Low Pressure Limitation of the Orbitron lonization Gauge

Abstract

It has often been supposed that the orbitron gauge is an instrument to measure pressures of less than 10-10 Torr, since its theoretical lowpressure limit is about 5×10-13 Torr. In order to check this suggestion, some orbitron gauges of optimal design parameters have been studied at pressures below 10-10 Torr. It is found that a low pressure limit for orbitron gauges of less than 2×10-1 Torr is hardly achievable. This fact may be explained by space charges which are built up at very low pressures where the electron mean free path for electron-ion impact is in the order of some 108 cm. Due to this space charge, which appears as a potential barrier in the vicinity of the cathode, the number of electrons injected onto stable orbitals is drastically reduced. Hence, a major fraction of electrons fall onto the anode after only a few revolutions, thus giving rise to a higher X-ray limit than expected from theoretical predictions.