Vacuum breakdown in the systems of the electrodes with LaB6 cathode

Summary form only given as follows. Vacuum breakdown in the system of the electrodes with LaB₆ cathode in the experimental diodes and triodes with external copper anodes was investigated. Investigations were carried out on the diodes with negative anode voltage up to 60 kV. Maximum breakdown between the cathode and anode was determined by spasmodic increase of discharge number. Diodes were made with different fixed distances between anode and cathode: 1 mm, 1.3 mm, 1.6 mm, and 3.1 mm. On the results of the investigations with increasing length of the gap from 1 mm to 3.1 mm maximum breakdown field also increases from 1×10⁷ V/m to 1.95×10₇ V/m. A hypothesis was made that such dependence was connected to fact that in the diodes with larger distance between anode and cathode, coating of the anode surface is thinner. The surface of the anode was investigated by means of the scanning electron microscope (SEM) and X-ray Microanalyzer (XMA). The SEM and XMA observations showed that anode surface was coated by a solid amorphous layer with centers of forming crystals on a base of lanthanum. Triodes with LaB₆ cathode were made with graphite grid in the form of a disk with round holes, and with smoothly changing distance between anode and grid. Investigations were carried out with positive anode voltage up to 50 kV and negative cut-off voltage on the grid. Maximum breakdown field in the anode-grid gap was equal to (0.8-0.9)×10⁷ V/m and practically did not depend upon the distance between anode and grid in the limits from 4 mm to 6 mm. Such small breakdown field may be explained with small electrical strength of graphite