Estimating the Quality of Thermionic Cathodes for Microwave Vacuum Tubes Using the Low-Frequency Noise Parameters

Abstract

It is proposed to estimate the quality of thermionic cathode emitting surface in microwave vacuum tubes from the level and shape of low- frequency noise spectral characteristics. The proposed concept is based on the idea about the noise generation mechanism caused by migrating flow of activating atoms over the surface of weakly emitting defects. The noise is generated due to work function fluctuations caused by a randomly changing number of activating atoms on the defect surface. The cathode emitting surface contains both emission centers with a low work function (the current from them is taken predominantly when the noise is significantly damped by spatial charge) and low-emitting areas with an increased work function, which are in saturated state when there is no spatial charge. The latter ones make the predominant contribution to the noise and perform the role of emission defects. A procedure for preprocessing the measured noise spectral characteristics has been developed, which involves smoothing and averaging by means of median filtering and cubic spline interpolation algorithms. The procedure also includes subsequent decomposition into separate components using the previously modeled base components for defects of different sizes. The decomposition technique has been implemented as a computer program based on a synthesis of spectral characteristics by summation of base components in various combinations. For a group of similar dispenser cathodes made of W-Re sponge impregnated with Ba compounds and coated with a thin Os film, the quantitative content of defects as a function of their dimensions — so-called cathode images — has been determined. The found defect areas increase with the operating time and are proportional to the square of cathode currents. It is shown that a larger defect area obtained from the noise spectral characteristics before long-term tests corresponds to more significant degradation of cathode emission parameters after the test. Thus, the measurement of low-frequency noise and processing the results after comprehensive experimental confirmation on large batches of cathodes can be regarded as a tool for predicting and evaluating the reliability of vacuum tubes.