The ATS-6 geosynchronous satellite carries two cesium ion thrusters as part of the technology experiment package. Their effectiveness in controlling the spacecraft potential was shown during normal operation of the thrusters. Auroral particle detectors were used to estimate spacecraft potential, amount of cesium propellant backflow to the spacecraft, and electron signature of the thruster. Operation of the thruster clamps the spacecraft to about -10 v in the presence of a wide range of ambient particle flux. Some positive ions from the thruster are returned to the spacecraft during this period. The plasma bridge neutralizer of the thruster is also effective in controlling the spacecraft potential. Laboratory simulation tests of the ATS-5 ion thruster thermionic emitter neutralizer showed that negative differential charging on the spacecraft surface probably prevents the emitter from completely discharging the spacecraft in flight

Goldstein, R.

English

NASA Technical Reports Server

Contcmts
1. Introduction 122
2. ATS-6 Plasma Emitter Flight Te,_t_ 122
3. ATS-5 Neutraltzee Emitter Laboratory
Simulation Tests 12¢;
4. Summary and Concluston_ 128
Acknowledgment 129
Reference_ 129
6. Active Control of Potential of the
Geosynchronous Satellites ATS-5 and ATS-6
RaymondGoldstein
Jet PropulsionLaboratory
Pasadena,California
Abstract
The ATS-6 geosynchronous satellite carries two cesium ion thrusters as part
of the technology experiment package. Their effectiveness in controlling the Space-
craft potential has been shown during normal operation of the thrusters. The Uni-
versity of California at San Diego auroral particles detectors were used to estimate
spacecraft potential, amount o£ cesium propellant backflow to the spacecraft, and
"electron signature" of the thruster. Operation of the thruster clamps the space-
Craft to about -10 v in the presence of a wide range of ambient particle flux. Some
positive ions from the thruster are returned to the spacecratt during this period.
The plasma bridge neutralizer o£ the thruster is also effective in controlling the
spacecraft potential. Laboratory simulation tests of the ATS-5 ion thruster
thermionic emitter neutralizer showed that negative differential charging on the
spacecraft surface probably prevents the emitter froll, completely discharging the
spacecraft in flight.
This paper presents the results of one phase of research carried out at the Jet
Propulsion Laboratoryj California Institute of Technology, under Contract
NAST-100, sponsored by the National Aeronautics and Space Administration.
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I. INTHI)flI!CTION
The effectiveness of the cesium ion thru,_tors and their neutralizer_ carried
an the geosynehronous satellites ATS-5 and ATS-6 to control the spacecraft poten-
tial under a wide range of environmental conditions has recently been
reported, 1, 2, 3 Some variation of these devices is a likely candidate to be used
on a spacecraft explicitly for active potential control. It Is therefore important
to understand details of the operation of these devices and their effects on the _.
spacecraft. For spacecraft conducting scientific measurements, for example, of
partich=s and fields, the q,Jestion arises as to the direct effect of the discharge
device on the measurements. That is, it is desirable to know in what way, if any,
the device will contaminate the measurer, lent with spurious contributions. In the
following, an analysis of some of the data from the University of California at San
Diego auroral particles detectors on ATS-60 taken during operation of a thruster,
is described and discussed. It is shown that Operation of the discharge device has
a marked effect on the character of the measured data.
While the plasma devices on ATS-6 appear capable of clamping the spacecraft
potential even during severe environmental conditions, the ATS-5 thermionic
emitter has not been entirely successful. The spacecraft usually can only be par-
tially discharged, and even then, the potential generally does not remain constant.
In order to understand the cause of these effects, some laboratory simulation
studies were performed using a flight spare ATS-5 thruster. The tests and results
are described below.
2. ATS-6PLASMAEMITTERFLIGHTTESTS
2.1 Ion Measurements
The configuration of ATS-6 is shown in Figure 1. The features of interest
here include the two ion thrusters, one of which is located on the north and the
other on the south facing surfaces on the Earth Viewing Module (EVM). The UCSD
detectors are located on the Environmental Measuring Experiment (EME) at the top
of the spacecraft. The detector of concern in the investigation discussed here is
capable of rotating so as to accept electrons or ions in a N-S almost vertically
oriented plane. A 10..m diameter mesh antenna lies between the EVM and EME
(approXlmatt, iy 13 m apart). Each of the solar array booms is approximately 17-m
long.
During the flight operation of the north-facing thruster in 1974, it was observed
that higher tt_an normal count rates were observed in the low energy ion detector2
char, nei_, Furthermore, during the period of time in which the detector was
122
O0000002-TSB14
.t I L J ! 1 [ I
UCSDAURORAL
PARTICALSEXPERIMENT
/ EAST-WESTSENSOR
HEAD
MAGNETOMETER/ ENVIRONMENTAL
/ /MONITOR EXPERIMENT
SOLARPANEL _ / / , --.. ,(ONEOFTWO) / / "
_''' 'f_f'lT _ • • ,,i
/_I//'' _/ ;" " " " _30FTDIAMETER
"!_ __/(_,'.' PARABOLICON ENGINE _,:;.- .'vlr_r;_i
=_--I/A IONENGINE
_" _.! .Li'Tf_?_l_'\ ", (HIDDEN)
=\\
_y (NORTH),,,_ _ _ .X
•Z (EARTH)
Figure 1. ATS-6 SpacecPaft Configuration
rotated, a strong angular dependence of the counts Was also observed. In Figure 2,
this angular dependence is shown _or ions in the 11.8 eVchannel, corresponding to
the peak in the low energy counts. /_ strong maximum is seen near the due north
(0°) direction, with a weaker maximum near the "straight up" (90 °) direction.
The thruster beam was directed downward at an angle of 60° with an energy of
550 eV. Hence, the primary beam ions would not likely travel to the detector, par-
ticularly since the local magnetic field was roughly along the beam direction. A
more likely source of the ions detected is the cloud of thermal energy charge ex-
change ions formed near the beam boundary, especially near the thruster exit.
These low energy ions are greatly influenced by the electric fields resulting from
differential charging on spacecraft surfaces. Whipple 4 has shown in fact, that the
solar array surfaces of ATS-6 can often be the order of one or two hundred volts
negative with respect to the rest of the spacecraft. In addition, the differential
charging also appears to produce a negative potential : well" in the vicinity of the
solar panels. SuCh potentials would be sufficient to attract ions to the upper por-
tion of the spacecraft. In fact, the 0 ° angle of Figure 2 corresponds to the detector
looking approximately at the north solar panel. The data of Figure 2 theretbre
123
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10 4 1 I I I I I I I I I
103
COUNTSIs z02
-20_ 0° 20° 40° 60° 80° i00" 120_140° 160' 180'
N'S DETECTORANGLE
Figure 2. Angular Dependence of 11.8 ev ion Count Rate
During N ion Thruster Operation on ATS
probably corresponds to low energy ions produced in the thrust beam andthen bent
around the EME by the negative ditferentlal charging and potential well in the
vicinity of the solar panels.
Analysis of the energy distributionshows that the low energy ions observed
during thruster operation have a peak around I0 eV but have a very nonMaxwellian
distribution. Such characteristics are normally not seen in the environmental
plasma. This is further evidence that these ions are introduced by the thruster
and are not truly environmental in origin. Since the thruster couples the space-
craft to about -10 V with respect to plasma potential, thermal thruster _ons reach
the spacecraft with energy of 10 eV, explaining the peak measured at that energy.
Of course, any low energy environmental ions preseflt would also appear at that
poteutial. However, no low energy environmental ions were seen prior to thruster
operation.
Operation of the plasma bridge neutralizer alone is sometimes but not always
accompanied by a similar low energy ion peak. TLese data have not been analyzed
in great detail, and it is not understood why this ,_riabllity exists.
i J
l
124
00000002-TSC02
I I I I [ I ! I '
! 2.2 El_cCroJt_|_a_urementR
i The electron detector normally shows a group ot low energy electrons to be
present under almost all conditions. These electrons, which peak around I0 eV,
i correspond to secondaries and photoelectrons produced at the spacecraft and
trapped by the negative poterlttal well surrounding it. These low energy electrons
4
persist even during daylight charging events ot a few hundred voRs negative.
During OperatiOn of the ton thruster the count rate for these electrons drastically
! decreases, indicating their ability to escape from the spacecraR vicinity. 2 The
i i/
character of the remami,,g low energy electrons is quite different, however, from
i_I! thosewithoutth_steroperation.
i This difference is ShoWn it1 FigUre 3, wher.e the phase space density (effec-
tively the count _ate divided by the square of the energy} is plotted for the low
14
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! , 103
e-
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!:,
:_ _ 102
_ LU
t/1
"i'i" ¢_
v', I0-
_ o THRUSTEROFF
• THRUSTERON
_' I ..... I I I I l
•,_ i 0 5 10 15 20 25 30
' : ENERGY.eV
!.
,-.-='., Figure 3. Phase Space Density of Low Energy
_ Electrons Before ;,,,d During Ion Thruster Oper-
ation on ATS-6
'/
125
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00000002-TSC03
I I I I I I ! I
/.
/
i
energy electrons before and during thi'uster operation. Note that a Maxwellian
distribution should give a straight line, the effective temperature deterrnitting the
slope. The thruster-oft electrons follow a straight line quite closely, indicating
a-near Maxwellian distribution (t_mperature = 5 eVL It should be noted that the
lowest energy point is _ubject to relatively large error because of energy scale
calibration difficulties. The th:'uster-on electrons, aside from being fewer in
number, show a distinctly different distribution. They are no longer Maxwellian.
It is not likely that electrons from the thruster plasma have arrived at the
detector, because of the adverse potentiaLgradients. A more probable explanation
is that modification of tl-e potential barrier by the thruster plasma has allowed low
energy envir6nment_l electrons to reach the detector. Normally these electron_
are repelledby th_negativepotentialbarrier and do not reach thespacecraft. The
electronsdetectedwiththe thrusteron are thusprobably a mixture ofspacecraft
generatedsecondariesand true environmentalelectrons. Since thespacecraft
WaS Clamped at -,I0V by thethruster,the environmentalelectronswere retarded
inenergy by thatamount.
Operationofthe neutralizeralonehas a similarbut lessdrasticeffecton
measured electronsas does fullthrusteroperation. The neutralizerdoes not have
as significantan effecton thepotentialbarrier.
The th_'usteroperationdoes not have a cleareffecton theangulardependence
ofthe low energy electroncounts. Sincemost of theseelectronsoriginateat the
spacecraft,thisisnot surprising.
3. ATS-5 NEUTRALIZEREMITTERLABORATORYSIMULATIONTESTS
A seriesof testswere performed inthe Xerox-Electro OpticsSystems 2 ft×
5 ftlongvacuum chamber to studythe characteristicsof theATS-5 thermionic
emitterunder conditionssimulatingdifferentialcharging on thespacecraft. A
schematic diagram of thetestsetupis shown inFigure 4. A night spare ATS-5
ion thrusterwas mounted along the centerlinenear one end of thevacuum chamber.
The end ofthe thrusterprotrudedthrougha holeinan electricallyisolatedmetal
platesimulatingthe spacecraftsurface. This arrangement resembled closeiythe
actualmounting of thethrusteron the (cylindrical}spacecraft, A second metal
plate,2 R from the first,was Used tosimulate a plasma sheathor potentialbarrier.
Power supplieswere provided tobiasthe two metal platesas shown. Currents
were measured as indicatedinthe figure.
Power was appliedto thethermlonicemitterneutralizerappropriateto give
176f)°C,the nightnominal temperature of the emitter. With no biasingabout
4.2_ ,a emission was obtained. Itwas thennoted thatup to 200 v negativeon the
126
_-;......................... t _ _......._ _.... _ " ,. ......1,..................., _ _=_.................. __, ,;_l .....;__;__..:...._ _; _i_o.... o............._; iii i___._-
00000002-T8C04
NEUTI_ALIZIrR
FILAMEJg]'_
AtS-5_oN \1
THRUSTER--",
vAC.
CHAM_R"-_
SIMULATEO
SICSURFACE ULATED
_J SHEATH
Figure 4. Schematic Diagram of ATS-5 Emitter Laboratory Test
simulated sheath had little(< a few perc._nt)effecton the emission current. How- 1
ever, even a small negative bias on the spacecraR surface produced a marked i
reduction in the emission current. This effect is shown in Figure 5, which is 1
taken from an X-Y plotter output.The totalemission is the totalcurrent leaving l
the filament. For comparison, the current intercepted by the thruster shell
(identifiedas ATS-5 Ion Thruster in Figure 4) is also shown. Note that almost all
of the current escapes the thruster and littleis returned directly to the thruster
body even though the filament is recessed a few inches from the opening.
Differentialcharging on the spacecraR surface is thus very effective in sup-
pressing emission from the filament. In the case of the flighttests described in
References I and 2, the spacecraR was charged to a high negative potential in
eclipse. Both the surface of the spacecraR (largely covered with solar celts and
hetme is dielectric)abd the frame should be at about the same potential. However,
ifthe filament is turned on the spacecraft frame becomes tess negative while the
surface remains at the original potential The filament then experiences a nega-
tive differential potential which suppresses the emission, as in Figure 5. The
filament is not able to reduce the potential further and the potential may remain
apprnximately constant or increase in magnitude. The details of the environment
particle fluxes .rod energy distribution, as well as the geometry of the spacecraft
determine the actual voltage history of the spacecraft during this process.
127
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• il¸
,i¸ ¸
i=_:
_,i_i _ _" , ', , , _ , ,
/\4
I \
,.CURRENTTOTHRUSTERSHELL(x ._)
_A _
-
i
r
::: i 1 TOTALEMISSION
_ i' 0 I I I | I I I r
_-_i; 0 5 10 15 20 25 30 35 40
i_:,, NEGATIVESIC SURFACEPOTENTIAL
Figure 5. i_missionSuppressionby Negative SrrfacePotential
i i
? 4. SlSti.'tiARt ._,_1}CO_i'i.l,SlO_
The chailgein characterotlow energy electronsand ionsdetectedat.the
.....:: spacecraftduring ionthrusteroperationhas been examined. The resultindicates
! thatuse ofa plasma devicefor spacecraftpotentialcontrolproduces a signature
i-: atoti-boar_particledetectors. Iriorder tominimize or understandcontamination
t,. effectsa carefulcallbration,inthe laboratoryor in flight,would be necessary.iJ
:: Inaddition,mounting of thedischargedeviceon a boom well away from thespace-
__ craR isprobablydesirable. Dielectricsurfaces,which can cause differential
=!i:. charging,aggravatethe contaminationproblem and shouldbe avoided on the
_-,_ spacecraft.
Labol-atory simulation studies have shown that differential charging on space-
i : craftsurfaces i_very effectiveininhibitingelectronemission from theSITS-5
< neutralizerfilament. This effectprevents the emitter from completelydischarging
_ the spacecraft under those conditions, and explains poor results of attempts to con-
trol the spacecraft potential in flight. Dielectric surfaces should be avoided for
' electronemitterstobe effectivepotentialcontrollers.Mounting on a boom away
;! from the Spacecraftwould alsominimize interactiveffects.
12O
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00000002-TSC06
Acknowledgment
The author is grateful to E.L. James of Xerox-Electro Optics Systems and
J.R. Beattie and L.C. Pleas for assistance and suggestions in carrying out thc
measurements on the ATS-5 emitter characteristics.
References
dll.l_l
1. Bartlett, R.O., DeForest, S.E., and Goldstein, R. (1975)Spacecraft charg-
il_g control demonstration at geosynchronous altitude, AIAA Paper 75-359.
2. G01dStein, R., and DeForest, S.E. (1976) ActiVe control of Spacecraft poten-
tials at geosynchronnus oi'htts, in Spacecraft Charging by Magnetos_heric
Plasmas, RoSen, Editor, AIAA0 New York in cooperation with MIT Press,
Cambridge, Mass. and London, England, pp. 169-181.
3. Purvis, C.K., Bartlett, R.O., and DeForest, S.E. (1976) Active control of
spacecraft charging o_ ATS-5 and ATS-6. Pre_ented at USAF/NASA Space-
craft Charging Technology Conl'erence, U.S. Air Force Academy. Also
this volume.
4. Whipple, E.C., Jr. (1976) Observation of photoelectrons and secondary elec-
trons reflected from a potential barrier in the vicinity of ATS-6,
J. Geophys. Res. 81:(No. 4):715-719. 1
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Active control of potential of the geosynchronous satellites ATS-5 and ATS-6

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