An update on the life testing of commerical, high current density impregnated tungsten cathodes is presented. The B-type cathodes, operated at a current density of 2 A/cm2 and a cathode temperature of 1100 C have now been run satisfactorily for more than four years. The M-cathode, at the same current density but at an operating temperature of only 1010 C, have been tested for more than three years. The M-cathodes show no degradation in current over their present operating life whereas the current from the B-cathodes degrade about 6 percent after four years of operation

Elmer, P.

Forman, R.

English

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TEST STUDIES ( NASA) EB41 	 C kTHODE	 $A02/!!F A01
X180-18302
NASA Technical Memorandum 81441
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G3/33 47353
I
THERMIONIC CATHODE LIFE TEST STUDIES
Ralph Forman
Lewis Research Center
Cleveland, Ohio
and
Paul Elmer
W atkins -Johnson
Palo Alto, California
Prepared for the
1980 Tri-Services Cathode Workshop
sponsored by the Department of the Air Force
New York, New York, April 15-17, 1980
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THERMIONIC CATHODE LIFE TEST STUDIES
by Ralph Forman
NASA-Lewis Research Center
Cleveland, Ohio 44135
and
Paul Elmer
Watkins-Johnson
Palo Alto, California
SUMMARY
A NASA-Lewis Research Center Program for life testing commercial, high-
current density impregnated tungsten cathodes has been in progress since
1971. This report is an update of this program first presented at the
Cathode Workshop in Washington in 1978. B-type cathodes, operated at a
current density of 2A/cm2 and a cathode temperature of 1100°C, have now
w
been run satisfactorily for more than four years. M-cathode, at the same
current density but at an operating temperature of only 1010°C, have been
tested for more than three years. The M-cathodes show no degradation in
current over their present operating life whereas the current from the
B-cathodes degrade about 6% after four years of operation.
I
rifornla. The objectives of this continuing program are: 1) to demonstrate
ability of specific cathode types to, produce current densities of 2A/cm2
a minimum design life of 20,E tours of continuous operation without
failure and 2) to competitively evaluate the performance of different cathode
types by endurance testing, while operating under identical electrical, geo-
metrical and vacuum conditions that realistically duplicate the operating
conditions	 present in a transmitter tube. To accomplish the latter
objective, a cathode life test tube was designed which is essentially the
same as a high power microwave tube. 2 The only difference between the cathode
life test unit and a high power microwave tube is that the former uses a solid
metal drift tube in place of the rf interaction circuit employed in the "al.-ter.
Five different cathode types have been evaluated on this program. They
are the "B", "S", "Tungstate," "M" and Litton type "B" cathode. The "B" type
is an impregnated tungsten cathode originally developed by Philip Metalonics.3,4
Its distinguishing features are that it uses a 5:3:2 (Ba0:Ca0:Al 203) impregnant
mixture and the density of the porous tungsten base is about 80%. The "S"
cathode is an impregnated tungsten cathode manufactured by Semicon Associates.5
Its unique features are that it employs a 4:1:1 (Ba0:Ca0:Al 203) impregnant
mixture with a porous tungsten base density of about 82%. The "Tungstate"
F3
E
cathode is a type developed at General Electric Company 6, ^ and consists of
a mixture of approximately 90% tungsten, 9% tungstate compound Ba 5Sr (WO 6 ) 2
i	 and 1% ZrH2 , which is pressed and sintered into a matrix at high temperature.
The "M" cathode was orig;nally developed by Philips, 8 as a standard impreg-
nated "B" type cathode with a sputter deposited coating of Osmium and Ruthenium.
As a result, the work function of the surface is lowered so that operation at
lower temperatures is possible. The Litton type "B" cathode uses the 5:3:2
(BaO:CaO:Al 203) impregnant mix and was fabricated by Litton for the CTS 200
watt 12.2 GHz tube program. They were installed on life test in the period
1975-1976, a time corresponding to the CTS satellite launch period.
Life test data on the first four cathode types, covering the period up
to the early part of 1978, have been published. 2 They showed the Tungstate
cathodes failed within 7000 hours and the "S" cathodes were all removed from
test after about 20,000 hours or less. The "B" and "M" cathode types were
still running satisfactorily after about 30,000 hours and 20,000 hours respec-
tively. "B" and "M" type cathodes have continued to be tested and the results
as of December 1979 are illustrated in Fig. 1. The older "S" type information
is included in the figure. The "B" type cathode has been run for more than
40,000 hours and shows steady degradation. It is down 6% whereas we have set
10% as the failure point. The "M" cathode has been tested for 30,000 hours
and still shows no degradation. The unusual feature of the "M" cathode data
is the fact that initially its performance seems to improve with time. This
was characteristic of all the "M" cathodes tested. All the other cathode types
which have been tested showed degradation with time.
Fig. 2 illustrates the reproducability of the data obtained from the
4"B" cathodes designated "P", tested on this program. The "S" and "M" curves
of Fig. 1 are shown in this figure for reference. Cathode P-4 and P -6 have
been operated for more than four years. Cathode P-7 failed in about three
years. Cathode P-5 was operating satisfactorily for more than three years
life when an accident occurred and the tube containing this cathode developed
a vacuum leak. It is very encouraging to note that, even though the number
of samples were limited to four, individual results are well correlated with
each other and are different than those obtained for the "S" and "M" cathodes.
Fig. 3 illustrates the data obtained on the Litton "B" type cathodes,
designated L. Included in the figure are the "M", "B" and "S" cathode curves
of Fig. 1. The life test studies are continuing on these cathodes and the
results correlate quite well with the "B" type cathode data; an indication
that the impregnant mix does control the life characteristics of the tungsten
impregnated cathode.
The a,-)st interesting data in this series are those for the "M" type
cathodes. Fig. 4 illustrate the data obtained on the four "Vi" cathodes tested.
Initially they all showed an increase in emission as is illustrated in t ,-
figure. To obtaih a better understanding of what this means, it is nece; --drl)
to describe the procedure used for setting the parameters initially in Che
experiment. After activation and initial processing the anode voltage on the
cathode life test vehicle is adjusted (approximately 10 W to give a space
charge limited anode current of 0.620 A. With a cathode area of 0.3" cr „2,
this corresponds to 2A/cm 2 . The tube is then put on life test and the current
at this initial anode voltage is measured as a function of time. All the other
cathodes showed steady degradation but the M-cathodes all showed an increase
in current. This is an unexpected phenomenon because the current is adjusted
to be space charge limited. Since anode voltage and geometrical spacing remain
5constant with time during life, the cnly other possible variable is surface
area of emission or surface coverage of active material. One possible ex-
planation for the phenomenon is that with M-cathodes the surface coverage
of active material (Ba on Os or some compound formation between Os and Ba),
responsible for copious emission, increases early in life with stabilization
taking place in about 5,000 hours. In the case of the "B" and "S" type
cathodes a similar explanation  has been offered to explain the reverse effect
of decreasing current with time by assuming the partial monolayer coverage
decreases with time.
II
6REFERENCES
1. D. H. Smith, "Long Life Endurance Testing of Cathode Assemblies
for Use in Microwave High Power Tubes," Cathode Workshop,
Washington, D.C., January 31, 1978.
2. R. Forman and D. H. Smith, "Thermionic Cathode Life-Test Studies,"
IEEE Transactions on Electron Devices, Vol. ED-26, pp 1567
3. "The Philips Impregnated Cathode," Philips Metalonics Technical
Bulletin, PM 101-IM-MV-863-205.
4. R. Levi, "Improved 'Impregnated Cathode'," J. Appl. Phys., Vol. 26,
p. 639, 1955.
5. "Semicon Dispenser Cathodes," Semico: Associates Technical Bulletin,
April 26, 1967.
6. R. J. Bondley, "Longer Life or Higher Power from High Emission Cath-
odes," Space/Aeronautics, pp. 85-89, December 1965.
7. R. J. Bondley, W. T. Boyd, R. G. Lock, T. J. Nall, and M. J. Slivka,
"High Current Density Cathodes," General Electric Co., Schenectady,
NY, January 1969. Army Electronics Command, Fort Monmouth, NJ,
ECOM-2289-F. AD-850447.
8. P. Zalm, and A. J. A. van Stratum, "Osmium Dispenser Cathodes,"
Philips Tech. Rev., Vol. 27, pp. 69-75, 1966.
i
61
CATHODE CURRENT AT REF
VOLTAGE, A
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CATHODE CURREN T AT REF
VOLTAGE, A
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. 68
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0	 10 000	 20 000	 30 000	 40 000
LIFE TEST HOURS, hr
Figure 3. - Cathode current at fixed anode referei ice voltage versus time for three
Litton type "lt" cathodes.
.68
.48
0 10 000	 20 000	 30 000	 40 000
LIFE TEST HOURS, hr
Figure 4. - Cathode current at fixed anode reference voltage versus time for four
"M" type cathodes.
II
__	 __..	 -_
1. Report No. 2. Government Aron No. 3. RKiptont's Catalog No.
NASA TM-81441
4. Title W4 Subtitle 5. Report ate
THERMIONIC CATHODE LIFE TEST STUDIES
6. Performing Organization Code
7. Author(s) s. Performi.ig Orponimion Report No
Ralph Forman and Paul Elmer E-317
10. Work Unit No.
9. Performing Orgeniaation Name and Addraaa
National Aeronautics and Space Administration 11. Contract or Grant No.
Lewis Research Center
Cleveland, Ohio 44135 13. Type of Report and Period Covered
Technical Memorandum12. Spomoring Agency Name and Addrwa
National Aeronautics and Space Admia:stratiou 14, Sponsoring Apvicy code
Washington, D.C. 20546
15, SupplenMntary Notes
Ralph Forman, Lewis Research Center; Paul Elmer, Watkins-Johnson, Palo Alto,
California.
15. Abstract
A NASA-Lewis Research Center Program for life testing commercial, high-current density
impregnated tungsten cathodes has been in progress since 1971. This report is an update of
this program first presented at the Cathode Workshop in Washington in 1578.
	
B-type cathodes,
operated at a current density of 2 A/cm 2 and a cathode temperature of 11000 C, have now	 j
been run satisfactorily for more than four years.	 M-cathode, at the same current density
but at an operating temperature of only 10100 C, have been tested for more than three years.
The M-cathodes show no degradation in current over their present operating life whereas the
current from the B-cathodes degrade about 6 percent after four years of operation.
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17. Key Words (Suggested by Author(s)) 1s. Distribution statement
Traveling wave tubes Unclassified - unlimited
Cathodes STAR Category 33
Life tests
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19. seewity Claaif. lot this report) 20	 Security Cusvf. It 	 this pope)	 21	 No. of Pages
	 22
Unclassified Unclassified
' fol sale by the National Technical Infolmation Service Spline f ,cc V rZ n ,j .111b1


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