91-023

TIE DEVELO 'RET OF TIE CATIODES-CCIMENSATORIS FCOSTATIONARY PLASMA THRUSTERS (SIr) IN USSR

B.A.Arkhipov , K.N.Kozubsky.

Enterprisr- "Fakel", Kaliningrad (obl), USSR.

Abstract

Performnnce study and developnent of the

plasma cathodes In USSR began in the late

60-s. The first flight tests were carried out on system "EOL", based on SPT. RiL-.ults of the studies

".eteor"satellite as a part of SRT propulsion of physical, fluid, thermal, starting and life

system "EOL". time characteristics are shown in this paper.

Phisical, hydraulic, thermal, starting and

lifetim performances have been studied for the

last years. It ras shown, that the main Diagramof th_- cathllJE'c-co nsa 1tor

contribution to the process of elrctirof, production

is nude by thermoemission and space ionization of In the Flg.1 the diagram of CC design is

plasma generating gas flowing through cathode (Xe, shown. Gas is supplied through the tuh .5, and

Ar and other inert gases and their mixtures). then through the central channel 6 of tlhernionic

A significant research phase was devoted to emitter 1. Thermionic emitter 1 is heated by heater

the issues, concerning cathode lifetime. Namely, 2 to the temperature, that provides necessary

the problems of emitter poisoning (LaB ), heater electron emission, sufficient for stabli, discharge

y ad r h b s . keeping between Internal surface of the thermionicdurability and reliability have been solved. emitter 1 and Ilgniter . ,r anode (i-.n't shown on

During the research work the following emitter e) of th e plasma source. Atl .r thecd o the firure) of the plasma source. Atrer the

parameters have been determined: the conditions establishing of the stea-state me heter 2 andunder which the firing and dshae e ahllshing of the steady-state nte heater 2 andunder which the firing and discha je I,nitrl 3 re d.iconnected and CC ",'or!-:. inself-nmintanance in the operation spac' n- 5 10 auto-mode, in which the necessary temperature level

1/cm3,electron generation mechanisms, conditions of the thermionic emitter 1 is maintained due to

for the aclive zone e i.stence in the cathode space. the energy, supplied from discharge.

On the basis of this research work a number of The system of coaxial thermal s, -reens 4

cathode-compensators, operating at the discharge serves for decrease of the therml flow from

currents being in the rate of 1.0...60A (K-2, K-5, thermionic emitter 1 to external part..

K-50, K-50D) were developed.The cathode-compensators have passed the

necessary scope of ground tests. Electric

propulsion thrusters for spacecraft orbit control

are equipped with them.During the period since 1972 hitherto "100

cathode-compensators K-2 and K-5 were subjected to

flight tests and put into operation. They had shown

high efficiency, operational stability and

reliability.The cathode-compensators K-50 and K-50D are

being used at the development of advanced electricpropulsion thrusters.-

Notation

CC Cathode Compensator- ~

SPT Stationary Plasma Thruster

Ud Discharge Voltage, V / /

Upp Plasma Potential , V

Ul Igniter Voltage, V

Id Discharge Current, A

Nph Permanent Cathode Heating, W

Nh Heating Power, W

Tw Temperature of Cathode Wall

Qc Flow Rate into Cathode, mg/s

Fig.1. Diagram of the cathode-compensator

Introduction 1- thermolonic emitter,

Performance study and development of the 2- heater, 3- ignitor,

plasma cathodes were started in USSR in the late 4- thermal screens,Gn-s. The first space tests of cathodes wereperformnd on MIETOR spacecraft in the propulsion 5- gas feeding tube,

91-023

F; rki , ;: ' .r ,, , and diffu i fo, g, .s "i; f t he sp*.ed

Cathode ompensators a a_g.isdisc large lit where

Eo =v / il the itr ,engtth of electric field

CC is a gas-discharg. n i ith h-llow !)et. n t:haninl .;is amal catlirod- wall,cathode, which provides high density of .t' 7 . - i un.l fr j. ith tr;,velllng t iiw-, millicurrent. II coll d -s wit h at l om of :

Let us describe briefly thae hys;ical i c,(fi1:i-O o tr of u,, i

processes, taking place In the chatiber of CC, anrd I

arc discharge environment. vit - theriial velocity of ion:,During discharge ignition the greatest impact , - c -sectio t of collision of ion with

has the thermionic emission from the cathode. 1'Current density in this case may be determined bh n,-trl,Richardson-Dashman formula. Electrons, generated Stbsi i tut itnby thermionic emission, enter the chamlnr of hollow Tio= 'in dicathode (cylindric channel) with energy: into the formula 1, w can derive the following

t W - epk neces.;ary condition of discharge self-keepingvwhere: (auto-mode) ill the hollow cathode:W - energy of "tail" electrons of Max vll

distribution, W > epk; ' ve>(noR ) 2. v >/ev e1 2 1 (2)S' i - n tt i o' 1

ek - clectronic work function rf (.A h . ate .-.ial.Thus, in proesa. of . I ng of di silar,-If voltage Ui is applied to th- ignition electrode, T

aulto d the plasma Is ene-ratd inside the disharthen in absence of plasma electric field will

au ut -de te Plasmra is a l-ers dpend uinde the fllow

penetrate into the cathode. t . Plw l

- parires depend upon the Howpenetrate into the cathode. rate, the potential difference between cathode andelectrons fanld they gain ext ra emr ic - eU i ,,n ae, the tenperatur and material of cathode, theelectron and they gain extra e i kint of prop.ellant and geometry of chamber.

then total electron energy is: asl.suI in fi. " t '; , t ' :I

= Et + E = W + elU - ek 'io - 6'30 a ,

For ionization of atoms it is necessary, that U = 10...20 V,S>pi, where (xeno ononization potential). Ions out 1"'I is about 1,generated in cathode chamber due to electron impact R = 0.25 m,are moving to the walls, and electrons - to the inignition electrode or to the anode. If e

= 110

15 cm

2

concentration of neutral atoms is designated as no, T = 2...5 V (V,=1'i-i cm s)and concentration of electrons -n , then quantity anld substituting these values Into the forrlaa (2),of ions, generated In channel by the electron we derive, that for discharge self-keeping theimpact (per second), may be determined as: neutral gas densll In the discharge chamber n

N=TRin l< OeoVe>none' should be more than 5"1016 I/cm

3. Estimatng the

where: gas temperature as equal to 2000 K and using gasRn - internal radius of channel, equation, one could obtain that it Is necessary to

S nnreach the pressure In the chamber Pg of more thanI - channel length, 10 Torr. Flow rate 0.1 nm/s corresponds to thisOeo -cross-section of atom ionization by pressure. During experiments It was established,electrons, Ve - electrons velocity. If gas density that lower limit of xenon flow rate in thein channel is low and ion mean free path is auto-mode d, corresponding to this pressure, 1ssuperior to the channel dimensions (Al > Rin), about 0.08 mg/s.hence ions will reach walls without collisions Inside the cathode channel the plasma densityIn a period Is unevenly distrihuted along the axis. In the

T-R /VI =R /2cU/M catlode chamlter the active discharge zone IsSRinVi =RIn/ ' created, in which temperature and density of plasma

where: are superior compared to other discharge zones.Location of active- zone depends mainly upon the

U - characteristic potential on channel axis. flow rate and discharge parameters for the certain0 geomtry of chamber.

Within time Ti the quantity of generated ions per Now consider the steaivd-state mode ofchannel length unit Is discharge In case, when active zone Is located

inside the cathode chamber. 4The i uasi-neutralS=- -=R '<~c V >n neT (1) plasma of high density (10 ...10 1/cm3) is

1 ogenerated in the chamber, that is confirmed by thetests, and thie dynamics of ions mot l,11 isFor discharge self-keeping the following condition determined by diffusion and motion laws. Dbv

s necessary: 2 radius In case, when electron telrperature TeR In eo >noe > in e T1 differs from Ion tetiperature TI, is determined by

or the equallon:

i T > n ne"

Rd=l'( kT T /4Tn 1e(T +TeId e 15e e 3i

i.e. quantity of generated ions should be rore than F abt 15 3absorption of them on cathode vwlls, that is eprovided with sufficient gas density. If gas To = 2...5 el and TI about 0.1 eV Debye radius Rddensity provides, that 2l <

< Rin, then ions will Is about 10 Ia, that is much less than Rin and

characteristic dirmensions of active zone. Themove towards the walls because of diffusion. It isasrumed, that lonlzation process is single-stage applied electric field slightly appears In the

plasma and it is possible to suppose, that In the

2

91-023

,tliv .. ' I'..- '- . . Is propellant utilization coefficient

particles of both charges tak.-; place In bhth a::lt kpu I/ 1 ,and radial directions. DuC to ;Imil:r char -d

particles (for example, electrons) gradient, redischarge current,particles- move along the axis with vloci, d - discharge crr

determined by the equation: I the current equivalent tI o lo,, r;de.

v= (Da/n)dn dz, 1t is the quantity of electrons, supplled

where coefficient of artlipolr diffusion ' : by cathode per one atom of gas, and d.-t,.rmines the

S=(b ) b fficiency of cathode rlp.'rl li'.

Da =(b e'be D i e Optimum flow rat, of xrnon for cathode of iK

be,bl De,D - diffusion and mblllty coefficients of type is Q-=0.15...0.20 mgs, Kp beign I ;...20 ;.

electrons and ions, connected up hy Einstein discharge current ld 2.0 A.

equation Maximun K value at In cathod.: oprratin withb/D~l/kT P

On the boundary of the a'-tlve zone,: charge'd thrusltr is about 50. Durllrg ch!; :crtter ling z 'f

particles concentration falls rapi)dly, I'" t b.th flow of cathode K-2 the ran"' of xenon flow rate

reconbination and "blowi. i.u! t <f '!- t by 0.05... 0.4 mg/s was studied. At flow rate Qc> 0.0S

neutral flow. Also ort tc l;, ; ", IthI

pressures in steady state ,ould b. ,ua, : h i/s cathode operates In steady-state auto-nviek

Ppr= p n sP -,eadf: th long t Imr: and with st,.Al, parameters.

p g i o Voltage--Flow Rate characteristics, obtained

P =nekT , P inkTe, P -lTo dring op.rat I n of cathod K 2, connr,-ct'di in di d,''Se shri-m, and m.asured with target, are a;ho-n In

where : . " In th,: <li-c:.i rge ciio i'-n t rany 1.0... '1.0 ?P gas pressure,P - gas pressure, .. at low flow rate (Q about 0.1 nu/.) th ,Pe P p - pressure of *.!, It 'cns, ions and neutral

ep ,PI' - pressur of l i d n l sat urat lon electron c'uren.t was nut observed. At

particles. Id 0.S...60 A cathiod. oprates in steady-state

When Ppl > Pg, I.e. when pressure of plasma isequal or superior to the kinetic pressure of gas, at,- In ranr, O.C .. S A l ;,lol",l les tgnthe active zone is located Inside the cathode to app-ar, dlschar-e par,saters "flw ", and at

channel and with fall of flow rate moves to.au'ds Id < 0.6 A the steady state auto-mode was not

the bottom of chamber. Such mode Is observed with m established. However, at low power of permanent

I',ss than 0.8 mg/s. At Ppl < Pg plasma of actIv,- cathode heating (Np) (about 15 W) discharge may be

zone moves towards the channel outlet beca'se5.' pihgas pressure, and dicha- e in thl. case

ay " stabilized, discharge voltage and plasma potential

even on the cathode outlet, and in this case for my be lowered. In this mod cath;ode can operate

our comitions Qc C> 0.8 g/s- teady at Id 0.5...0.6 A also.our condltlons Qc> O.S mg/s. d

Integral _characerist ics o f _ca th o

co pensators 50Ud,Id'IA

Experimental data on pressure in the channel 145-

of cathode vs flow rate for structure temperature /Ud d=2A

200C and 370°C and In auto-mode with dlscharg-

current 2.0 A (structure tenperature 370 C) a~-- 0 Ud.d3A-

show n n Fig. 2.35

40 id.2;Tw570 > 30

Id-0; T -370*

i 20 -

So15 U p ,Id = A

10 UpP,Id-2A /

0 0,1 0.2 05 O, 5 Up Idd 3 A /

FLOW RATE , m/s

0 0,,i 0 03 0.4

Fig.2. Pressure in th,. 'hannl .,f '.th cathod' FLOW RATE, mq/s

versus flow rat::

Pressure difference et irrmt ins, perforred for

flow rate m = 0.3 mg's and wall ti-nrrrature Tw

about 2000 K, give the sanm ,rder of value (abou! r- 1 V,,!ta--Flow Rate Ch!irctri stlcs

50 Torr). To meet the requiremnts of orbit malntenance

Increase of pressure in auto mode whe-, and station-keeping of spacecrafts the start times

compared to the "hot' cathode case may he 0...20 min are needed, depending upon the on-board

attributed to the pressure '-f electrons In th power set capatllltles and mission.

active zone of hollow catI hd,. In SPT on gaseous propellant the transit ,n

One of the most stgn!fiant cathode paramnters processes are mainly connected with thermrl

91-023

processes in cathode, heating tim. of cathod sed on die a r of , rk!n inemitter being the determinant. Principally two discharge C

d oln'rt r;'ing 1.0.. .. 0u A ftpe:. w ,

ways of SPT starting are possible: the first - from K-5, K-Sc

) was developed.the "cold" cathode state (-70...1500

C), starting Design of K-5 and K-50 cathodes corresponds totier s t

being about some tens of scor:ds; at use Fig. 1 and differs from design of K-2 In dimensionsof permanent heating of cathode. In Fig. -1 the only.data st S P vs starting power Nh and permanent The main characteris ics of rm2ntioned cathodesheating power Nph are shown. In flow rate range Qc are given in Table 1.

0.2....1.5 rm./s, and magnetic field Induction range Table 1:B -0.007...0.03T, that is characteristic for SPTf, . - - ---- - -the. starting time of SPT doesn't depend upon B and Technical parameter K-2 K-5Q but critical value of flow rate exists (Q =0.08 .c

c I"ohv.ant X,:ng/s), at which Ts t begins to grow rapidly. Start Flow rate, g/s 0.11...0.5 0 ). .. 0.5failures appear at Qc<

0.OS mg/s. Discharge current, A 1.2...2.5 1.2...5.0Lifetime, hours 2.700 4.000Nurmber of switchings on 70,000 10.000Start preparing tilu. s Go 60Power consumption during ,

N60 85w preparing, W 150 150Niw Ud-W20NO Dimnsions, inn 60x20%25 GOx20x25

0 N----U ----------------------------------------4 . .o.N2. ------------------------------ ..20 4 Technical parameter K-;, K 7020-------------------------------------------------------

--- --- --CX Prop..!lant

S 0Flow rate, mg/s ).....7n 0 10 20 30 0 50 Discharge current, A 10...60Peainent C tde latig, W Lifetime, hours 3, 000

Number of switchings on 5, Q00Start preparing time, s 60Povrr consumption during

Fig.4. Dynamic (Starti!ng) Characteristics preparing, W 170Curve in Fig.5 characterize the dependencep 't ensions, 75x29x34

is t < " - - - - - - - - - - - - - - _vs minimum Initial gover of cathode heater (at in forced modeinitial temperature 20 C), which allows to select Conclusion:either time, under the certain starting power, iftime of readiness for start is within 0...20 min, CC K-2, K-5 have passed all necessary groundor power, if time Is fixed within those limits, tests. SPTs for orbit correction of spacecraft are

The obtained results show the high starting supplied with them.capabilities of cathode with permanent heating, SPT In 19S7-1988 two experimental spacecrarftsperformance stability and possibility of meeting were launched, and SPT operation with cathode K-2any requirements on launching of SPT on gaseous was demonstrated on their board in modes of activepropellant. spacecraft altitude stabilization relative to one

During development of the CC for SPT a number of the spacecraft axis. The permanent readinessof studies was performed, regarding the cathodes was provided by thermal stabilization of thelifetime. In particular, the questions of emitter emitter of cathode K-2 at power consuption Nphpoisoning (LaBS), heater reliability and durability 40...50 W per cathode.were resolved. Heat balance of cathode was Based on SPT with cathodes K-2, plasma sourcesconsidered. were developed and tested in space for active

experiments, particularly, one of these sources wasused in international experiment PORCUPINE.

Totally 108 of cathodes K-2, K-5 were operatingIn space for 20 years.

0. CC K-50, K-50D are used for developirent ofS I ' advanced electric propulsion engines with power of

E Qc=0,2 mg/s up to 25 kW.

S V Thus the state of the cat hode-compensatorU = 90 V development in the USSR is high enough.

S6 x- References

- A1. Artsimovlch L.A., Morozov A.I., Kozubsky K.N.,Rylov Yu. P. et.al. Stationary plasma thruster

2 x tSPT) development and its tests on board the-- "---- "Meteor" artificial earth satellite. Kosmicheskie

Issledovanlya, 1974, 12, iss. 3. pp. 451-468.0 50 80 110 40 170 220

Perinllt Ctbode eg, 2. Galeev A.A., Sagdeev R.Z., Morozov A.I.,Kozybsky K.N. et .al. "PORCUPINE" project,experiment 12. "Investigations on problems ofsolar-earth physics", M., 1lZMIAN SSSR, 177,pp. 152-160.

Fig.5. Start-up Time vs Cathode Heater Power

4