Carbon and carbon-coated electrodes for multistage depressed collectors for electron-beam devices—A technology review

The efficiency of multistage depressed collectors (MDC's) for electron-beam amplifier tubes is strongly affected by the secondary electron emission characteristics of the collector electrode surfaces. Specifically, to recover the maximum kinetic energy from the tube's spent electron beam and therefore to achieve high collector efficiency, the electrode surfaces must have low secondary electron emission properties. Copper, the most commonly used MDC electrode material, has relatively high emission properties. While several methods for reducing these emission properties have been applied with some success, graphite or carbon electrodes are currently being considered as much more promising electrode materials. Furthermore, experimental studies have demonstrated that the already low emission properties of these carbon forms can be reduced to the lowest levels observed thus far by ion texturing their surfaces under the proper conditions. Recently, a method of applying a highly textured carbon film on a copper substrate was developed that has a particularly significant potential for the MDC electrode application. This paper reviews important aspects of the carbon and carbon-coated MDC electrode technology developed at the NASA Lewis Research Center as well as related technology developed under NASA sponsorship. The following topics are included: electrode material properties, surface treatment procedures, surface physical characteristics, secondary electron emission measurements, and MDC fabrication methods. The performance of MDC's with untreated graphite electrodes, ion-textured graphite electrodes, and textured, carbon-coated copper electrodes is also discussed.