Lifetime and Stability of Carbon Cold Cathodes
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ABSTRACT We present the results of tests measuring the life of carbon cold cathodes and determining what conditions limit the life of the cathode. Cathode life and stability are important for a broad range of applications, including displays, electron sources for satellite thrusters, x-ray tubes and using different gas Cathodes were tested in vacuum chambers microwave devices. environments as well as in sealed and gettered glass envelopes. We measured emission current half-lives of 10,000 - 20,000 hours or more in sealed display devices, depending on operating conditions. The presence of a significant oxygen or water partial pressure degrades the life of the cathode. After removing the gases the decay rate was restored to near the original value. A similar partial pressure of hydrogen gas has little or no effect on the life of the cathode. Initial results of operation in a xenon environment indicate carbon cold cathodes are much more robust compared to microtip cathodes. These results will be discussed with respect to the various applications considered for carbon cold cathodes. INTRODUCTION Cold electron sources are desired for a broad range of applications. These applications include displays, x-ray sources, microwave devices, and satellite thrusters. Although cathode life and stability are important for each of the applications, the cathode operating environment and conditions are different. Displayrs: Generally, the cathode is operated in a evacuated glass envelope sealed with passive getters. Hot filament sources work well for CRT applications, but for flat panel displays with high information content, cold sources are needed. For matrix addressed displays, the cathodes are operated in a pulse mode with very low duty factors - 1% or less. Device lifetimes should be 10,000 - 20,000 hours. Decomposition of phosphor materials and general outgassing coupled with poor getter pumping speeds due to unfavorable device geometries can compromise the life of the cathode. Flicker and non-uniform aging across the display will not be tolerated. X-ray sources and microwave devices: Cold cathodes are desired to achieve greater efficiency, device portability and mechanical durability. Generally, these devices are also evacuated glass and metal tubes with passive getter pumping. Device geometries and materials are much more "vacuum friendly" compared to many display devices. High frequency pulsed or DC operation may be required. Satellite thrusters: The orbits of satellites are maintained and adjusted by electric thrusters mounted on the satellite. Hall or ion thrusters are two types of electric propulsion systems that are commonly used on satellites. The hollow cathodes used to thermionically emit electrons used in these thrusters require propellants and heaters which place lower limits on their 97 0 Mat. Res. Soc. Symp. Proc. Vol. 558 2000 Materials Research Society
size, power scalability and efficiency. Cold cathodes are desired for small thrusters. For a field emission cathode to be compatible with Hall or ion thruster s
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