Technique for in vacuo passivation of ZrAl alloy bulk getters

We have developed a method for passivating previously activated ZrAl alloy bulk getters. Such passivation is essential for avoiding embrittlement of the getters during discharge cleaning operations in tokamak fusion devices. The method consists of exposing the getters (at room temperature) to a controlled amount of oxygen, followed by pulse discharge cleaning to restore the vacuum vessel walls to clean conditions. We have tested this method on the Tokamak Fusion Test Reactor (TFTR) and present measurements of the decrease in the hydrogen and oxygen pumping speeds as functions of the quantity of oxygen adsorbed. These measurements are in good agreement with previous laboratory results. Using this method followed by 20 h of pulse discharge cleaning, we have reduced the getter pumping speed by more than a factor of 2×10³ and have recovered the same clean vacuum conditions as existed prior to the oxygen exposure.