New ionization gauge geometries providing stable and reproducible sensitivities

A primary cause of inaccuracies in Bayard–Alpert (BA) gauges has been previously identified as the unstable and nonreproducible distribution of electron emission from hot cathodes. Because of uncontrollable variations and instabilities in electron emission density pattern along the length of a hot cathode, the assumptions required to apply the fundamental ionization gauge equation, p=i₊/(Si₋), are not well satisfied in BA gauges or other ionization gauges employing grids. A new class of ionization gauge geometries is described which avoids these difficulties by closely satisfying the assumptions. New geometries are defined so that regardless of where on the hot cathode an electron is emitted, the electron has the same probability of causing ionization as any other emitted electron. Computer simulated electron and ion trajectories are presented for an optimum simple geometry. Testing of hardware has shown that our computer model is a good predictor of actual electron trajectories. Extensive testing including life testing is being carried out but the limits of performance of this new design have not yet been ascertained. These data indicate it should be possible to duplicate the pressure range of the BA gauge in a simple ionization gauge that is fully compatible with existing gauge controllers and which provides considerably better stability and reproducibilty of gauge sensitivity than does a BA gauge.