Ion-pumped vacuum systems for downhole applications

Many physics experiments used for nuclear diagnostics measure low energy particle emission as a function of time. Transmission of these particles from source to detector typically must occur in a rarefied or vacuum environment. In instances where the experiment is buried in the earth, ion pumps have been successfully used to produce vacuum pressures on the order of 10-4 Pa (10-6 torr). In this application, ion pumps offer significant advantages compared to other pump types. Because of their simplicity, ion-pumped vacuum systems are inherently reliable. Only one coaxial cable is required to both power the system and monitor the pressure. Ion pumps remove gas through an ionization process. Since the vacuum pressure is directly proportional to ion current at these pressures, no additional instrumentation is required for pressure measurement. Some of the specifics associated with these systems and some of the steps taken to assure system reliability are discussed