By the use of oxide-cored cathode (J. Phys. Soc. Jap. 14, 180 (1959)), focussing mechanism of various types of electron guns is investigated. Independent of the type of the gun, an image of the tip of cathode is formed on a fixed plane if suitable bias voltage is applied. At this bias voltage the illuminated area on the plane is minimum, the current density is maximum and observed brightness reaches nearly the ideal value calculated by Langmuir's formula. Near the cut-off bias voltage, space charge affects the brightness of electron beam. Data useful for the design of electron guns are given.
Thermoelectron microscope of emission type is confirmed to be a powerful tool for the study of the mechanism of deformation of metals at high temperatures. The sample, prepared in the form of a ribbon, about 2mm. wide, 0.06mm. thick, and 10mm. long, is elongated at a temperature between 800 and 1100°C by the use of a simple device. The heating is made by directly passing a d. c. current through the sample. This heating method gave no harmful effect on the image up to a magnification of 200× and up to a heating current of 10 amps. The tensile stress produced in the ribbon did not cause any trouble in the sharpness of the electron image. The formation of slip bands, the development of rupture along grain boundaries and through grains, and the process of recrystallization and grain growth were clearly observed on a fluorescent screen. Although only little has been made on the metallographic analysis so far, it was clearly and continuously noticed that the fracture occurred at 850°C only along grain boundaries and recrystallization did not take place in the crystal grains when the rate of elongation was about 0.3%/min., but that the fracture proceeded accompanied by recrystallization in the grains when the rate of elongation was about 5%/min.
Cesium thermionic converters are made for experiment and their characteristics are observed. This is a part of the study on oscillation phenomena and dissipation of electrodes in connection with the work function of electrodes in plasma. The tubes have tungsten as cathode and cesium coated nickel as anode, forming parallel electrodes in one and concentric electrodes in the of herin cesium vapor. A testing circuit for the tube with cathode of small heat capacity is developed The open circuit voltage of these tubes is affected by photoelectron emission of the anode. The output power and oscillation amplitude decrease as the evaporation of cesium from the anode increases. The oscillation occurs only at and near a certain temperature of the cathode; it resembles the twin oscillation of discharge tube with thermionic cathode. If argon at 1mmHg pressure is introduced into the tube, auxiliary discharge in front of the cathode caused by the presence of argon can stop the oscillation.
Stability and maximum allowed current of field emission cathode are checked experimentally. This experiment is made with diode tubes which have a tungsten emitters of 0.1μ_??_0.4μ tip radius under the pressure of the order of 10-10 Torr. The results are as follows: (1) Emitter suffers emission slump by gas contamination at room temperature, but its surface can be kept clean at 1300°C or higher. (2) Emitter is apt to deform at high temperature by migration effect; it is very difficult to keep the condition balanced and form of the emitter constant at the same time. (3) Maximum allowed current of emitter is several 10mA in pulse operation (duration 4μs, duty cycle 60 pps) and several mA for direct current. (4) In practical use, this emitter is effective in pulse operation at high temperature.
Pure grade commercial alkaline earth carbonate contains some impurities such as NaNO3, MgO, SiO2, Al2O3, etc. which are mixed up with the raw material in manufacturing process mainly by the use of alkaline earth nitrates, precipitation solution and ballmill. These impurities except MgO have detrimental effects upon emission and spark characteristics of the cathode, for they are apt to react with the oxide coating; observation under electron microscope reveals sintering and fusing on the cathode surface. On the other hand, the cathode prepared from pure alkaline earth carbonate suffers high evaporation of activated matter and is liable to form undesirable interface layer.
Activation rate ra and evaporation rate re were measured on both carburized and uncarburized thoriated tungsten filaments in which thoria content and grain size were varied purposely. In the case of uncarburized filaments, ra and re rise remarkably with thoria content, while theinfluence of grain size is rather small. The dependence of ra, and ra on thoria content disappears. by carburization. Brown-Tube type electron emission microscopes were used to observe the activation and deactivation processes of thoriated tungsten ribbons. The eruption of thorium found previously by Ahearn and Becker is also seen on uncarburized samples. Positions of the eruption are not always. on grain boundary; its frequency increases slowly with thoria content. In carburized ones, thereappears no eruption. On the basis of the results of these observations and further on the assumption that theamount of thorium per an eruption increases with thoria content, the abovementioned behaviors, of ra and re may be explained reasonably. For carburized ones, the established theory holds.
A new type cathode composed of “impregnated cathode” and “molded cathode” is investigated. In this cathode, excess Ba is supplied from inner impregnated porous tungsten to outer molded cathode. Studies on the optimum manufacturing conditions suitable for this composite cathode indicate that the two parts-molded part and impregnated part-should be made under the following conditions: (1) Molded part; (Ba·Sr)CO3: Ni=3:7 by weight, compressing pressure: 10 ton/cm2, sintering condition: 900°C 10min.+1050°C 10 min. (2) Impregnated part; specific gravity of W: 10_??_12, impregnants: 3 BaO·Al2O3·1/2CaO, etc. Zero field emission at 830°C(Br. ) and the work function of the cathode are 2_??_3 A/cm2 and 1_??_1.1 ev respectively. In D. C. operation while the current of 0.6 A/cm2 was being drawn at 820°C (Br. ), no significant decay of emission was seen for over 10, 000 hrs. This composite cathode is expected to have an extremely long life because the Ba supply to the molded part from the impregnated tungsten is made at a relatively low temperature.