RESEARCHING OF SELF - HEATED HOLLOW CATHODES START EROSION CHARACTERISTICS

Nikolay N. Koshelev, Andriy V. Loyan

SCIENTIFIC AND TECHNOLOGICAL CENTER OF SPACE POWER AND ENGINES AT JSHARKOV AVIATION INSTITUTE

35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit

20-24 June 1999 Los Angeles, California

For permission to copy or to republish, contact the American Institute of Aeronautics and Astronautics, 1801 Alexander Bell Drive, Suite 500, Reston, VA, 20191-4344.

RESEARCHING OF SELF - HEATED HOLLOW CATHODES START EROSION CHARACTERISTICS

Nikolay N. Koshelev, Andriy v. Loyan

SCIEbJTIFIC AND TECHNOLOGICAL CENTER OF SPACE POWER AND ENGINES AT KHARKOV AVIATION INSTITUTE

Address: 17, Chkalova st. Kharkov, 3 10070, Ukraine;

tel.: +380 (572) 44-2599;

fax: +380 (572) 62-5065;

e-mail: loyan@ai.kharkov.ua

Abstract

In the report possible ways of SHC start erosion measurement were analyzed, optimal method was determined. Method of start erosion estimate is presented. Results of experiments and calculation are shown. Discussing of results which we get are

carried out and ways to decrease of start erosion were fixed.

It was demonstrated that decrease of start erosion is necessary.

Introduction

At present the electric propulsion systems (EPS) having high specific impulse and lingering resource are wide researched and used in systems of orbit correction and spacecraft (SC) repositioning. But all EPS types has long pre-start time of preparation because of cathodes long heating and quasi- stationary transition times. Cathode is most thermal and inertial loaded unit, therefore cathode will definite EPS resource. Significant cathode inertial makes a frame for any EPS using ~possibilities in orbit correction and SC repositioning programs and in plasma systems for SC different electric charging neutralizing.

Using of self-heated hollow (SHC) cathodes - is the one from most radical

methods, for inertia problem solving. In this devises heating of emitter is existed by plasma discharge and emitter surface interaction. It is necessary to note that change of usual heated cathodes to SHC will allows to make propellant gas economy, additional units and blocks mass decreasing and make start of EPS more reliable. But this interesting perspectives now takes place in high-dynamics systems. It is important to note that SHC operating regime in this devices could be different from EPS lingering operating regimes. So (for example) in neutralizing systems SHC block could have impulse regime.

1

Investigation of a cathode erosion

Obviously that there are many methods of a cathode erosion determination. But majority of them has number of deficiencies, which does not allow to use them in a cathode start erosion determination. The methodic which have been based in by material, plasma-forming gas and discharge spectral line lighting intensity measurements is proposed by authors. We note that method without contact allows to measure of any cathodes elements erosion in real time regime. Scheme of exp eriment is shown in Fig. 1.

FhmA Fig. 1. Scheme of experiment

It is well known that in all time moment in investigated cavity will be mass of lightning erosion material: m(z)=ll, *V.ma.So We could write m(Ti), where ri = i . AT , AX - time between measurements, after that:

$m(Ti) _ we could calculate mass of i=l cathode erosion material:

m(Tn>= ?m(Ti); 1 i=l

or for erosion speed we have:

or with taking into account (1):

maP(Tn) = na-V.ma

AZ 3

Concentration of atoms could be described by plasma lightning intensity measurements:

k=

where 1 - length of beam in plasma;

K = ~cr&)f(E)&& - coefficient of &a

process velosity. Solving 3 and 4 we have:

m ap(zn) = ma V la(T) 5 ftv Aki ‘7. ne(r).k(r) z’ k=l

CAk k=O

In accordance with 5 for measurement of erosion speed enough to carry-out the quick probe measurement (T, and n,) and researching of spectral line, This method is optimal (from authors point of view) because of number facility minimum using and reliable realization. But and in this case quick-measure equipment creation is very difficult problem. That is why we propose next method for make investigation without probe current-voltage characteristic measurement. The method is based by parameters:

m 3P(Tn ) - start erosion;

m 3p - erosion in stationary regime;

2

moP(?r) = I(T)-IT, *K 6

i 3P(T) I(r) * ne CT> * K(z)

concentration of ion could be written from Ohm’s low:

3=-e-(neFe--ZiViFi)

In X proection we have:

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j=e~*n,V, =enepE=ene e E 8 mevea

In this case ion current not for account, because of low movement of ions in so type of a discharge. Field intensity could be calculated with proposition that cathode voltage decreasing does not depend from discharge current and gas consumption:

EIU-Ay, I h

where: AT - cathode voltage decreasing; h - gap between electrodes. Taking into account parameter v,, we have:

m3P(T) _ I(r) J u(Q -AV ?%e . p

m3P 1 J(t) u- 4 mxe(z)

Where P=P(Oa oea El;

After analysis _ of p we can say that in discharge so type Te = 2 - 3 eV and p increase not more then 3 times. Taking in to account time of process and low thin of investigated plasma we following equation CEU-l be written:

~~PCT> ,iT(z> J u(t) -4 mxe g

m3P 1 J(t) u- AP mxe(z)

Analysis of some SHC starting experimental data was carried out in accordance with 9. You can see in Fig. 2. - Fig. 4.

It is obviousiy that in current range from 5 up to 25 A (for SHC start ) exists 2 peaks of erosions desorpsion of activator material. First peak coincides with first peak of Ba ions lightning intensity and time of this effect not more then 0 -10 sec. Second peak more than first one and coincides cathode erosion in non optimal regime of plasma - forming gas consumption.

Analysis starts characteristics shows that value of first peak could be more then second one when SHC starting, but with increasing of stationary discharge current, erosion maximum increases in 3 - 3,5 times, with first peak increasing in 50%. Moreover with increasing of second peak value, the endures and form is increased also. It is important to note that the time of start erosion recovering to stationary level has less dependence from stationary operating current: l 90-llOsecfor5- 10Acurrent; l 100 - 130 set for current more then 15 A.

But we think that integral erosion characteristic in first and second “stage” is to be researched more important. Mass of erosion material could be calculated from:

22 . m3p = J m3pWz

Zl

Where ~1, 22 - time of start and finish of experiment. In table. 1. You can see start erosion data. It is obviously that most significant influence into start erosion when starting more 10 A does erosion which is determinated by gas consumption decreasing from optimal regime in 30 -50 %. In this case first erosion is not big (10 %) from summ. That is why most erosion- dangerous stage is the second stage. As a conclusion: 1. For small start currents it is necessary to

guarantee of emitter quickest heating for transition form discharge in spots to distribution one.

3

2. In any case it is necessary to guarantee of quickest stationary gas consumption recovering for erosion decreasing.

Table. 1. Start erosion data

In the article investigation results of Cathode M50. which have been tested is diaphragm erosion in impulse regime are shown in fig. 5. Cathodes diaphragm have presented. been consisted from W-MO soldering.

: . . , . . : .

,,<i*.

Fig. 5. Cathode M50. Thermocouple (TC) WReG/WRe20,

which have been to SHC surface by spot welding through the foil Nb. TC’s contacts

were taken away from the vacuum chamber for noise disturbance decreasing.

In first stage of experiments we have carried-out diaphragm transition to stationary regime tests. Temperature dependence from starting time when SHC

M50.02 have been started to stationary current 8 A. is shown in Fig. 6. Existing of 3 heating stage is obviously:

4

1. Upt0200C0,- maximum velocity of characterized erosion maximum with heating and small erosion, because of arc sharp gas consumption decreasing. discharge spots operating regime.; 3.

2. From 200 Co up to 600 Co - regime of From- 600 Co up to operating temperature regime.

arcing in a second sort spots, which is

Fig. 6. SHC diaphragm heating .

Diaphragm after 840 starts is shown in fig.7. You can see erosion form like parabola.

Diaphragm after 15 10 starts is shown in fig.8. (740 hours of start regime)

Analysis of fig. 8a. have shown more difficult nature of destruction. 1. Global parabola erosion exists like in fig.

7. 2. Significant different form fig. 7. is in

existing of small parabola which takes place in field near emitter, and significant smooth in diaphragm field.

3. Detail ,analysis of diaphragm front view (fig. 8b.) You can see that small parabola was created by internal parts of diaphragm erosion.

I.e. plasma discharge interaction with diaphragm walls allows to material desorption by gas flow. After this, (when gas ~flow is very extended in vacuum) the erosion material covers in a back surfaces. From the one hand it allows to significant changing of hall geometry from designed one, discharge parameters. From other hand it allow to see “imaginary ” increasing of cathodes

Fig. 7. SHC diaphragm after 840 starts.

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Availability of very developed crystal Moreover, we think that the double “jump” surface of diaphragm confirms of lingering of electric potential allows to crate the small and significant temperature influences. parabola.

Fig. 8. SHC diaphragm after 840 starts. a) thrashing; 6) front view.

It is well known that relation of start and SHC could be used like cathodes unit for stationary regimes erosion could be EPS for solving of orbit correction and SC described as 1:lO. So 740 hours of start repositioning problems. And for conclusion regimes corresponds to 7500 hours of about SHC resource ability - is necessary to stationary regime of operation. Therefore carry-out additional experiments. out the additional experiments.

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30 40 lFo 80 fO0 z;c

Fig. 2. Dependence of discharge parameters 1 -voltage,2-current,3-gas consumption,4-intensity

G,c u,s

40

8

2

of spectral lightning;5-erosion

Fig. 3. Dependence of discharge parameters l-voltage,2-current,3-gas consumption,4-intensity of spectral line lightning;5-erosion

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20 eb 60 eb 40 zgc Fig. 4. Dependence of discharge parameters

l-voltage,2-current,3-gas consumption,4-intensity of spectral line lightning;5-erosion

References

1. Characterization of high - performance electric propulsion system / N.V. Belan, S.A. Lobov, A.V. Loyan, A.I. Oranskiy // Second European Spacecraft Conference, Netherlands, 27 - 29 May, 1997, p. 473 - 479.

2. Marepnarrbr Kon@epenunn IEPS - 95, MOCKBa 1995.