3 -505 - IEPC-95-75

3 LIFETIME OF THE STATIONARY PLASMA THRUSTER

I .M n :I "Permen-l CDesign FureauTaeKalin~naraa, Russia

Staion'ustsa r\'ZPT)a -hsten suorzs fly oprtdsinc 1:-72 o boa- of severalIRussian spacecraft- 'Meteor', -Meteor- Priroda, since 1982 on some soececraft of -Kosmos', and

I .AIXEL's exoenience shc that SPIT lifetime depends primarily upon the erosion of thei.ha Bra~ ede and t' *:ahod life.s Cwrrently operated thrusters, SPT711 and SPT110C3 nave demonstrated the foilowina oerforrnance. thruEA/oower - 40 mN/680 VV and 80 mN/1360 VW.:et-q, pC;if.C im pulse - 1 503.-1100 r., effi-ency - 45-50% [31 and life, rnaintaining all principalparameters -3500 and 7000 hlours resoeciiveiy. SPT100 had oacced the life testina at FAI<EL

fa~h;:[~mwt:h acumulated 70OCI hou*rs and facility C;f jet Propulsion Latcratcr'; (USA) [53, theaccumnuiated life ic 5700 hours at 6000 cycles on one cathode. SPT 100 is still operable.

Currentlv ooeraied Russian sc'acecraft did not recuire from SPT the fifetime, in excess of 700-200C Iur. EL", Pe~ .rform:n; 'north-soutfh taln pion the txatlcnary o-rbit, and,navrno increased the iife of geoctationary spacecraft to 10 -15 years, lifetime recuirement is now"Pc- a:- -"OCC-500C ho-ursz, at ntriioned thrust and --wer.

1. EFFECTS ON THE SPT LIFETIME3Miieiffc: mr SPT iifetm rr #zie Vo grouos:

- fect_ Dn anooe unit tife. e. isoiator eotge ero-sion:- flc~i~tr~s: - . a.edischarge cnamb.er Passage crosc section area (miS) ratio,

-acnceieratrma voizage riJ)v

piu- e d veru nce- an'ie .containing 90% of ion current:3 - ~materi of dica-echamber isolator.;-test conaitions

2- cathrYe 'ife.

- urnty g'-are of xenion (esp-_zialiv by oxygen and water ,, dir-ected into cathode-- encn ,",ateohrough cathode;'3 - test conditions.

Currently, the investigation of phenomena, causing a higher erosion of -.pare, non-operatingI athode, s bing _c-djt.

Ths ei!be ciscus~ed in ceparate report.32. FLOW STRESS AND DISCHARGE V.,OLTAG E

nese e~ff i- wers diccoverad at the very beginning of SPT development both by' ctudy ofsmpiec ana in vthe lfe tesis of SPT. Fig. 1 showz the change in the rate of erion (V) with the

diaag voltage (U) for diffarent materialr.

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!t ;s known that each matenal has its own voltage threshold (Up), below which it does noterooe because of insufficient energy of chargec particles. At the voltage close to the threshold, SPTevidently would have very low efficiency, thus being not suitable for most applications.

The dcharge chamber eage rate verun the discharge voltage and the flow stress can beexpressea as follows:

V = c mS ( -Up)(1- ) , (1)

where c - factor of the discharge chamber matenal and the plume aperture angle;

n - the thrust efficiency of SPT.

The erosion rate is the most in the beginning of operation. Then it decreases according to thespecific relationship, Fig. 2 [6]. The lifetime, the firing time (t), of SPT is determined according to thefollowing equation:

S;;' (632 b-1') I (c m/S (U - Up) (1 - )), (2)

where 8 - thickness of the inner wall of the discharge chamber,b - width of the passage of the discharge chamber,K - cef.tcient.

Ac &li geometrical dimensions in SPT are directly connected with the diameter, T can beexpressed in the following way:

t= r. - (c m/s (U - Up) (1 - )), (3)

where. u is the diameter of the discharge chamber.

This reiationship shows satisfactory usefulness till 1200 hours of the accumulated operationtime of SPT00, Fig. 3, that corresponds to the half erosion angle of 220, the angle of flowdivergence, containing the 90% of the ion current. At this moment the direct impingement at thedischarge chamber surface by the charged ions becomes negligible. However, isolator is still beingsputtered. but at the rate by 10-12 times slower The physical process changes. Paper [7] assumedthe erosion by electrons, heated by near-wall conductivity.

Therc i1 another one version - ions, formed at the discharge chamber outlet, contribute to theerosion. This process exists from the beginning of life, but contributes significantly less than erosionby the accelerated ions.

Erosion of dischargt chamber after 7000 hours life testing show Fig. 7.After 2C0 the SPT life is determined by the following expression: .

S= K; d ((a - 1)/.O.9) / (c m/S (U - Up)(1 -)) (4)

where K2 - coefficient,a - cone angle of discharge chamber edge;o0.9 - angle, containing 90% of the ion current.

The total time of SPT operation is determined as follows:

= T+ 1 =(K 1 d + K2 d ((a - 1) / 09)) / (c m/S (U -Up) (1 -,)). (5) 3For SPT, at the specific parameters m/S = 0.1 mg/sec cm2, U = 300 V:

TE = 12d + 58d = 70d, (6)

I

1 -507-

e imDuI.se. aSe'Pe y R100. i - 2 0- I7 11 s- tnat m7,3po, ds t: 7000&ri ~ ~ ~ ~ ~ ~ ~ 6" Caycrnz-iutcmcr ha~o a G wc exceedr. the requiremnn ;,-rr W~2h *:at m uver~g c 'v a geo0stnonYary sacecraft" 'Vth aCtive Vle Cf 1.5 vearsz

3I E 2CS ON THG LIFETIME BY THE SPrT DIMGNSIONS AND THE VALUES OFPARAMETERS r"!S AND U.

CA VEL devLped tjhe Smur !irnc (Fig a- at +' a~e ~r 5t20mmII2.drrecfc

35t

Life of the whole model line was discussed in1 .

U The goometrical similarity shows

=.2, then Fi+ l/i "i 1Icl = 2 (7)

wner6 a., Si -ne aiameter or the discharqe chiamber ano the Cross section of the acelerating

U t e p rasari ., tha ~ ~ 3 iiaa;jooso h PT modsijlin~eat 01M /e- and J

"A! k1e 6 200 0 0 2 700 60 00 111000___

________ _______ i 500 10__ ________ 30 2700 1650 1_2000__ 000___18000_ 000

The 11(9 of SPT 100 has been test demonstrated.* :ncraasing the discharge voltage, the lifa cr. and the total thrust impulse PF decrease,de~r9a::-g the flow stress and Vie diScharge voi01tce - OPPOS"te. Changlng the 'low Strss 'within theran.ae as wiji oa described fo0rzv,, tne. tota thrust impulse is aimost the -ame. and cr. criangec

*r" - 2 :mnaesthe excoe :hrC va~uez a! 3the tota! !hnj-,t irncuize and thle EPTtima -,,iz~h the increase of Th at cznztant U=.303 V anid U at constant rm1C,=...I mgis within

r3r,3 - .tte d"ecrease of m.1 an,. U-

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'r'--a7in9 tha P.er a, z nomrnai C;Y ;nCrsar-iin etrair-T or .r-o enmoIU

~: t~ ~ me t~m te ' ~ ~ D~ rio~a !~rnf. U \ ter 'U = 50 V, i ., w hen n3oc~r nreae~ ncea~ng /~thrust increasec proportionafly to the power criange, or b

n~e~gU, in ' -ZU Z /'ud,vner f.- act: try wh.,cr,, U t.ad 1:- increased.

.-craaSing V", -.ver relative lo I . norninal level,~-me e, however, !t IS not desired !-Daci~,reaza by more than 2 timer., aria to decrease U beaow 150 V, as iower these levels thie5z~ec~~cOh~rcte~:tc! c he tr,!%u'ter are wcrce

Ta~p? 3Parar~e-- Sr-35 SPT50 £2PT70 -,,T'C IS- 140 SP-1200I cT8PPI0 C C!2T280

_N .W ~ 0 50 150 .3000 60 1,2000 25000~ 920 4. 918 3672 150Prf m . kNs 6 1 200-1 560000l 2000 6000O 18000 50000

Jer;,),s 1300 11400 11450 1500D 1550 1600 16503

1200 100 270 1550 160000U

-60 2 -40 4820 962S_____ ____ ____ ____ ____ __ 9 J 1 0 4076 0 140 28 0

0~0 7 00 00 2710 00 000 240003501";rnn) h.. Bo35!0N 100 14570 100 15 10002 210

0.5 ' 2.5 1 A.0 10 20 403P ,kNz 100 350 100 30900 27000 80000

45100 4700 10000 150000023500- i s m i n /~5

0 0 5-10 0 00 ) 0 -

00~r i i~1 00 1200 '' 1200 1250Ho 1270 1 - o

The appication areas for these thrusters are DfI ctured in Fia. 5.

- 509 -

I . PLUME DIVERGENCE

Plume Jiverence is very important. It is usually descnribed by the cone angle, containing 90%c he Icn curent SPT100s cnaractenstical ion distnbution is pictured in Fig. 6. Cone angle forSPI 00 s 45 It is almost imoossible to select such ExH parameter of the field, so that eaccnargec par'.:ce was accelerated in the same direction - paralel to the walls of the dischargechamber However, the investigation on plume divergence decrease should be conducted. Thesuccess in tnhs area would be significant contribution to the SPT life extending effort.

3 5. DISCHARGE CHAMBER MATERIAL

Variua4 requirements are imposed on the material of the discharge chamber. It must beinsulative at temperature of 1200-1800 "C, temperature resistant, thermocycling resistant, vacuumtight and not to evaporate. And to satisfy the lifetime and repeatability requirements it must beerozion resistant and not to cause contamination on thruster components and on spacecraftInvestigation held at MAI and other entities demonstrated that the most erosion resistant under ionimpact are isolators with minimal ratio of the molecular mass to the density n

Such material may be boron nitride and boron nitride - based materials. The sputtering of thismaterial is low contaminating as well.

S6. THt : : TEST CONDITIONS

Ch :: ri:'omons are: static Pressure inside the vacuum chamber of s 5.10- torr and dynamicof < 5.!05 ton-rr by xenon. At the pressure over 510-5 ton-the thrust drops at the same input powerT he experience shows that vacuum chamber dimensions shouid be as follows: diameter over 2.5 mkencth ver 3nm. The residual gas in the vcuum chamber must be composed of inert gases: xenon,kryptn, helium

3 To pr .i. all above conditions, cryogenic pumcing systems are the best: he!;um or neon withsmal! helium pumps.

Very i :portant is to operate the thruster till its end without opening the vacuum chamber, sothat I.olator was not saturated with water vapor and air, affecting the thruster Performance in thebeginning of each test cycle after opening the vacuum chamber.

In thi conditions SPT100 parameters are stable both at 'FAKEL' and at JPL for more than7000 -ours of operation and 6000 cycles (Fig. 8)

S Wiie .:onducting the life test with oil pumps, SPT100 performance continually deterioratedand -nechanical cleaning of the discharge chamber of the thruster had to be performed. Fig 9indicates the thrust change within the life period. (e marks the time ahen the isolator was cleaned)

Z. PARAMETER REPEATABII IT WITHIN IFETIME PERIOD

,thout having accomplished the life test, it is very hard to predict the parameters. Paer [9)pre: -nts the attempt to obtain the thrust versus time relationship. Life test at 'FAKEL' and JPLderr;.., t- that conclusion was wrong.

Fig. 8 indicates that parameters are even more stable beyond 1500 hours of operation andremain d so till 7000 hours of operation. The testing is going on.

- 510 - I

*r nce 5C ur r~tnurust change rrnax/r.mn is i Sc , then oetwean 51)C an: 7000

8. THE WAYS TO INCREASE LIFETIMEThe fe teSts at :FA.EL and at JPL demonstrate- that re_:uir-: 5000 4ours of oceration withzmc.e caracteristics are prcvided.

,,:t tne -amne time the fo~cowing ways of extending the lifetime are woried out.3- selection of such configuration of the magnetic and electrical fields so that to prcvide thebetter -ccusing of piume (lifetime and efficiency would surely be improvec a

- selection of better relative to erosion mater~i for the discharge chamber edoe:- 'FEN Ix' design is beiria reveiopea, allow~ing to move the chamber forward at least once asitz =eCge is worn out.

9. METHODS OF LIFETIME DEMONSTRATION5

N4atura!!%' the most easyv and reliable rnethc-- it trie !re= !ife et fthe thriinter :or -7 ulvocnsycctern in zhe condizions maximally close to real.3

Hiovev, dire:. U10- test has zuch dizadv'antages az iong Lime required and extremely high_-o t .

IHen-e tevera! vther methods were considered and submited. -Among othert, these areLa.-t~tsting, higher strazz testing and testing with propailant zimulatorz.. :KE::z s pnanca demon.-trated total -70000 hourz both with £FT7O and SpTi00, atIdjifferent !engrh of testing A-, earlier phase of SPT70 and SPT1OO development all defets weredizcz~ared duning the firstk 250 hourso testing. It' thruater passed this phase withoutno~o~nthen they Vl demnonstrated h~ghly !stabL!e pamete.-s.

Suort ey;erence aflcwr tc ccnclude that f.:r each SP7 mrode! (SPT70 or SPT10O) !t ssuffiCient to conduct the fuil life testing no more than two thructers. 3 thrusters are needed to test fordetermmnaton of reiatvonship baetween the SPT perfrmance and the erosion gemetry of thediscrharge chamber, testing for no longer than 300 hours - required and sufficient time to check outthe erosion rate - and 500 cce.The tthruiters undergo the "measuremrent of the geometry of thedizcnarge chamber and X-i-ay analyzis of the cathode. The quaiity onria" are ac follows: thecL-nstar! re_!atiionsh~r the discharge chamber gemeetrca! rarametem. versus time and cathodeIemitter geometry, and c:onstant output and specific parameters, leakage and eectr&ical parametersas ceclared in the rpctfication, Such method could be recommended for use.3

Higher strezs mrodes and use of cheaoPer proptellant.Other accelerating methods were use of cheaper propellant, e.g., ar-on, material of the3discharga chamber with less ero.-ion resistance, or both altogether. Ho-wevjer, usage of mock-upmatenal and propellant would affec-t the SPT perforance and requires comparative testing andcon-.&quaniy more expenszes.

Method of higher stress on the thruster, e.g., higher xenon flow -tresc or higher disch~argevoltaoe twac considered as. well.

I - 51~1 -

STnan~gG 6;ffarant n afz, 3 r tnz mat6riai-- ara rn~r. &t azariai 6a rSome ni*-: ter'r

ztla oiiracz ;i?6 tesztng.

achieved life is more than sufficient for geostationarv sacecraft 'nortn-south' station-keepina at3 pace:raft !Ife up t.: 15 s,:

The lIfe of SPT rmodels Of higher p:%r - suf foient "o Provlde fo-r ,hj=-re~ ro itatgeostationary orbit. Smaller spacecraft with lower mass may use mooeis SPT50 or SPT7O. the life of

w iclh i suffr~ent toproviAde tr ttonkei The samne thmustert. msy be use- ;In atitudemaintenance system.

The life test orccedures of JPL and 'F4KE;L are sufficiently correct. The around test results:-c l with 2?;T70 an dS F7 I c reAlF environment ;,-.-I *ja

i A ~ugr~zva, .K, kin a;an v A Morozov Ph-ysicai orccasaas ardch~aracteris tics of

2. A-kc o~cs: ;. .A A -r> es, A C e s p 1B.B-.oco).Los. Teop~tF [f p c eU zHeprox~c bix VcTaHoB c., KA',A. MocKza. M ,a uKc'crocej-e. I 984.

3. B.A.-hioov. A.S.Bober V lKim. et al SPT eiectnc Droulsion systemn for soacecraft orbit3 man~eujveh.-,; RGrC-E? 9247. 1zt Rus:;an - Ger... rofr~e~Eadter pt~s

E3S.. cv .. 3:rKN K:Sk .AMasennikcv ef a! SPTIOO mcdule 1Ufet~m= testrecuits. AlA-9-2854. 30th Joint Proouision Conference.

S . C.EGarner, j.RBros~hv, J.EPolk, L.O Plees. Cvciic endurance test of a SPT100. AIAL 94-2856.301, Joint P,-.rion Co nference.

1 ~ ~ ~~~~~ ~~~ 5. .!A~lmo et -l .e:rmr \fpamaprmees P~ Plume ad its effect nsoDacec-r-aft comoonents.

37 S.AoArx14nois .I0 -w&o) H.A.Mmrtic imtos. A~.vloD0308. -(Kbd-4303IAi~?Ae~C~LKa ~OA AeT~t~ ?2MCi-OZt !-1TOKa. TIQ CIi5~~ Z. 18 hi.

1002

3 .~ £ ~ N A ~O--L~ t ! ~.cmer 3 : n of electrnc thrustersr~~ IE~~O 3 m3, 22l nternatirnai Elatc Provuision C--nzrenc.~

9. B-A.Fle-rn)OCor &K AD. OCoeKRC -rk AerpaaL iif ir Lrpi; p ec:Y-pCzi±LxCrlIA. PKT, :.yCr ! ). H0l-iT C9-7p. 2617

1o .em of trne Zzai~cnary' Placrna T~~~r pae r ,--ulzion

-12-

- -Z

--7-A1, 39 6 ,C

: %7 ~Ja -x 2o 16'1 6) on SPT-70 and SPT-1003

a) on the samples of materialsFi.1Isltreoonae

7.

o3

50

* K ~ ____ 0 __62

_m 7?>- 2 02

J ~a,27 p \ 2J

. I i - 5 0 f

J100. zo _10 0

Fig. 2. SPT-100 erosion rate vs. time ~X

'g. 2 S- T-i00 No 22 erosion profies wereexperinmenizltjeteminec curing 4000 hours of lfetvtn

llife testing00 N 05 rosir prfilc'PTaft2-- 000 ouis o

3 /Parametric Series. I1P-0

2 SPT-70, 3 SPT-100) 4 SPT-140,5 SPT.200, 6 SPT-280

Z;0 U xSovj 300 45

I 0,5

0 20 00 600 100 WOO /2o /t9- AsecSum thrust pulse

Fig. 5. SPT usage' fields

CI I 7S

I 0.8C0.2

0 4

0

o 0 0 0 0 0 0

Probe Angle w.r.t Thrust Axis (degrees)UFig.6. SPT-100 current densit roie

30 and 600 hour of operation in JPL

chme

-514-

Fi 7, SPT 100 No 0S ,os on afler 7000 hours life testing.

80__ _ _ _ _ _ _ _ _I

0 30 00 100 :ou 30 u~ :500 'Qoo 4so 0 0 Ouu3,o 0000 ~5~ 7:ri

a) Test at Fakel

.10090 N-

o 30 .00 00200 250 000 2500 .000 .500 000 500 00 00

b) Test at JPL

Fig. 8. Thrust Change of SPT-100

,_ _ _ _ _ _ _ _ _ _ _ _ _ _

Lo70-r

50j

Fig. 9. Thrust change of SPT-iOO in the "oil' varuum