261 IEPC-93-025

EXPERIMENTAL INVESTIGATION OF AHOLLOW CATHODE DISCHARGE

A. Salhi', RL M. Myerst, and P. J. TurchitThe Ohio State University

Columbus, Ohio 43210

AbstractI. Introduction

An experimental investigation of an orificed, hollow cath-ode was undertaken to obtain more detailed informa- The combination of low temperature and adequatetion than that previously available, in order to com- lifetime exhibited by low work function hollow cathodespare with recent theoretical modeling. Three diagnos- has led to their use in numerous plasma applications.tic techniques were applied: single Langmuir probes, The hollow cathode has been widely used in ion thrusterspectroscopy and pyrometry. The measurements were technology since 19621. After the development and useaimed primarily at the determination of plasma prop- of the hollow cathode in the SERT II as a neutralizererties, such as electron temperature, electron number by Rawlin and Pawlik' and as the main cathode of thedensity and plasma potential within the hollow cathode. SERT II thruster by Kerslake et al.3, all ion thrustersIn this experimental study, both argon and xenon were have adopted hollow cathodes. In addition to ion en-used in an attempt to establish the effect of propellant gines, hollow cathode technology has been applied suc-on the hollow cathode characteristics. Furthermore, ori- cessfully to a steady-state 100 KW class MPD thruster 4.fice diameters of 0.76 mm, 1.21 mm, and 1.27 mm were Moreover, these devices were used as high current den-used to determine the effect of geometry on cathode dis. sity ion sources to heat plasmas in controlled thermonu-charge. Langmuir probe diagnosis was performed along clear reaction experiments5 . In this case, hollow cath-the cathode center line. These measurements allowed odes offered, through adequate selection of the operat-determination of the axial distributions of plasma con- ing conditions, independent optimization and control ofditions. In addition to plasma properties, it was essen- the discharge parameters. Hollow cathodes were alsotial to measure the cathode external and internal surface utilized as a spectroscopic source to study gas spectratemperatures to identify the different emission mecha- and to determine transition probabilities.6' 7 Recently,nisms and emission current density profile. It was found the hollow cathode has found other applications such asthat the hollow cathode discharge was governed primar. plasma contactors' where this device provides a space-ily by the discharge current. Both the surface electron craft with the ability to emit or collect charged particlesemission region and the length of the internal plasma from a surrounding plasma environment.column scaled with the cathode inner diameter. Fur-thermore, the gas temperature was deduced to be higher Because of their wide range of application, it be-than the wall temperature. comes essential to understand fully the physical pro-

cesses governing hollow cathode discharges. This knowl-edge would allow the successful design of the hollow

Nomenclature cathode and optimization of its operation. In spite ofnumerous experimental investigations' - z3 and theoret-ical workl 4"'- conducted in this area, the hollow cath-ode still imposes several challenges (e.g., the proper se-e electron charge, 1.602 x 10- 1'9 Coule electron cha , 1.602 x lection of the operating conditions to meet mission re-k Boltzman's constant, 1.381 x 10- JIK quirements). It is the intent of this paper to address

m electron mass, 9.1 x 10-i 1 kg these questions through the identification of the primarymN ion mass, kg mechanisms of hollow cathode arc discharges.T, electron temperature, KV floating potential, VV, plasma potential, V II. Experimental Apparatus

*Graduae Student and Rearch Assocate, Departmentof Aeronautical & Astronauticl Engineering, Student Member An experimental program was carried out at theAIAA. NASA-Lewis Research Center's Electric Propulsion Lab-t Propuion Engineer, Sverdrup Technology, Inc., NASA- oratory (EPL). All tests were run in Bell Jar 6 (BJ6),Lewis Rmearch Center Group, Member AIAA.

'Profesor, Depatment of Aeronautica & Astronautical n- a 0.53 m diameter, 0.36 m long stainless steel vacuumgineerin. Senior Research Scientist, USAF Phillips Laboratory chamber (figure 3). BJ6 was evacuated using a turbo-Member AIAA. molecular pump, in conjunction with a roughing pump,

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IEPC-93-025 262

which was capable of pumping 260 liters/sec of air at 99.8% alumina tube (maximum operating temperaturea range of 1 to 10-" Pa (7.5x10 - 3 to 7.5x10-9 Torr). 1950 C). The probe current-voltage characteristics wereThe gas feed system consisted of two independent gas displayed and recorded on a digital oscilloscope (Nicoletinjection ports, one feeding gas into the hollow cathode 310).with the other feeding gas into BJ6. In this fashion thepressure in the chamber could be varied independently The experimental spectroscopic setup consisted of aof the cathode flow rate. The gas system consisted of 500M Czerny-Turner and an optical spectrometric mul-a number of precision leak valves that allowed fluctua- tichannel analyzer (OSMA) used for fast spectral datations in flow rate to be eliminated. A Hastings linear acquisition. The OSMA, a computer - controlled multi-mass flowmeter was used to establish the desired flow channel image detector, had the capability of detecting,rates. The gas feed system was also equipped with a measuring and manipulating spectra at high acquisitioncapacitance manometer, rated for 100 Torr maximum rates. The OSMA consisted of a computer console, apressure, that allowed measurement of the pressure up- detector controller (ST-120) and an IRY detector head.stream of the hollow cathode. Ultra high purity grade The detector head consisted of an optoelectronic image(99.999 %) gases such as argon, xenon were used as pro- device with the necessary electronics for its optimizedpellant in this facility. They were delivered from high manipulation. It was a self-scanning photodiode arrayspressure (250 psig) bottles, with 1024 pixels arranged linearly on a 25.4 mm long

single line. The detector was characterized by its wideA power supply capable of delivering 600 volts was spectral range, high dynamic range, geometric accuracy,

used with a current regulator. This setup allowed oper- thermal and temporal stability and the lack of read-ation at currents ranging from 1 to 30 A and voltages out lag problems. The detector received power, ther-between 10 and 60 V depending on the anode size, flow mostated and timed signals from an ST-120 detectorrate and distance between the electrodes. The facility controller. The ST-120 coordinated data collection withwas also equipped with 1 kV ignitor required for arc ig- the experiment, digitized and averaged data, set expo-nition. sure time, stored and transmitted data to the computer

and provided a real-time display of the free running de-The hollow cathode assembly (figure 4) consisted of tector readout. The spectrometer received light through

an insert, cathode body, an orifice plate, a heater and a single mode optical fiber. This fiber had a core diam-a radiator shield. The insert was 3.81 mm inner diam- eter of 200 pm, a cladding diameter of 250 jpm, a jacketeter, 5.33 mm outer diameter and 25.4 mm long tube. diameter of 1000 pm, a numerical aperture of 0.2 andIt was made of low work function material (1.8-2.0 eV): an aperture half angle of 11.5 degrees. To improve fibertungsten impregnated with barium compounds (4BaO- to spectrometer coupling an achromatic lens was posi-CaO-A 20 3 ). The low work function allowed the oper- tioned between the fiber and the entrance slit. In thisation of the cathode at low temperatures (1000 C). The fashion, the fiber beam was adjusted to the focal ratioinsert was housed in a molybdenum-rhenium cathode of the spectrometer. The alignment of the optical fiberbody: 5.59 mm inner diameter, 6.35 mm outer diame- with the spectrometer entrance slit was achieved by ater and 63.5 mm long tube. A 2% thoriated tungsten 0.5 mW compact He-Ne laser.orifice plate was electron-beam welded at one end of thecathode body. It had 5.84 mm outer diameter, 1.24 mm The internal surface temperature of the hollow cath-thickness and came in different orifice sizes. The ori- ode was determined using pyrometry by coupling a quartzfice sizes used in this research were 0.76, 1.21 and 1.27 rod to a two-color photodiode. The two-color photodi-mm diameters. The orifice outlet was designed with a ode is a silicon/germanium "sandwich" detector. The45-degree chamfer. An 8-turn coiled heater is used to high performance silicon detector mounted over the ger-heat the cathode during activation and ignition proce- manium detector responds to radiation from 400 nm todures. The heater consisted of a coaxial tantalum wire 1000 nm while the germanium responds to longer waveand tube insulated by magnesium oxide tubing. The length 1000 nm to 1800 nm that pass through the siliconheater was shielded by a 0.013 mm thick, 30.5 mm wide photodiode. This kind of detector is suitable for two-and 457 mm long tantalum foil wrapped around it. To color temperature measurements from 500 C to 2000 C.complete the electric circuit a 152 x 102 mm tantalum A clear fused quartz rod was used to transmit radiationplate was used as anode. from cathode surface to the detector. The quartz rod

was designed to withstand maximum working temper-Two different plasma diagnostic techniques were used ature of 950 C and 1200 C in a continuous and short-

in the experimental investigation of the hollow cathode term operations respectively. It had a constant index ofdischarge: single Langmuir probes and visible light emis- refraction for a wide wavelength band and exhibited asion spectroscopy. The probes used were single Lang- fairly constant transmission efficiency more than 90% inmuir probes in which a thin tungsten wire 0.127-0.254 the range of wavelengths between 200 nm and 2000 nm.mm in diameter and 1-2 mm long was supported by a The 1 mm diameter quartz rod used, was supported by

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a 99.8% alumina tube. To reduce the sight of light of the 0.6 to 1.4 eV for xenon were measured along the cath-plasma radiation, the quartz rod was cleaved at 45 de- ode center line using single Langmuir probes. In allgrees, coated by a tantalum film few microns thick and measurements, the probe traces exhibited behavior con-covered by a layer of ceramic adhesive. A small window sistent with Maxwellian distribution of the plasma elec-was left uncoated so that the incoming radiation through tron speeds. The electron temperature was found to bethe window could be reflected by the 45-degree surface nearly constant with the discharge current (figure 1).and transmitted through the fiber to the detector. The This behavior was found at all distances from the cath-pyrometric probe had to be manually moved rapidly in ode orifice. Similarly, the gas flow rate had very littleorder to minimize the exposure time so that the optical effect on the electron temperature. The experimentalproperties of the material would not degrade. In this data showed substantial variation in electron tempera-case, the high sampling rate required was achieved by ture along the cathode center line and an increase of elec-automating the reading of both probe position and ra- tron temperature with the orifice diameter. The effect ofdiation intensity signals. This task was accomplished propellant was illustrated in figure 2. The lower electronby the use of a multiple-channel A/D card. The use of temperature obtained in the case of xenon is consistenta computer allowed fast data reading, processing and with the low ionization potential of xenon compared tostoring, argon.

The cathode external surface temperature was mea- In attempt to shed some light on the state of thesured using type R thermocouples (maximum working plasma within the hollow cathode, spectroscopy was em-temperature 1600 C). In an attempt to determine the ax- ployed under a wide range of operating conditions. Theial temperature distribution on the cathode surface five particular question was whether or not the plasma wasthermocouples were embedded between the heater turns in local thermodynamic equilibrium (LTE) and the con-through small holes (1 mm in diameter) in the radiation sequent physical sense of the temperature determinedshield. Ceramic ( 99.8% alumina) tubes were used as in- using relative intensity measurements. Using argon IIsulation for its high working temperature. The presence spectral lines in the range 400-500 nm with excitationof the heater coil and the radiation shield surrounding energies from 19 to 23 eV, and xenon II in the spectralthe cathode tube minimizes the thermal losses from the range 400-700 nm with excitation energies from 14 tocathode surface. Thus, the uncertainty in temperature 18.5 eV, Boltzmann plots were constructed. In mostmeasurements due to thermocouple-surface contact, was cases, deviations from a straight line were less thanreduced considerably. 15%. Using relative-intensity measurements, temper-

atures from 0.91 to 1.3 eV for argon and 0.98 to 1.09 eVfor xenon were calculated for discharge currents varyingfrom 1 to 20 A. The results obtained showed a weak de-pendence of the "electron temperature" on the operat-III. Discussion of the results. D n of te r s ing conditions, and agreement with the average electrontemperature obtained using Langmuir probe.

Plasma propertiesElectron number density

In the case of the single Langmuir probe, a saw-tooth voltage pulse at frequencies of 1,000 to 10,000 Hz The electron number density was determined usingwas applied to the probe which allowed the entire probe cylindrical Langmuir probes. Due to the small size ofcharacteristic to be obtained in tenths of a millisecond. the cathode insert, a few millimeters in diameter, allIn all measurements more than five probe traces were measurements were performed along the cathode cen-recorded, analyzed and averaged. Due to the high prob- ter line, with the probe inserted from the back of theability of contamination, special care was taken in clean- cathode. The probe diagnosis was conducted using ar-ing the probe before each measurement by drawing satu- gon and xenon under a wide range of discharge cur-ration current to heat the probe tip until probe emission rents, mass flow rates and orifice sizes. In the regionsis observed. In addition, the probe was kept under a neg- where the plasma density reached 1014 cm- 3 or higher,ative bias (-50 V) for one to five minutes between pulses. it became difficult to obtain the electron saturation cur-

rent without severely perturbing the discharge. TheseElectron temperature perturbations were usually reflected in a rapid increase

of the cathode surface temperature and fluctuations inElectron temperature was measured using both di- discharge current and voltage. In this case, the probe

agnostic techniques for both argon and xenon. Under bias was offset to avoid reaching the electron saturationoperating discharge currents ranging from 1 to 20 A, current and consequently, the calculation of the plasmaelectron temperatures from 0.6 to 2.2 eV for argon and density was performed using the ion saturation current

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by applying Lafromboise's theoretical results20 . It is In all cases, the plasma potential for both argon andimportant to point out that the difference between elec- xenon decreased with the discharge current. This behav-tron number densities obtained based on Lafromboise's ior was related in part to to the increase in the electronresults and the thermal electron saturation current (for number density and the decrease in the internal plasmadensities less than 1014 cm- 3 ) varied from 1% to a max- column length. The plasma potential decreased rapidlyimum value of 20%. as we moved away from the cathode orifice. The plasma

potential for argon was higher, in most case, than thatIn general, plasma densities of 101 - 1015 cm- were of xenon but the difference in voltage decreased with

calculated under the range of operating conditions ap- the increase in discharge current (figure 6). In addition,plied in these experiments. The electron number den- this comparison showed clearly that the length of xenonsity reached a maximum value in the vicinity of the ori- internal plasma column was much longer than that office and decreased as we moved away from the orifice argon. Even though the cathode with 1.21 mm orificeto values of the order of 1012 cm - 3 at approximately diameter operated at lower discharge voltages, the es-two insert diameters. Farther away, the plasma density timated plasma potential was higher than that of thedropped drastically to values difficult to measure by the cathode with 0.76 mm orifice size. This high potentialprobe setup. The data (figure 5) indicated the existence was attributed mainly to the length of the conductionof three different zones within the cathode. It is believed path of the internal plasma column.that the active zone where the ionization takes placestarted at about one insert diameter from the orifice, Cathode internal pressurethe second zone, a diffusion sheath", extended also toabout one insert diameter and the last zone described The cathode internal pressure was measured upstreamthe neutral gas. The xenon plasma region within the of the cathode using a highly accurate (0.1% accuracy)cathode was larger than that of argon. This difference capacitance manometer rated for 100 Torr. Cathodeis consistent with the fact that xenon has a lower ion- internal pressures from 5 to 30 Torr were measured de-ization potential than argon. It was found that larger pending on flow rate, discharge current and orifice size.orifice diameter induced higher electron density. This In an attempt to determine the dependence of the cath-effect appears to be related to the higher electron tern- ode internal pressure on the operating conditions, theperature measured in the case of the larger orifice size. pressure was plotted against the discharge current andOn the other hand, this difference may be very small flow rate. The results showed that the pressure increasedin reality due to the errors associated with measure- linearly with the discharge current for a fixed gas flowments and probe theory. While the electron density was rate. Furthermore, it was interesting to notice that thestrongly dependent on the discharge current it appeared ratio of the slopes of those straight lines was equal toweakly affected by the flow rate. It is important to point the ratio of the gas flow rates. That is, the pressure isout that the maximum values of the electron densities proportional to the mass flow rate which is consistentmeasured are similar to the one calculated using the law with the theoretical formulation of the mass flow rateof mass action for typical operating parameters of the of a choked flow. The effect of the propellant on thecathode. pressure is illustrated in figure 7. By comparing xenon

and argon, the pressure ratio was found to be equal toPlasma potential the square root of the ratio of the atomic weights, which

is also an indication of the validity of choked flow at theUnder the conditions presented here, the hollow cath- orifice. Finally, to establish the effect of the orifice size,

ode operated at total discharge voltages from 10 V to the pressure was plotted against the discharge current40 V. The maximum plasma potentials measured within for two different orifice sizes. It was found that the pres-the hollow cathode did not exceed 35 V. In general, lower sure variation with discharge current was substantiallydischarge voltages were obtained with the cathode oper- magnified as the orifice diameter was decreased.ating with xenon. Also, an increase of orifice size causedlower discharge voltages. Wall temperature

In the vicinity of the orifice where a full probe trace In an attempt to understand fully the hollow cath-was not obtained for the reason mentioned in the pre- ode operation, it is important to investigate the cathodevious section, the plasma potential was estimated from wall temperature and its dependence on the operatingthe following equation2 l: conditions. Also, the knowledge of the surface temper-

ature is vital to identify the important mechanisms ofkT r m, electron emission from the cathode surface. These dif-

Vp = V} - - In } (1) ferent aspects of the cathode surface emission are related2' to the lifetime of the cathode which represent a central

question for the suitability of hollow cathodes for space

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applications. IV. ConclusionsIn this study, there has been a special interest in deter-mining the temperature of both external and internalsurfaces of the hollow cathode. In order to obtain the The experimental work conducted on orificed, hol-temperature profile on the external surface of the cath- low cathode arc discharges has shown that the cathodeode, type R thermocouples were attached to the surface operation, for a given geometry, is determined primarilyat several positions through the radiator shield and the by the discharge current. Beyond some minimum val-heater of the hollow cathode. This setup allowed con- ues, the mass flow rate has little effect on the cathodestruction of figure 8 which describes the temperature operation. On the other hand, the propellant has directdistribution under various operating conditions. It was impact on the discharge and ignition voltages. From the-evident that the wall temperature reached a maximum oretical modeling s (for cylindrical cathodes), the innerin the vicinity of the orifice plate and then decreased diameter is an important parameter that represents therapidly as we moved further upstream. In general, there scaling length for the surface emission and the internalwas a difference of 300 to 400 C in temperature along the plasma column. Because the hollow cathode operatescathode surface. The cathode temperature depended using thermionic emission, larger diameters would allowstrongly on the discharge current while it appeared in- lower emission current densities and therefore, lower wallsensitive to the gas flow rate. However, at very low mass temperatures. The latter would substantially increaseflow rate, the cathode temperature increased substan- the cathode lifetime.tially. This effect is related directly to the increase ofthe discharge voltage and subsequently the occurrenceof fluctuation patterns of the discharge luminosity and Acknowledgementsvoltage. The effects of propellant and orifice size on thecathode temperature were apparently not significant.

This work was supported by NASA-Lewis Research Cen-The insert temperature was determined using the py- ter. We would like to thank David C. Byers, James S.

rometry technique described in Section II. The knowl- Sovey, Maris A. Mantenieks, Vincent K. Rawlin, Michaeledge of the insert temperature is extremely important in J. Patterson, Timothy R. Verhey, and George C. Soulas,establishing the surface plasma-wall interaction, surface NASA-Lewis Research Center, for their assistance, sup-cooling capability by electron emission, surface work port and invaluable suggestions. The technical assis-function and the upstream limit of the active zone. The tance of Fred Jent and his team is sincerely appreciated.insert temperature profile was compared to the exter-nal surface temperature. It was found that both profileswere similar and the temperature difference was small References(less than 5%). This observation is consistent with thefact that the cathode wall was very thin and that thecathode was shielded on the outside. In this fashion, 1. R. C. Speiser and L. K. Branson. American Rocketthe radial heat loss through the wall was extremely low. Society, Paper No. 2664-62, 1962.Therefore, the outer temperature, which was easy to 2. V. K. Rawlin and E. V. Pawlik. AIAA Paper No.measure, could be used to predict the insert tempera- 67-670, 1967.ture and its axial distribution. 3. W. R. Kerslake, D. C. Byers and J. F. Staggs. AIAA

Paper No. 67-700, 1967.The analysis of the thermionic current density al- 4. M. A. Mantenieks. Preliminary test results of a hol-

lowed identification of the part of the surface that con- low cathode MPD thruster. IEPC Paper 91-076, Octo-tributed mostly in the emission and comparison with ber 1991.previous predictions of scale size of the internal plasma 5. J. S. Sovey and M. J. Mirtich. A hollow cathodecolumn. It was found that the current density fell dras- hydrogen ion source. NASA TM-73783, 1977.tically beyond one insert diameter from the orifice. The 6. K. Danzmann and M. Kock. Improved Ar(II) transi-scale size of the emission surface was comparable to the tion probabilities. J. Quant. Spectrosc. Radiat. Trns-scale size of the plasma internal column described ear- fer, 29(6):517-520, 1983.lier. 7. B. van der Sijde. Configuration temperature in a

Furthermore, the work function of the impregnated hollow argon arc and transition probabilities of the Ar-tungsten insert is known to be in the range of 1.8-2.0 gon I spectrum. J. Quant. Spectrosc. Radial. Trans.eV 2 . The deduced effective work function based on fer, 12:703, 1971.thermionic emission (Richardson equation) and the mea- 8. J. D. Williams. An experimental investigation ofsured cathode wall temperature fell in this range. hollow cathode-based plasma contactors. NASA CR.

187120, May 1991.

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9. G.V. Babbkin, V. G. Mikhalev, E. P. Morozov, and 16. A. Lorente-Arcas. A model of the discharge in theA. V. Potatov. Experimental investigation of the plasma hollow cathode. Plasma Physics, 14(6):651-659, 1972.in multichannel cathode. Journal of Ap plied Mechanics 17. A. Salhi and P. J. Turchi. A first-principles modeland Technical Physics, 17:767-770, November-December for orificed, hollow cathode operation. AIAA Paper No.1976. 92-3742, July 1992.10. D. B. Fradkin and A. W. Blackstock et al. Experi- 18. A. Salhi and P. J. Turchi. Theoretical modelingments using 25-KW hollow cathode lithium vapor MPD of orificed, hollow cathode discharges. IEPC Paper No.arcjet. AIAA Journal, 8(5):886-894, 1970. 93-024, September 1993.11. L. M. Lidsky, S. D. Rothleder, D. J. Rose, and 19. D.E. Siegfried and P. J. Wilbur. Phenomenologi-S. Yoshikawa. Highly ionized hollow cathode discharge. cal model describing orificed, hollow cathode operation.Journal of Applied Physics, 33(8):2490- 2497, August AIAA Journal, 21(1)5, January 1983.1962. 20. P. M. Chung, L. Talbot and K. T. Touryan. Elec-12. D.E. Siegfried. Xenon and Argon hollow cathode tric probes in stationary and flowing plasmas. AIAAresearch. NASA CR-168340, page 76, January 1984. Journal, 12(2):133-144, February 1974.13. D.E. Siegfried and P. J. Wilbur. A model for mer- 21. F. F. Chen. Electric Probes, in R H. Huddlestonecury orificed, hollow cathodes: Theory and experiment, and S. L. Leonard. Plasma diagnostic techniques. Aca-AIAA Journal, 22(10), October 1984. demic Press Inc., New York, Chapter 4, pp. 178,1965.14. J. L. Dekroix, H. Minoo, and A. R. Trindade. 22. R. Forman. Surface studies of thermionic cath-Establishment of a general rule for a hollow cathode odes and the mechanism of operation of an impregnatedarc discharge. Journal de Physique, 29(6):605-610, July tungsten cathode. NASA TN D-8295, September 1976.1968.15. J. L. Delcroix, H. Minoo and A. R. Trindade.Gas-fed multichannel hollow cathode arcs. Rev. SciInstrum., 40(12):1555-1562, 1969.

SArgon * L 2 mm o w Rate* 0.5 A eq.Orf. Dia. 0.76 mm Dischorge Current 15 AS Rate - 0.93 A Equiv. L - 6 mm O

r i f ic e Dimeter 1.21 mmo

2.4L - 9 mm

5 2.4 -.U

S2.0 T T

2 5 6 I 10 11 12 1 14 I I 2 ' 4 S ' 7 ' *

2 c

2 3 ' S 5 7 11 9 10 11 12 13 14 13 5 1 2 3 4 5 a 7 a 9

Discharge Current (A) Distonce from Orifice (mm)

Figure 1: Effect of discharge current on electron temper- Figure 2: Axial distribution of electron temperatureature along the cathode center line at different distances along the cathode center line in argon and xenon dis-from the orifice. charges.

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imm Pe" Gape

bi:o: da ?Won W~ "

Figure 3: Schematic of hollow cathode test facility

Figure 4: Schematic of hollow cathode assembly

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IEPC-93-025 268

Ia" .. - 1

, Xenon * I . 5 A Flow Rate 0.50 A eq. Aroni0'* Flow Rote = 0.5 A eq. I g A 14 Orifice Diameter - 1.21 mm * xenon

Orifice Diameter - 1.21 mm

• 1014 : 10

1013

z ,.W 1011 "

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23

along the cathode center line at different discharge cur- pressure in argon and xenon plasmas.rents.

28 -1 - -

Flow Rte - 0.5 A eq. * Argon Xenon * I , 2 A2, Discharge Current - 5 A , Xenon 00 Fow Rote - 0. 9 3 A eq. , I - 5 A

Orifice Diameter * 1.21 mm - Orifice Diameter , 1.21 mm , .U ItOA

20 -

2 T '00o ,u

o 1E I * aw Y

4

0 1 2 3 4 5 6 7 I 10 11 0 4 1 12 Is 20 24 28 32 36 40

Distance from Orifice (mm) Distance from Orifice (mm)

Figure 6: Axial distribution of plasma potential along Figure 8: Axial distribution of cathode external surfacethe cathode center line in argon and xenon plasmas. temperature at different discharge currents.

8