Hot-Cathode Magnetron Ionization Gauge for the Measurement of Ultrahigh Vacua

Abstract

In order to extend the low-pressure limit of the conventional hot-cathode ionization gauge it is necessary, at a given emission current, to increase the ratio of the ion current to the x-ray photo-current. This ratio may be increased by increasing the sensitivity of the ionization gauge. The gauge may be modified to permit the electrons to travel in long paths before they are collected by the positive grid or anode. Under these conditions the probability of the electrons' colliding with an ionizing a gas molecule will be enhanced, and the sensitivity of the gauge will be improved with no increase in x-ray photoemission. To accomplish this a magnetron with a cylindrical anode was selected for the ionization gauge and operated in a magnetic field with an intensity of 2.5 times the cutoff value. Two end plates maintained at a negative potential relative to the cathode collect the ion current generated in the magnetron and prevent the escape of electrons. Positive ion emission from the cathode is suppressed by mounting a hairpin filament on the axis of the cylindrical anode and well removed from the region of the negative ion collector. A very low level of electron emission is used to prevent instabilities in operation and to give a maximum ratio of ion current to x-ray photocurrent. Mearurements of sensitivity and x-ray photocurrent indicate that the magnetron gauge is linear down to a pressure of 4×10−14 mm Hg. Ability to measure low pressures with the gauge appears to be limited by the sensitivity of the circuit measuring the low ion currents. The ion pumping speed was found to be 0.003 liter/sec under normal operating conditions. Magnetron gauges have been built with ceramic metal envelopes.