Effect of External Magnetic Field and Radiation upon the Accuracy of Total and Partial Pressure Measurements

Cold cathode discharge gauges use crossed electric and magnetic field. The slope of the gauge current versus prcs~ure characteristic curve on a log-log plot may bc stccpc’r a~ pressures ranging 10. ’- 10” ’ mbar and the pressure reading strongly depends on the applied magnetic field. The trajccmrq of ions in the quadrupole mass filter is also inllucnccd h) strong external magnetic fields (- 100 Gauss) by altering the spiral path. These undesired side cffccu can occur l~trticol~l~ if the cold cathode gauges and quadrupolc mass spcc‘tf-onj~Ic‘r heads are situated too close to bending magnets in the \torag< ring. Secondary electrons can be generated if radiation s\rikl%s the tubes used to mount gauges and anslyscr heads. Kead~ngs from the gauges and mass spectrometer heads can bc affcctcd by the creation of extra ions. In the Elettra storage ring all above mentioned vacuum instruments had to bc installed (due to spatial problems) in places where there is a strong cxlernsl magnetic field (- 1000 Gauss) and also the radiation retlcctcd from the photon absorber is not negligible. In some cases the, simple p-metal shielding was not adequate. Su~tablc modification of vacuum component mounting and correction of pressure readings has led to more reliable total and partial pressure measurements. 1