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IGC100- Bayard-Alpert Gauge Filaments: Tungsten or Thoria? www.thinkSRS.com Bayard-Alpert Gauge Filaments: Tungsten or Thoria? Introduction Careful attention must be applied to the choice of filament material when selecting a Bayard-Alpert ionization gauge for a vacuum application. The optimum choice of filament material is very application dependent, and interactions of the gas with the cathode material must be carefully considered. Filaments, in both wire and ribbon shapes, are required to (1) supply a stable electron emission current with reasonable energy input and filament temperature, (2) have reduced chemical reactivity with the rarefied environment being measured, (3) have a reduced evaporation rate at the operating temperature (i.e. long life), (4) have a vapor pressure at least one tenth of the lowest pressure that has to be measured and (5) have low levels of ionic and neutral molecule outgassing compatible with the lowest pressure measurements. It is commonly believed that the filament in a Bayard-Alpert gauge (BAG) presents a reliability problem (especially when compared to Cold Cathode Gauges) and it is often true that the operating life of hot cathode ionization gauges is determined by filament lifetime. However, unless damaged by ion bombardment, high pressure operation, chemical reactions or severe contamination, filament lifetimes are usually in the thousands of hours (usually years). Several attempts have been made to replace hot filaments with cold electron emitters in BAGs1; however, no commercially available gauges have resulted from those efforts yet. Filament Materials Filaments are based on thermionic phenomena 2- the emission of charged particles from the surface of a conductive material or compound at high temperature. The emission of charged particles depends on the work function3 of the material, defined as the energy, measured in eV, required to move an electron from Fermi level outside the surface. The filament materials used in BAGs can be grouped into two classes: (1) pure metals and (2) oxide-coated cathodes. This section only concentrates on the materials commonly encountered in commercially available gauges. For information on less common materials, consult the excellent primer by Gear4 and the work by W. Kohl5. Stanford Research Systems, Inc. 1/5 www.thinkSRS.com IGC100- Bayard-Alpert Gauge Filaments: Tungsten or Thoria? Among pure metals, tungsten (W) is used on a large scale in ionization gauge heads operated in medium and high vacuum environments. The operating temperature of tungsten cathodes is between 1900