IEPC-9 5 -116 '

P'l.ASMA FEATURE.S lN O,.ASISTEADI)Y IMPD) (1NNE.S

iIRt)KA/ TA1.\llA.\RA. K\A/1 N(lI MiliSL'n). ll,'ll /1A\C"

YOI'HI K.-AY.YA :sa!i TAKAO( Y)OSHKAA\\

Abstract

Plalsm;i diagnostics and flowficld analysis in quasi-stc:id\ MPI) ch;ir nels. ;ii;aoi icalizingclectronm;inetic only blowin, acceleration, i.e., one-dimensional flo Aield. \\ C ;;e .T. Thedischarec current concentrated near the downstream end of the disLchi;re ch.ileCr, andparticularli the current fractions for molecular gases we\ r much l.tr :'cr ithlm : iosc formonatomic gases regardless of particle weight. Ho eve\cr. the current concent iI. lon near theinlet, which ,was expected from the one-dimensional MHD tlowield anal\ is. \\is hardlyobserved. The electron temperature optically measured ;areed with thit calculatecd. Theaxial variation of the velocity inferred from the electron number densit measured showedthat the plasma was expected to he drastically accelerated only near the outlet. although theanalyzed velocity profile had two acceleration zones near the inlet and the outlet.

Introduction

The quasi-steady MPD arcjet is a promising propulsion device which utilizes principallyelectromagnetic acceleration of the interaction between discharge current of kiloamperes andazimuthal magnetic field induced by the discharge current. Various propellants. such as lightgases of H. and He. heavy gases of Ar. Xe and SF,, and waste ones in space stations etc.can he used because the thrust depends only on the discharge current and does not basicallyon propellant species for electromagnetic acceleration. However. in an MPD dischargechamber. complicated chemical reactions including dissociation and ionizaition are expected tooccur together with the acceleration process. Furthermore. for th, .ccclcration theory (oMPD arcjets. there exist two components of electromagnetic force. i.e.. the blowing andpumping forces. These forces generate a complicated flowfield in a; discharge chamber.Consequently. it has been recognized that the performance characteristics on the dischargevoltage, thrust, thrust efficiency and electrode erosion depend on propellant species andelectrode geometryll]-[5].

One or two dimensional analyses of MPD arcjet flow\fie., ha',e ic n nmade hmany researchers(6)-[S]. in which the electric field is determined hb the point of the sonicor the magnctosonic singularity. These calculated results are significant for optimum desi.gnof MPD arc.et chambers and for understanding of the discharge feature and the accelerationmechanism.

In the present study, plasma diagnostics and flowfield analysis in quasi-steady MPDchannels, almost realizing only electromagnetic blowing acceleration, i.e.. one-dimcnsionaiflowfield, are carried out to understand plasma feature and acceleration process in anacceleration zone simplified. The discharge current distributions on the electrodes areexamined using segmented electrodes. Optical measurement is conducted, and the electrontemperatures and the electron number densities are estimated. The experimental results arecompared with analvtical ones.

* Associate Professor, Faculty of Engineering Science, Osaka University,1-3. Machikaneyama. Toyonaka. Osaka 560. Japan.** Graduate Student of Osaka University.+ A-\sistant Engineer, Osaka Uni.crsit..++ Professor, Osaka University.

- 797 -S[xp rin iental Apparatus

I ', i [) ,., -.

Mii5I L ."

:. T. i L hL' i . i ; it ! 1 .r r . . .'.. .' "

n" c ie d i ' I'- . C l

]' l. Ihe n. m d'Ji.

of copper is di' idcd iAll,into 14 anodc p ts. c h Fi. 1 C(ross section ot qusi - , si J 'I) ' icOt ,k hich is' ...11: ,1\ channel M C-1.

ilsulated to (nc :iiithlcr.I

;and the scinicnt N\o, is locaitcd outside thgedicharge chamber. lh current entrii cachanode scment is mieuricd with a Rogo\v ski coil to examine current rir;ctios om the aInodc.

i.e.. to infer the currcnt pattern in the interclectrode region. Fhc cilinilical c;ithoidc .5 mm1in diameter 2,S( mm long is made of thoriated tungsten. The downstreamn ends of bothelectrodes are provided with insulators which prevent current from concentrating on theirsurfaces and from spreading out of the discharge chamber.

Gases are iniected xwith a cathode slit / anode slit ratio of 5() 50 into the dischargechamber through a tasl acting valve (FAV) fed from a high pressure reservoir. The risetime and width of the gas pulse, measured with a fast ionization gauge. are 0.5 and 6 msec.

respectively. The mass flow rates are controlled by adjustment of the reservoir pressure andthe orifice diameter of the FAV. Argon. hydrogen and nitrogen are used as propellants.The mass flow rates ire 1.37 gs for Ar. (.40 g/s for H, and 0.74 -gs for N,. The masstlow rate for each gas is set up at a corresponding critical current of about 10 kL. which isderived theoreticall\ from tin e rule of mini:num input power or \Alft\ ns critical ionization. clocity.

The main power-supplyig pulse forming network is capabh of storing 62 kJ at 8 kV.:n:d it delivers a single nor.reversing quasisteady current of nmaximum 27 kA w ith a pulse

.ith of ().6 msec. A .acuum tank 0.6 m in diameter 5.75 :n in length. where the MPD,hannel is fired. is c' acuated to some 1(0 Pa prior to each discharge.

Discharge current is measured with a Rogowski coil calibrated with a known shunte'istance. Voltage measurement is performed with a current probe (lIvatsu CP-502). which

detects the small current bled through a known resistor (I) k_) ic!,eween the electrodes.

Emission spectroscopic measurement is conducted as reliable pl:smna diagnostics in MPDlischarge chambers. Light comes from the plasma through a slit 0.5 mm in width between.-cments. The emission is collected by a lens and is introduced into a (.5-mimonochrometer through an optical fiber. The monochrometer of diffraction-grating-type

HAMAMATSU C50'-)5 is provided with 150- and 2.4(00-groves mim grating plates. animage intensificr and a 1024-channcl diode array detector, achieving spectral resolutions of0.S and 0.05 nm. respccti.cly. per detector channel. Electron temperatures and electronnumber densities are determined using the datai. The elcctr :: temperature is determinedusing a relative intensit. me::hod of spcc:ral lines, i.e., b.\ means of Boltzmann plotting with.A\rl or NIl spectral lines. The electron dc sit\ is estimated romn the Stark width ofhvdrogen H, line 4-S) I nm. in which :a mixture of argon or nitrogen rand a f!w pr-rcent

Csed hvdrogen is used. Line-of-sight measurement is conducted, and horizontally-;i\ rageiphysical properties are calcu!ted directly from 'he horizontall\ - integrated spectral intensitiesS mea:sured: i.e.. Abel transformatiuns arc not canied out.

i

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Experimental Results and Discussion

(Currcnt Distribution

Ficure 2 shows the current fractionsdstributCed on the se:imcntcd anode of MC'-I. " 1 Ar '0-in which the mainctic Reynolds nunmber i st ,to aboult 0). The curreni fractions on theanode ,sement No.14 are highest for allg.ases: i.e.. the current concentrates near the °downstream end. Furthermore, the current, .fractions of No.14 for H, and N. are muchhigher than those for Ar recardless of particle 0

wveiht. and in other words for the monatomic , 80ases the current is distributed in the f 6n

intermediate recion of the anode although forthe molecular eases the current hardly exit in Ethe region. These are explained from MIPD 0flowfield analyses as follows[7].[8). The 1 0 5 6--- 0 2 13 4ionization process of molecular gases is slower___than that of monatomic ones owing to a time N2 INAlag due to dissociation process. Therefore, the 80ionization process of molecular gases starts 60near the arcjet exit. Also, the current 40fractions of No.14 decreased with increasing

. ., , 20 _

discharge current, and the current isdistributed in more upstream segments. 0 1 2 3 4 5 6 7 8 9 1011 121314Hence, these features on current conduction Segmented Anode Numberare found to depend strongly on gas speciesand discharge current lcvels[9]-[12]. Fig.2 Current fractions on anode of MC-1

for Ar. H, and N, at 10 kA.The magneticElectron Temperature and Electron Number Reynolds numbers are set to about 10.Density

Figures 3 and 4 show the axial variations of the electron temperature for Ar and N..respectively, in which the magnetic Reynolds numbers for Ar are about 10 and 20 at 10 and15 kA. respectively, and for N, about 10 and 35 at 10 and 15 kA, respectively. The

2.0 2.01. Ar 10kA .8 Ar 15kA1.0 1.8

>1.6 >1.6a) a

1.4 ' A - 1.4

S 1 .2 " 2 1.2 c § .

S 1.0 1 1.0E E1 0.8 i 08

o 0.6 o.r= 6mm o 06 r= 6mm20 r=11mm . r=11mm

a . 0.4 r=16mm a 0.4 r=16mmw - r=20mm a - r=20mm

0.2 c r=23mm 0.2 c r=23mm

0.0 4 8 12 16 20 24 28 0.0 4 8 12 16 20 24 28

Axial Position . cm Axial Position , cm

(a) Ar at 10 kA. (b) Ar at 15 kA.Fig.3 Axial variations of electron temperature for Ar. The magnetic Reynolds numbersfor Ar are about 10 and 20 at 10 and 15 kA, respectively.

I

- 799 -

1 c-- - -- - - - "-- -- ----- -~ ----* ' , -c , :J 5 -, ,

*: :

t*

S06i 06S0r= 6rm - 6mm

S0 4 r 16mm 4 16rmm

o 2: r23rm ,m r 23mm

00 4 5 12 16 20 24 28 00 8 '. 1 . 24 28

.:al Position cm Aic;a P : , :

(a) N. at 10 kA. (h) N at 1i k.-.

Fie.4 Axial variations of electron temperature for N. The mlagnetic Rc\nolds numbers

for N. arc about 10 and 35 at 10 and 15 kA. respectively.

10' -- 10''1Ar 10kA Ar 15kA

r= 6mm - r= 6rmmr=11mm r=11mm

A r=16mm r=16mmE1016 r=20mm 1 0

6 v r=20mm- r=23mm = r=23mm

- C0 0

o10'5 Z 10 5

U

10'0 4 8 12 16 20 24 28 10 4 8 12 16 20 24 28

Axial Position , cm Axial Position, cm

(a) Ar at 10 kA. (h) Ar at 15 kA.Fig.5 Axial variations of electron number density for Ar. The magnetic Reynoldsnumbers for Ar are about 10 and 20 at 10 and 15 kA, respectively.

experimental errors are within 0.2 eV for Ar and within 0.3 cV for N,. The electrontemperatures for both gases are axially kept about 1.2-1.4 cV regardless of dischargecurrents. Since the profiles hardly depend on radial positions, the flowficlds in the MPDchannel used are expected to be nearly one-dimensional ones.

Figure 5 shows the axial variations of the electron number density for Ar. The profileshave peaks at an axial position of about 20 cm. As shown in Fig.6. the electron numberdensity for N- has the characteristic as well as that for Ar although the peak location movesupstream. This characteristic is related closely to dissociation and ionization processes andacceleration process in the channel. The electron densities for Ar and N. range from2 '- 1 to 3- 10" cm - .

I

- 800-

. 8 2 - 'I.. 28 0 0 8 ' '"*4 ;"8

'A osiion , cr" A i PC ' "

(;i) V at 10 kA. (I) N .1 15 kAFie.6 Axial variations of electron number density for N. The maIenic Reinoldsnumhers for N. arc about 10 and 35 at 10() and 15 kA. respectivel.\.

Flowfield Analysis

The present analsis is carried out using a popular onc-dimcn.ial I MHD flow modcl[13]. Particularl, we assume as follows: (1) the gas flows etwccen pralllc[ clectrodes:(2) the flow. electric field and magnetic field arc perpendicular to one anothcr: (3) themagnetic field is induced by the discharge current: (4) viscosity, heat conduction. convectionand radiation, and Hall effect are neglected: (5) the electron temperature and the translationaltemperature of hcavy particle species are considered: (6) the velocities for all species aresame: (7) non-equilibrium dissociation and ionization are considered: (S) The electric field isdetermined from the singularity of the sonic point. The length :ind width of the electrodechannel are 28 and 3 - cm. and the gap between the electrodes is 2 cm. The mass -lowrates are same as 'iose for the experimen:s.

Calculation Results and Disct:,sion

Figures 7-10 show the axial variations ofthe current densit'. temperatures, number 40- -densities, degreec of dissociation andionization and velocity for Ar and N, at 10kA. respectively. The sonic points for both 20igases exist just near the inlet. The electron 0 10itemperature at the sonic point for Ar is15.000 K. the heavy species temperature 600 2 o-1 1 2 2 I,, i r c . -1 4 8 12 16 20 24 I28K. the degree of ioniza:ion 0.0038, andelectric field 610 V'm. The physical values o 100 --for N- are 18.300 K. 700 K. 0.0035 and 1038 80 N?- 10kA

V/m. respectively, and the degree of 6dissociation is 0.)0. Uo

40

The calculated current .istributions show 20'that the current concentrates near the inlet and '

0 3 12 16 20 24 2outlet. For Ar, there is conduction of small 0 4 8 12 16 20 24 2A..y al Pjsition. cmcurrent in the intermediate region as weil as

the current distributions measured as shown Fie.7 (Clcul:tcd current fractions of MC'-Iin Fig.2 although current hardly flows for for Ar and N a.t 10 kA. The nmagneticN,. The current concentration near the Reynolds numbers arc set to about 10.

I

- 801 -

2 C - - 20 - . . .3 .r >,2- N> 10

-- . n 1-

n 0

00 .4 8 ;2 6 20 24 28 0 4 6 1' 24 23

Aial Pos ton,. cmn

(a) Tcnipcraturcs for Ar. (;a) cmpcrturcs lor N..

S----- --------------- 10

Ar 10kA N: 10kA

E 10 n.

S101 10

S- z

1 0 110

0 O 4 8 12 16 20 24 28 4 8 12 16 20 24 28

Axial Position, cm Axial Position, cm

(h) Number densities for Ar. (b) Number densities for N,.

. -- 1. ------ r-------1_ -1.0 -0

SAr 0I N. 10kA

SkA lonizationS10kA DissociationS10.6 06 .... .

I------ 15kA

S0.4 0.4j

. _:5

0 0.2

1- , 0o

0.4 0. 4

0.0 4 8 12 16 2024 28 0.0 4 8 12 16 20 24 28

Axial Position, cm Axial Position, cm

(c) Degree of ionization for Ar. (c) Degrees of reactions for N .

physical properties for Ar at 10 kA physical properties for N- at 10 kAwith Rm=10. with Rm=l10.

.

- 802 -ilLcl d!",Ie'; CC w i th tle currlent t IInc eurcd h, si c\pcicd lto hC !K',_ c oi ,.t11; cci U .;usi ll ;' at

us rrlo;" ;!e '1od . i.e.. not n :,Also. a !h:C iluid modcl ot electrons, i 's indneutral 1iii c ,s cids to be used e;it ' :h !e i;lei't.l u leCrmcli I 1 . sincC the expeim Ient:il NM l)L[c t1inn l , I \it 1 n l l I lijis 1e io' Ie

eIltem nt \n.1 cI d ifltn tilng electrodes nrutsildc themain di.,ch.i~.c zone. the current concn:ilitil onll

Inear the inict is cxpccted to relax.

-A shown in Figs. t(a) and 9(a), the ccctrontCllipcriatur s I rl- ;aibout 1- 1.5 C\ In !he 4 : 1 ,intcncrm diiat rceion of the chainnel ,s \.ciil I,thosc nicas urcd zas shown in Fies.3 and 4although therl exist drastic increases neiar Fie. I. ) CalculIatcd axial 'ta I:Ii ons ofthe inlet and outlet hecause of current velocities for Ar iind N at 10 kA. Theconcentration. The degrees of dissociation magnetic Re nolds numbers arc set toand ionization, as shown in Figs.S(c) and about 10.9(c). agree with the electron temperatures.In the particle number densities shown in Figs.S(h) and 9(h). the electron densities increaseintensivelV near the inlet. and they are almost kept constants in the interm dieate region otthe channel. These profiles disagree with those measured as shown in Figs.5(a) and ((a).Furthermore. the electron densities calculated for N, are much lower than those measured.This is due to the current concentration near the inlet. For the calculations the plasmagenerated near the. inlet is accelerated extremely just near it and near the outlet owing tocurrent concentration, as shown in Fig.10. However, in the experimental MPD channel thegas is gradually ionized through the channel, and the plasma generated is expected to hedraticall\ accelerated only near the outlet. The velocity roughly inferred from the electrondensity measured shows this feature of acceleration. Consequently. the analvzcJ currentconcentration near the inlet causes the difference of the acceleration process. that is. thelarge differences of the velocity and the degrees of the chemical reactions w hic'i .rc icrlatedto the difference of the clc:tron number density.

Cu.r.ent Distributions of MPID channel MC-11

The coaxial MPD chranncl GMC-11, as shown in Fig. i, is Gas orsdesigned so that theexperimental results approaches 345- C67 _-odethe analytical ones. The anodehas a length of 200 mm. andthe inner diameter of 50 mm is 1 - lsame as that of MC-I. Thecathode is 26( mm in diameter. InsulatorThe gap between the electrodesis 12 mm. and i: is about half 50rof that of MC-I. MC-II is Fig. 11 Cross section of quasi-,:cadN MiPDnot providcd with floating arc channel MC-Il.electrodes. and gas is injecteduniforml\ from the upstream end of the dischar:c chamber. Current d .ntribu:ioi(- on theelectrodes can be measured with the segmented anode and cathode.

Figures 12 and 13 show the current distributions on the anode and cathode ,of (MC-II for

I

- 803 -

. . . . 1 ......I. .. .. . II

80| .1 -<sr -- -*. .. 2'" m C' :, e sur ace : -- h -.. :

1 o . c-surf.ace" 60 d '.: -"

40

: 100 --- 100Ar. Rm=10 - N,, Rm=10

6 80- o 80LS:Cathode surface - :Cathode surface

60 c :Anode surface 601 L :Anode surface

40- 40r

20- 20

23...4--5 6 7 8 9 0 1 2 3 4 5 6 9 10

100. 1------- 00------Ar, Rm=20 N- , Rm=20

80- 80'-S :Cathode surface :Cathode surface

60- o :Anode surface 60 1 :Anode suface-*! : " -.3- 0 - _40-

1 i , *o.II .- 8 I 20

1 2 3 4 5 6 7 8 9 0 2 3 .- 5 ' 3' 9 'O

Segment Number Segment :'um-er

Fig. 12 Current fractions on anode and Fig. 13 Current fractions on anode and

cathode of MC-1I for Ar at S kA vwith cathode of MC-II for N, it S kA with

,various magnetic Reynolds number,. various magnetic Reynolds numbers.

The mass flow rate is controlled to The mass flow rate is controlled to

var\ the magnetic Reynolds number. var\ the magnetic Reynolds number.

Ar and N,. The current fraction on the anode intensively increases downstream: howevcr,

that on the cathode gradually increases except for Ar at high magnetic Reynolds numbers.

The current fractions on the cathode for Ar at high magnetic Reynolds numbers slightly

increase near the inlet compared with those in the intermediate region of the cathode. Sincethe current pattern on the anode is slightly different from that on the cathode, a little axialcurrent flows in the arc channel However, we can recognize the above experimental resultswith MC-I even by using MC-1I.

Conclusions

Plasma diagnostics and flo tfield an:i sis in the quasi-steady MPD channels. ;i!mios,

rcalizin, onlv electromagnetic blolwing acceleration. i.e.. one-dinmensional tlow i d. wcrc

nide. The discharge current -onccntraed near the do\wnstream end. :tnd paricularly the

current t;actions for molcular gascs \\wre miuch larger than those to: ma;Iaontic g;.ie

regardless of particle weight. However,. the current concentration near the inlet. which was

I

- ou'+ -

expected from the one-dimensional MHD flowfield analysis, was hardly observed. Theelectron temperatures for Ar and N_ optically measured were axiall\ kept about 1.2-1.4 eV

regardless of discharge currents. The electron temperature measured aiLccd with that

calculated. The electron number densities for both gases ranged from 2 1' to 3 10cm . The axial variation of the velocity inferred from the electron number densi\ lmeas;ured

showed that the planma was drastically accelerated onl\ near the outlet ot the channel.

although the analyzed velocity profile had two acceleration zones nc r thc i:: e and theoutlet.

References

1. H. Tahara. Y. Kaga\a and T. Yoshikawa." Ouasistcad\ MagnceoplasnmaJ\ namic Thruster

with Applied Magnetic Fields for Near-Earth Missions." J. Propulsion and Power. Vol.5.1989. pp. 7 13- 7 17 .2. H. Tahara. F. Takiguchi. Y. Kagaya and T. Yoshikavwa." Perforniance Characteristics andDischarge Features of a Quasi-Stcad Applied-Field MPD Arcijt." Proc.AIDAAAIAADGLRJSASS 22nd Int. Electric Propulsion Conf.. Vol.l. Paper 911-1173. 1991.3. T. Yoshikawa. Y. Kagaya and H. Tahara." Continuous Operational Tests of a Quasi-Stcady MPD Arcict System." Proc. AIDAA AIAA DGLR JSASS 22nd Int. ElectricPropulsion Conf.. Vol.1, Paper 91-075. 1991.4. H. Tahara, Y. Kagaya. Y. Tsubakishita and T. Yoshikawa." Performance Characteristicsand Discharge Features of a Quasisteady Applied-Field Magnctoplasmad namic Arcjet,"H nd German-Russian Conf. Electric Propulsion Engines and Their Technical Applications.Paper A2-13. 1993.5. H. Tahara. Y. Kagaya and T. Yoshikawa." Effects of Applied Magnetic Fields onPerformance of Quasisteady Magnetoplasmadynamic Arcjet." J. Propulsion and Power. Vol. 111995, pp.337-342.6. H. Matsumura. H. Tahara, Y. Tsuhakishita and T. Yoshikawa." Elcctromagnetic Particle-In-Cell Simulation of Plasma Thruster Exhaust Plumes." Proc. Int. Symp. Simulation andDesign of Applied Electromagnetic System, Paper F38, 1993, pp.693-696.7. H. Tahara, H. Yasui. Y. Kagaya and T. Yoshikawa." Experimental and TheoreticalResearches on Arc Structure in a Self-Field Thruster," AIAA-87-1093. 1987.8. M. Sumida and K. Toki," Real-Gas Effect on the Magnctoplasmadynamic Arcjet." J.Propulsion and Power, Vol.7, 1991, pp.1072-1074.9. T. Tsubaki, H. Tahara, Y. Kagaya, Y. Tsubakishita and T. Yoshikawa." CurrentDistributions of a Self-Field Magnetoplasmadynamic Arcjet," Proc. Int. Symp. Simulationand Design of Applied Electromagnetic System, Paper F39. 1993, pp.6 9 7 - 7 ( 0 .10. T Yoshikawa, H. Tahara, Y. Kagaya and Y. Tsubakishita," Performance Characteristicsand Current Distributions of Quasisteady Magnetoplasmadynamic Arciets." 1 nd German-Russian Conf. Electric Propulsion Engines and Their Technical Applications. Paper A2-15,1993.11. H. Tahara, T. Tsubaki, Y. Kagaya, Y. Tsubakishita and T. Yoshikawa," DiagnosticExperiment and Numerical Analysis of One-Dimensional MPD Flowficlds,"AIDAA/AIAA/DGLR/JSASS 23rd Int. Electric Propulsion Conf., Paper 93-197, 1993.12. H. Tahara, Y. Kagaya and T. Yoshikawa," Current Distributions in a QuasisteadyMagnetoplasmadynamic Arcjet," J. Propulsion and Power, Vol.9, 1993, pp.7 7 8 - 7 7 9 .13. T. Shoji and I. Kimura," Analytical Study on the Influence of Nonequilibrium forCurrent Flow Pattern and Flow Field of MPD Arcjets," AIAA-90-2609. 1990.

I