Long-term changes in the sensitivity of quadrupole mass spectrometers

We routinely use quadrupole mass spectrometers (QMS) to monitor vacuum conditions, gas purity, and plasma–wall interactions in the Tokamak Fusion Test Reactor (TFTR) at Princeton. Two QMS systems have been operating on TFTR continuously for a 2 year period. Both QMS systems are absolutely calibrated at weekly intervals using a six-part standard gas mixture. The calibration procedure is based on the use of transfer standards (ion gauge and capacitance manometer) that are calibrated against a primary standard (spinning rotor gauge) on an external vacuum system. We have identified variations in the efficiency of the QMS ionizer and drifts in the sensitivity of the electron multiplier ion detector to be the major reasons for the observed changes in overall QMS sensitivity. Weekly variations in sensitivity greater than 100% have been observed following system bakeout at 150 °C and with the use of rhenium filaments which were initially in the QMS ionizer. Operation of the QMS system with tungsten filaments and at constant temperature has yielded more stable operation with weekly sensitivity changes generally being less than 10%.