Improvements in MDC and TWT overall efficiency through the application of carbon electrode surfaces

An experimental program was conducted to investigate secondary electron emission losses in multistage depressed collectors (MDC's) in order to determine their effects on the overall traveling-wave tube (TWT) efficiency and identify techniques for minimizing them. Two representative TWT's and several computer-modeled MDC's were used. The experimental techniques used enabled the measurement of both the TWT overall and the collector efficiencies. The TWT-MDC performance was optimized and measured over a wide range of operating conditions using geometrically identical collectors that utilized different electrode surface materials. Comparisons of the performance of copper electrodes to that of various forms of carbon, including pyrolytic and isotropic graphites, were stressed. The results indicate that a very significant improvement in the TWT overall efficiency was obtained in all cases by the use of carbon rather than copper electrodes and that the extent of this efficiency enhancement depended on the characteristics of the TWT, and TWT operating point, the MDC design, and the collector voltages. Ion-textured graphite was found to be particularly effective in minimizing the secondary electron emission losses. Experimental and analytical results, however, indicate that it is at least as important to provide a maximum amount of electrostatic suppression of secondary electrons by proper MDC design. Such suppression, which is obtained by ensuring that a substantial suppressing electric field exists over the regions of the electrodes where most of the current is incident, was found to be very effective. Experimental results indicate that, with proper MDC design and the use of low secondary electron yield carbon electrode surfaces, degradation of the collector efficiency can be limited to a few percent. Consequently, further dramatic improvements in collector efficiency should not be expected if electrode surfaces with significantly lower secondary electron emission yield than those presently available are developed.