Integrated Liquid Bismuth Propellant Feed System∗

Kurt A. Polzin,† Thomas E. Markusic,† and Boris J. Stanojev‡NASA-Marshall Space Flight Center

Huntsville, AL 35812

Colleen Marrese-Reading†NASA-Jet Propulsion Laboratory

Pasadena, CA 91109

A prototype bismuth propellant feed and control system was constructed and operated in conjunctionwith a propellant vaporizer. An electromagnetic pump was used in this system to provide fine control ofthe hydrostatic pressure, and a new type of in-line flow sensor was developed in an attempt to providean accurate, real-time measurement of the mass flow rate. High-temperature material compatibility wasa driving design requirement for the pump and flow sensor, leading to the selection of Macor for themain body of both components. Post-test inspections of both components revealed no degradation ofthe material. In separate proof-of-concept experiments, the pump produced a linear pressure rise as afunction of current that compared favorably with theoretical pump pressure predictions, with a pressureof 10 kPa at 30 A. Preliminary flow sensor measurements have been made at a bismuth flow rate of 6mg/s ± 6%. A real-time controller was successfully used to control the entire system, simultaneouslymonitoring all power supplies and performing data acquisition duties.

I. INTRODUCTION

Operation of Hall thrusters with bismuth propellanthas been shown to be a promising path for devel-opment of high-power (140 kW per thruster), high-performance (8000s Isp at >70% efficiency) electricpropulsion systems[1]. The use of bismuth also alleviatesseveral logistical issues that would normally be associatedwith development and deployment of a high-power Hallthruster operating on xenon, which is the traditional propel-lant option. The cost of propellant for testing and perfor-mance of deep-space missions is not nearly as prohibitivesince bismuth costs far less than xenon ($75/kg comparedto $2000/kg). Also, since it is a solid at room tempera-ture, vaporized bismuth can be condensed using simple,water-cooled plates, essentially ‘cryopumping’ the propel-lant at room-temperature and obtaining equivalent pump-ing speeds of millions of liters/sec. Finally, while Hallthrusters operating on xenon and other Noble gases havedifficulty operating at high-voltages (>1 kV), thrusters us-ing bismuth have achieved very high I sp because they canoperate at voltages approaching 10 kV.

Presently, there are several efforts underway both in theU.S. and abroad that aim to validate the high performanceof bismuth-fed Hall thrusters and understand the physicalmechanisms that allow for high-voltage, high-power, high-

∗Presented as paper AIAA-2006-4636 at the 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Sacramento, CA,July 9-12, 2006. This material is declared a work of the U.S. Governmentand is not subject to copyright protection in the United States.†Member AIAA.‡Senior Research Engineer, Madison Research Corp.

performance operation[2–4]. The work described in thispaper is part of the Very High Isp Thruster with AnodeLayer (VHITAL) program[2]. Our effort was focused onthe design and construction of a propellant managementsystem that could deliver liquid bismuth to a Hall thrusterwhile simultaneously monitoring the propellant flow rate.This is a critical element of the VHITAL program sinceperformance cannot be accurately assessed without preciseknowledge of the mass flow rate. Previous performancemeasurements[1] used a pre/post-test propellant weighingscheme that did not provide any real-time measurement ofthe mass flow rate during thruster operation, leading to rela-tively high error bars on both Isp and thrust efficiency. TheVHITAL propellant management system was designed toobtain more accurate, temporally resolved flow rate mea-surements. The overall system also served as a test bedwhere hardware and control algorithms could be evaluatedfor future propellant feed system development efforts.

In the next section, we define the problem while pay-ing special attention to bismuth-specific issues that compli-cated our efforts. The feed system hardware that is actu-ally in contact with molten bismuth is described in Sect. IIIwhile the electronics that control and monitor the hardwareare discussed in Sect. IV. Section V contains data show-ing the performance of the electromagnetic pump and flowsensor.

II. DEFINITION OF THE PROBLEM

In the VHITAL thruster, the bismuth propellant must un-dergo three phase transitions: solid to liquid, liquid to gas,and gas to plasma. The role of the propellant managementsystem we assembled is to deliver liquid bismuth to the

42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit9 - 12 July 2006, Sacramento, California

AIAA 2006-4636

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

POLZIN et al.: Bismuth Propellant Feed System

vaporizer while simultaneously monitoring the mass flowrate. Thus, the system must both melt the propellant andpossess the means to transport it.

In our preliminary analysis we identified the followingbismuth-specific challenges associated with the develop-ment of the VHITAL propellant management system:

• The high density of bismuth (O(104) kg/m3) and lowpropellant mass flow rate (O(10−5) kg/sec) resultsin a very low volume flow rate (O (

10−9 -10−10)

m3/sec) that is challenging to continuously monitorusing in-line flow sensors. The high density of thepropellant also causes gravitational influences on thefeed-line pressure – as the propellant level in the tankrecedes during testing – which must be compensatedfor by varying the pumping pressure.

• The high melting temperature of bismuth (∼275oC) makes makes it difficult to employ many “off-the-shelf” components (e.g., valves and electronics).Consequently, many of the feed system componentsrequire custom design and fabrication.

• The relatively low electrical conductivity of bis-muth makes it nearly impossible to measure the flowrate electromagnetically, as was previously done forlithium[5]. Flow sensing elements that are in di-rect contact with the flow must be electrically in-sulated, but the high temperature of molten bismuthprecludes the use of some of the most attractive in-sulators. Finally, the feed system and any electricalconnections must be shielded from condensing bis-muth vapor (from the thruster exhaust) as this canelectrically short some of the feed system compo-nents – rendering them inoperable.

III. FEED SYSTEM HARDWARE

The propellant management system we designed totackle the difficulties listed in the previous section operatesin the following manner. A propellant reservoir contain-ing solid bismuth is heated until its temperature is abovebismuth’s melting temperature. The liquid bismuth can bemade to move using a combination of two different tech-niques. The application of gas pressure to the reservoirforces molten bismuth through the system. The secondmethod, which exploits the fact that bismuth is an electri-cally conducting fluid, employs an electromagnetic (EM)pump that is integrated into the propellant feed line down-stream of the reservoir. An in-line flow sensor capable ofmaking real-time measurements completes the propellantfeed system. The entire system is shown in Fig. 1. In whatfollows, each of these four major components are discussedin greater detail.

ToVaporizer

GasPressure

HeaterBracket

FlowSensor

EM Pump

BiFlow

Reservoir

Slot for Cartridge Heater

FIG. 1: Assembled bismuth propellant feed system.

A. Propellant Reservoir

The reservoir is designed to allow for the storage andmelting of high purity bismuth. It is fabricated from 316Lstainless steel and has a removable lid for loading bismuth.An inlet tube welded into the body allows for gas pres-surization that forces liquid bismuth out of the reservoirthrough an outlet tube. Two three-inch long, 1/4” diam-eter cartridge heaters are used to heat the reservoir. Thecopper plates bolted to the steel structure are employed touniformly distribute heat to the reservoir.

B. Gas Pressurization System

The gas pressurization system (shown photographicallyin Fig. 2 and schematically in Fig. 3) is designed to providegas pressurization to the reservoir within the (continuouslyadjustable) range of 0-200 Torr. The system is designedto operate inside a vacuum chamber, allowing for ease ofintegration with other thruster subsystems.

The pressure vessel (PV) is a 0.7 liter, fiber wound cylin-der rated to 4500 psi. The hand-operated regulator, whichreduces the high pressure in the reservoir to the lower pres-sure required for the electro-pneumatic regulator (EPR) ac-tually limits the pressure rating of the system to roughly200 psi. Two solenoid operated valves (SOV1 and SOV2)are used to isolate the propellant reservoir from the gaspressurization system and vent the pressurized lines di-rectly to vacuum.

The EPR is the heart of the gas-pressurization system,using a “bang-bang” arrangement of solenoid valves. The(propellant reservoir) pressure is set to the desired value byalternately opening and closing the high pressure and vac-uum valves. An on-board control system uses a feedbacksignal from an on-board pressure transducer to determinethe proper sequencing of the SOV open/close operations.External control is maintained using a 0-10 VDC signal

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POLZIN et al.: Bismuth Propellant Feed System

EPR HOR

PV

SOV1

Hand OperatedFill Valve (HOV)

SOV2

FIG. 2: Photograph of the gas pressurization system.

EPRIn

Out

Vent

HOR

Pressure Vessel (PV)

SOV1

Supply Valve

HOV

SOV2 VentValve

Fill

Supply

Ventto

Vac

FIG. 3: Schematic representation of the gas pressurization system.

that allows for continuous, remote adjustment of the reser-voir gas pressure.

C. Electromagnetic Pump

For the sake of brevity, we refer the reader to Ref. [6]for a review of electromagnetic pump theory and a sur-vey of the current state-of-the-art. We employed glass-micaceramic (also known as Macor) in the construction of themain body of our EM pump because it is stable and non-reactive with liquid bismuth near 300 oC temperature. It isalso inexpensive and easily machinable compared to otherceramics.

The ease of fabrication afforded by the use of Macor(versus, for example, aluminum nitride) allowed us to pur-sue an aggressive design for the EM pump. Samariumcobalt magnets were employed, with a magnet separationof 2.9 mm and a channel height s of 2.0mm. This resultedin an on-axis magnetic field strength B of 0.64 T and gaveB/s = 3.2 T/cm.

The major components of the bismuth EM pump are

shown in Fig. 4. The CAD model is included to show a sec-tioned view that reveals the inner construction. The mate-rials used to fabricate the pump were: Macor (pump body),iron (magnet yoke), 316L stainless steel (end caps and feedlines), Inconnel (electrodes), and samarium cobalt (mag-nets). The electrodes were bonded to the Macor body us-ing high-temperature epoxy and the feed lines were weldedto the end caps. The feed lines utilize Swagelok VCR fit-tings for attachment to the rest of the feed system and twoParker metal c-rings seal the joints between the end capsand the ceramic body. Two small (1” long, 1/8” diameter)cartridge heaters (not pictured) were used to heat the endcaps and feed lines where they joined to the pump to en-sure the bismuth would remain molten while it traversedthe pump.

D. Flow Sensor

We have developed a new type of flow sensor, which wecall the ‘hotspot’ flow sensor[5]. The device operates inthe manner illustrated in Fig. 5. A pulse of thermal energy(derived from a current pulse and associated joule heating)is applied near the inlet of the sensor. The flow is ‘tagged’with a thermal feature that is convected downstream by theflowing liquid metal. A downstream thermocouple recordsa ‘ripple’ in the local temperature associated with the pass-ing ‘hotspot in the propellant. By measuring the time be-tween the upstream generation and downstream detectionof the thermal feature, the flow speed can be calculated us-ing a time of flight analysis.

The primary advantage of this technique is that it doesn’tdepend on an absolute measurement of temperature, but in-stead relies on the observation of thermal features. Thismakes the technique insensitive to other externally gener-ated low-frequency thermal fluctuations. The hotspot inthe upstream flow is generated by pulsing current directlythrough the liquid metal; by doing so we exploit the intrin-sic resistivity of the fluid and obviate the need for a sepa-rate resistive heating element. In order for the hotspot flow

EM Pump AssemblySwagelokconnector

magnet yoke electrode

Interior Detailpump body(sectioned)

magnet yoke magnetelectrode

c-ringseal

FIG. 4: Photograph of assembled EM pump and a CAD drawingsectioned to show the pump’s inner detail.

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POLZIN et al.: Bismuth Propellant Feed System

flow in flow out

I(t)T (t)

FIG. 5: Principle of operation of the hotspot flow sensor.

Hotspot Flow Sensor Assembly

heater clamps

CAD Drawing of Flow Sensor

liquidinlet sensor

body

electrodeheating thermocouple

liquid outlet

FIG. 6: Photograph and CAD drawing of assembled flow sensor.

sensor to provide useful results, the spatial integrity of thehotspot must be maintained until it reaches the thermocou-ple location. The hotspot will tend to decrease in magni-tude as it propagates, due to thermal diffusion. Thereforewe have designed the device such that the thermal diffusiontime scale is much longer than the convective time scale.

As with the pump, the flow sensor was constructedaround a Macor central body (see Fig. 6). The fluid con-nectors at the ends of the sensor and the heating electrodeswere fabricated from 316L stainless steel. The downstreamtemperature measurement was made using a 0.002” diam-eter butt-welded type E thermocouple that penetrates theMacor body, with approximately 1 cm separation from the

hotspotpulse circuit

cRIO system

adjustable-output power supplies

fixed-outputpower supplies

FIG. 7: Control system electronics suite for the VHITAL feedsystem.

heating location. The fluid connectors were sealed to theMacor body using Parker metal c-rings while the elec-trodes and thermocouples were sealed using high tempera-ture epoxy. Heaters were clamped to the fluid connectorson the flow sensor to inhibit propellant freezing. The flowchannel has a diameter of 0.022”, which results in a bis-muth flow speed of ∼0.5 cm/sec at a mass flow rate of ∼10mg/sec.

IV. CONTROL SYSTEM

The VHITAL propellant management control systemmust not only control all the components described in theprevious section, but must also perform all the data sam-pling tasks required during operation of the feed system.Tasks include operation, adjustment, and switching of thevarious power supplies connected to the experiment, mon-itoring of any analog voltages generated by components inthe system, operation of the hotspot flow sensor circuitry,and performance of temperature measurements to monitorthe overall state of the feed system. This last task is mademore difficult by the need to sample the flow sensor ther-mocouple temperature as quickly as possible to achieve anaccurate measure of the flow rate.

The electronics suite developed for the VHITAL feedsystem (see Fig. 7) is centered around a compact Recon-figurable I/O (cRIO) real-time embedded control system.This system performs all communications and data sam-pling required for feed system operation. The entire systemhas been tested in conjunction with the feed system hard-ware and operated within the expected parameters. We pro-ceed first with a discussion of the embedded control systemarchitecture and then describe the electronic subsystemsthat operate in conjunction with the embedded controllerto form the complete control system.

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POLZIN et al.: Bismuth Propellant Feed System

FIG. 8: Control system architecture and data flow (from ni.com).

FIG. 9: cRIO real-time controller/FPGA architecture (fromni.com).

A. System Architecture

In general, the embedded control system can be operatedas either a stand-alone system or in conjunction with an ex-ternal computer. The latter mode of operation is the one wehave implemented in the VHITAL system. In this mode,the embedded controller is passive and receives all com-mands from the external computer (in our case, connectedusing a fiber optic network converter for electrical isola-tion). The controller executes all the commands it receivesand simultaneously monitors the feed system using if/elselogic to prevent hardware damage. A schematic showingthe top level control system architecture and the flow ofcommand and data signals between the various componentsis shown in Fig. 8.

The user interface was designed in Labview and is usedboth to issue commands to the embedded controller andto display data from the control and data acquisition sys-tems in real time. In principle, any standard computer withnetworking capability can operate as the user interface ma-chine. The embedded controller hardware combines smallformat PC capabilities with a fast Field ProgrammableGateArray (FPGA). The cRIO system is designed for rapid pro-

totyping and fast signal conditioning operation using theFPGA clock for data acquisition. This system was selectedfor the VHITAL application because of the compact size ofthe cRIO unit and the various available I/O modules.

B. The cRIO Chassis

The cRIO chassis accommodates four modules, whichcan be selected depending on the numbers and types of I/Osrequired for an application. Each module has between 4and 32 channels, depending upon the function of the mod-ule. The following is a complete description of the cRIOmodules we used:

• NI cRIO-9002: real-time controller with 32 MBDRAM, 64 MB compact flash

• NI cRIO-9201: 8-channel, 10 V, 500 kS/s, 12-Bitanalog input module

• NI cRIO-9481: 4-channel, sourcing digital outputsingle-pole single-throw relay module

• NI cRIO-9472: 8-channel, 24 V Logic, 100 s, sourc-ing digital output module

• NI cRIO-9211: 4-channel, 14 sample/s, 24-Bit, 80mV thermocouple input module

Each module connects to the real time controller throughthe FPGA module as shown on Fig. 9.

C. Power Supplies

The control system uses two separate power subsystems.A Vicor dual output AC/DC converter is used to providepower to the electronics. These include the cRIO module,optical network converter, and cooling fan. In addition, thissupply is switched using the cRIO-9481 module to energizethe solenoid valves SOV1 and SOV2 in the gas pressur-ization system. Five variable Lambda ZUP output powersupplies are used to provide adjustable power to the feedsystem. These supplies were selected because they have asmall form factor, are easily packaged into a cluster, andcan be controlled through a serial connection. The suppliesare interconnected and adjusted by the real-time controllerusing the RS-485 communication protocol. The suppliesmonitor their current and voltage outputs and send thesedata back to the controller using the same communicationsprotocol. The following is a list of the specifications andprimary function of each power supply:

• Magnetic pump - 6 V/66 A

• Component cartridge heaters - 80 V/2.5 A

• Reservoir cartridge heaters - 120 V/3.6 A

• Flow sensor capacitor charge - 120 V/1.8 A

• Gas pressure regulator control signal - 10 V/20 A

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POLZIN et al.: Bismuth Propellant Feed System

D. Flow Sensor Pulse Circuitry

The ‘hotspot’ pulse circuitry is designed to store energyin capacitors and then release that energy in a fast pulse tothermally tag the liquid bismuth flowing between the elec-trodes in the flow sensor through Ohmic heating. Measure-ments of the temperature in the flow are obtained using athermocouple connected to the cRIO-9211 module. Theflow rate is calculated by taking the time difference be-tween the initiation of the heating pulse and the detectionof the temperature “peak” as it is convected downstream.

In the pulse circuit, an SCR is used to switch the energyfrom the capacitor bank into the flow sensor. The SCR isoptically isolated from the control system through an op-tocoupler, thus minimizing the risk of exposing high cur-rent to the controller. The embedded controller initiates acurrent pulse by triggering the SCR gate through the opto-coupler. To stop current from flowing, a serial commandis sent to the power supply that charges the capacitor bank,setting it to output zero power. This drops the leakage cur-rent below the ‘keep-alive’ threshold of the SCR, allowingthe solid-state switch to open. While this active shutdownprovides total isolation between the charge circuit and thereal-time controller, it is also a relatively slow process.

V. COMPONENT PERFORMANCE

The entire system was tested at NASA’s Jet PropulsionLaboratory (JPL) in conjunction with a propellant vapor-izer. Once primed, the pump operated successfully and wecould drive current pulses between the electrodes in thehotspot sensor. We have successfully validated the flowsensing concept by acquiring hotspot flow sensor data us-ing hot (>300 oC), flowing bismuth. To date, encouragingbut non-repeatable measurements have been obtained withthis system. Post inspection of the pump and flow sensorrevealed no cracking of the Macor components. Tests thatquantified the performance of the pump and flow sensor aredetailed below.

A. Pump Pressure

The EM pump was previously evaluated[6] by measur-ing the hydrostatic pressure developed by the pump as afunction of input current I . In order to eliminate the diffi-culties associated with handling liquid bismuth, these testswere conducted using a substitute liquid metal – gallium.The theoretical pressure P developed by an EM pump isindependent of particular properties of the conducting fluidand given by the equation

P =B

sI,

where B is the magnetic field strength in the channel ands is the width of the conduction current path through theliquid metal.

12

10

8

6

4

2

0

EM

Pum

p P

ress

ure

[kP

a]

302520151050Pump Current [A]

� = 1.6 mm

� = 2.4 mm

FIG. 10: EM pump pressure versus current (the solid lines aretheoretical curves based on the value of s given, the dashed linerepresents a linear curve fit of the data).

Applied current levels ranging from 10-30 A were testedand the results are shown in Fig. 10. As expected, the datashow that the pressure developed by the EM pump is lin-ear with current. A linear curve fit (dashed line) yieldsa value for B/s equal to 334±16 Pa/A. The process ofcomparing this to the predicted pump performance is com-plicated by the ambiguity in the definition of the channelheight s. The pump was constructed by drilling an axial,2.4 mm hole through which the liquid metal flows. How-ever, the electrodes penetrate through the sides of the pumpand are somewhat smaller in height (1.6 mm). The value ofs is dependent on the (unknown) current density distribu-tion. However, the geometry of our EM pump allows us tobound the effective value of s between 1.6-2.4 mm. The-oretical lines for each of these extremes are shown in Fig.10. The measured data show good quantitative agreement,lying roughly midway between the two theoretical curves.

The data demonstrate that the bismuth EM pump is capa-ble of delivering the required hydrostatic pressure (O(10 3)Pa) for electric propulsion applications. Furthermore, thepump is seen to operate at a level consistent with the the-oretical maximum performance. This implies that lossesassociated with stray conduction currents are negligible inthe present design.

B. Flow Sensor Measurements

The flow sensor was operated and quantified in conjunc-tion with a propellant vaporizer presently under develop-ment at JPL[2]. The heating pulse current output as a func-tion of time is plotted in Fig. 11. For the an initial capacitorcharge of 15 V, the pulse peaked just under 80 A with awidth of roughly 400 µs. While the magnitude of this out-put will change with the initial capacitor voltage, the wave-forms will not change in shape.

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POLZIN et al.: Bismuth Propellant Feed System

100

80

60

40

20

0

Hot

spot

Cur

rent

[A]

1.20.80.40.0

Time [ms]

Cap. Voltage15 V

FIG. 11: Current output of the hotspot pulse circuit for an initialcharge voltage of 15 V.

318

317

316

315

314

313

Tem

pera

ture

[C]

76543210

Time [s]

6 mg/s (± 6%)

FIG. 12: Flow sensor measured bismuth fluid temperature as afunction of time. Flow rate is estimated based upon the sensorgeometry, fluid density and hotspot time-of-flight.

Temperature measurements obtained downstream of thelocation where the heat pulse was introduced are presentedin Fig. 12 for hot, flowing bismuth. Time is measured fromwhen the current pulse is initiated. While the tests werenot repeatable, the data shown do indicate a temperaturepeak passing the thermocouple location ∼4 seconds afterthe heating pulse. A flow rate of 6 mg/s is estimated basedupon the sensor geometry, fluid density and hotspot time-of-flight. The uncertainty in this estimate of ±6% is basedon the possible flow rates corresponding to the times theleading and trailing edges of the hotspot pass the thermo-couple.

VI. SUMMARY AND CONCLUSIONS

We have presented a design for a bismuth propellantmanagement system that delivers liquid bismuth to a vapor-

izer while simultaneously monitoring the mass-flow rate inreal time. This system has been assembled as a series ofmodules, allowing it to serve as a test bed for hardware andcontrol algorithm development. We can draw the followingconclusions from this development effort.

• High-temperature material compatibility was adriving design requirement for the bismuth EMpump and flow sensor. Macor was chosen for the in-sulating body material to minimize shunt conductioncurrent losses. Post test inspection after exposure tohot, flowing bismuth revealed no degradation in ei-ther component.

• The EM pump produced a hydrodynamic pressure inthe liquid metal of 10 kPa when operating at 30 A.A measure of pump pressure as a function of inputcurrent showed good quantitative agreement with thetheoretical pump output.

• The flow sensing technique employed by the hotspotflow sensor was successfully demonstrated using hot,flowing bismuth during operation of the full feed sys-tem in conjunction with a propellant vaporizer. Aflow rate of 6 mg/s with an uncertainty of ±6% wasestimated based on data from the flow sensor. Theflow rate measurements were not repeatable possiblydue to pressure fluctuations in other system compo-nents. Additional experiments of the integrated sys-tem are required but preliminary results are encour-aging.

• A cRIO real-time controller, coupled to a remotecomputer/user interface through a fiber optic net-work converter, was used to control all feed systemelectronics and simultaneously perform all data ac-quisition. We demonstrated operation of the entiresystem (power supplies, pulse circuit, data acquisi-tion) and were able to control and monitor the feedsystem using our remote computer interface.

Acknowledgments

We appreciate the management support of Mike Fazahand Jim Martin and the program office support of Dr.Michael LaPointe throughout the duration of this effort.We also acknowledge the contributions of Doug Daven-port, Doug Galloway, Tommy Reid, Keith Chavers, Ron-dal Boutwell and Jeff Gross to this effort. The VHITALprogram was supported by NASA’s Exploration SystemsMission Directorate (Project Prometheus) and funded un-der contract NAS7-03001managed by John Warren. Partialsupport for this work has been provided by NASA-MSFC’sTechnology Transfer Office.

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[3] J. Szabo, C. Gasdaska, V. Hruby, and M. Robin, “BismuthHall thruster with ceramic discharge channel”, Proc. 53rdJANNAF Propulsion Meeting, Monterey, CA, Dec. 2005.

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in a bismuth Hall thruster”, 29th Intn’l Electric PropulsionConference, Princeton, NJ, Oct. 31-Nov. 4, 2005. IEPC-2005-129.

[5] T.E. Markusic, K.A. Polzin, B.J. Stanojev, C. Dodson, and A.Dehoyos, “Liquid metal flow sensors for electric propulsion”,Proc. 53rd JANNAF Propulsion Meeting, Monterey, CA, Dec.2005.

[6] T.E. Markusic, K.A. Polzin, and A. Dehoyos, “Electromag-netic pumps for conductive-propellant feed systems”, 29thIntn’l Electric Propulsion Conference, Princeton, NJ, Oct. 31-Nov. 4, 2005. IEPC-2005-295.

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