Performance of Zr–Al getter pumps under transient load conditions

Testing of the pump limiter concept in the Impurity Study Experiment (ISX-B) tokamak will involve the use of Zr–Al nonevaporable getter pumps capable of handling intermittent pulses of hydrogen and/or deuterium in the presence of carbon and oxygen impurity concentrations of several percent. To study the pumping characteristics under these conditions we have installed a Zr–Al cartridge pump in a vacuum chamber equipped with a fast gas puff feed system, a quadrupole residual gas analyzer, and a high speed ion gauge for transient pressure measurements. In this paper we report on the performance of the pump over a wide range of gas loads up to that sufficient to provide tens of monolayers of coverage of the getter surface. With flow rates up to 13 Torr l/s, pumping speeds for hydrogen were measured to be 1200–1500 l/s at pressures up to 10 mTorr. The measurements were carried out with gas pulses ranging in length from 50 ms to over 1 s and under conditions that provided a constant pumping speed for impurity species.