Experiments, evaluating the performance of low-pressure thermionic converters, were conducted with cesium, potassium, and sodium-metal vapors. The results of the investigation are useful in the selection of favorable conditions for the design of thermionic reactor fuel elements, including RF output for special applications

Richards, H. K.

English

NASA Technical Reports Server

April 1969	 Brief 69-10090 
•
ICY C
AEC-NASA TECH BRIEF 
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
-	 the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Performance of Low-Pressure Thermionic Converters Is Evaluated 
A report describing a series of experiments to eval-
uate the performance of low-pressure thermionic con-
verters has been published. The results of the investi-
gation should help in the selection of favorable 
conditions for the design of thermionic reactor fuel 
elements, including rf output, if desired, for special 
applications. 
The experiments with a low-pressure thermionic 
converter were conducted with cesium, potassium, 
and sodium-metal vapors. These metals provided for 
the neutralization of the electron space charge and the 
reduction of the collector work function. Although 
cesium is usually used as an ion producer, the results 
for potassium were very good. Even sodium vapor pro-
duced a relatively high power output, though less than 
cesium and potassium. Since the work functions of 
potassium and sodium are higher than that of cesium, 
they would permit a higher collector temperature 
without excessive electron back emission, a factor of 
importance for space application where heat rejection 
becomes a major problem. 
A tantalum emitter was brought to the desired tem-
perature by electron bombardment. A molybdenum 
collector and guard ring were situated opposite the 
collector. The emitter-collector distance could be 
varied several millimeters by means of a micrometer 
control. Although most experiments were limited to 
low pressures of 10 -5 to iO_2 mm Hg, some results 
were obtained at pressures up to 1 mm Hg. The high-
pressure data included ignited modes. 
The investigation included the dc electron current, 
output voltage, and power as a function of emitter
temperature, vapor pressure, emitter - collector dis-
tance, and effect of the different alkali metals. Cov-
ered in the investigation was the production of high-
frequency oscillations (rf). The frequency, rms voltage, 
and power output were reported in detail as functions 
of the dc collector voltage, emitter temperature, 
emitter - collector distance, vapor pressure, and alkali 
metal. in addition, a dc- rf interaction was observed 
and analyzed. For this purpose, the rf and dc sections 
were separated by capacitance- inductance net-
works. Photos from an oscilloscope showed the ampli-
tudes and wave shapes of the rf output. 
The results were also discussed in relation to their 
applications for the design of thermionic reactor fuel 
elements. Special consideration was given to the rela-
tive merits of different alkali metals for neutralization 
of the space charge and reduction of the collector 
work function. 
Notes: 
I. The report is published in "Experimental investiga-
tion of Low-Pressure Alkali-Metal Thermionic 
Converters for DC and RF Power," ANL-7377; 
Argonne National Laboratory, September 1967. 
This report is available from the Clearinghouse 
for Federal Scientific and Technical Information, 
Springfield, Va. 22151; price: $3.00 (microfiche 
$0.65). 
2. This information may be of interest to persons and 
organizations concerned with the design and de-
velopment of thermionic converters.
(continued overleul) 
This document was prepared under the sponsorship of the Atomic 
Energy Commission and/or the National Aeronautics and Space 
Administration. Neither the United States Government nor any per-
son acting on behalf of the United States Government assumes any
liability resulting from the use of the information contained in this 
document, or warrants that the use of any information, apparatus, 
method, or process disclosed in this document may not infringe pri-
vately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000090 2020-03-16T17:51:59+00:00Z
3. Inquiries concerning this report may be directed 
to:
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B69-10090 
Source H. K. Richards
Reactor Physics Division 
(A RG- 10276)
Patent status: 
Inquiries about obtaining rights for commercial use 
of this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439 
Brief 69-10090
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Performance of low-pressure thermionic converters is evaluated

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