the impact of harness impedance on hall thruster discharge

National Aeronautics and Space Administration

www.nasa.gov35thInternationalElectricPropulsionConference,IEPC-2017-023


TheImpactofHarnessImpedanceonHallThrusterDischargeOscillations

IEPC-2017-023

LuisR.PiñeroNASAGlennResearchCenter

Cleveland,Ohio,USA

October8-12,2017

35th InternationalElectricPropulsionConferenceGeorgiaInstituteofTechnology•Atlanta,Georgia•USA




Outline

• Background• DischargeOscillations• HallThrusterTestSetup• DischargeCircuitAnalysis• FilterOptimization• DischargeHarnessModeling• HarnessDesign• DischargeRippleSensitivity• Conclusions




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




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


40µs/div

12.5 kW / 600 V / 20.8 A


40µs/div

DischargeOscillations

• Hallthrustersexhibitcharacteristicdischargecurrentandvoltageoscillationsduringsteady-stateoperation• Currentoscillationsarecausedbypropellantionization,magneticfieldgradientsandplasmainstabilities• Lowfrequencycomponentsor“breathingmode”oscillationscansignificantlyimpactPPUdesign• OscillationsoftheHERMeS thrustershowdifferent“modes”




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)


10.4 kW / 500 V / 20.8 A


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


100µs/div

6.3 kW / 300 V / 20.8 A


40µs/div

12.5 kW / 600 V / 20.8 A


40µs/div

• AEPSneededanswerstoseveralquestionsincluding:- Howtospecifydischargevoltageripple?- Howmuchdischargefilterisneeded?- Whatspecificationsarecriticalforharness?- Isdischargevoltagerippleaproblem?




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




+ 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)




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)




6 mVacuumHarness

Thruster

DischargeFilter(BOB)

DischargeSupply

DischargeHarnessModel

• DischargesupplywasmodeledasDCvoltagesourceandlargeoutputcapacitorsasinlaboratorypowersupplies

• Thrusterwasmodeledas:- DCcomponentrepresentedbyresistor- ACcomponentrepresentedbycurrentsourceprogrammedwithdatacapturedwithdigitaloscilloscope

• Dischargefiltercomponentswereincluded

• Asimplelumpedelementmodelwasusedforharnessthatincludesaresistor,inductorandcapacitor- Thismodelisvalidbecausetheharnessismuchshorterthatwavelengthofoscillations- Thevaluesusedmatchmeasurementsof6mharnessinVF-5




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




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




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




Conclusion

• Theprimarycauseofdischargevoltageoscillationsisthevoltagedropgeneratedbyhighfrequencydischargecurrentoscillationsflowingthroughthefiniteimpedanceoftheelectricalharness

• Theinductanceofthedischargeharnessmustbeminimizedtoreducevoltageoscillations• Increasingthedischargefiltercapacitorisnotaneffectivemethodtoreducevoltageripple• Asimplelumpedelementmodelcanbeusedtopredictdischargevoltageoscillationsif

highfrequencydischargecurrentcomponentsareincluded• Inductancecanbereducedusingseveraltechniques• Fortestinginvacuumfacilities,aparallel-platebusbar waseffectiveandeasytoimplement• Dischargevoltagerippledoesnotimpactthrusterperformance• Otherpossibleimpactsstillrequireinvestigation

the impact of harness impedance on hall thruster discharge

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