Fifty-megawatt high-power klystrons in the KEK electron linac have used a larger diameter cathode of 85 mm. Stable operation requires a careful manufacturing process of the cathode and a low-gradient gun design. Therefore, the klystron manufacturing, including a cathode processing, has been improved so as to obtain stable performance. This paper presents the klystron improvements associated with using the BI cathode and klystron operation statistics for the past 17 years.

This paper describes the investigation and application of scandate cathodes in Beijing Vacuum Electronics Research Institute (BVERI). The data of emission performance and lifetime were acquired both in closed-spaced diodes and in microwave tubes. Evaporation rates and gas poisoning properties have been determined. From the results of a series of diode experiments and the feedback information of practical usage in tubes, there are three key factors that in our opinion limit the practical application of scandate cathodes in general. These are no obvious saturation onset of I/U characteristics, weak ion bombardment resistivity and non-uniform emission. The authors conclude that to a certain extent, impregnated scandate cathodes (I-Sc) can be used as alternative electron sources instead of M-type cathode in microwave tubes.

In this paper, scandate cathodes with scandia doped tungsten matrices are studied. Attempts are made to clarify some interesting phenomena presented in the whole process. The sub-micron porous tungsten matrix with uniformly distributed Sc₂O₃ was formed by quasi-spherical scandia doped W particles. During impregnation the reaction between Sc₂O₃ and barium aluminates impregnant results in the formation of compounds, preferentially of Ba₂ScAlO₅. But after the subsequent ultrasonic water cleaning, Ba₂ScAlO₅ is dissolved and mostly removed from the surface region. In the activation process, Ba and Sc re-aggregate to the surface. A uniformly distributed Sc and Ba layer on surface of tungsten substrate after proper activation has been verified by the results of high resolution scanning Auger microscopy (SAM) and Auger electron spectroscopy (AES) depth profile. For such cathodes typical emission of more than 30 A/cm² space charge limited current density at 850 °C_b has been achieved. However, the uniform distribution of Sc and Ba on surface still seems not be able to remedy the abnormal Schottky effect. We propose that a surface multi-layer containing Ba, Sc and O, formed after proper activation on the W base, is the reason for copious emission and abnormal Schottky effect of such cathodes.

The paper describes the preparation and emission property of scandia and Re doped tungsten matrix impregnated cathode. By an easy and reproducible way, solid–liquid doping combined with two-step reduction, powders of tungsten particles covered with scandium oxide were obtained. Compared with scandia mixed tungsten powders prepared by mechanically mixing, scandia and rhenium doped tungsten powders had smaller particle size, for example, scandia (3 wt%) and Re (5 wt%) doped tungsten powders had the average size of about 50 nm in diameter. Based on this kind of powder, scandia and Re doped tungsten matrix with the sub-micrometer sized tungsten grains and a more uniform distribution of Sc₂O₃ were obtained in this paper. Scandia and Re doped tungsten matrix impregnated cathode had shown excellent emission property and good emission uniformity. The space charge limited current densities of more than 58A/cm² at 900 °C_b could be obtained and the work function of this cathode was as low as 1.18 eV.

Electron emission and surface properties of the Sc₂O₃ mixed matrix impregnated cathode coated with Ir, Os, Pt and Mo are studied. The cathode is composed of porous body of a metal matrix (5 wt% Sc₂O₃ in W) and an impregnant of 4BaO·CaO·Al₂₃, on which metal films of about 500 nm are evaporated. Sc atoms do not appear on the film surface even after prolonged heating at 1150°C, and the coated cathodes do not show the emission anomaly usually seen for Sc₂O₃ mixed matrix cathodes. The Mo film, however, alloys easily with W in a short heat treatment time and the Sc atoms are distributed nonuniformly over the surface. The Mo coated cathode does show the emission anomaly. A simple coating with Sc film on the ordinary W matrix cathode reveals neither the surface properties nor the emission properties of Sc₂O₃ mixed matrix cathodes.

Scandia doped tungsten powders were prepared by spray drying combined with two-step hydrogen reduction. The particle size of doped tungsten powder, powder morphology and doped tungsten matrix were characterized by scanning electron microscope, X-ray diffraction and laser diffraction particle size analyzer, respectively. The reduction behavior of Sc₂O₃ doped tungsten oxide and the effect of Sc₂O₃ on the property of tungsten powder were studied by the temperature programmed reduction. The experimental results showed that the precursor powders prepared by spray drying had spherical shape. The addition of Sc₂O₃ could decrease the reduction temperature of tungsten oxide. The scandia doped tungsten powder had sub-micrometer size in the range of 0.1 to1 μm and scandium distributed evenly in the powder. By using this kind powder, sub-microstructure cathode matrices with semispherical grains and homogenous distribution of scandium were obtained.

Scandia pressed cathodes were prepared by powder metallurgy method using mixed powder of scandia doped tungsten and barium-calcium aluminates obtained by different mixing methods. The element distribution uniformity greatly affected the emission property. The powder prepared by ball-milling had small particle size and uniform distribution of different elements. This kind of powder was favorable for the chemical reaction among Ba, Ca aluminates, scandia and tungsten to take place sufficiently to form active substance which can improve the emission property. The emission current density of the cathode prepared with ball-milling powder was about 8 times higher than that prepared by manually mixing method, 4 times higher than Os coated M-type cathode. The higher content of the active substance on the cathode surface prepared by ball-milling led to the better emission performance of the cathode.