Secondary Electron Suppression with Carbonized Aerogels

GRC has been a leader in developing technology to increase the efficiency of Traveling Wave Tubes (TWTs) for Radio Frequency (RF) communications. TWTs are used by most space missions for communications and radio science at microwave frequencies. A major method to improve the efficiency of TWTs is to recover the remaining energy in the spent electron beam in an electron collector. GRC has been developing ways to achieve this for many years, both by developing CAD techniques for electron collector design, and by developing materials to reduce the secondary electron emission which can reduce the ability to recover the electron beam energy. The use of graphite for electron beam collector surfaces was developed at GRC, and is used on many NASA missions, including Kepler, the Lunar Reconnaissance Orbiter, and the SCaN Testbed. Texturing the collecting surface has been demonstrated to reduce secondary electron emission, and was used on the Cassini Ka-band TWT copper collector, which had similar performance to untextured graphite. However, textured graphite surfaces lacked the robustness needed to survive launch vibrations. Carbonized polyimide aerogels promise the necessary robustness along with reduced secondary electron emission, allowing further improvement of TWT efficiency, achieving a very good return on the low investment made. The goal is to reduce the DC power required for high data rate RF communications by increasing TWT efficiency through the improvement of energy recovery from the spent electron beam.

When graphite replaced copper for electron collector surfaces, a considerable increase in TWT efficiency was obtained. Replacing the state-of-the-art graphite collector surfaces with a robust lower secondary electron emission surface will further improve TWT efficiency. This has the potential to enhance many new NASA missions, since most plan on using TWTs as high power RF amplifiers for high data rate communications. Even in 1990 saving one watt of DC power was considered to be worth three to four thousand dollars for a LEO mission, and a million dollars for a deep space mission. Beyond NASA's interests, there are several commercial applications where the proposed efforts will have considerable impact both in terms of efficiency and cost. First, communication satellites may carry over a hundred TWTs, and would obtain considerable energy savings. Second, these electron collector materials could also be used on the ground to reduce the power consumption of the tubes used by TV broadcasters and in the klystrons used in scientific accelerators.

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