. They are selected based on their performance under UHV conditions especially minimum out-gassing during pre-evacuation and known degassing rate while regeneration. During reactivation and regeneration the cryo-surface carrying the adsorber bed is heated up to temperatures in the range of ∼150 deg C to activate the sorbent for the better cryoumping effect. Thermal outgassing studies of various samples including activated carbons, Al2O3-TiO2 coating, indigenous cryo-adhesives, indium wire and G-10 were carried out and are discussed in the paper. The measurements were quantitative and will be helpful for estimating outgassing and degassing gas loads during pumping and bake-out conditions.