the impact of harness impedance on hall thruster discharge
TRANSCRIPT
National Aeronautics and Space Administration
www.nasa.gov35thInternationalElectricPropulsionConference,IEPC-2017-023
National Aeronautics and Space Administration
TheImpactofHarnessImpedanceonHallThrusterDischargeOscillations
IEPC-2017-023
LuisR.PiñeroNASAGlennResearchCenter
Cleveland,Ohio,USA
October8-12,2017
35th InternationalElectricPropulsionConferenceGeorgiaInstituteofTechnology•Atlanta,Georgia•USA
National Aeronautics and Space Administration
www.nasa.gov35thInternationalElectricPropulsionConference,IEPC-2017-023
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Outline
• Background• DischargeOscillations• HallThrusterTestSetup• DischargeCircuitAnalysis• FilterOptimization• DischargeHarnessModeling• HarnessDesign• DischargeRippleSensitivity• Conclusions
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Background• GRCandJPLstarteddevelopmentofahighpowerHallthrusterandPPUin2012• Buildingblockfora40kW-classsolarelectricpropulsion(SEP)system• HallElectricRocketMagneticShielding(HERMeS)thrusterwasdeveloped
- Power=12.5kW- Thrust>600mN- SpecificImpulse~3,000s
• HP-120/800Vpowerprocessingunit(PPU)wasdeveloped- Input=100– 140V- Dischargeoutput=300– 800V- Peaktotalefficiency>95%- Vacuumcompatiblebrassboard
• TransitionedtoAerojetRockedyne underAdvancedElectricPropulsionSystem(AEPS)contract• Development,qualificationandfabricationofflighthardware
- Thruster- PPUwithintegraldigitalcontroller- Xenonflowcontroller- Harnesses
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10.4 kW / 500 V / 20.8 A
Discharge Current: 5A/div(ac)Discharge Voltage: 5V/div(ac)
100µs/div
6.3 kW / 300 V / 20.8 A
Discharge Current: 1A/div(ac)Discharge Voltage: 5V/div(ac)
40µs/div
12.5 kW / 600 V / 20.8 A
Discharge Current: 5A/div(ac)Discharge Voltage: 10V/div(ac)
40µs/div
DischargeOscillations
• Hallthrustersexhibitcharacteristicdischargecurrentandvoltageoscillationsduringsteady-stateoperation• Currentoscillationsarecausedbypropellantionization,magneticfieldgradientsandplasmainstabilities• Lowfrequencycomponentsor“breathingmode”oscillationscansignificantlyimpactPPUdesign• OscillationsoftheHERMeS thrustershowdifferent“modes”
National Aeronautics and Space Administration
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PSD
Idisc
h(a
rb U
nits
/Hz)
0 100 200Frequency, kHz
12.5 kW / 600 V / 20.8 APS
D Id
isch
(arb
Uni
ts/H
z)
0 100 200Frequency, kHz
10.4 kW / 500 V / 20.8 A
0 100 200Frequency, kHz
PSD
Idisc
h(a
rb U
nits
/Hz) 6.3 kW / 300 V / 20.8 A
DischargeOscillations
• Lowestfrequencypeaksoccuratapproximately5to50kHz• Evenwhenoscillationsappeartobeverysinusoidal,therearerelativelylargepeaksatmuchhigherfrequencies
10.4 kW / 500 V / 20.8 A
Discharge Current: 5A/div(ac)Discharge Voltage: 5V/div(ac)
100µs/div
6.3 kW / 300 V / 20.8 A
Discharge Current: 1A/div(ac)Discharge Voltage: 5V/div(ac)
40µs/div
12.5 kW / 600 V / 20.8 A
Discharge Current: 5A/div(ac)Discharge Voltage: 10V/div(ac)
40µs/div
• AEPSneededanswerstoseveralquestionsincluding:- Howtospecifydischargevoltageripple?- Howmuchdischargefilterisneeded?- Whatspecificationsarecriticalforharness?- Isdischargevoltagerippleaproblem?
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CDIS
Thruster
BOB
VDPS+-
VDIS+-
+-
VF
DischargeSupply
IDIS
Thruster
BOB
VANDVDPS+-
VDIS+-
+-CHV
RHV LHVRHA LHACHA
VF
CDIS
DischargeSupply
ZHA ZHV
IDIS
• TypicalHallthrusterexperimentaltestsetupincludesadischargesupplyandabreak-out-box(BOB)
• Vacuumharnessinductancecanimpactdischargeoscillations• Rule-of-thumbinductancevaluefortypicaltwisted-pairharnessisapproximately380nH/m
• Z=√R2 +(XL - XC)2- XL =2πƒL andXC =1/(2πƒC)
• Electricalharnessesinambientandvacuumhaveafiniteimpedance• ThisisparticularlyimportantinlargevacuumfacilitieslikeVF-5(4.5mdia)andVF-6(7.5mdia)
HallThrusterTestSetup
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+ VHA -
Thruster
BOB
VDPS+-
VDIS+-
VAND+-CHV
RHV LHVRHA LHACHA
VF
CDIS
I CD
IS(A
C)
DischargeSupply CDPS
+ VHV -
ZHA ZHV
DischargeCircuitAnalysis
• Dischargecurrent hasDCandACcomponents(IDIS(DC) &IDIS(AC) )- DCcomponentisprovidedbythedischargesupply(IDPS(DC))- ACcomponentisprovidedbydischargefiltercapacitor(ICDIS(AC))anddischargesupply(IDPS(AC))
• ACcurrentsgenerateavoltagedroponharnessimpedances(VHA &VHV)• DischargefiltercapacitorreducesvoltagerippleattheBOBcausedbydischargesupplyoscillations(IDPS(AC)) and
ambientharnessimpedance(ZHA)• Dischargecurrentoscillations(IDIS(AC)) andvacuumharnessimpedance(ZHV)generatevoltagerippleatthruster
anode(VAND)• Voltagerippleatthethrusteranode(VAND)willbehigherthanvoltagerippleatdischargefiltercapacitor(VDIS)
IDIS(AC)IDIS(DC)
IDPS(AC)IDPS(DC)
2 ms/div
Discharge Current: 5A/div(ac)
Discharge Voltage (BOB): 5V/div(ac)Anode Voltage: 5V/div(ac)
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Thruster
BOB
VANDVDPS+-
VDIS+-
+-CHV
RHV LHVRHA LHACHA
LDIS
VF
CDIS
IDIS(DC+AC)IDPS(DC)
I CD
ISDischargeSupply CDPS
DischargeFilterOptimizationTest
• Testwasconductedtodetermineminimumamountofcapacitancerequiredtocontroldischargevoltageripple
Discharge Supply Current: 1A/div(ac)
Thruster Current: 2.5A/div(ac)
Capacitor Current: 2.5A/div(ac)
200 µs/div
• Dischargefiltercapacitor(CDIS) wasgraduallyreducedfrom200µF• A30µFcapacitorisminimumrequiredtocontrolvoltageripple• Capacitorchangesdidnotaffectdischargecurrentoscillations
• Aninductor(LDIS) wasaddedto:- MinimizeACcomponentfromdischargesupply(IDPS(AC))- Forcedischargecapacitor(CDIS) supplyACcomponent
• Dischargevoltagewassensedatthrusteranode(VAND) andcomparedtodischargevoltageatBOB(VDIS)
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6 mVacuumHarness
Thruster
DischargeFilter(BOB)
DischargeSupply
DischargeHarnessModel
• DischargesupplywasmodeledasDCvoltagesourceandlargeoutputcapacitorsasinlaboratorypowersupplies
• Thrusterwasmodeledas:- DCcomponentrepresentedbyresistor- ACcomponentrepresentedbycurrentsourceprogrammedwithdatacapturedwithdigitaloscilloscope
• Dischargefiltercomponentswereincluded
• Asimplelumpedelementmodelwasusedforharnessthatincludesaresistor,inductorandcapacitor- Thismodelisvalidbecausetheharnessismuchshorterthatwavelengthofoscillations- Thevaluesusedmatchmeasurementsof6mharnessinVF-5
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SimulationResults
12.5 kW / 600 V10.4 kW / 500 V
Simulated Anode Voltage: 5 V/div (ac)
10V
-25V
0V
0.0 ms 1.0ms0.5 ms
Anode Voltage: 5 V/div (ac) Discharge Current: 5 A/div (ac)
-25V
0V
40A
-10A
0A
Simulated Anode Voltage: 5 V/div (ac)
10V
-25V
0V
Anode Voltage: 5 V/div (ac) Discharge Current: 5 A/div (ac)
-25V
0V
0.0 ms 0.4 ms0.2 ms
40A
-10A
0A
• Simulatedandmeasureddischargeanodevoltagesareverysimilar• P-PandRMSripplevoltagesfromsimulationarewithin14%ofmeasuredvalues• Dischargeanodevoltagerippleiscauseexclusivelybyvoltagedroponharnessbydischargecurrentoscillations• Plasmadonothaveasignificantcontributiontovoltagerippleandbehavelikecapacitiveload• Tominimizevoltageoscillations,thrusteranodeharnessinductancemustbeminimized
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0
3
6
9
12
15
18
21
24
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-8 -6 -4 -2 0
Vol
tage
Rip
ple,
VP-
P
Vol
tage
Rip
ple,
VR
MS
Cap to Thruster Distance, m
Anode RMSCDIS RMSAnode P-PCDIS P-P
LowInductanceHarnessDesign
• ThelongestharnessrequiredforAEPS(8m)willgeneratevoltagerippleof3.6%P-Pand0.7%RMSatfullpower
0
3
6
9
12
15
18
21
24
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 2 4 6 8
Vol
tage
Rip
ple,
VP-
P
Vol
tage
Rip
ple,
VR
MS
Harness Length, m
Anode RMSCDIS RMSAnode P-PCDIS P-P
Option Pros Cons1. Minimize length Low complexity Limited by test setup2. Twisted / braided wires Low complexity Limited effectiveness
Impacts harness design (flexibility and cost)3. Capacitor distance to
thrusterEffectiveInsensitive to lengthFlight heritage
Mechanical, thermal and vacuum impacts on capacitorCan impact discharge output filter design
4. Coaxial cable Low inductance Difficult to implement for high powerLimited flexibility
5. Parallel-plate busbar Low inductanceEasy to fabricateScalable
RigidDifficult to implement in spacecraft
• Parallel-platebusbar wasimplementedinVF-5andVF-6atNASAGRC- Inductancewasreducedto<20nH/m- The4.5mbusbar inVF-6hasonly90nH inductanceor4.4%ofatypicaltwisted-wireharness
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RippleSensitivityTest
• Atestwasconductedtoassesstheeffectofdischargevoltagerippleonthrusterperformance
Time, us
V AN
D, V
IDIS , A
Discharge Current, 10A/div(ac)
Anode Voltage, 50A/div(ac)
174VP-P
LTUNE
• Nosignificantchangeinthrusterperformance(thrust,efficiency,currentoscillations,etc.)• Impactonlife,plumeandEMIstillhastobeassessed
• A30 µH“tuning”inductorwasaddedintheBOBafterthedischargefiltercapacitorsCDIS toartificiallyincreasedischargevoltagerippleatthethrusteranode
BOB
VDIS+-
CDIS
VDPS+-
DischargeSupply Thruster
+-
VFIDIS
LDIS
• Anodevoltageripple~174VP-P whileoperatingat12.5kW/600V
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Conclusion
• Theprimarycauseofdischargevoltageoscillationsisthevoltagedropgeneratedbyhighfrequencydischargecurrentoscillationsflowingthroughthefiniteimpedanceoftheelectricalharness
• Theinductanceofthedischargeharnessmustbeminimizedtoreducevoltageoscillations• Increasingthedischargefiltercapacitorisnotaneffectivemethodtoreducevoltageripple• Asimplelumpedelementmodelcanbeusedtopredictdischargevoltageoscillationsif
highfrequencydischargecurrentcomponentsareincluded• Inductancecanbereducedusingseveraltechniques• Fortestinginvacuumfacilities,aparallel-platebusbar waseffectiveandeasytoimplement• Dischargevoltagerippledoesnotimpactthrusterperformance• Otherpossibleimpactsstillrequireinvestigation