Refractory-metal heater base wound with a thin refractory metal wire increases the longevity of oxide-coated cathodes. The wire-wound unit is impregnated with the required thickness of metal oxide. This cathode is useful in magnetohydrodynamic systems and in electron tubes

Kerslake, W.

Vargo, D.

English

NASA Technical Reports Server

February 1965	 Brief 65-10032 
NASA TECH BRIEF 
-it 
NASA Tech Briefs are issued by the Technology Utilization Division to summarize specific 
technical innovations derived from the space program. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical information, Springfield, Virginia, 22151. 
Wire Winding Increases Lifetime of Oxide-Coated Cathodes 
Tantalum Ribbon
	 I 
Ribbon Plus Wire Bent 
Into Hairpin Shape
CI 
The problem: Increasing the lifetime of cathodes 
used in rocket ion engines. Tests on a number of 
different cathodes under development for use in rocket 
ion engines showed that oxide-coated cathodes are the 
most efficient producers of electrons. A cathode in 
such an engine must be sufficiently hot to emit elec-
trons and hence will lose material by evaporation. The 
cathode will also be coupled to or immersed in the 
plasma discharge and it will therefore also lose mate-
rial because of sputtering erosion due to positive-ion 
bombardment. The oxide coatings must therefore be 
sufficiently thick to maintain a source of active 
material over relatively long periods of time. The 
thickness of such coatings is limited, however, by the 
ohmic resistance of the oxide material, which when 
subjected to the emission current, causes overheating 
and destruction of the cathode. Research was there-
fore required to find a means of reducing the resis-
tance of the thick coatings and thereby increasing the 
lifetime of the cathodes. 
The solution: Winding the refractory-metal heater 
base of the cathode with a thin refractory-metal 
wire and impregnating the wire-wound unit with the 
required thickness of metal oxide.
,Tungsten Wire (100 Turns) 
trunurrturuturrr, 
2. 
Finished Cathode Coated 
With BaO
4. 
How it's done: The heater base, consisting of a 
0.013- by 0.36- by 5.1-centimeter tantalum ribbon is 
wound with approximately 100 turns of 0.007-centi-
meter tungsten wire. The wire-wound ribbon is then 
bent into a hairpin shape, and its entire surface is 
coated with a paste consisting of barium carbonate in 
water or in a solution of nitrocellulose and butyl 
acetate. The cathode is conditioned and partially 
activated by heating it in steps to approximately 
1300° K and holding this temperature for 5 to 20 
minutes without a discharge. During the heating the 
water and organic constituents of the paste are either 
evaporated or decomposed, and the barium carbonate 
is converted to barium oxide. Final activation of the 
cathode is accomplished by exposing it to the ion-
chamber discharge. 
Notes: 
1. Suggested applications for this cathode are in mag-
netohydrodynamic systems and in high-power, 
long-life electron tubes. 
2. Further information concerning this invention is 
described in AIAA Paper No. 64-683, "Cathode 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States Govern-
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes 
any warranty or representation, express or implied, with respect to the 
accuracy, completeness, or usefulness of the information contained in
this document, or that the use of any information, apparatus, method, 
or process disclosed in this document may not infringe privately-owned 
rights; or B. Assumes any liabilities with respect to the use of, or for 
damages resulting from the use of, any information, apparatus, method, 
or process disclosed in this document. 
https://ntrs.nasa.gov/search.jsp?R=19650000032 2020-03-11T17:17:13+00:00Z
Durability in the Mercury Electron-Bombardment 
Ion Thrustor," by William R. Kerslake, August 
31-September 2, 1964. Inquiries may also be di-
rected to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio, 44135 
Reference: B65-10032
Patent status: NASA encourages the immediate 
commercial use of this invention. Inquiries about 
obtaining rights for it commercial use may be made 
to NASA, Code AGP, Washington, D.C., 20546. 
Source: Don Vargo and William Kerslake 
(Lewis 154) 
Brief 65-10032	 Category No. 03


Wire winding increases lifetime of oxide coated cathodes

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