The use of discharge tubes in electric circuits

Abstract

Experience in the manufacture of high power microwave tubes (multi-MW range) has shown that the materials which constitute the tube are as much important as the materials which constitute the thermionic cathode itself. Indeed, where the oxide coated cathode is concerned, it has been recognized since the time of Wehnelt that the residual gas and the cathode performance are related. For a long time, the influence of the gas evolved of the electrodes and the tube envelope during operation, on cathode emission, has been recognized, too. All these phenomena have been designated “ambient phenomena” where “ambient” means the gases or vapours in the vicinity of the cathode and, in a wider sense, the sources of these gases and vapours which present the constructional parts of the tube. Since in the tube practice it is not always convenient to point out the nature of the ambient gases in function of the materials and the treatments they undergo, it has been found useful to examine the influence of the materials on an oxide coated cathode in a test diode, a technique which is in principle well known for controlling the cathode and tube materials of receiving tubes (“standard diodes”). The diode in question is adapted to the needs of pulsed tubes and allows—thanks to this fact and contrary to the usual standard diodes—to measure the cathode performance under the operating conditions except the anode voltage which is lowered. After a rapid description of this diode, it is shown that the influence of the ambients reveals three different behaviours of a I23 plot, where the available useful current (under space charge limitation) is limited either by apparition of the sparking phenomenon or by saturation. The influence of the operating conditions (d.c. or pulsed operation, pulse length, repetition frequency) on the current where the sparking occurs are demonstrated and the results obtained with certain anode materials are indicated; that is: Iron, Kovar, Titanium, Molybdenum, Copper, Platinum and Nickel. The best anode materials were platinum and nickel; in this latter case, in pulsed operation a current density of more than100Acm2 has been obtained, the limitation occuring not by saturation, but by sparking. Finally, the influence of the glass used as the diode envelope is shown. In conclusion, the diode reveals as a relatively simple control device. Nevertheless, when the nature of the ambients must be pointed out, more improved devices should be applied, such as the mass-spectrometer, a subject of the next paper.