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Results for “THERMIONIC CATHODE”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

X-ray Diffraction Studies of the Structure and Thermochemistry of Alkaline-Earth Oxide-Coated Thermionic Cathodes

NASA LeRC has a broad, active cathode technology development program in which both experimental and theoretical studies are being employed to further development of thermionic cathodes for use as electron sources in vacuum devices for communications and other space applications. One important type of thermionic cathode under development is the alkaline-earth oxide-coated (BaO, SrO, CaO) cathode. Significant improvements in the emission characteristics of this cathode have been obtained through modification of the chemical composition and morphology of the oxide coating, with the best result thus far coming from the addition of In2O3 and Sc2O3. Whereas the In2O3 produces a finer, more uniform particle structure, the exact chemical state and role of the Sc2O3 in the emission enhancement is unknown. The purpose of this cooperative agreement is to combine the studies of the surface chemistry and electron emission at NASA LeRC of chemically modified oxide coatings with a study of the thermochemistry and crystal structure using X-ray diffraction equipment and expertise at Clark Atlanta University (CAU). The study at CAU is intended to provide the description and understanding of the structure and thermochemistry needed for further improvement and optimization of the modified coatings. A description of the experimental procedure, preliminary X-ray diffraction test results, together with the design of an ultrahigh vacuum chamber necessary for high temperature thermochemistry studies will be presented.

Karikari, E. K.↗
NASA NTRS2002

Power-Efficient, High-Current-Density, Long-Life Thermionic Cathode Developed for Microwave Amplifier Applications

A power-efficient, miniature, easily manufactured, reservoir-type barium-dispenser thermionic cathode has been developed that offers the significant advantages of simultaneous high electron-emission current density (>2 A/sq cm) and very long life (>100,000 hr of continuous operation) when compared with the commonly used impregnated-type barium-dispenser cathodes. Important applications of this cathode are a wide variety of microwave and millimeter-wave vacuum electronic devices, where high output power and reliability (long life) are essential. We also expect it to enable the practical development of higher purveyance electron guns for lower voltage and more reliable device operation. The low cathode heater power and reduced size and mass are expected to be particularly beneficial in traveling-wave-tube amplifiers (TWTA's) for space communications, where future NASA mission requirements include smaller onboard spacecraft systems, higher data transmission rates (high frequency and output power) and greater electrical efficiency.

Wintucky, Edwin G.↗
NASA NTRS1979

Thermionic cathode life-test studies

A NASA-Lewis Research Center program for life testing commercial, high-current-density thermionic cathodes has been in progress since 1971. The purpose of the program is to develop long-life power microwave tubes for space communications. Four commercial-type cathodes are being evaluated in this investigation. They are the 'Tungstate', 'S' type, 'B' type, and 'M' type cathodes, all of which are capable of delivering 1 A/ sq cm or more of emission current at an operating temperature in the range of 1000-1100 C. The life test vehicles used in these studies are similar in construction to that of a high-power microwave tube and employ a high-convergence electron-gun structure; in contrast to earlier studies that used close-space diodes. These guns were designed for operation at 2 A/sq cm of cathode loading. The 'Tungstate' cathodes failed at 700 h or less and the 'S' cathode exhibited a lifetime of about 20,000 h. One 'B' cathode has failed after 27,000 h, the remaining units continuing to operate after up to 30,000 h. Only limited data are now available for the 'M' cathode, because only one has been operated for as long as 19,000 h. However, the preliminary results indicate the emission current from the 'M' cathode is more stable than the 'B' cathode and that it can be operated at a true temperature approximately 100 C lower than for the 'B' cathode.

Forman, R.↗
NASA NTRS1999

Novel Low-Cost, Low-Power Miniature Thermionic Cathode Developed for Microwave/Millimeter Wave Tube and Cathode Ray Tube Applications

A low cost, small size and mass, low heater power, durable high-performance barium dispenser thermionic cathode has been developed that offers significant advancements in the design, manufacture, and performance of the electron sources used in vacuum electronic devices--such as microwave (and millimeter wave) traveling-wave tubes (TWT's)--and in display devices such as high-brightness, high-resolution cathode ray tubes (CRT's). The lower cathode heater power and the reduced size and mass of the new cathode are expected to be especially beneficial in TWT's for deep space communications, where future missions are requiring smaller spacecraft, higher data transfer rates (higher frequencies and radiofrequency output power), and greater electrical efficiency. Also expected to benefit are TWT's for commercial and government communication satellites, for both low and geosynchronous Earth orbit, with additional benefits offered by lower cost and potentially higher cathode current loading. A particularly important TWT application is in the microwave power module (MPM), which is a hybrid microwave (or millimeter wave) amplifier consisting of a low-noise solid state driver, a vacuum power booster (small TWT), and an electronic power conditioner integrated into a single compact package. The attributes of compactness and potentially high electrical efficiency make the MPM very attractive for many commercial and government (civilian and defense) applications in communication and radar systems. The MPM is already finding application in defense electronic systems and is under development by NASA for deep space communications. However, for the MPM to become competitive and commercially successful, a major reduction in cost must be achieved.

Wintucky, Edwin G.↗
NASA NTRS1976

Surface studies of thermionic cathodes and the mechanism of operation of an impregnated tungsten cathode

The surface properties of conventional impregnated cathodes were investigated by the use of Auger spectroscopy and work function measurements, and these were compared with a synthesized barium or barium oxide coated tungsten surface. The barium and barium oxide coated surfaces were prepared by evaporating barium onto a tungsten surface that can be heated to elevated temperatures. Multilayer or monolayer coverages can be investigated using this technique. The results of this study show that the surface of an impregnated tungsten cathode is identical to that observed for a synthesized monolayer or partial monolayer of barium on partially oxidized tungsten, using the criteria of identical Auger patterns and work functions. Desorption measurements of barium from a tungsten surface were also made. These results in conjunction with Auger and work function data were interpreted to show that throughout most of its life an impregnated cathode operating in the range of 1100 C has a partial monolayer rather than a monolayer of barium on its surface.

Forman, R.↗