STABLE AND REPRODUCIBLE BAYARD-ALPERT IONIZATION GAUGE

This is the third in a series of related reports on (1) nonstable behavior of widely used ionization gauges, (2) causes of nonstability and nonreproducibility in widely used Bayard-Alpert (BA) gauges, and (3) a stable and reproducible BA gauge design with approximately a tenfold improvement in both stability of gauge calibration after thousands of hours of operation, and reproducibility gauge-to-gauge compared to older design BA gauges and inverted magnetron gauges. Computer simulation of electron and ion trajectories utilizing a program named SIMION was used to optimize the design. A grounded conducting shield of closely controlled dimensions completely surrounds the cathodes and anode. The anode has partial end caps on both ends. independently tensioned, thoria coated iridium ribbon cathodes are precisely positioned so that the flat emitting surfaces face imaginary axes laterally displaced from the anode axis by a small amount, The cathodes are relatively short compared to the anode. A 0.040 in. diam ion collector and an emission current of 100 muA help extend the upper pressure measuring limit to above 1 X 10(-2) Torr and provide an x-ray limit of 1.6 x 10(-10) Torr. With a 0.005 in. diam ion collector and an emission current of 4 mA, the x-ray limit is 1.4 x 10(-11) Torr and the upper pressure measuring limit is above 1 X 10(-3) Torr. This new technology, called Stabil-Ion technology, provides sufficient stability and reproducibility to justify storing accurate calibration data for a specific Stabil-Ion system or averaged data from a set of nominally identical systems in electronic memory. Thus, the typical nonlinearities in sensitivity and purposeful changes in emission current or gas type do not significantly affect the accuracy of pressure indication. Consequently, accurate real time readout of pressure and stable process control are provided by the new design.