A new type of photocathode driven by a laser

Abstract

To meet the needs of high-frequency, miniaturized vacuum microwave devices and identify suitable cathode materials and laser systems, a new photocathode for microwave vacuum electronic devices has been studied. Based on our previous work, we have proposed a strategy to develop this new type of cathode. First, a tungsten sponge diffusion barrier layer was used as the evaporation source of the emission material, instead of the traditional nickel tube heating method. This serves as the emission material by heating a layer of the emissive material storage chamber to provide a controlled photoemissive layer for replacing the evaporation loss of the active material, thereby extending the life of the cathode and enabling recovery from poisoning and exposure to the atmosphere. Then, the photocathode with Os and Osnanoparticle films were prepared. This n-type photocathode demonstrated some features that were different from those of traditional photocathodes. To overcome the drawback of chemical methods used to clean the copper, we have proposed a strategy to develop a new method. A tungsten copper alloy substrate was heated by a high-frequency heating coil crucible fired at 1900 K. The copper in the tungsten copper alloy matrix was thereby heated and it evaporated rapidly. Then the copper was deposited onto a condenser. The results show that this technology leaves no residual on the substrate surface, occurs over a short time interval, and produces little environmental pollution. A 50-nm-diameter OS metal nanoparticle film was prepared on the surface of the photocathode substrate by using a DC magnetron sputtering metal target method, and a Cs$_3$Sb photocathode emission experimental diode with a titanium pump was prepared. The degree of vacuum in the tube can reach 5$\\times$10$^{-8}$ Pa. The emission performance of the Os film deposited on the substrate and the Os nanoparticle film photocathode was tested. In the experiment, the maximum laser power values for stable emission were found to be 0.416 and 0.193 W, respectively, and the photoemission current densities were 40.8 and 53.9 mA/cm$^2$, respectively. The quantum efficiencies were calculated to be 1.80$\\times$10$^{-3}$ and 5.13$\\times$10$^{-3}$, respectively. It can be inferred that the light absorption rate of the Os-nanoparticle-film-coated photocathode is a factor of 2.16 higher than that of the Os metal film, and the quantum efficiency is a factor of 2.85 greater. Analysis indicates that the main reason for improvement of the quantum efficiency of the Os nanoparticle film photocathode comes from the improvement of the light absorption rate.