NASA Technical Memorandum 102099 

Arcjet Cathode Phenomena 

Francis M. Curran and Thomas W. Haag 

Lewis Research Centel' 

Cleveland, Ohio 

and 

John F. Raquet 

United States Air Fora? Academy 

Colorado Springs, Colorado 

- - 

tHASA-Tn-ro2099) ABCJET CATHODE PHENGBEHA nag-271 21 

IlraS, Levis Research Center) 17 pCSCL 200 

Unclas 

G3/34 0225952 

Prepared for the 

1989 JANNAF Propulsion Meeting 

Cleveland, Ohio, May 23-25, 1989 
ARCJET CATHODE PHENOMENA 

Francis M. Curran and Thomas W. Haag 

National Aeronautics and Space Administration 

Lewls Research Ceriter 

Cleveland, Ohio 44135-3191 

and 

John F. Raquet* 

United States Air Force Academy 

Colorado Springs, Colorado 80840-5701 

ABSTRACT 

Cathode tips inade from d number of different materials were tested in a modular arcjet thruster 

in order to examine cathode phenomena. 

collected during testing, indicated that all of the tungsten-based materials behaved similarly 

despite the fact that in one of these samples the percentage of thorium oxide was doubled and another 

was 25% rhenium. The mass loss rate from a 2% thoriated rhenium cathode was found to be an order of 

magnitude greater than that observed using 2% thoriated tungsten. Detailed analysis of one of these 

cathode tips showed that the molten crater contained pure tungsten to a depth of about 150 microns. 

Problems with thermal stress cracking were encountered in the testing of a hafnium carbide tip. Post 

test analysis showed that the active area of the tip had chemically reacted with the propellant. 

A 100 hour continuous test was run at about 1 kW. Pclst test analysis revealed no dendrite 

formation, such as observed in a 30 kW arcjet lifetest, near the cathode crater. The cathodes from 

both this test and a previously run 1000 hour cycled test displayed nearly identical arc craters. 

Data and calculations indicate that the mass losses observed in testing can be explained by 

evaporation. 

the observed decrease in mass loss rate with time. 

Periodic disassembly and examination, along with the data 

An increase in the redeposition rate with cathode recession was postulated to explain 

From measurements of the cathode crater size, estimates of the emitting area were made. These 

were used with realistic bounds on the crater temperature, material work function, and the electric 

field at the cathode to study the cathode emission process,. From this, simple calculations Suggest 

that both field and thermionic emission are involved. 

INTRODUCTION 

Research and development of arcjet thrusters for space propulsion began over three decades ago. 

In this era, numerous missions requiring primary propulsion were under consideration and so a 

majority of this early effort focused on 30 kW class hydrogen arcjets. It was assumed, at least at 

the outset, that this power level would be provided by the nuclear power plant being developed under the SNAP-8 program and that the problems associated with long term hydrogen storage would be solved. 

The program also included analysis and testing of smaller 1 to 2 kW devices, on hydrogen, for 

auxiliary propulsion. By the early 1960's it became apparent the anticipated advances in space based 

power availability were layging and that missions compatible with arcjet technology had not 

developed. For these reasons. the NASA-sponsored arcjet program was discontinued. A comprehensive 

review of the effort was presented by Wallner and Czika in 1965.l 

power arcjet in 1983 for application to missions, such as north-south stationkeeping (NSSK) on 

geosynchronous communications satellites. At about the same time, the Air Force started research on 

30 KW class devices for high thrust applications. It was recognized early in both of these programs 

that hydrazine and ammonia were the most viable propellants as the long term storage of hydrogen was 

as yet prohibitive from the standpoint of system mass. For compatibility with modern satellites the 

low power work has been centered around the 1.2 kW level with the decomposition products of hydrazine 

as the propellant. 

will require a new system design and an amnia system wollld eliminate the need for a gas generator 

and offer a higher specific impulse than does hydrazine due to its lower molecular weight. 

To be of value in the proposed missions, both the high and low power arcjets will be required to 

operate reliably over long periods of time. In a typical NSSK mission scenario. the low power arcjet 

life requirement has been estimated at 600 hours taken in approximately 1 hour increments over a 

Recently, interest in the arcjet has been rekindled. NASA began a program to develop the low 

The high power devices have been used primarily with ammonia since any mission 

*1988 Air Force Cadet Summer Research Program. 

1 
10 year peric~d.~~j For the SP-100 reference mission a long-term life will be required. and el ctrode 

were successfully completed at the 30 kW level, as was a 150 hour test of a 2 kW device.6 All of 

these tests were performed using hydrogen as the propel'lant. Ammonia was tried briefly at high power 

in a slightly modified engine but no long term testing took place. Tests with propellants other than 

hydrogen at low power met with little suc~ess.~ A more recent endurance test was undertaken at the 

Jet Propulsion Laboratory using a 1960's design arcjet with ammonia and severe cathode degradation, 

in the form of dendrite formation, was observed after 573 hours.8.9 

erosion in high power arcjets is currently in progress.10 

Some parametric testing of arcjet cathode erosion characteristics has been done at low power on 

hydrazine. These tests indicate that increasing both chamber pressure and cathode diameter decrease 

erosion in short term tests.3 

1.26 kW suggest that, after an initial burn-in period, the cathode reaches a steady state 

configuration and that erosion may not be a severe problem in this power range.11 

erosion is a concern. During the earlier arcjet program. endurance tests of both 5004 and 720 f hours 

An investigation of cathode 

Results from a recently completed long-term cyclic lifetest at 

I n a l l of the above mentioned tests, thoriated tungsten welding rods have been used for 

cathodes. 

effects. For this report, cathode tips were fabricated from a number of different candidate 

materials. These were run in low power thrusters under conditions typical of those anticipated 

on-orbit. 

presented. A 100 hour continuous lifetest was also run for this study. 

and from the 1000 hour cyclic lifetest were examined and these results are also presented. 

APPARATUS 

Little work has been performed to characterize erosion over time or to examine material 

The tips were removed periodically for analysis and the results of the analyses are 

The cathodes from both this 

Two areas uf low power' arcjet cathode have been investigated. The first area deals with a 

direct comparison of different types of cathode material. Six different cathode tips were machined 

to identical geometries in an effort to compare erosion rates, operating voltage, and starting 

characteristics of each. The second area of cathode phenomena deals with the metallurgical changes of a chosen material after a prolonged continuous lifetest. Grain structure, cathode tip geometry, 

and thorium oxide depletion were the main concerns. 

shown in Figure 1. This radiatively cooled thruster was designed to allow convenient and repeatable 

aisembly. A 2% thoriated tungsten nozzle was used. The constrictor was 0.64 mm diameter by 0.41 mm 

long. The converging and diverging ends of the nozzle were conical with half angles of 300 and 20°. 

respectively. The anode cathode gap spacing was set by inserting the cathode into the thruster until 

contact was made with the anode and then withdrawing the desired amount. 

herein, a gap of 0.58 mm was used. Gdseous propellant was injected tangentially into a cylindrical 

arc chamber just upstream of the nozzle. 

flow field. 

Cathode materials comparison tests were conducted with a laboratory arcjet whose schematic is 

For the tests described 

Gas entered through two 0.34 mm dia holes to form a Vortex 

Each arcjet cathode tip consisted of a cylindrical rod 3.18 mm in diameter with a 300 half angle 

conical tip at one end (Figure 2(a)). These tips were approximately 10 mm in length, with the 

conical tip at one end and an attachment pin at the other. Due to the short length of each sample, 

an extension rod was used to hold the cathode tip material within the arcjet thruster (Figure 2(b)). 

The extension rod was 3.18 mm in diameter, and 20 cm long, and made from 2% thoriated tungsten. 

four prong receptacle at one end was used to grip the attachment pin of each sample. Tolerances were 

held such that this receptacle provided a spring loaded friction fit allowing for easy interchanges 

of cathode tip samples. Figure 2(c) shows the assembled cathode as it was put into the thruster. 

The cathode used in the 100 continuous hour test differed from that of the material comparison study 

in that a single piece, 3.2 mm did by 190 mm long, thoriated tungsten rod was used. 

30" conical tip at one end, and was directly interchangeable with the cathode assembly used for 

material comparisons. 

boron nitride front insulator. Longitudinal grooves along the outer surface of the insulator allowed 

passage of propellant to the chamber injection holes and permitted regenerative preheating of the gas. 

The thoriated tungsten nozzle, gas injector, and front insulator were all contained within an 

anode housing. 

graphite gaskets. 

also served as a thermal radiator for the tungsten nozzle. 

extension rod and electrically isolated the propellant feed tube. 

fitting was used to clamp and seal the cathode in position. 

into a stainless steel anchor located deep within the insulator. The anchor was bored through to 

allow the cathode and its insulating sheath to pass axially down the arcjets center. 

A 

It also had a 

In each arcjet test assembly the cathode was centered coaxially within the arc chamber by a 

Each component was positioned within the barrel of the housing and separated by 

The walls of the housing not only directed and preheated incoming propellant, but 

The rear half of the arcjet consisted of a boron nitride insulator which anchored the cathode 

A modified compression type gas 

The propellant feed tube was threaded 

2 
t 

A coiled spring made from Inconel X-750 and a boron nitride compression plunger were located in 

the rear insulator. Force from the spring was used to axidlly compress internal graphite gaskets 

around the gas injector and nozzle located in the front of the arcjet.. Machining tolerances were set 

to accommodate radial thermal expansion over the entire hedting and cooling cycle. The anode housing 

was joined to the rear insulator assembly using two stainless steel flanges and four bolts. The 

flanges were designed to flex axially to maintain uniform pressure over this joint. 

Testina was Derformed in one of two seDarate vacuum facilities. deDendinq on availability. Long 

duration runs were normally carried out in a 0.64 m diameter by 0.64 m iong bell jar. 

mechanical roughing pump with a capacity of 21000 L/min was used, which maintained an ambient 

pressure in the bell jar of approximately 100 Pa (0.75 torr) during arcjet operation. Other test 

was performed in a tank thdt wds 1.5 meters in diameter, 5 meters long, and serviced by four 

30,000 L/sec oil diffusion pumps. Pumping speeds were such that ambient pressure inside the tank 

approximately 5~10-~ Pa during arcjet operation. The thruster to be tested was placed within a 0 

diameter by 0.9 m long port extension at one end of the taink. 

Propellant for all tests consisted of a hydrogen/nitrogen gas mixture at a volume ratio of 2 

to simulate fullv decomDosed hvdrazine. A two channel mass flow control system was used to meter 

A singie 

ng 

was 

9m 

1 

and 

regulate both gaies separately: The gases were then mixed in the propellant feed line on the way to 

the thruster. Each channel could be operated from a central console, or remotely through an external 

set point. The maximum flow rate available was 10 L/min for each gas, at a feed line pressure of up 

to 9.6~105 Pa. 

A pulse width modulated power supply with fast current regulation was used to power the arcjet 

A 4 kV pulse could be applied to the arcjet once every 

thruster. 

12 amps could be drawn at 120 V dc. 

the power supply to initiate arc breakdown. 

second until ignition occurred. 

flowrate, and inlet pressure. Arc current was measured using a Hall-effect current probe attached at 

a vacuum teed through on the test chamber. 

calibration wds checked prior to each test run. Arc voltage was also monitored at the vacuum feed 

through and reduced with a 1O:l voltage divider to avoid transient spikes to the recording equipment 

during startup. A digital multimeter was also used to monitor the arc voltage. An optical isolation 

amplifier was used to eliminate noise interference. All experimental data was recorded using an 8 

channel strip chart recorder. The digital displays of various instruments were recorded periodically 

to obtain precise data points. 

An open circuit voltage of 175 V dc was available with this unit and a maximum current of 

A high voltage starting pulse generator was incorporated into 

Experimental data obtained during firing included arc voltage, current, propellant mass 

The probe provided excellent noise isolation and its 

EX PE R I M E N TAL PROC E C)UR E S 

A major- goal of this investigation was to compare the suitability of a number of different 

cathode material samples for use in an arcjet. Each sample was machined to the same shape, and run 

at as close to identical conditions as was practical. The criterion for comparison were mass loss, 

steady state voltage, and arc stability. 

The sequence began with 2 hours of operation followed by another 2 hour run and a final 5 hour run. 

After each of these cycles the thruster was disassembled for examination of the cathode tip. 

of 3 mass loss data points. 

hardware over the duration of testing, an effort was made to proceed with all samples through the 

completion of a given stage of testing before going on to the next. Exceptions to this rule were 

established when the first 2 hour burn-in revealed some ma.teria1 samples which were at higher risk of 

not completing the intended test sequence. 

section. 

Each sample was tested in stages so that some indication of long-term trends could be inferred. 

Cathode mass loss was determined during disassemblies between each stage of testing, for a total 

As a means to reduce effects from possible cumulative changes in test 

This will be discussed in greater detail in a following 

Prior to testing. each tip sample was weighed using am electronic analytical balance. The scale 

was calibrated before each weighing and each cathode sample was thoroughly cleaned. 

were taken and averaged for each sample. 

one sample was weighed on two separate dates. 

difference in weight from the two measurements was only 0.01 mg. 

photographs were also taken of each cathode tip prior to testing. 

procedure on every occasion. 

check was made prior to each run. 

instrumentation was allowed at least 1 hour to warm up. 

Three weighings 

Scanning Electron Microscope (SEM) 

In order to determine the reproducibility of this method, 

The total weight of each tip was about lg and the 

Each cathode sample was assembled in the common arcjet described previously using the same 

The arc gap was set to 0.58 mm for each cathode and a pressure leak 

The thruster was installed in a vacuum facility and all 

The propellant gas flow rates were set to 

3 
give a 2:l hydrogen to nitrogen mixture ratio at a mass flow rate of approximately 0.05 grams/ 

second. After each operating period the 

thruster was allowed to cool briefly before being removed from the vacuum facility and disassembled. 

A post-test pressure check and arc gap measurement were made to verify-the integrity of the arcjet. 

A common test profile was intended for each cathode tip. In the initial 2 hour run, the arcjet 

was started at 12 A, and then turned down to 11 A (approximately 1.1 kW) after about 1 minute. The 

current was then left at 11 A for the remainder of the 2 hours. Current, voltage, and pressure 

measurements were recorded at 15 minute intervals throughout the run. After disassembly, each tip 

was weighed and photographed using the procedures described earlier. 

The arcjet was reassembled for the second stage of testing. This began with two brief operating 

periods in which transient behavior was observed prior to steady state arc attachment (approximately 

2 minutes). About 15 minutes of cooling time was allowed between restarts. A 2 hour 11 amp test run 

followed in which voltage, gas flow, and pressure were recorded at 15 minute intervals. After this 

second 2 hour run,, the cathode tip was once again weighed and photographed. 

were recorded'at half-hour intervals. 

and photographed a final time. 

This was given 5 minutes to equilibrate prior to ignition. 

The third stage of testing consisted of a 5 hour run at 11 A. Thruster operating parameters 

This run concluded the test and the cathode tip was weighed 

Following final 

lengthwise cross-sec 

the grain boundaries 

for elemental analys 

surface analysis, each cathode sample was encapsulated and ground to reveal a 

ion along its center axis. The surface was then chemically etched to bring out 

s on selected samples. 

Both energy dispersive x-ray analysis (EDS) and Auger spectroscopy were used 

RESULTS AND DISCUSSION 

Cathode Tip Evaluations 

General Observations. As indicated in the preceding section, a controlled test sequence was 

attempted with each of the cathode tips in order to study the erosion characteristics of the various 

materials. The materials used are given in Table I. The full series of tests was possible with all 

of the tungsten based materials. In these tests, the arcjets ran stably and little anomalous 

behavior was noted. 

abnormal amount of sparking was observed at the start of the test and this continued after the arcjet 

voltage had stabilized and the arcjet was running in high mode (i.e., with the anode attachment in 

the divergent section of the nozzle). Because of this, the test was stopped for an evaluation of the 

tip after 2-1/2 minutes. The results will be discussed below. Thermal stress cracking was a problem 

with the hafnium carbide tip and this test was also terminated after a very brief period as described 

below. Previously, a hafnium carbide tip was run at this laboratory in support of another 

program.12 

In the arcjet thruster the difference in potential between the 

electrodes is equal to the summation of the voltage drops across the cathode fall zone, the body of 

the arc, and the anode fall zone. The same anode geometry and operating conditions were used in each 

of the cathode tip tests and, as mentioned previously, normal, high mode behavior was observed in all 

the tests. Thus, any major differences in operating voltage should be attributable to differences in 

tbe cathode fall zone at the cathode tip. This assumes the gap setting was repeatable from test to 

test which was an initial concern in these experiments. Table I1 shows the operating voltages of the 

cathode tips at the end of the various test periods. From the Table it is obvious that there was 

very little variation in voltage between the tips made from the three tungsten-based materials 

(cathodes 1 to 4). A general trend of a gradual increase in voltage with time is displayed. This 

was caused by the gradual recession of the cathode tip to a steady-state condition, which has the 

effect of lengthening the arc. 

be discussed in more detail in following sections. 

identical results. 

The operating voltage of the thruster with the rhenium based cathode rose more rapidly than in 

the tests with the tungsten based cathode tips. In these tests, the current had to be decreased as 

the power into the arcjet structure became excessive. This rapid rise in voltage suggests a higher 

rate of cathode recession, this was observed in the SEM photographs and will be discussed in the next 

section. 

As previously discussed, only a single test of a hafnium carbide tip was performed in which 

steady state operation was-obtained. The operating time in this test w2s approximately 1 and 1-11,? 

hours and it was run at 10 A rather than 11 to decrease the heat load at the tip. In this test 

stable arcjet operation was observed but the voltage increased very rapidly, about 22 volts, over the 

course of the test. As in the case of the rhenium-based cathode this indicates rapid changes in the 

cathode tip structure. The initial voltage level, however, was similar to voltage levels observed in 

The test sequence was altered for the tip made of 2% thoriated rhenium. An 

The results of this test are reported here for completeness. 

Voltaqe-Current Characteristics. 

Tip recession has been documented in previous testing1l,l3 and will 

The two tips of 2% thoriated tungsten gave almost 

4 
other arcjet testing under similar conditions. 

as the fall zone, were not much different for this tip than in the case of the standard metal 

cathodes. 

This suggests that conditions near the cathode, such 

Mass Loss Measurements. Many investigations of cathode phenomena in free burning arcs at 

pressure levels between vacuum and atmospheric have been perf~rmed.l~-~~ 

under many conditions the arc current is emitted from one or more small molten spots on the cathode 

surface and that these spots move rapidly across the surface. Porto, et al., suggest that is likely 

that, for cold cathodes, these spots exhibit the same basic characteristics regardless of the ambient 

pressure.l6 In general, these studies have indicated that erosion rates are dependent on both 

pressure and current but do not suggest a dependence on operating time. 

Hardy and Nakanishi20 show that cathode mass loss rates in free burning. high pressure arc discharges 

are linear with time in most cases. This is not surprising in that the motion of the cathode 

attachment region produces constant cathode surface conditions. 

however. As noted earlier, the arc is forced to seat at the cathode tip by the propellant 

tlowfield. Thus, the erosion characteristics for the arcjet may be quite different than those 

observed in the case of the free burning arc. Figure 3 shows the mass loss, as a function of time, 

for the various arcjet cathode tips tested. 

shown in Table 111. From previous data a mass loss rate for the 1000 hour lifetest cathode was also 

estimated and included in the Table. 

samples made from 2% thoriated tungsten displayed nearly identical erosion characteristics. 

enough data were collected to allow a full statistical analysis. The results of these two separate 

tests, however, taken with the previously noted similarity in current-voltage characteristics, 

indicate that the tests were run with reasonable repeatability. 

that the mass loss rates for the other two tungsten-based tips were higher than those observed in the 

two 2% thoriated tungsten based tips only over the first 2 hours of testing. After this, the mass 

loss rates for all the tungsten-based materials were virtually identical. A rough idea of how the 

erosion rate decreases over long periods of time can be extracted using the averaged mass loss data 

from the 2% thoriated tungsten cathode tip tests and that obtained in the 1000 hour lifetest. This 

analysis is based on the assumption that the dominant cathode cooling mechanism is radiative. The 

mass loss rate for the 1000 hour test cathode, as shown in Table 111, was estimated by subtracting 

the averaged mass loss obtained with the two 2% thoriated tungsten tips at the 9 hour mark from that 

of the total mass loss measured in the 1000 hour test to obtain the mass lost after this point in the 

lifetest. This is certainly to be considered only an approximation as the cathode tip shape and the 

mass flow rate were slightly different in the lifetest thari in the cathode tip tests. However, both 

of these would be expected to have little affect on the mass loss rate and so the data in Table 111 

are likely reasonable. From Table 111, it is evident that the mass loss rate decreases significantly 

with time. The mass loss rate estimated at 1000 hours is nearly an order of magnitude below the 

initial mass loss rate for cathodes 1 and 2. In fact. this may be an overestimation of the mass loss 

rate at the end of the test. Given the observed trend, the lack of data points at intermediate time 

intervals probably leads to an overestimate of the erosion rate at the 1000 hour point. This is 

consistent with the postulate that the cathode erodes quickly back from the original sharpened tip to 

a near stead state shape from which the mass loss rate is very small, as suggested in earlier 

studies.ll~~~ The decrease in the mass loss rate with time also indicates that for the case of the 

arcjet, in which the arc is forced to the tip by the flow field, the erosion phenomena cannot be 

simply described by a plot of grams/coulumb or grams/hour as in the case of the free burning arc 

experirnent~l~~~~ noted at the start of this section. 

than that of the other tungsten-based materials. For this sample the mass loss rate after 4 hours of 

operation was seen to be an order of magnitude above those measured for the tungsten-based cathode 

tips. 

of 2% thoriated tungsten. As will be discussed in the next two sections, the thorium oxide likely 

does not play a significant role in the emission process and the observed mass loss rates can be 

explained solely by evaporation. 

displayed a mass loss rate significantly higher than that of the tip containing 25% rhenium implies 

that it must have been running at a higher temperature. 

rhenium has a significantly higher work function than does pure tungsten (5.1 versus 4.5 VI. 

work would be necessary, however, to confirm the postulated difference in operating temperature 

between rhenium and tungsten tips. 

the arc cathode attachment occurs from a small molten crater at the cathode tip and that the 

dimensions of this crater are nearly constant with time. 

the total erosion rate would decrease with time. 

nearly steady state configuration, the redeposi tion rate increases leading to a lower net erosion 

rate (i.e.. a lower mass loss rate). 

Detailed Cathode Phenomenology 

analysis as part of the test and analysis sequence. 

It has been shown that 

In fact, data presented by 

This is not the case in the arcjet. 

The mass loss rates for the tungsten-based materials are 

From the Figure and the Table it can be seen that the two 

Not 

Figure 3 and Table I11 also show 

Figure 3 also shows that the mass loss of the thoriated rhenium cathode is significantly higher 

The melting point of the 2% thoriated rhenium tip is expected to be 200 to 300 'C below that 

If this is postulated, then the fact that the rhenium-based cathode 

This could be explained by the fact that 

Further 

Evidence will be presented in a following section to show that 

This being the case, it is unlikely that 

This suggests that as the cathode erodes back to a 

Tungsten Based Alloys. As discussed previously, all four of the tips were subjected to SEM 

All of these displayed similar phenomena so, for 

5 
brevity, one of the 2% thoriated tungsten tips was chosen for detailed analysis and this is presented 

here. Figures 4(a) to (c) show a sequence of SEM photographs of the tip taken after 2. 4, and 9 

hours of operation, respectively. For comparison, Figure 4(d) shows a SEM photograph of the second 

2% thoriated tungsten tip after its first 2 hour run. 

the shape of the tips is very similar in each case. The arc appears to have originated from a small 

molten crater at the tip. 

At the conclusion of testing the cathode shown in Figures 4(a-c) above was sectioned, polished, 

and etched and then subjected to SEM analysis with both energy dispersive (EDS) and Auger 

spectroscopy. The entire tip in its polished condition is shown in Figure 5(a), and Figure 5(b) 

shows the tip after etching to bring out the grain boundaries. 

were analyzed via EDS and found to be composed of thorium oxide. 

presence of nodules was not unexpected. Figure 5(b) shows, however, that no nodules were present at 

the cathode tip. In fact, EDS showed no thorium oxide within 100 to 150 microns of the cathode tip. 

From the recrystallization pattern near the tip, this also appears to be the approximate depth to 

which the material was molten. Behind this area, the grain boundaries. though irregular, appear to 

be homogenous and unaffected throughout the bulk of the material as do the dark nodules. 

suggests that,operation does not cause gross changes in the bulk material and that there is little 

migration of thorium oxide toward the tip. 

figure 5(c) which presents a more magnified view of the region near the crater on the polished 

cathode tip shown in Figure 5(a). This photomicrograph shows that no thorium oxide nodules are 

present in the resolidified region near the crater. Thus, there is no indication that arcjet 

operation causes migration of thorium oxide toward the molten crater. 

magnification a number of occlusions were observed on some of the cathode tips. 

is shown in Figure 6. 

region of the tip. 

diameter. 

photograph. It is likely that these pits were sites of thorium oxide nodules which vaporized due to 

the extreme temperature at the tip. This temperature must be at least the melting point of tungsten 

at which the vapor pressure of thorium oxide is greater than 1 torr.21 

2% Thoi-iated Rhenium. As described in a previous section, the 2% thoriated rhenium tip test was 

first terminated after only 2-112 minutes after excessive sparking was observed in the plume. An SEM 

photograph of the tip at this point is shown in figure 7(a). From this it is obvious that a very 

extensive portion became molten very rapidly. The crater on the tip after 4 hours of operation shown 

in figure 7(b), is similar to those observed on other tips, but it does appear to be deeper as might 

be expected from the higher mass loss rate and operating voltage (indicating an increased arc length). 

Hafnium carbide has recently been suggested as an alternate cathode12 material 

because of its high melting point (ca 3890 'C) and low work function of about 3.5 V. Numerous 

attempts were made to run a tip ground from hafnium carbide. In an initial experiment with a 

slightly different tip geometry, the tip ran well for some time, but cracked on restart. Another tip 

cracked almost immediately on start up. Finally, in this study, a tip was run briefly without 

cracking. Visually the tip maintained its deep blue 

coloration except at the tip where its color had turned to gold. 

showed that the tip had become molten. 

and 8(b) under high magnification. Figure 8(a) shows the border of the molten area. Large crystals 

seem to have formed right at the edge of this area. A region near the center of the molten area is 

shown in figure 8(b). This appears to be resolidified with boundaries similar to those seen in the 

metal tips. Analysis of the composition in this region, however, indicated that a major chemical 

change had taken place as this area was found to now be composed of hafnium nitride. This suggests 

that a chemical reaction between the propellant and the hot cathode had taken place. While it may be 

possible to lessen or eliminate the stress cracking by using a hafnium carbide cermet, it is doubtful 

that this would change the tip chemistry and this probably means that hafnium carbide is not a viable 

cathode material. 

From these photographs, it can be seen that 

A number of the dark nodules observed 

These were sintered cathodes so the 

This 

A clearer view of this is shown in the photomicrograph of 

During the course of the SEM photography, one final observation was made. Under high 

An example of this 

This photograph was taken early in the test cycle and shows the resolidified 

A number of pits are seen and these are approximately one to two microns in 

This is approximately the same size as the thorium oxide nodules seen in the earlier 

Hafnium Carbide. 

This was taken from the thruster for analysis. 

SEM photographs of this region 

Different regions of this tip are shown in Figures 8(a) 

Long-Term Cathode Operation 

The analysis of the cathode used in a long term continuous ammonia arcjet test run at the 30 kW 

power level showed the formation of dendrites that grew from the rim of the cathode tip crater 

outward toward the anode.1° The cathode from a recent 1000 hour1500 cycle low power arcjet lifetest 

showed no evidence of dendrite formationll. It was of interest to determine if dendrites would form 

at low power during long-term, continuous operation. To study this, a 100 hour continuous test of a 

low power (-1 kW) arcjet was run. The cathodes from both this test ana the 1000 hour cycled test 

were sectioned and analyzed. 

Different photographs of the cathode tip run in the 100 hour test are shown in Figure 9. 

SEM photograph of the tip under low magnification (Figure 9(a)) shows that a small arc crater had 

The 

6 
" 

formed on the tip. A photomicrograph of this crater, taken looking straight at the cathode tip, is 

shown in Figure 7(b). The cathode crater observed is almost identical to those that have been 

observed in other arcjet tests.11*13 The tip, after sectioning, polishing, and etching is shown in 

Figure 9tc). As in the case of the short term tests, a resolidified afea can be seen in the region 

of the cathode tip. The grain structure immediately behind (or underneath) this area does not appear 

different than that observed in the bulk. As in the shorter cathode tip tests, no thorium oxide 

nodules were seen in the region of the arc crater. 

Similar views of the cathode tip used in the 1000 hourlcyclic are presented in Figures 10(a) 

to (c). This tip was originally bullet-shapedll and polished, rather than ground with a straight 30" 

angle. From Figures 9 and IO it can be seen that there is a significant difference between the final 

shapes of these two cathode tips. 

raised slightly from the surrounding surface. 

in the case of the 1000 hour test was in a depression on the tip. 

difference is not clear. 

of the tip from the 100 hour test at the 100 hour point and future testing would be required to 

address this issue. The arc craters observed in both tests, however, were nearly identical in size 

and appearance. Measurements of these craters and a similar crater from an earlier test13 give 

diameters between 0.15 and 0.20 mm. All of these cathodes were initially 2% thoriated tungsten. 

thermionic in nature. 

from figure 9(a). the crater on the tip from the 100 hour test is 

figures 10(a) and 10(b) show that the cathode crater 

The explanation for this 

It is possible that the lifetest cathode was similar in appearance to that 

Questions exist as to the actual emitting area and as to whether the emission is purely 

The Richardson-Dushman equation for thermionic emission is:22 

jt = AT2exp [-+elkT) (1) 

where jt is the current density, k is Boltzmann's constant, e is the electron charge, 4 is the 

thermionic work function of the emitting surface (in volts), and A is a constant dependent on 

material. 

tungsten is certainly possible and early calculation, in fact, used the boiling point of tungsten to 

determine the emission from the tip.23 

contributes to the current density. 

The temperature in the molten pool is unknown but a temperature above the melting point of 

It is also possible that Shottky-enhanced emission 

The equation for this ca.n be written as:22 

pl.389 A] 

T j = j, exp 

in which E is the electric field strength in volts per centimeter and jt is the thermionic emission 

when E = 0. For the purposes of this analysis, the exponential term on the right-hand side of this 

equation will be called the enhancement factor and labeled y. Using the above equations some 

calculations were made to bound the problem. In these, a value 

of 60.2 was used for A.22 

emitting, a current density of approximately 4.6~104 A/cmZ is required for a total current of 11A. 

If only the base of the crater is emitting, the current density would have to increase by about two 

orders of magnitude. 

tungsten. The upper limit for the thermionic work function idas set at that of solid tungsten. 3.1V 

was used as the lower limit as it produced nearly the correct value of the emission current density. 

The low limit on the field strength (106 V/cm) used in the calculations of field enhanced emission 

was chosen because values in this range have been calculated for some cases.24 The upper limit of 

lo7 V/cm was chosen because it was what was found to be necessary to yield the right order of 

magnitude for the current density in the worst case used in the thermionic emission calculation. 

From Table IV it can be seen that the current density obtained using the work function of solid 

tungsten at the melting point and with no field emission is dbout two orders of magnitude below that 

required for the case of emission from the entire crater. Using same work function and setting the 

temperature to tungsten's boiling point yields more than the required current density. Also from 

Table IV. this same result can be obtained using a work function well below that of solid tungsten. 

While a work function of 2.6V is typically assumed for thoriated tungsten, evidence presented earlier 

shows that the molten crater appears to be pure tungsten. 

tungsten is unknown, the assumption of a work function in the 3.1 to 3.5 V range is speculative. 

be required to obtain the necessary current density in the case in which the entire crater is 

emitting at the melting point of tungsten and assuming a work function of 4.5 V. 

elevated temperature, however, the calculations indicate that this current density can be achieved 

with a field strength of less than 106 V/cm. 

cathodes in high pressure arcs, as noted earlier, however, calculated values in the 106 V/cm range 

have been reported.24 Cathode fa1 1 voltages in high pressure discharges typically range between 

1 and 10 volts.25 Thus, if field enhanced emission is contributing to a significant extent, the 

sheath thickness would have to be on the order of 10-5 to 

from the above, it is probable that the observed cathode spot is molten during operation and 

that electrons are being emitted across the entire surface of the spot. 

emission mechanism is not conclusive, it is likely that some combination of thermionic and field 

The results are shown in Table IV. 

If it is assumed that the entire crater of a diameter of 0.0175 cm is 

The temperature limits used were set by the melting and boiling points of 

Because the work function of molten 

Again, the values given in Table IV indicate that an ellectric field strength of lo7 V/cm would 

By assuming an 

Little experimental data exists on field strengths near 

Cm.1 

While the evidence for the 

7 
emission is involved and that the spot temperature is somewhat above the melting point of pure 

tungsten. 

as :26 

An equation for the evaporation rate at fixed metal vapor pressure has been given by Dushman 

W = Pv/ d22nRT/M (3) 

W is the evaporation rate in grams/cm2-sec, Pv is the vapor pressure in atmospheres. M is the 

molecular weight of the material, R is the universal gas constant and T is the absolute temperature, 

in degrees K. 

about 3.9~10-~ g/cm2-sec for tungsten at its melting point. A rate of about 6glcmf-sec is obtained 

at the boiling point of tungsten. Taken across the molten area, these produce total evaporation 

rates of about 9.4~10-8 g/sec and 1.4~10-~g/sec, respectively. 

compared to the mass loss rates observed in the testing, while the former is on the same order of 

magnitude as the mass loss rates observed in the cathode tip tests. Thus, since the tip is at or 

above the melting point of tungsten evaporation could account for the entire mass loss. The fact 

that the mass loss rate was observed to decrease with time, while the cathode emission phenomena 

remained essentially constant, suggests that redeposition of material on the cathode becomes more 

important as the cathode tip recedes. 

appearance of the two cathodes from the long term tests. 

lifetest was cycled and accumulated more than 500 starts. 

occurred over the entire tip including the area on the shoulder well behind the tip crater (see 

Figure 10(a)). 

continuous test in which a single start was performed. 

ignites upstream of the cathode tip and then is rapidly forced there by the gas flow as the tip heats 

to a temperature that can support the emission. This phenomenon may be due to the observed depletion 

of thorium oxide at the tip. 

cathode material, rather than pure tungsten for ease in starting. 

required to provide a better understanding of this phenomenon. 

Using vapor pressure data tabulated by Margrave,*l this equation gives an eva oration rate of 

The latter value is very high 

This issue is important in understanding arcjet life and one final difference was noted in the 

As stated, the cathode from the long term 

It was evident that surface melting had 

This surface melting phenomenon was not as extensive in the case of the 100 hour 

From this, it is postulated that the arc 

This would suggest the continued use of 22 thoriated tungsten as the 

Further experimentation would be 

CONCLUOING REMARKS 

In this study a variety of materials were tested as arcjet cathodes in a modular thruster. 

Periodic disassembly of this thruster allowed a comparison of mass loss rates and SEM analysis of the 

tips. At the conclusion of testing, the cathode tips were sectioned, polished, etched and examined 

in order to analyze cathode phenomena. The cathodes from both a 100 hour continuous test and a 

1000 hour/500 cycle test were also analyzed for comparison. 

rod. 

high and low powers. 

repeatability. 

doubled and in which 25% rhenium was added. 

lxhibit similar mass loss rates. Calculations were performed which indicated that the mass loss 

rates observed can be explained solely by evaporation. 

nearly identical for the tungsten-based cathodes, indicating similar conditions existed in the tip 

region during operation. 

indicates that thorium oxide does not play a role in determining the steady state operating 

characteristics of the arcjet. 

crater was pure tungsten to a depth of about 150 microns adding strength to this conclusion. 

Additionally, the data indicate that rhenium can be added in some quantity, also without altering the 

arcjet cathode condition. 

much higher that of the tungsten based compounds. 

have been molten and to have reacted with the nitrogen in the propellant to turn to hafnium nitride 

in the molten region. It is doubtful, therefore, that hafnium carbide is a viable candidate in any 

form for use in the arcjet thruster. 

form under these conditions. 

photographs of the cathode after testing showed no sign of dendrite growth. 

100 hour test and a previously run 1000 hour, cycled test had nearly iaentical arc craters. 

indicates long term stability of the controlling emission mechanisms, however, insufficient evidence 

was available to establish those emission mechanisms with certainty. 

to bound the problem, however, and these suggested that the electron current was due to a combination 

of field enhanced and thermionic emission. 

The baseline cathode tip used in this study was made from standard 21. thoriated tungsten welding 

This material has been used almost exclusively throughout the history of arcjet testing at both 

A second cathode made from this material was also tested and demonstrated 

Tungsten-based compounds were tested in which the thorium oxide concentration was 

All the tungsten-based cathode materials were found to 

The operating voltages were also found to be 

These findings, plus the observation of the lack of thorium oxide migration 

Detailed analysis of one of the cathode tips showed that the arc 

The tip made from 2% thoriated rhenium was found to have a mass loss rate 

The tip made from hafnium carbide was found to 

A 100 hour continuous lifetest was run at about 1kW to determine whether or not dendrites would 

This phenomena was observed in a long term test of a 30 kW arcjet. SEM 

The cathodes from the 

This 

Simple calculations were made 

a 
Comparison of the mass loss rates of the 2% thoriated tungsten cathodes from the materials tests 

and from the 1000 hour lifetest indicated that the mass loss rate decreases by at least an order of 

magnitude over this period of time. Since there was evidence indicating that the emission mechanism 

does not change with time, it was concluded that the redeposition rate increases as the tip recesses 

With time. The reason for this is not presently known. 

References 

1. Wallner, L.E., and Czika, J., Jr., "Arcjet Thrustor for Space Propulsion," NASA TN D-2868, 1965. 

2. Knowles, S.K., Smith, W.W., Chun, S.I., and Feconda, R.T., "Low Power Hydrazine Arcjets: A 

System Description for Near Term Application," 1986 .JANNAF Propulsion Meeting. Vol. 1, 

K.L. Strange and D.S. Eggleston. eds.. CPIA-PUBL-455.-VOL. 1, Chemical Propulsion InfOtmatiOn 

Agency, Johns Hopkins University, Laurel, MD, 1986, pp. 399-408. 

3. Knowles, S.K., "Arcjet Research and Technology, Phase 11, Final Report," NASA CR-182276, Rocket 

Research Co., Redmond. WA (to be published). 

4. John, R.R., Connors, J.F., and Bennett, S., "Thirty-Day Endurance Test of a 30kW Arcjet Engine," 

Paper No. 630274, AIAA 1963. 

5. Todd, J.P., "Thirty KW Arcjet Thrustor Research," Report No. FR024-10338 (APL-TDR-64-58), 

Giannini Scientific Corp., Mar. 1964. 

6. McCaughey, O.J.. Geideman, W.A., and Muller, K., "Research and Advanced Development of a 2 kW 

Arcjet Thrustor." Plasmadyne Corp., Santa Ana, CA, GRC-1646. 1963, (NASA CR-54035). 

7. Shepard, C.E., and Watson, V.R., "Performance of a Constricted-Arc Discharge in a Supersonic 

Nozzle," Physico-Chemical Diagnostics of Plasmas, T.P. Anderson, R.W. Springer, and 

R.C. Warder, Jr., eds. Northwestern University Press, 1964, pp. 261-272 (AIAA Paper 63-380). 

8. Pivirotto, T.J., King, D.Q., Brophy, J.R., and Deininger. W.D., "Performance and Long Life 

Duration Test of a 30 kW Thermal Arcjet Engine," Final Report for the Period Dec. 28. 1984, to 

Nov. 30, 1986, Air Force Astronautics Laboratory, Air Force Space Technology Center, Space 

Division, Air Force Systems Command, AFAL-TR-87-010. 

9. Pivirotto, T.J., and Deininger, W.D., "Analysis of a Used Pair of Arcjet Electrodes," Proceedings 

of SPIE, Volume 872, Propulsion, Jan. 1988, pp. 119-162. 

10. Deininger, W.D.. and Pivirotto, T.J., "High Power, NHl3 Arcjet Cathode Lifetime Evaluation 

Experiments and Plans," Proceedings of SPIE, Volume 872, Propulsion, Jan. 1988, pp. 127-134. 

11. Curran, F.M. and Haag, T.W., "An Extended Life and Performance Test of a Low Power Arcjet," AIM 

Paper 88-3106, July 1988 (NASA TM-100942). 

12. Lawless, J.L., "Demonstration of Advanced Arcjet Cathode Materials," Phase I, Final Report, 

SPI-29-3. NASA CR-185111, Space Power Inc., San Jose, CA. 1988. 

13. Curran, F.M. and Haag, T.W., "Arcjet Component Conditions Through a Multistart Test," AIAA Paper 

87-1060, May 1987 (NASA TM-89857) 

14. Kimblin, C.W.. "A Review of Arcing Phenomena in Vacuum and in the Transition to Atmospheric 

Pressure Arcs," IEEE Transactions on Plasma Science, VOL. PS-10, No. 4, Dec. 1982. 

15. Daadler, J.E.. "Cathode Spots and Vacuum Arcs," Physica, 104C. 1981. 

16. Porto, D.R., Kimblin, C.W.. and Tuma, D.T., "Experimental Observation of Cathode Spot Phenomena 

in the Transition from a Vacuum Metal-Vapor Arc to a Nitrogen Arc." J. Appl. Phys.. Vol. 53, 

No. 7. July 1982. 

Atmospheric Pressure Arcs," J. Appl. Phys., Vol. 45, No. 12. Dec. 1974. 

17. Kimblin, C.W., "Cathode Spot Erosion and Ionization Phenomena in the Transition from Vacuum to 

18. Juttner, E., "Characterization of the Cathode Spot," IEEE Transactions on Plasma Science, 

19. Hantzcshe, E., "Theory of Cathode Spot Phenomena," Physica, 104C, 1981. 

20. Hardy, T.L. and Nakanishi, S., "Cathode Degradation and Erosion in High Pressure Arc Discharges," 

Vol. PS-15, NO. 5, OCt. 1987. 

NASA TM-83638. 1984. 

9 
21. Margrave J.L., ed.. The Characterization of Hiqh-Temperature Vapors, James Wiley, New York, 1967. 

22. Cobine. J.D., Gaseous Conductors, Dover Publications, New York. 1958. 

23. Buhler. R. D., "Electro-Thermal Propulsion." UCLA Short Course X458 DE. July 1962. 

24. Bade. W.L. and Yos, J.M., "Theoretical and Experimental Investigation of Arc Plasma-Generation 

Technology," Part 11, Vol. I. Air force Systems Command, ASD-TDR-62-729, AVCO Corp.. Sept. 1963. 

Cathode # 

1 

2 

3 

4 

5 

6 

..r 

L:. C'nkelnburg, W.. and Maecker. H., Electric Arcs and Thermal Plasma, Translation from Handbuch der 

P'rysik. Bd. 22, Gasentladungen 11, 1956. 

Material 

2% Th02/w 

2% Th02/w 

4% Th02/w 

2% Tho2R5% Re/W 

2% Th02/Re 

HfC 

:6. Dushman, S., Scientific foundations of Vacuum Technique, John Wiley, New York 1962 

Hours of Operation 

2 

4 

9 

1030 

Table I. Cathode Tip Sample Materials 

Mass Loss Rate (mg/hr) 

0.09 0.09 0.15 0.20 

0.09 0.09 0.11 0.09 

0.06 0.05 0.06 0.07 

Cathode 1 Cathode 2 Cathode 3 Cathode 4 Lifetest cathode 

__-- 

---- 

---- ---- ---- 0.01 ---- ---- 

Table II. Arcjet Operating Voltage at Constant Current* and Mass Flow Rate 

Operating Time I Cathode 1 I Cathode 2 I Cathode3 I Cathode 4 I Cathode 5 I I I I I 

2 Hours 98.6 98.9 loo. 1 102.2 105.8 1 4HOm 

9 Hours 

102.2 102.2 102.1 103.9 127.2 (lo amps) 1 102.5 1 106.2 1 104.1 1 107.3 1 

* All tests but final 2 hour run on cathode #5 run at 11 amps 

Table 111. Mass Loss Rates for Tungsten-Based Cathodes with Time 

I I I 1 

10 
Table IV. Cathode Spot Current Density Calculations 

T (K) 

3650 

6270 

3650 

6270 

$ (V) E (V/cm) 

4.5 

4.5 

3.1 

0 

1x106 

iX107 

3.1 1.5~ 1 O6 

PROF 

TUBE 

0 

1x106 

1.5x106 

iX107 

0 

1x106 

iX107 

1.5~10~ 

0 

1x106 

1.5x106 

iX107 

470 

56x105 

2.1~104 

52x106 

PROPELLANT 

TUBE ANCHOR -... FRONT 

INSULATOR REAR -..‘-., ’.. INSULATOR -,. 

- 

Y 

1 .o 

3.3 

4.4 

45 

1 .o 

2.0 

2.4 

9.1 

1 .o 

3.3 

4.4 

45 

1 .o 

2.0 

2.4 

9.1 

- 

- 

- 

- 

- 

j (A/cm2) 

470 

1.6~10~ 

2.1~103 

2.1~104 

//- 

(B) SLOTTED HOUER. 

4.1 x lo4 

1.3~10~ 

1.7~10~ 

1.8x 106 

‘ANODE 

HOUSING 

,’’ INSERT 

-ANODE 

’ INJECTION 

DISK 

’- PROPELLANT 

FEED TUBE 

FIGURE 1. - CUTAWAY VIEW OF ARCJET THRUSTER. 

CD-Be-40822 

azD 

(A) CATHOM TIP. 

(C) CATHODE AS!BBLY. 

FIGURE 2. - CATHODE TIP AND SLOTED HOLDER. 

co-ev-40sn 

11 
5- 

Y F 

cn 4- cn 

s 

4- CATHODE3 

0 CATHODE5 

a 

0 CATHODE1 

0 CAlHOl F2 

&?I CATHOD, 3 

0 2 4 6 8 10 

OPERATING TIME (hours) 

FIGURE 3. - CATHODE MASS LOSS VERSUS TIME. 

CD-89-40824 

(A) IWO PFRCENT THORIATEII TUNGSllN I If‘ AI 11 II 

TWO HOURS 01 OPLRATION. (70X). 

~~ 

(C) TWO PERCENI THORIATED TUNbSII N IIP AI II II 

NINE HOURS OF OPLRATION. (70x1. 

(1%) IWO I’I.RCENT THORIATED TUNGS71 N TIP AFTER 

FOUR HOURS OF OPERATION. (150~). 

(I)) SI LONI) IWO I’LIKtNI IIIORIATEL) IUNGSTEN TIP 

AFTER TWO HOURS 01 OPERATION. (150~). 

FIGURE 4. - SEM PHOTOGRAPHS OF TUNGSTEN BASED CATHODE TIPS AT VARIOUS POINTS IN TESTING. 

CD.Me?S 

ORIGlNAL PAGE 

BLACK AND WHiTE ?f-‘OTT)GR4PH 

12 
(C) POLISHED TIP (2OOX). 

FIGURE S. - PHOTOMICROGRAPHS OF SECTIONED CATHODE TIP IN POLISHED MD ETCHED CONDITIONS. 

CD-09-40826 

I 

I 

FIGURE 6. ~ RECRYSTALLIZED ARFA OF CATHODE TIP 

WIIH PITS (1200x1. 

(A) AFTER TWO AND ONE-HALF MINUTES OF (11) AFTER FOUR HOURS OF OPERATION. 

OPERATION. 

FIGURE 7. - TWO PERCENT THORIATED RHENIUM CATHODE TIP. 

CD-89-40820 
(A) NEAR BORDER OF MOLTEN AREA. (B) MOLTEN (RECRYSTRALLIZED) AREA. 

FIGURE 8. - SEM PHOTOGRAPHS OF HAFNIUM CARBIDE TIP. 

CD-09-40829 

(A) SEN PHOTOGRAPH OF TIP. 

(B) P :IOMICROGRAPH OF ARC CRATCR. (C) PHOTOMICROGRAPH OF TIP AFTER SECTIONING 

AND ETCHING. 

FIGURE 9. - CATHODE TIP FROM 100 HOUR CONTINUOUS TEST. 

CD-EP-40830 
(A) SEM PHOTOGRAPH OF TIP. 

(B) PHOTOGMICROGRAPH OF ARC CRATER. (C) PHOTOMICROGRAPH OF TIP AFTER SECTIONING 

AND ETCHING. 

FIGURE 10.- CATHODE TIP FROM 1000 HOUR CYCLIC LIFETEST. 

u)-89-40831 

15 
NASA TM-102099 - 

4. Title and Subtitle 

Arcjet Cathode Phenomena 

17. Key Words (Suggested by Author(s)) 18. Distribution Statement 

~ 

7. Author@) 

Francis M. Curran, Thomas W. Haag, and John F. Raquet 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland. Ohio 44135-3191 

2. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington. D.C. 20546-0001 

3. Recipient's Catalog No. 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-484 1 

10. Work Unit No. 

506-42-3 1 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

5. Supplementary Notes 

Prepared for the 1989 JANNAF Propulsion Meeting, Cleveland, Ohio; May 23-25, 1989. Francis M. Curran 

and Thomas W. Haag, NASA Lewis Research Center; John F. Raquet, United States Air Force Academy, 

Colorado Springs, Colorado 80840-5701. 

6. Abstract 

Cathode tips made from a number of different materials were tested in a modular arcjet thruster in order to 

examine cathode phenomena. Periodic disassembly and examination, along with the data collected during testing, 

indicated that all of the tungsten-based materials behaved similarly despite the fact that in one of these samples 

the percentage of thorium oxide was doubled and another was 25% rhenium. The mass loss rate from a 2% 

thoriated rhenium cathode was found to be an order of magnitude greater than that observed using 2% thoriated 

tungsten. Detailed analysis of one of these cathode tips showed that the molten crater contained pure tungsten to a 

depth of about 150 microns. Problems with thermal stress cracking were encountered in the testing of a hafnium 

carbide tip. Post test analysis showed that the active area of the tip had chemically reacted with the propellant. A 

100 hour continuous test was run at about 1 kW. Post test analysis revealed no dendrite formation, such as 

observed in a 30 kW arcjet lifetest, near the cathode crater. The cathodes from both this' test and a previously 

run lo00 hour cycled test displayed nearly identical arc craters. Data and calculations indicate that the mass 

losses observed in testing can be explained by evaporation. An increase in the redeposition rate with cathode 

recession was postulated to explain the observed decrease in mass loss rate with time. From measurements of the 

cathode crater size, estimates of the emitting area were made. These were used with realistic bounds on the 

crater temperature. material work function, and the electric field at the cathode to study the cathode emission 

process. From this, simple calculations suggest that both field and thermionic emission are involved. 

Electric propulsion 

Arcjet thruster 

High enimissivity coatings 

Unclassified - Unlimited 

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