THE USE OF A HIGH-RESOLUTION QUADRUPOLE GAS MASS-SPECTROMETER SYSTEM FOR SELECTIVE DETECTION OF HELIUM AND DEUTERIUM

For controlling and monitoring of fusion deuterium plasmas and for hel ium leak-testing of vacuum vessels as well as for experimental investi gations of possible effects of selective gas pumping during cryopumpin g, it is necessary to detect helium in a deuterium atmosphere, which s tands as a model gas for deuterium/tritium mixtures. To cope with that , a novel quadrupole gas mass spectrometer (BALZERS GAM 400) has been procured and tested, which allows for base-line separation of helium ( mass 4.0026 amu) and deuterium (4.0282 amu) in a high-resolution mode and also offers the conventional operation mode (mass range up to 128 amu) by two different ion-source parameter sets. Thus, it is possible to analyse multi-component mixtures quantitatively with a high He/D-2 resolution and a sensitivity for of her gases in the 10 ppm-range. The mass spectrometer is placed outside the test vessel and equipped with two different gas inlet systems from the test vessel to cover the who le process pressure range between 10(-4) mbar and atmospheric pressure . The performance characteristics of the gas analysing module are pres ented. Experience gained by using the analyser to monitor concentratio n shifts due to the selectivity in cryopumping of gas mixtures is also reported. (C) 1998 Elsevier Science Ltd. All rights reserved.