Effects of CsI Coating of Carbon Fiber Cathodes on the Microwave Emission From a Triode Virtual Cathode Oscillator

We discuss the effects of cesium iodide (CsI) coating of carbon fiber cathodes on the microwave emission from a triode virtual cathode oscillator. As compared with the uncoated cathode, the CsI-coated carbon fiber cathode significantly improved the diode performance and, most notably, lengthened the microwave pulse, from 150 to 200 ns. The light emission from the diode, the diode perveance, and the diode gap change were introduced to explain the observed extension of microwave pulse. After CsI coating, the carbon fiber cathode exhibited the absence of strong plasma, a slow plasma expansion velocity, and an almost unchanged diode gap during the main voltage pulse. It was found that heavy plasma ions, slow plasma expansion velocities, and long microwave pulses tend to be closely tied. These results show that, given a proper diode design, the carbon fiber cathodes with CsI coating have great promise for generating long-pulse microwave radiation.