impact of anode layer thruster plumes on satellite

Copyright © 1997 by Frank S. Gulczinski III. 1Published by the American Institute of Aeronauticsand Astronautics, Inc. with permission.

IMPACT OF ANODE LAYER THRUSTER PLUMES ON SATELLITE COMMUNICATIONS

Frank S. Gulczinski III* and Alec D. Gallimore†

Plasmadynamics and Electric Propulsion LaboratoryDepartment of Aerospace Engineering

Douglas O. Carlson‡ and Brian E. Gilchrist§

Radiation LaboratoryDepartment of Electrical Engineering and Computer Science

The University of MichiganCollege of EngineeringAnn Arbor, MI 48109

Abstract

Microwave interferometry was used to investigate the impact of the plume of a D-55 anode layer thruster onsatellite communications. A 17 GHz microwave signal was transmitted through the plume in order to measurephase shift and signal attenuation at various axial positions and discharge voltages. At 0.25 m from the thrusterexit plane, a centerline phase shift of 37° was measured with a half-angle at full width half maximum of 22° forthe nominal 300 V discharge. At 1.00 m, the phase shift had decreased to 20° and the half-angle to 13.5°. Signalattenuation measurements showed attenuations of -1.0 dB to -1.4 dB near centerline, but in a double peakedstructure that appeared to reflect the annular nature of the plume. Increasing the discharge voltage had nomeasurable effect on either the phase shift or signal attenuation. Electron number density distributions werecalculated by Abel inverting the signal phase shift. For the 300 V discharge, the centerline number densitydecreased from 1.0*1011 cm-3 at 0.25 m to 2.1*1010 cm-3 at 1.00 m. Analysis of the distributions indicates thatelectrons are not focused by increasing discharge voltage.

* Graduate Student, Aerospace Engineering, Student Member AIAA† Assistant Professor, Aerospace Engineering and Applied Physics,Senior Member AIAA‡ Graduate Student Research Assistant, Electrical Engineering andComputer Science§ Associate Professor, Electrical Engineering and Computer Scienceand Atmospheric, Oceanic and Space Sciences, Member AIAA

Introduction

The anode layer thruster is a Russian developed Hall-effect thruster that is being tested and considered foruse on western satellites for missions including station-keeping, repositioning, and orbit transfer. It is veryattractive for these roles because it is capable ofoperating over a wide range of specific impulses,which allows one thruster to perform several missionsover the lifetime of the satellite. However, there existsthe possibility that signals transmitted to and from thesatellite will pass through the thruster’s exhaust plume.Due to the plasma density and plasma frequency of theplume, the signal may be phase shifted or attenuatedresulting in communications degradation. This studyis part of the extension of previous microwaveinterferometry work at the University of Michigan on

arcjets1 and stationary plasma thrusters2 to the anodelayer thruster.

In addition to determining phase shift and signalattenuation at several axial locations in the near andfar field, a range of thruster discharge voltages will beinvestigated. Experiments using cylindrical and planarLangmuir probes to investigate the ion current densityin the plume of a D-55 anode layer thruster at theCentral Research Institute of Machine Building(TsNIIMASH) in Kaliningrad, Russia indicated thatincreasing the thruster’s discharge voltage is the mosteffective method for focusing the ionic components ofthe plume.3 Similar results were found by Manzellaand Sankovic.4 This study will attempt to determine ifthe same results occur with the electron-driven RFsignal phase shift and attenuation. In order to confirmthat ionic plume focusing is observable in the

2

University of Michigan’s vacuum chamber, radialFaraday probe measurement sweeps were taken at twodownstream locations perpendicular to the thruster exitplane and several thruster discharge voltages. Theresulting ion current density measurements arepresented in Appendix A.

Based on the observed phase shift, Abel inversion canbe used to determine the electron number density in theplume from the relationship:

( )n rn x

x rdxc

r

R

= −−

∫

λπ

∂φ ∂2 2 2

(1)

Number density distributions for the various operatingconditions and axial locations investigated arepresented in this report. This data is compared withLangmuir probe measurements in Appendix B.

Experimental Description

The experimental measurements described here wereperformed at the Plasmadynamics and ElectricPropulsion Laboratory (PEPL) of the University ofMichigan in its 6 m by 9 m stainless steel vacuumchamber. Primary pumping is provided by six watercooled oil diffusion pumps, each rated at 32000 l/s onair. These are backed by two 2000 cfm blowers andfour 4000 cfm mechanical pumps, giving the facilityan overall measured xenon pumping speed of 30000l/s. The pressure during testing was 6.2*10-5 Torr(corrected for xenon). A more complete description ofthe experimental facility can be found in Gallimore, etal.5

FrequencyConverter

NetworkAnalyzer

DAQComputer

D-55ALT

Vac

uum

Cha

mbe

r W

all

Axial

Radial

Figure 1: Experimental Setup

The microwave system, shown in Figures 1 and 2 hasbeen designed to work in the Ku band (12-18 GHz).6

Here, measurements were taken using a 17 GHz signaltransmitted through the plume of the thrusterorthogonally to its axis at a transmitted power of lessthan 0.1 mW. The measurement system consists of acomputer controlled Hewlett Packard 8753B networkanalyzer connected to a microwave up-down frequencyconversion circuit and two lens corrected hornantennas, each with 7° to 8° beam widths and aseparation of 1.6 m. The network analyzer wascalibrated using a Hewlett Packard model 85033Dcalibration kit with a calibration traceable to NIST.The antenna beams are described by a Gaussiandistribution function with a standard deviation of0.024 m.6 The total phase noise of the microwavesystem is ±0.2° peak-to-peak, due primarily to a 20 dBdifference between the transmitted and received signalsat the network analyzer.

Figure 2: Microwave Interferometer

The microwave interferometer was mounted on PEPL’smulti-axis positioning system. This system permits 0.9m travel perpendicular to the thruster exit plane (axial)and 1.5 m travel parallel to the exit plane (radial). Thetable movement produces an additional vibrationalnoise proportional to the velocity and acceleration ofthe table. During these tests, the table was acceleratedat 0.01 m/s2 and traveled at a radial speed of 0.01 m/s,producing an additional ±1.4° of noise. The support

3

structure was covered with thin graphite sheeting toprevent sputtering.

The thruster used in this experiment, shown in Figure3, is a model D-55 anode layer thruster developed byTsNIIMASH which is on loan from the Jet PropulsionLaboratory. The D-55 has nominal dischargeoperating conditions of 300 V and 4.5 A (1.35 kW)with xenon as its primary propellant. A more completedescription of the D-55 can be found in Garner, et al.7

Figure 3: D-55 Anode Layer Thruster

Radial sweeps of the thruster plume were taken at axialdistances of 0.25 m, 0.50 m, and 1.00 m from the exitplane, and at discharge voltages ranging from 200 V to350 V in 50 V increments. Due to the asymmetry ofthe microwave interferometer mounting structure,radial measurements were taken from -0.533 m to+0.978 m, measured from the thruster centerline.

Experimental Results

The results for signal phase shift at the three axiallocations are given in Figures 4 through 6:

-500 -400 -300 -200 -100 0 100 200 300 400 500

Distance from Thruster Centerline [mm]

0

5

10

15

20

25

30

35

40

Phas

e Sh

ift [

Deg

rees

]

Discharge Voltage200 V250 V300 V350 V

Figure 4: Phase Shift at 0.25 m Axially

-500 -400 -300 -200 -100 0 100 200 300 400 500


0

5

10

15

20

25

30

35

Phas

e Sh

ift [

Deg

rees

]



-500 -400 -300 -200 -100 0 100 200 300 400 500


0

5

10

15

20

25

Phas

e Sh

ift [

Deg

rees

]



The phase shift at a discharge voltage of 300 V for allaxial locations is given in Figure 7:

-500 -400 -300 -200 -100 0 100 200 300 400 500


0

5

10

15

20

25

30

35

40

Phas

e Sh

ift [

Deg

rees

]

Axial Position0.25 m0.50 m1.00 m

Figure 7: Phase Shift for 300 V Discharge Voltage

The signal attenuation data for the three axial locationsis given in Figures 8 through 10:

4

-500 -400 -300 -200 -100 0 100 200 300 400 500


-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2Si

gnal

Atte

nuat

ion

[dB

]


Figure 8: Signal Attenuation at 0.25 m Axially

-500 -400 -300 -200 -100 0 100 200 300 400 500


-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

Sign

al A

ttenu

atio

n [d

B]



-500 -400 -300 -200 -100 0 100 200 300 400 500


-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Sign

al A

ttenu

atio

n [d

B]

Discharge Voltage200 V250 V

300 V350 V


A summary of the attenuation at a discharge voltage of300 V for all axial locations is given in Figure 11:

-500 -400 -300 -200 -100 0 100 200 300 400 500


-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Sign

al A

ttenu

atio

n [d

B]

Axial Position 0.25 m0.50 m1.00 m

Figure 11: Signal Attenuation for 300 V DischargeVoltage

Data Analysis and Discussion

The results shown in Figures 4 through 6 indicate awell defined region of signal phase shift within theplume of the anode layer thruster that must beaccounted for by satellite designers when positioningthrusters and antennas. This is especially true if thesatellite is intended to send or receive signals that areencoded such that they are phase sensitive. For all ofthe axial cases, the difference between the variousvoltage cases was within the total phase noise of±1.6°. Therefore, the raw phase data alone does notindicate whether increasing the thruster dischargevoltage focuses the plume with respect tocommunications signals.

Examining the evolution of the phase shift regionmoving axially away from the thruster in Figure 7 wenote that, for the 300 V case, the shift at thrustercenterline decreases from 37° at 0.25 m to 20° at 1.00m. Using the full width at half maximum as areference point, the phase region increases in width asthe plume diffuses outward. However, converting tocylindrical coordinates we note that the half-anglemeasured from the thruster centerline to the edge of thephase shift region decreases from 22° at 0.25 m to13.5° at 1.00 m.

Examining the signal attenuation data in Figures 8through 10, we note that the ratio of attenuation tonoise is less than the ratio of phase shift to noise sincethe plume has a much larger effect on the signal’sphase than on its magnitude. Also note that there isvery little difference between the various voltage cases,especially considering the level of noise involved.

As seen in Figure 8, at 0.25 m from the thruster exitplane, there are two strong peaks in the signal

5

attenuation of -1.2 dB and -1.4 dB at ±0.06 m from thethruster centerline. There is a gap between these twopeaks, the magnitude of which is larger than thetypical noise observed in the attenuation measurements(which can be seen at radial distances greater than 0.2m). This leads to the conclusion that what is beingseen is the effect of the annular structure of the anodelayer thruster plume on RF signals. It is believed thatthe difference between the two peaks is due to the noisein the system. It appears that this structure is alsoevident at 0.5 m (Figure 9) though the peaks areweaker (-1.0 dB and -1.2 dB) and wider, and thusappear to be converging. It could conceivably beargued that this structure can also be seen at 1.00 m inFigure 10, but given the low signal to noise ratio at thispoint it is not clear that this is the case. Overall, asshown in Figure 11 for the 300 V discharge case, thereis a signal attenuation of approximately -1.0 dB to -1.4dB near the thruster centerline, which would proveespecially detrimental to satellites that have lowtransmission power levels.

Electron number density distributions in the plumewere determined by an Abel inversion technique.First, a double transformation (Fourier, then Hankel)was performed on Equation 1 to avoid computationalissues associated with noise introduced by thederivative of the raw data and the singularity at theintegral end point. Next, filters were used to removedata noise and the antenna pattern from φ(x)6 beforethe Abel inversion technique was applied to give thenumber density profile. The resulting profiles areshown in Figures 12 through 14 for the three axiallocations. The bumps in number density at large radialdistances are the result of the filtering anddeconvolution process.

0 100 200 300 400 500


0.00x1000

2.00x1010

4.00x1010

6.00x1010

8.00x1010

1.00x1011

1.20x1011

Ele

ctro

n N

umbe

r D

ensi

ty [

#/cm

3 ]

Discharge Voltage200 V250V300 V350 V

Figure 12: Electron Density at 0.25 m Axially

0 100 200 300 400 500


0.00x1000

1.00x1010

2.00x1010

3.00x1010

4.00x1010

5.00x1010

Ele

ctro

n N

umbe

r D

ensi

ty [

#/cm

3 ]

Discharge Voltage 200 V250V300 V350 V

Figure 13: Electron Number Density at 0.50 m Axially

0 100 200 300 400 500


0.00x1000

5.00x1009

1.00x1010

1.50x1010

2.00x1010

2.50x1010

Ele

ctro

n N

umbe

r D

ensi

ty [

#/cm

3 ]

Discharge Voltage200 V250V300 V350 V

Figure 14: Electron Number Density at 1.00 m Axially

Figure 15 contains a summary of the results for adischarge voltage of 300 V.

0 100 200 300 400 500


0.00x1000

2.00x1010

4.00x1010

6.00x1010

8.00x1010

1.00x1011

1.20x1011

Ele

ctro

n N

umbe

r D

ensi

ty [

#/cm

3 ]

Axial Position0.25 m0.50 m1.00 m

Figure 15: Electron Number Density for 300 VDischarge Voltage

In examining these number density profiles, one mightexpect that there would be structure seen similar to theion profiles shown by Borisov3, Manzella andSankovic4, and in Appendix A since, in order toneutralize the exhaust beam of the anode layer thruster,the electrons must follow the ions. However, since theelectrons are much more mobile than the ions, one

6

might conclude that any such structure would be lessdefined.

After analyzing the data in Figures 12 - 14, it appearsthat this theory is not valid. Ion focusing is seen as anincrease in the peak value of the ion current densityand a narrowing of the profile (Figures A1 and A2) asdischarge voltage is increased. Though there arespreads in the electron number density profiles, they donot follow the trends seen in ion focusing and appear,in fact, to be random. Previous interferometry workwith this system estimated an uncertainty of ±17% anda repeatability of ±10%.2 The measurements in thisstudy fall within these ranges. It is theorized that theincreased mobility and subsequent diffusion of theelectrons results in the formation of a neutralizingcloud, the density of which is independent of dischargevoltage, through which the ions pass as they travelaway from the thruster. This allows quasineutrality tobe maintained even though the electrons do notconform to the ion distribution patterns.

Examining the evolution of number density with axiallocation in Figure 15 we see, as expected, that due todiffusion the magnitude of the peak density decreasesand the width of the profile increases as the distancefrom the exit plane increases.

Summary

Microwave interferometry facilitates the measurementof phase shift and attenuation allowing satellitedesigners to determine proper positioning of antennasand thrusters so as to minimize communicationsdegradation. This is especially critical if there is lowtransmission power available on the satellite or if thesignal is encoded in such a manner that it is sensitiveto phase shift. Thruster discharge voltage appears tomake little or no difference in the phase shift andattenuation of a communications signal passingthrough the plume in the Ku band. The electronnumber density profiles do not indicate that theelectrons follow the ions and thus are not focused by anincrease in discharge voltage. For all parametersexamined, increasing the distance from the thrusterexit plane broadened their profile and decreased theirpeak values.

Acknowledgements

The research contained herein was sponsored by theAir Force Office of Scientific Research under Dr. MitatBirkan; this support is gratefully acknowledged. Theauthors wish to thank Dr. Shawn Ohler of MIT

Lincoln Laboratories, who designed and built themicrowave interferometry system used here while atthe University of Michigan. They also wish to thanktheir fellow researchers at PEPL and the Radiation Labfor their invaluable assistance in performing theseexperiments, especially Mr. Sang-Wook Kim forassistance with the Langmuir probe measurements andMr. Matthew Domonkos for the operation of the D-55and assistance with the Faraday probe measurements.The authors would like to thank the Jet PropulsionLaboratory for the loan of the D-55, and SashaSemenkin and Sergei Tverdokhlebov of TsNIIMASHfor their assistance in the initial and continuedoperation of the D-55. Mr. Frank Gulczinski issupported by the United States Air Force Palace KnightProgram.

References

1 Ohler, S.G., Gilchrist, B.E., and Gallimore, A.D.,“Non-intrusive Plasma Diagnostics for ElectricPropulsion Research,” AIAA 94-3297, June 1994.2 Ohler, S.G., Gilchrist, B.E., and Gallimore, A.D.,“Microwave Plume Measurements of an SPT-100Using Xenon and a Laboratory Model SPT UsingKrypton,” AIAA 95-2931, July 1995.3 Borisov, B.S., et al., “Experimental Study of ExhaustBeam of Anode Layer Thruster,” IEPC 95-051,September 1995.4 Manzella, D.H. and Sankovic, J.M., “Hall ThrusterIon Beam Characterization,” AIAA 95-2927, July1995.5 Gallimore, A.D., et al., “Near and Far-Field PlumeStudies of a 1 kW Arcjet,” Journal of Propulsion andPower, January-February 1996.6 Ohler, S.G., Gilchrist, B.E., and Gallimore, A.D.,“Non-intrusive electron number density measurementsin the plume of a 1 kW arcjet using a modernmicrowave interferometer,” IEEE Transactions onPlasma Science, June 1995.7 Garner, C.E., et al., “Experimental Evaluation ofRussian Anode Layer Thrusters,” AIAA 94-3010, June1994.

7

Appendix A

Far Field Ion Current Density for the D-55 AnodeLayer Thruster

In order to investigate the detectability of plumefocusing in the PEPL vacuum facility, far field ioncurrent density measurements were taken in the plumeof the D-55 anode layer thruster using a Faraday probe.The probe and techniques were the same as those usedby Marrese, et al. in their investigation of the D-100anode layer thruster.A1 The voltage read by the Faradayprobe is related to the ion current density by the simpleformula:

JV

A Rionprobe

probe shunt

= . (A1)

The probe area was 4.34*10-4 m2 and the shuntresistances were 9.94 Ω and 20 Ω for the 0.25 m and0.50 m cases, respectively.

The ion current density measurements are presented inthe following figures. For the 0.25 m case,measurements were not taken at radial distancesgreater than +0.15 m at 300 V or at any position at 350V due to probe failure.

-300 -200 -100 0 100 200 300


0

50

100

150

200

250

300

350

Ion

Cur

rent

Den

sity

[A

/m2 ]

Discharge Voltage200V250V300V

Figure A1: Ion Current Density at 0.25 m Axially

-500 -400 -300 -200 -100 0 100 200 300 400 500


0

10

20

30

40

50

60

70

80

90

100

Ion

Cur

rent

Den

sity

[A

/m2 ]

Discharge Voltage200V250V300V350V

Figure A2: Ion Current Density at 0.50 m Axially

Comparing the results of Figure A2 to those ofManzella and SankovicA2 (taken on a constant radiusarc 0.6 m from the thruster exit) we find that the PEPLmeasurements are slightly higher but that they followthe same trends in profile shapes. Given the numerousdifferences between the two tests (thruster, probe,facility pressure, etc.) we feel that this adequatelydemonstrates the detectability of thruster focusing inthe PEPL vacuum facility.

The enclosed currents from Figures A1 and A2 aregiven in Table A1:

Axial DistanceDischarge Voltage 0.25 m 0.50 m

200 V 2.44 A 3.01 A250 V 2.48 A 3.06 A300 V 2.47 A 3.13 A350 V X 3.16 A

Table A1: Enclosed Current for Faraday ProbeMeasurements

ReferencesA1 Maresse, C.M., et al, “D-100 Performance andPlume Characterization on Krypton,” AIAA 96-2969,July 1996.A2 Manzella, D.H. and Sankovic, J.M., “Hall ThrusterIon Beam Characterization,” AIAA 95-2927, July1995.

8

Appendix B

Comparison of Microwave Interferometry andLangmuir Probe Electron Density Measurements

In order to compare the microwave interferometryelectron density measurements shown in Figures 12and 13 to probe measurements, a cylindrical Langmuirprobe was swept to various locations within the plume.The probe was a rhenium coated design, previouslyused at PEPL to examine the far field of a 1.35 kWarcjet.B1 The probe used a 9.94 Ω shunt and wasdriven by a function generator and bipolar powersupply, a method previously used by Kim, et al.B2 Theprobe had a radius of 0.0021 m and was 0.051 m long,giving a collection surface area of 6.87*10-4 m2.

The resultant Langmuir probe traces were analyzedusing a thin sheath, ion saturation model. This modelwas justified because the probe radius was much lessthan the electron mean free path (3*103 m) and muchgreater than the Debye length (4*10-5 m).Measurements were taken 0.25 m and 0.50 m axiallyfrom the thruster. No attempt was made to align theprobe with the flow vector, since only order ofmagnitude measurements were desired.

The electron temperature profiles are given in FigureB1.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500


Ele

ctro

n T

empe

ratu

re [

eV]

Axial Position

0.25 m

0.50 m

Figure B1: Electron Temperature for 300 V DischargeVoltage

The electron number density profiles, along with thecorresponding microwave interferometer results, aregiven in the following figures:

0.00E+00

1.00E+11

2.00E+11

3.00E+11

4.00E+11

5.00E+11

6.00E+11

7.00E+11

8.00E+11

0 50 100 150 200 250


Ele

ctro

n N

umbe

r D

ensi

ty [

#/cm

3 ]

Method

Microwave InterferometryLangmuir Probe

Figure B2: Comparison of Electron Density Results at0.25 m Axially

0.00E+00

5.00E+10

1.00E+11

1.50E+11

2.00E+11

2.50E+11

3.00E+11

3.50E+11

4.00E+11

0 100 200 300 400 500


Ele

ctro

n N

umbe

r D

ensi

ty [

#/cm

3 ]

MethodMicrowave InterferometerLangmuir Probe

Figure B3: Comparison of Electron Density Resultsat 0.50 m Axially

The Langmuir probe measurements indicateconsistently higher electron number densities than themicrowave interferometer. They do, however, agreewithin the order of magnitude that was sought in thisexperiment.

ReferencesB1 Gallimore, A.D., et al., “Near and Far-Field PlumeStudies of a 1 kW Arcjet,” Journal of Propulsion andPower, January-February 1996.B2 Kim, S.-W., Foster, J.E., Gallimore, A.D., “Very-Near-Field Plume Study of a 1.35 kW SPT-100,”AIAA 96-2972, July 1996.

impact of anode layer thruster plumes on satellite

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