Cesium-filled thermionic energy converters are considered as electrical energy sources in future spacecraft requiring tens to hundreds of kilowatts of electric power. The high operating temperatures necessary for a large specific power and high efficiency impose stringent constraints on the converter fabrication. The converter physics for reducing operating temperatures and cesium plasma losses are being studied to achieve high reliability without sacrificing the power performance of the converters. Various cesium parameters which affect the converter performance are: (1) electron temperatures, (2) plasma ion densities, and (3) electric potential profiles. These were investigated using a Langmuir probe in a simulated converter. The parameters were measured in different cesium discharge modes

Shimada, K.

English

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N A T l O N A L  A E R O N A U T I C S  A N D  S P A C E  A D M I N I S T R A T I O N  
Technical Memorandum 33-551 
Probe Measurements of a Cesium Plasma in a 
Simulated lhermionic Energy Converter 
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NATIONAL AERONAUTICS AND S P A C E  ADMINISTRATION 
Technical Memorandum 33-551 
Probe Measurements of a Cesium Plasma in a 
Simulated Thermionic Energy Con verter 
K. Shi'mada 
J E T  P R O P U L S I O N  L A B O R A T O R Y  
C A t l f O R N I A  I N S t l T U T l t  O f  T E C N N 0 1 0 0 Y  
P A S A P E H A ,  C A L I f O R N I A  
?RECEDING PAGE BLANK NOT FIUIL 
PREFACE 
The work described in this report was performed by the Guidance and 
Control Division of the Jet Propulsion Laboratory. 
JPL Technical Memorandum 33-55 1 iii 

ABSTRACT 
Cesium-filled thermionic energy corlverters are  being considered as  
candidate electrical energy sources in fuSAre spacecraft requiring tens to 
hundreds of kilowatts of electric power. The high operating temperatures 
necessary for a large specific power and high efficiency inevitably impose 
stringent constraints on the converter fabrication to achieve the desired 
reliability of the power system. The converter physics for reducing oper- 
ating temperatures and cesium plasma losses a r e  being studied to achieve 
high reliability without sacrificing the power performance of the converter s. 
Various cesium parameters which affect the converter performance are: 
(1) electron temperatures, (2)  plasma ion densities, and (3) electric potential 
profiles. These were investigated using a Langmuir probe in a simulated 
converter. The parameters were measured in different cesium discharge 
modes. 
JPL Technical Memorandum 33 -55 1 
IN  TR ODU C TION 
To increase the reliability and to relax the engineering constraints so 
that the practical converters can be more readily fabricated, a reduction of 
operating temperatures i s  being considered. An investigation of a new mode 
of operation or  a new improved converter design i s  essential to meet the 
high reliability and performance of a low-temperature converter. 
In this article results of recent investigations of a cesium plasma using 
a Langmuir probe in a simulated thermionic converter a r e  presented. The 
results include: (1) electron temperatures, (2) plasma ion densities, and 
(3) electric potential profiles in various discharge modes. 
JPL Technical Memorandtim 33-55 1 
11. TEST VEHICLE 
The diode used for probing the cesium plasma i s  a simulated thermi- 
onic energy converter with parallel plane electrode geometry. This device 
differs from a practical converter in that it has: (1) a sapphire window, 
( 2 )  a large interelectrode distance D, which Is 5 . 2  mm and 2. 1 mm during 
the measurements for cesium temperatures up to 444 OK and to 533 "K 
respectively, and (3) a movable Langmuir probe. The 1 -mm-diam probe 
wire, similar to that of Bullis (Ref. l ) ,  protrudes through a hole in the col- 
lector at i ts  center. A micrometer mechanism makes i t  possible to move 
the probe across the entire interelectrode spacing between the collector and 
2 
the emitter. The active area of the probe (0. 0078 cm ) is parallel to the 
2 
collector. The active area of the collector i s  1. 25 cm ; hence, the probe 
occupies 11160 of the current-collecting area of the diode. The probe area 
i s  the exposed cross section of a tantalum wire. For a case with D = 5.2 mm, 
the cylindrical side of the probe was insulated by tantalum oxide, 0.005 cm 
thick, formed directly on the wire by a thermal oxidation process. In con- 
trast, the probe was insulated with a layer of plasma- sprayed alumina for a 
case with D = 2. 1 mm. The probe assembly i s  attached to the tube envelope 
through stainless steel bellows and the probe may be moved by a micrometer 
screw with res?ect to the tube envelope where the collector i s  mounted. 
During the measurements, the diode envelope was heated by electric 
heating tapes. The cesium reservoir was kept a t  least 50°K below the tube 
envelope temperature so that the cesium pressure could be determined by 
the cesium reservoir temperature. 
JPL Technical Memorandum 33 -55 1 
111. MEASUREMENTS 
The plasma parameters were measured with a circuit (F ig ,  1) consist- 
ing of: (1) the diode loop for driving it a s  a cesium gas discharge tube, and 
(2)  the probe loop for measuring the probe current flowing in the probe- 
collector circuit through a variable probe t i as ,  The probe characteristic s 
were displayed on an X-Y recorder a s  the probe potential was swept between 
-6 V and +1  V with respect to the collector, , A family of characteristic 
curves was obtained for different locations of the probe, and each family 
was taken for different discharge modes of the diode. Depending upon the 
diode current, the conduction occurred in four different modes of cesium 
discharge: (1) extinguished mode, (2)  anode glow mode, (3) ball-of-fire 
mode, and (4) plasma mode. The f i rs t  two modes occur at low currents 
before the cesium vapor breaks down and the remaining modes, which a r e  
accompanied by the characteristic plasma column in the interelectrode space, 
occur the reafter. The diode operations were further defined by measuring 
the emitter temperature TE and the cesium reservoir temperature TR. The 
emitter temperature was measured through a sapphire window using an 
optical pyrometer at the blackbody hole in the emitter. 
The location of the probe tip was measured from the plane of the col- 
lector by noting the turns of the micrometer screw required to protrude the 
probe into the interelectrode space. Thus, the probe tip i s  flush with the 
collector surface, showing no protrusion, when d = 0, and the tip i s  
1.04 mm from the collector when d = 1. From the measured probe charac- 
teristic s ,  the temperatures of electrons, plasma ion densities, and the 
potential profiles in the interelectrode gap were determined. 
Also, the visual observations of the cesium discharge a r e  incorporated 
with the analysis to obtain comprehensive interpretations of these results. 
JPL Technical Memorandum 33 -55 1 
RESULTS 
To investigate various modes of cesium discharge, the diode was 
initially operated at relatively low temperatures at which the diode exhibits 
all modes, depending upon the magnitude of current. A volt-ampere curve 
shown in Fig. 2 was obtained with TE = 1173OK and TR = 404OK. This 
curve exhibits a break at  a voltage where the cesium vapor ignites. The 
volt-ampere curve i s  divided into two regions: the lower-c:wrent unignited 
region, and the higher -current ignited region. Within the unigni ted region, 
the cesium discharge occurs in two modes, i. e. , the extinguished mode and 
the anode glow mode, depending upon the diode current. The anode glow 
mode i s  accompanied by a rapid increase of the diode current a s  a result of 
an increased electron-sp~ce-charge ceutralization by those ions generated 
in the anode glow. As the applied voltage increases,  the energy supplied to 
the stream of electrons becomes sufficiently large to cause a propagation of 
visible glow (ionized zone) toward the emitter which in turn results in the 
cesium ignition. 
In an ignited condition the voltage drop in the plasma, which i s  in con- 
tact with the collector, becomes small since the net space charge in the 
plasma i s  nearly zero. Consequently, the diode becomes equivalent to a 
diode having i ts  virtual collector at  the edge of the ball-shaped plasma. The 
emitter and plasma a r e  separated by a thin dark space. Current conduction 
occurs with the interelectrode gap partially filled with a ball-of-fire plasma 
(Ref. 2)  at  the magnitude of current shown a t  (3) in Fig. 2. As the current 
increases further, the plasma merely spreads to a larger  area,  maintaining 
the current density constant. The voltage across the diode remains practi- 
cally constant between point (3) and (4) in the volt-ampere curve. This 
region is ,  therefore, similar to that in a normal glow discharge (Ref. 3) in 
inert gases a t  low pressures. Further increase of diode current requires an 
increase in  the diode voltage to produce an intense plasma. The plasma 
mode, which occurs between We ball-of-fire mode and an intense-plasma 
mode, i s  identified a s  the maximum power point in practical converters. 
Similar observations were made by Hansen (Ref. 4) in a thermionic converter 
having an interelectrode spacing of approximately 1 mm. 
JPL Technical Memorandum 33-55 1 I. .;k 
The probe characteristics, showi~rg the functional dependence of probe 
current on the probe voltage with respect to the collector, were also meas- 
ured at four different operating points associated with four different discharge 
modes. A typical result i s  shown in Fig. 3,  where the positive current 
indicates that the net current to the probe i s  an electron current, and the 
negative current indicates that the probe i s  collecting more ion current than 
electron current, and/or the probe i s  emitting electrons. An increased 
electron emission from the probe, because of an increased heating from the 
hot emitter, becomes more evident when the probe i s  located closer to the 
emitter. The middle part of the probe curve i s  predominantly prescribed 
by the 'hermalized group of electrons a s  i s  shown in the semilog plots 
(Fig. 4). 
Curves shown in Fig. 4 for three different values of TR with 
T~ = 1123 "K indicate that the logarithm of electron current i s  linearly 
dependent upon the probe voltage V except for values where the local break- 
P 
down or the back emission from the probe disturbs the probe current. From 
the slope of the curve, which i s  proportional to e/kTe, the electron tempera- 
ture T was determined. 
e 
The electron temperature Te was practically independent of distance d 
(Fig. 5) and varied between 17,000 and 3000°K for cesium reservoir tempera- 
tures between 404 and 444°K. At higher cesium temperatures the electron 
temperature decreased to a s  low ah 2. 1 times that of the emitter tempera- 
ture: for example, T = 2572°K when TE = 1273°K with D = 2. i mm. 
e 
Cbeerved functional dependence did not agree with other published results 
(Refs. 1 and 5) ,  which were obtained at higher cesium tempe rcs or with 
diodes with srcaller gaps. The difference at low cesium temp .::;:!ure case 
appears to be caused by the fact that the current conduction is  drift-dominated 
in the present work in contrast to the diffusion-dominated conduction in 
others, in which electrons cool off toward the collector. It may be concluded 
that the cesium discharge at relatively low pressures and low electrode 
temperatures i s  more similar to a glow discharge in inert gases than to a 
cesium discharge in an ignited thermionic energy converter. 
JPL Technical Memorand-m 33-55 1 
The plasma ion densities were determined from the negative-saturation 
values of the probe characteristics. In this calculation the probe back- 
emission I w3.s graphically subtracted from the negative saturation to b 
obtain the ion current component Ii (Fig. 6) .  The results obtained at 
TE = 1173'K, for the ion density Ni,  are shown in Fig. 7 in semi-log 
curves. The ion densities increase toward the emitter (larger d), and i t  
varies exponentially as  expected (Ref. 6). The ion density, which was of 
the order of 10% of the cesium gas density, increased with the cesium reser-  
voir temperature T Re 
Lastly, the electric potential profiles in tire diode for four different 
modes of operation will be discussed. The electric potential ef the inter- 
electrode space was determined a s  the probe voltage a t  which the prob:! cur- 
rent equaled 11160 (probe area/collector area)  of the diode current. This 
method was used since the current flow was drift-domivted in this diode in 
which ar. LTE (local-thermodynamic-equilibrium) plasma did not exist. The 
electric potential determined by this method i s  plotted a s  a function of dis- 
tance in Fig. 8, after normalizing the r ~ t e n t i a l  to zero volts at d = 0 
(collector). The correction required for this normalization was approxi- 
mately 0.35 V, which can be considered a s  the work function difference 
between the collector and the probe. For comparison, the electric potential 
profiles a r e  also shown in Fig. 9 at higher cesium temperatures. The spac- 
potentials in the ignited mode here  were determined differently f r o r  the 
previo1.s case by determining the break in probe characterastics where the 
electron current saturated. 
JPL Technical Memorandum 33-55 1 
V. CONCLUSION 
The electron temperature varied between 17,000 and 3000°K for 
cesium reservoir temperatures between 404 and 444°K. The temperature 
higher than expected was a result of low cesium pressures at which the 
current condilction was drift-dominated. However, the electron temperatures 
a s  low as  2572.K were recorded when TE = 1273.K with higher cesium tem- 
perature at 533°K. The current conduction was diffusion-dominated in this 
case. 
13 3 The ion plasma densities were in a range between 10 /cm and 
12 3 10 /cm . The magnitude as well as  the functional dependence of densities 
an the location in space was a s  expected. 
The potential profile was obtained in four modes of discharge. The 
result showed clecirly a region 9f anode glow having a small plasma drop 
adjacent the collector. The similarity in potential prafiles between the ball- 
of-fire mode and the plasma mode was established; the difference between 
these two modes maii~ly lies in the size of the plasma and not the intensity. 
JPL Technical Memorandum 33 -55 1 
' 1 .. 
REFERENCES 
6. Shimada, K., "Space- Charge Distribution in a Neutral Plasma,' '  in 
Supporting Research and Advanced Development, Space Programs  
Summary 37-42, Vol. IV, pp. 56-59. Je t  Propulsion Laboratory, 
Pasadena, Calif., Dec. 3 1, 1966. 
1. Bullis, R. H., and Wiegand, W. J., IEEE Report on the Thermionic 
Conversion Specialist Conference, Cleveland, Ohio, pp. 258-265, 
Oct. 1964. 
2. Johnson, E. O., RCA Review, Vol. XVI, pp. 82-108, Mar. 1955. 
3. von Engel, A., Ionized Gases, p. 195. Oxford University P r e s s ,  
London, 1955. 
4. Hansen, L. K., and Warner, C., IEEE Report on the Thermionic 
Conversion Specialist Conference, Cleveland, Ohio, pp. 3 10-3 19, 
Oct. 1964. 
5. Reichelt, W. H., and Kruer,  W. L., IEEE Report on the Thermionic 
Conversion Specialist Conference, San Dlego, California, pp. 129-137, 
Oct. 1965. 
i 
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JPL Technical Memoracdum 33-55 1 V 
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JPL Technical Memorandum 33 -55 1 
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JPL Tec'mical Memorandum 33 -55 1 
NASA - JPL - C O ~ . ,  L A ,  C.llf. 


Probe measurements of a cesium plasma in a simulated thermionic energy converter

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