NASA/TP--2002-211464

/

High-Purity Aluminum Magnet Technology

for Advanced Space Transportation Systems

R.G. Goodrich

Department of Physics and Astronomy, Louisiana State University,

Baton Rouge, Louisiana

B. Pullam

National High Magnetic Field Laboratory, Florida State University, Tallahassee,

Florida

D. Rickle

National High Magnetic Field Laboratory, Los Alamos National Laboratory,

Los Alamos, New Mexico

R.J. Litchford, G.A. Robertson, and D.D. Schmidt

Marshafl Space Flight Center, Marshafl Space Flight Center, Alabama

January 2002
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NASA/TP--2002-211464

High-Purity Aluminum Magnet Technology

for Advanced Space Transportation Systems

R.G. Goodrich

Department of Physics and Astronomy, Louisiana State University,

Baton Rouge, Louisiana

B. Pullam

National High Magnetic Field Laboratory, Florida State University,

Tallahassee, Florida

D. Rickle

National High Magnetic Field Laboratory, Los Alamos National Laboratory,

Los Alamos, New Mexico

R.J. Litchford, G.A. Robertson, and D.D. Schmidt

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

National Aeronautics and

Space Administration

Marshall Space Flight Center ° MSFC, Alabama 35812

January 2002
Acknowledgments

This work was sponsored by the NASA Marshall Space Flight Center through the Space Transportation Research

Project Office (John Cole, Project Manager) of the Advanced Space Transportation Program and through the Technology

Transfer Office (Vernotto McMillan, Manager) as Project No. TIP0(_30. The NASA Principal Investigator was

Ron J. Litchford.

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.

Available from:

NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076 1320

(301) 621 0390

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

(703) 487 4650

ii
TABLE OF CONTENTS

1. INTRODUCTION ......................................................................................................................

2. MAGNET DESIGN ....................................................................................................................

3. MAGNETORESISTANCE TESTING--NATIONAL HIGH MAGNETIC FIELD

LABORATORY, TALLAHASSEE FACILITY ..........................................................................

4. SHAKEDOWN TESTING--NATIONAL HIGH MAGNETIC FIELD LABORATORY,

LOS ALAMOS FACILITY ........................................................................................................

5. CONCLUSIONS AND RECOMMENDATIONS ......................................................................

REFERENCES .................................................................................................................................

1

6

10

16

21

23

°°°

111
LIST OF FIGURES

°

2.

°

°

5.

6.

°

8.

9.

10.

11.

12.

13.

14.

15.

16.

Long-term development history of high field magnets (courtesy of NHMFL) ...............

Virial theorem prediction for magnet structural cold mass as a function

of stored magnetic field energy ........................................................................................

Schematic representation of coil structure and illustration of the fabrication

procedure ..........................................................................................................................

Completed magnet and Dewar flask endplates ................................................................

Cross-sectional view of magnet integrated with its cryogenic container .........................

Test article being lowered into the 20-T resistive magnet at the NHMFL

Talla_lassee facility ...........................................................................................................

Generalized schematic of the test arrangement at the NHMFL Talla_lassee facility .......

Large-bore, 20-T resistive magnet during LHe 2 fill process ............................................

Control and data acquisition instrumentation at the NHMFL Tallahassee facility ..........

Measured magnet resistance as a function of temperature at zero field condition ..........

Measured magnetoresistance as a function of magnetic field at 77 K.

Inset shows the variation between zero and 2 T ...............................................................

Published temperature dependence for the resistivity of nominal high-purity A1 ...........

Data collection instrumentation at the NHMFL Los Alamos facility ..............................

Cryogenic Dewar flask for the magnet tests at the NHMFL Los Alamos facility ...........

Sealed test article container prior to a pulsed test at the NHMFL

Los Alamos facility ..........................................................................................................

Measured magnetic induction along the magnet centerline for the highest

energy pulse carried out at the NHMFL Los Alamos facility ..........................................

2

4

7

7

8

10

11

12

12

13

13

14

17

18

19

20
LIST OF ACRONYMS AND SYMBOLS

A1

Cu

FS

FSU

He

LANL

LHe 2

LN 2

LSU

MHD

MSFC

NHMFL

RuO 2

SMES

TP

aluminum

copper

Fermi surface

Florida State University

helium

Los Alamos National Laboratory

liquid helium

liquid nitrogen

Louisiana State University

magnetohydrodynamic

Marshall Space Flight Center

National High Magnetic Field Laboratory

ruthenium oxide

superconducting magnetic energy storage

Technical Publication

vi
A

B

I

m

N

R

r

S t

t

V

P

cross-sectional area of coil

magnetic induction

current

mass

number of coil turns

radial distance, resistance

radius

material working stress

time

volume, voltage

magnetic field energy

magnetic permeability

density

NOMENCLATURE

vii
TECHNICALPUBLICATION

HIGH-PURITY ALUMINUM MAGNET TECHNOLOGY FOR ADVANCED

SPACE TRANSPORTATION SYSTEMS

1. INTRODUCTION

NASA's long-term vision for achieving routine economical access to space and rapid interplanetary

transport requires consideration of ultra high-energy-density space transportation architectures. In most

cases, this implies utilization of high-temperature working fluids that axe either partially or completely

ionized. As a result, virtually all high-energy-density propulsion system concepts of note require magnetic

fields to confine and manipulate plasma for power or thrust generation. Therefore, no advanced plasmabased space transportation concept will ever come to fruition without the availability of large-volume, high

field magnets that axe sufficiently lightweight for flight applications.

Given this premise, it should be noted that basic research on advanced plasma-based propulsion

systems is routinely focused on plasmadynamics, performance, and efficiency aspects while the critical

enabling technologies, such as flight-weight magnets, axe forever relegated to follow-on development work.

Unfortunately, when a critical enabling technology associated with any advanced system architecture is

known to have a low technology readiness level, it will tend to be perceived as an indicator of high technical

risk and will greatly hamper the acceptance of that architecture for flight development. Consequently, there

is a growing recognition that applied research on the critical enabling technologies needs to be conducted

hand in hand with basic research activities. The development of flight-weight magnet technology, for example,

is a prime example of applied research having broad crosscutting applications to a number of advanced

propulsion system architectures.

In general, there axe two fundamental conceptual design approaches dependent upon the choice of

the magnet winding material. One approach is to use a highly conductive (but resistive) winding material

such as copper (Cu) or aluminum (A1). The alternative approach is to use a superconducting winding

material. Each approach has its unique advantages and drawbacks.

Resistive magnets axe conceptually simpler and are not limited to ultra-low operating temperatures,

but they do suffer from some maj or disadvantages with respect to flight applications. For example, they are

normally limited to relatively low field levels; they tend to consume a large amount of electrical power as

a result of the resistive losses encountered with high current operation; they consequently require a mechanism

for rejecting large amounts of waste heat due to the joule dissipation losses; and they tend to be susceptible

to rapid increases in magnetoresista_nce as the field is gradually increased.
In contrast,magnetsbasedonsuperconductor windingsarecapableofmuchgreaterfieldintensities,

exhibithighfieldstability,andintheabsenceofjouledissipation,haveverylowelectricalpowerconsumption

andavoidthelargewasteheatrejectionproblem.However,thenecessityforultra-lowoperatingtemperatures

placesseveredemandsonthethermalmanagementsystem.It isnecessaryto provideastabilizingquench

pathintheeventhatalengthof superconductingwiremomentarilygoesnormalandbecomesnonconductive.

Thestateoftheartforlaxge-volumesuperconductingandresistivemagnetsisnowwellestablished,

primarilythroughappliedresearchcarriedoutin supportof majormagnetohydrodynamic(MHD) and

fusionterrestrialenergyprograms.1,2However,thistechnologicalcapabilitywasdevelopedwith little

considerationfortotalsystemweight,andthetechnologycannotbegenerallyappliedto aerospacesystems

which axeextremelysensitiveto systemweight.A limited amountof effort hasbeendevotedto the

developmentof flight-weightmagnetsby theU.S.Air Force,butthistechnologyremainsimmatureand

will requireadditionalinvestment. 3,4

The developmenthistoryof high field magnetshas indeedbeendramaticin recentyears,as

summarizedby theNationalHigh MagneticFieldLaboratory(NHMFL)in figure 1.It canbeseenthat

superconductingmagnettechnologyhasadvancedto the point wherecontinuousoperation>20T is

consideredroutine.Evenhigherfieldscanbesustainedin pulsedmodesof operation.Oneshouldnote,

however,thatthesesystemsaxerepresentativeof magnetsdevelopedforterrestrialapplicationsonlyand

aretooheavyforflight.

lOO

Figure 1.

1925 1945 1965 1985 2005

Year

Long-term development history of high field

magnets (courtesy of NHMFL).

2
Themajorchallengeassociatedwith thedesignof lightweight,large-volume,highfield magnets

canbereadilysummarized.Becausemagneticflux densityfallsoff withdistancefromthecoilas1/r2,very

largecurrentsarenecessaryto fill theworkingvolume,andthis,inturn,leadsto theexertionof extremely

largeLorentzforcesonthecoil.Therefore,alargeconfinementstructureisneededto supporthoseforces

thattheconductingcoilscannotwithstandthemselves.Theneedfor alargeconfinementstructuregenerally

impliesalargesystemweightwhenusingconventionalmaterialsandconstructiontechniques.

Theconfinementrequirementsmaybefundamentallyexpressedin termsof thestoredmagnetic

fieldenergy,Wr_z:

=fff 8 dv 8 v,

:::V 2¢t 0 2¢t0

(1)

where/,t o is the magnetic permeability and V is the enclosed volume. The virial theorem may then be used

as an estimate of the minimal mass due to structural requirements (i.e., ideal hoop tension to contain the

stored energy):

m > '°W_ _ p B2 V, (2)

st st 2,u 0

where m is the mass of the confinement structure, p is the material density, and st is the material working

stress. Some typical values for W_z/m = st p axe summarized in table 1.

Table 1. Virial theorem requirements.

Material Wm/m= st/p(kJ/kg)

Fiber-reinforcedcomposites

Stainlesssteel(304LN)

Aluminum(2219T851)

Titanium

Beryllium-copper

10-50

44

107

3O9

58O

Because practical systems can require extremely large magnet volumes, it is clear that the expected

range of stored magnetic field energy densities will be enormous. For example, a 10-T field has a stored

magnetic energy density of W_z/V = 40 MJ/m 3. This number is indicative of the challenges faced by the

designer in terms of the size of the device and its structural requirements. The challenges axe many times

greater when coupled with severe system weight constraints, and becomes ever more clear that fundamental

research on materials and manufacturing techniques coupled with innovative design strategies will be

necessary to obtain the practical flight-weight magnets needed for advanced space transportation systems.
Therangeof requiredstoredmagneticenergiesfor theseadvancedsystemsis anticipatedto fall

between103and106MJ.If thecoldstructuralmassisplottedasafunctionof W_ using equation (2) for the

materials listed in table 1, the result shown in figure 2 is obtained. From this estimate, the structural mass

alone is observed to be between 103 and 106 kg using traditional materials. This result clearly indicates that

the containment structure dominates the weight of laxge-scale magnets. From previous design experience

with laxge-volume magnets for terrestrial MHD and fusion power, for example, it is observed that roughly

50 percent of the total system weight is related to the support of the forces of electromagnetic origin by the

substructure and superstructure.

,2

109

108

107

106

105

104

103

102

101

10-1

MinimumVirialMass

0

Beryllium-Copper

Titanium

Aluminum (2219T851)

Stainless Steel

Aggregate (AI, Stainless Steel, Composites)

Bechtel SMES

/.s •

10 o 101 102 103 104 105 106 107

Stored Energy (MJ)

Figure 2. Virial theorem prediction for magnet structural cold

mass as a function of stored magnetic field energy.

To address such fundamental challenges, NASA Marshall Space Flight Center (MSFC), Louisiana

State University (LSU), and the NHMFL have initiated an applied research effort aimed at advancing the

technology readiness level of flight-weight magnets. This Technical Publication (TP) reports on the group's

initial effort to demonstrate the feasibility of cryogenic, high-purity A1, resistive magnet technology. Under

normal considerations, resistive magnets axe of little attraction for aerospace applications, but high-purity

A1 offers some unique advantages that may permit limited use for flight.

In the design of a high-efficiency magnet, several factors are involved in the choice of the winding

material. For instance, the current-carrying material should have low resistance over the range of operational

temperatures so that joule heating is minimized; the coil resistance should not vary appreciably in the

4
presenceof themagneticfield; andthematerialshouldhaveanextremelyhigh strength-to-weight ratio.

High-purityA1is of specialinterestasawindingmaterialfor resistivelightweightmagnetsbecauseit

displaysthesevaluablephysicalpropertiesatmoderatecryogenictemperatures.

First, high-purityA1hasan extremelylow resistivity at liquid nitrogen(LN2) temperatures

(0.254_tf_.cm)asaresultofreducedelectronscatteringbyimpuritiesandthermalvibrations;therefore,the

electricalpowerconsumptionrequirementsaxegreatlyreduced.Second,A1is alow-density(2.7g/cc)

materialandtheendproductmagnetwouldbeof lowertotalmasscomparedto similardesignsinvolving

Cu.Third,themagnetoresistanceofhigh-purityA1atlowtemperaturesi expectedto saturateatrelatively

lowmagneticfields(-_2T) anddoesnotincreaseindefinitely,asisthecasefor Cu.

In general,theresistanceto currentflow in ametalis dueto electronscattering,whicharisewhen

electronscollidewith impurities,imperfectionsin thelattice,or phonons(i.e.,quantizedthermallattice

vibrations).Whenamagneticfield is present,eitherasanappliedfield or asaself-inducedfield dueto

currentflow,theelectronswill alsoundergocyclotronmotionandtendto spiralthroughtheconductor.

Thismotiongivesriseto amagnetoresistanceeffect.

ThereasonforthelowfieldsaturationofmagnetoresistanceinA1atlowtemperatures(T-_4K) has

to dowith thedetailsof its Fermisurface(FS)geometryandthefactthatopenorbitsontheFSexistfor

morethanonedirectionof appliedmagneticfield.5In contrast,Cuhasonlyonefield directionfor open

orbitsandits magnetoresistanceincreasesinproportionto thesquareof theappliedmagneticfield.

Themagnetoresistance ofhigh-purityA1atmoremoderatetemperatures(say,20to 100K),however,

maynot beasfavorableandneedsto bemeasuredin thepresenceof appliedmagneticfieldsup to the

maximumanticipatedoperatinglevel.Thesemeasurementswill becrucialtothedesignofpracticalsystems

andshouldbeinvestigatedatanearlystageto determinethefeasibilityof thisapproach.It wouldalsobe

desirableto engagein furthertestingof a completemagnetcoil wherethecoil is energizedto full field

whilemeasuringresistivity,inductance,andstrain.

Followingthislogic, it wasdecidedto design,construct,andtesta 6-in-diameterby 12-in-long

high-purityA1solenoidmagnet.ThecoilwasconstructedinthemachineshopoftheDepartmentofPhysics

andAstronomyat LSU andtestingwasconductedin theNHMFL facilitiesat FloridaStateUniversity

(FSU)andatLosAlamosNationalLaboratory(LANL).Thesolenoidmagnetwasfirstwound,reinforced,

pottedinhighthermalconductivityepoxy,andbenchtestedintheLSUlaboratories.Acryogeniccontainer

for operationat77K wasalsoconstructedandmatedto thesolenoid.Thecoil wasthentakento NHMFL

facilitiesin TallaJ-lassee, FL, whereits magnetoresistancewasmeasuredin a 77 K environmentunder

steadymagneticfieldsashighas10T.In addition,thetemperaturedependenceof thecoil'sresistancewas

measuredfrom77to 300K.Followingthisseriesof tests,thecoilwastransportedto NHMFL facilitiesin

LosAlamos,NM,andpulsedto2Tusinganexistingcapacitorbankpulsegenerator. Thecoilwascompletely

successfulin producingthedesiredfield withoutdamageto thewindings.
2. MAGNET DESIGN

The high-purity A1 magnet was designed as a four-layer coil wound on a G-10 (phenolic material)

form using 1-mm-diameter, 99.999-percent-pure A1 wire potted in thermally conducting epoxy. The coil

has a diameter of 6 in, is 12 in long, and was designed to generate a maximum field of 2 T for up to 50 ms.

The inductance for this coil configuration was estimated to be 10-20 mH, and the total current needed to

obtain a 2-T peak field was estimated to be 1 kA. The two major design limitations were the large stresses

encountered at high field strength and the thermal heating associated with joule dissipation during a long

pulse.

As one goes to higher and higher purity metal, for example, the material becomes ever more soft

and ductile. This is intuitively straightforward, of course, since it is the impurities that pin dislocations and

stop slippage under strain. On the other hand, one wants the metal to be as highly pure as possible to reduce

the overall electrical resistance and minimize joule dissipation when high currents are being used. A suitable

design compromise is to wind the coils from the highest purity A1 wire available and to embed the coil in a

thermally conducting epoxy, layer by layer. In this way, one can obtain a low-resistance, low-dissipation

current path while relying on the epoxy to carry the hoop stress and the longitudinal compression stress in

the coil and to rapidly conduct heat from the windings to the surrounding LN 2 bath. The most difficult part

of this procedure is to not break the wire during the winding process because it is so soft. Once the coils axe

wound and potted, however, they should be robust and exhibit low field independent resistance.

Figure 3 is a schematic representation of how the coil winding and its support structure were formed.

First, the external surface of the G-10 form was machined with a spiral groove having a depth equal to onehalf the diameter of the wire in order to accept the innermost winding of the coil. Second, a thin layer of

epoxy was placed in the groove as the initial layer of the bare A1 wire was wound. This procedure made the

initial support for the first layer as well as assuring that adjacent turns of the wire would not touch to form

an electrical short.

Next, a strand of fiberglass string was wound between each turn to add strength to the structure and

further ensure the separation of the wires. This combination of wire and string was then coated with a high

thermal conductivity epoxy encapsulant (Emerson & Cuming Stycast ® 2850 KT). Following the application

of the epoxy, a thin layer of Dupont Kapton ® tape was wound around this initial single layer of the coil and

set in the epoxy. The purpose of the Kapton tape was to add strength against outwardly radial stresses on

the wire and to provide electrical insulation between the first and second layers. While not shown in the

figure, the Kapton tape was stretched and had an indented spiral structure with indentations between each

turn on the first layer, providing a guide for the winding of the second layer.

The next three layers of the four-layer coil were wound, reinforced with fiberglass string and Kapton

tape, insulated, and potted in epoxy in the same manner as the first layer. After completing the winding of

all four layers, a layer of fiberglass cloth was tightly stretched around the entire coil structure and set in

place with epoxy. This final layer of cloth provides additional overall strength to the coil structure and

provides protection against external damage to the coil.
G-IO Form

Q AluminumWire

Q FiberglassString

KaptonTape

Stycast2850KTEpoxy

FiberglassCloth

Figure 3. Schematic representation of coil structure and illustration

of the fabrication procedure.

In order to reduce the overall inductance and resistance of the coil, the four layers were wound

independently and were electrically connected in parallel. This type of connection reduces the voltage

required by the power supply by a factor of 4 over that required for a series connection of the four layers. If

for any reason it is determined that a series connection would be required for some applications, the individual

layers can be rewired and connected in series without having to rewind the coil. Figure 4 shows the completed

coil with the endplates for the Dewar flask sitting on a workbench in the LSU machine shop.

Figure 4. Completed magnet and Dewar flask endplates.

7
A cross-sectional schematicof themagnetintegratedwith its cryogeniccontaineris shownin

figure5.Theoutervacuumshellis sealedwith arubberO-ringoneachend,andtheinnershellis sealed

with two indiumO-ringson eachend.Theentiresetof shellscanberemovedto work on thecoil if

necessary.Also includedaxetwo 100-kf2rutheniumoxide(RuO2)resistancethermometerswith leads

comingoutof theelectricalconnector.Whentheassemblyis orientedsuchthatthenitrogenfill tubesaxe

pointedup,oneof thethermometersilocatedonthetopofthecoil andoneonthebottom.Twoofthenine

leadscomingoutoftheelectricalconnectoraxeconnectedtoasingle-turncoil wrappedaroundtheoutside

middleof theA1coil. Thevoltageoutputof thiscoil canbeusedto monitorthefield producedduringa

currentpulse.Thecoil diameteris =6 in and there is one turn. This voltage output as a function of time

needs to be calibrated against a coil output located at field center for use as a field monitor.

VacuumLiner

Solenoid

CurrentIn Windings CurrentOut

\\ \\

\\ \\

"\ 7"

\\ \\

StainlessSteel

G-10Phenolic

-- ..... :211 :::::::2 Flask_'_ -

Connector

Figure 5. Cross-sectional view of magnet integrated with its cryogenic container.
Theconnectionsto thenine-pinelectricalconnector,in clockwisefashion,areasfollows:

1: 100-kf_resistorontopof coil leadNo. 1

2: 100-kf_resistorontopof coil leadNo.2

3: Pickupcoil leadNo. 1

4: Pickupcoil leadNo.2

5: Unused

6: Unused

7: Unused

8: 100-kf_resistoronbottomof coil leadNo. 1

9: 100-kf_resistoronbottomof coil leadNo. 2.

Thecurrent-inandcurrent-outconnectionsareA1stripsto whichthecurrentleadsfromthepulsed

powersupplycanbeconnected.Theseleadsareconnectedto thecoil oneachendthroughfourA1rods

extendingfromtheconnectorthroughtheDewarflaskwallstothecoil connections.

TheDewarflaskpackagehadadiameterof 12.5in andlengthof 20in.Themeasuredtotalmassof

thecoilwindingsandformafterfabricationwas3.69kg,andthetotalmassofthemagnetsystem,excluding

theendflangesfor matingto alaboratoryvacuumchamber,was15.58kg.

9
3. MAGNETORESISTANCE TESTINGmNATIONAL HIGH MAGNETIC FIELD

LABORATORY, TALLAHASSEE FACILITY

Upon completion of the fabrication process at LSU, the magnet was taken to the NHMFL facility at

FSU in Tallahassee, FL, for measurement of its magnetoresistance in a high field environment. The

Tallahassee facility was equipped with a large-bore (192-mm-diameter), 20-T resistive magnet and a liquid

helium (LHe2) Dewar flask insert for maintaining a 4.2 K test environment. To conduct the test, the magnet

was placed in a specially built stainless steel container as a means of isolating it from the LHe 2 bath. This

container was then inserted into the cryogenic Dewar flask, and the He Dewar flask was finally lowered

into the bore of the magnet, as indicated in figure 6. A generalized view of the final test configuration is

shown in figure 7.

Figure 6. Test article being lowered into the 20-T resistive

magnet at the NHMFL Tallahassee facility.

10
Vacuum

Valve

Jortand Electrical

Connections

LHe2

m

TestArticle

and

ExchangeGas

Existing

NHMFL

Helium

Dewar

Flask

Activated

Charcoal

Vacuum

Jacket

PressureReliefValve

Figure 7. Generalized schematic of the test arrangement

at the NHMFL Talla_lassee facility.

Prior to assembling the system, a strip of Cu-clad tape was placed along the length of the coil to

serve as a thermal contact, and two 100-kf_ RuO 2 thermometers were glued to this strip using General

Electric vaxnish. The thermometer leads were brought out of the Dewar flask through vacuum-sealed

connectors, and then brought out of the container using twisted-pair Cu leads. Expanded views of the test

apparatus axe shown in figures 8 and 9. In figure 8, the large-bore, 20-T resistive magnet can be seen near

the bottom center. This photograph was taken while LHe 2 was being transferred from the storage Dewar

flask into the cryogenic test container. Figure 9 shows the control and data acquisition instrumentation at

the foot of the stairs leading to the measurement platform.

During routine operation of this facility, power failures may occasionally occur, which could cause

the field inside the bore of the A1 magnet to drop from 10 to zero tesla in a few milliseconds. In order to

protect the resistance measurement equipment, two large diodes were placed across the inputs of the coil to

short out any large (>1,000 V) inductive back-voltage that might appear during a magnetic power supply

failure.

11
Figure 8. Large-bore 20-T resistive magnet during LHe 2 fill process.

Figure 9. Control and data acquisition instrumentation at the NHMFL

Tallahassee facility.

Measurements were taken as a function of applied field along the axis of the coil at 77 K and as a

function of temperature at zero field during the variable temperature cool-down period. Because one can

anticipate a sharp temperature rise during pulsed operation of the coil, additional constant temperaturedependent measurements were taken between 77 and 100 K at the zero field condition. For all of the

resistance measurements, a 40-Hz ac signal was applied to a 200 mV/A resistor in series with the magnet

coil. Two lock-in amplifiers were used--one to monitor the voltage across the coil and the other to measure

the voltage across the standard resistor as a means of monitoring the current.

12
ThecorrectedmeasurementsobtainedattheNHMFLTallahasseefacility axepresentedin figures

10and11.Becausethediodeprotectioncircuitrycausesanunknownbiasintheresistancemeasurement,

all of theTallahasseeresistancevalueshavebeenscaledto matchthezerofield,77K readingsobtainedat

the LosAlamosfacility (describedin sec.4) whichavoidedthe complicationsarisingfrom thediode

protectioncircuitry.Sincethiscorrectionisindependentof theappliedfield,theactualvaluesaxeaccurate

atall temperaturesandappliedfields.

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

75

O0

i i

, i .... i .... i .... I '0'•'0'41

00

I I

I i i i i I i i i i I i i i i I i i i i

80 85 90 95 100

Temperature (K)

Figure 10. Measured magnet resistance as a function of temperature at zero field condition.

0.80

0.75

0.70

0.65

' ' ' ' ' I ' ' ' I ' ' ' I ' ' '

0.'660 .... I I .... I .... I ' ' '

01 i!

o.658 l

• • 00

0.657 • O0

0.656 O0

0.654065510000•0•00 0000

0.653_0,, ,0 0.51.... 11.... 115' , , O00

O0 o°

00000000 O•

_O_l I 0 i i i I i i i I i i i I i i i

0 2 4 6 8 10

B(T)

Figure 11. Measured magnetoresista_nce as a function of magnetic field at 77 K.

Inset shows the variation between zero and 2 T.

13
Thepublishedtemperaturedependencefor theresistivityof high-purityA1is shownin figure12.

All zerofield measurementswerefoundtobeconsistentwiththiscurve,indicatingthatthestraininduced

inthecoilduringthewindingprocessdidnotsignificantlychangetheknowncharacteristicsoftheresistivity.

Furthermore,this plotindicatesthattheresistivityof nominalhigh-purityA1shouldbe10timeslessat

80K thanat273K. Measurementsfrom theTallahasseeexperimentgivenin figure10,however,yield a

12.5timesreductioninresistivityasthetemperaturedropsfrom273to76K.Morespecifically,theresistance

ofthecoilis8.18f_at273Kand0.654f_at77K.Thisindicatesthatthewindingprocessdidnotsignificantly

strainthewire andthattheuseof 99.999-percent-pureA1wireimprovesthetemperaturedependencefor

resistivity,considerably.

3.0

2.5

2.0

E

d 1.5

:=L

1.0

0.5

I I I I I I I I I I I

/

High-Pu

I I I I I I I I I I

50 1O0 150 200 250 300

Temperature(K)

Figure 12. Published temperature dependence for the resistivity

of nominal high-purity A1.

The field dependence for coil resistance, which quantifies the magnetoresista_nce, is shown in figure

11. To make these measurements, the field was ramped up in 0.5-T increments, stabilized, and 1,000

voltage and current readings were acquired at a fixed field level following a 1-min delay to ensure the

attainment of thermal equilibrium. After reaching 10 T, the field was ramped downward in 0.5-T steps but

with stops intermediate to the upward ramp schedule. Using the 1,000 readings at each field value, the coil

resistance was calculated from the average measured voltage across the coil and the averaged measured

current through the coil.

In order for the facility resistive magnet to achieve 20-T field levels, two 8-MW power supplies axe

required. On the day of the scheduled tests, however, one of the two supplies had tripped on several occasions

during a previous testing period. As a precaution, only one power supply was therefore available for

subsequent tests. This limited the maximum applied field to 10 T, but this is still 5 times greater that the

maximum operational field of the test article magnet.

14
Themeasuredresistancedataup to the operationalfield of 2 T axealsoshownin the insetof

figure11.Notethatthedataaxenotsmoothin theinsetdueto limiteddigitizerresolution.Althoughthese

measurementsshowthatthemagnetoresistanceof high-purityA1is nonzeroat77K, it is verysmallin

magnitudeandincreasesbyonly19percentfromzeroto 10T.In theoperationalfieldregionof zeroto 2T,

thechangeis<1percent.

15
4. SHAKEDOWN TESTINGmNATIONAL HIGH MAGNETIC FIELD LABORATORY,

LOS ALAMOS FACILITY

A second series of shakedown tests were performed at the NHMFL Los Alamos National Laboratory

facility in Los Alamos, NM. This facility specializes in pulse field applications and has the needed equipment

and instrumentation at hand. In these measurements, the coil was mounted in an 8-in-diameter LN 2 Dewar

flask with styrofoam padding around the coil to limit movement. Measurements of the coil resistance were

made immediately preceding and immediately following each current pulse. A small multi-turn pickup coil

of known cross-sectional area and number of rums was placed at the center of the field region and connected

to a 200-kHz digitizer to record the voltage output and calibrate the field produced by the coil.

The output of the pickup coil is given by

dB

V = NA-- , (3) dt

where V is the voltage, N is the number of turns, A is the cross-sectional area of the pickup coil, B is the

magnetic induction, and t is time. Thus, the time-dependent variation in the magnetic field can be obtained

through direct integration of the measured voltage waveform.

The experimental setup at the LANL facility is shown in figures 13-15. Figure 13 shows the data

collection computer and digitizers in the electronics rack. Figure 14 shows the cryogenic Dewar flask for

the coil, which is cooled to 77 K, and the field measurement probe extending out of the top of the assembly.

The outside of the container box is made of 1-in-thick G-10 for containment of any explosion. Figure 15

shows the containment box after it has been sealed and readied for a test pulse of the magnet. All of the data

at the NHMFL LANL facility were acquired using this apparatus.

16
Figure13. DatacollectioninstrumentationattheNHMFL LosAlamosfacility.

17
Figure 14. Cryogenic Dewar flask for the magnet tests at the NHMFL Los Alamos facility.

18
Figure15. Sealedtestarticlecontainerpriorto apulsedtestattheNHMFL LosAlamosfacility.

19
The testing methodology was to begin with low voltage pulses and gradually build up to the final

voltage that produced a 2-T peak field. The testing sequence was to first charge the 4-mF capacitor bank,

discharge the bank through the coil, and record the output voltage waveform from the pickup coil. The

pickup voltage waveform could then be postprocessed using equation (3) to obtain the magnetic induction

waveform for the magnet. The sequence of voltage charges and the resulting peak values for magnet current

and magnetic induction axe summarized in table 2.

Table 2. Magnet performance characteristics.

BankVoltage Imax

(kV) (kA)

0.50 0.270

1.67 0.670

2.16 0.850

2.63 1.01

3.09 1.15

3.21 1.18

Bm_ TotalEnergy

(T) (kJ)

- 0.5

1.16 5.57

1.45 9.33

1.74 13.8

1.98 19.1

2.04 20.6

The coil inductance was also measured during this test sequence, yielding a value of 15 mH. The

measured magnetic induction waveform along the centerline of the magnet is shown in figure 16 for the

highest energy discharge. The field generated per unit of current flow at the peak was 1.73 mT/A. The

temperature rise for each pulse was 20 K, and the cooldown time following each pulse was <60 s.

2.0

1.5

1.0

0.5

0

0 20 40 60

Time (ms)

Figure 16. Measured magnetic induction along the magnet centerline for the highest

energy pulse carried out at the NHMFL Los Alamos facility.

2O
5. CONCLUSIONS AND RECOMMENDATIONS

The current technology readiness level of flight-weight magnets has been recognized as a major

area of deficiency with respect to highly advanced, plasma-based, propulsion system architectures. This

deficiency is perceived as a major technical risk and hampers the acceptance and ultimate development of

high-payoff, high-performance propulsion systems. In response to this critical need, MSFC, LSU, and the

NHMFL have joined forces to initiate an applied research project aimed at advancing the technology

readiness level of flight-weight magnets.

This TP reports on the design, development, and testing of a 6-in-diameter, 12-in-long, 2-T cryogenic,

high-purity A1 resistive magnet having attributes favorable to lightweight construction. Normally, resistive

magnets axe of little attraction for aerospace applications, but high-purity A1 offers some unique advantages

that may permit limited use for flight. These advantages axe its very low mass density, its low resistivity,

and the fact that its magnetoresistance is limited in high field environments. Following a detailed design

and manufacturing study, the solenoid was wound from 1-mm-diameter, 99.999-percent-pure A1 wire in

the machine shop of the Department of Physics and Astronomy at LSU and reinforced using thermally

conductive epoxy. The solenoid winding and form was then mated with a custom low-weight Dewar flask

designed and manufactured by the same organization.

The final package dimensions of the magnet had a diameter of 12.5 in and was 20 in long. The

measured total mass of the coil windings and form after fabrication was 3.69 kg, and the total mass of the

magnet system, excluding the end flanges, was 15.58 kg.

The magnet was then shipped to the NHMFL Talla_lassee facility where it was installed in the

20-T resistive magnet laboratory to evaluate its resistance characteristics. The tests in that facility included

measurement of the temperature dependence of coil resistance at zero field and measurement of the field

dependence of coil resistance at 77 K. It was found that the resistivity of 99.999-percent-pure A1 decreased

12.5 times as the temperature dropped from 273 to 76 K. This is a substantial improvement over nominal

high-purity A1, which is known to exhibit a decrease in resistivity of only 10 times. This test also indicated

that the winding process did not significantly strain the wire. Measurements of coil resistance demonstrated

a 19-percent increase in magnetoresista_nce as the applied field varied from zero to 10 T. In the critical

operational range from zero to 2 T, however, the rise in magnetoresista_nce was <1 percent.

A follow-on series of shakedown tests were performed at the NHMFL LANL. In this facility, the

magnet was pulsed with a 4-mF capacitor bank charged to gradually increasing voltages until the peak field

exceeded 2 T. The peak field obtained during these tests was 2.04 T at a peak current of 1.18 kA with a total

energy of 20.6 kJ. The coil inductance was also measured to be 15 mH. The field generated per unit of

current flow at the peak condition was 1.73 mT/A, and the temperature rise experienced by the solenoid

winding was no more than 20 K. These shakedown tests verified the structural integrity of the magnet and

demonstrated the level of performance achievable when using high-purity A1 as a winding material.

21
Basedontheresultsof this research,the authorsconcludethathigh-purityA1resistivemagnet

technologyrepresentsaviablesolutionfor specializedflight applicationswherepulsedmagneticfieldsaxe

essentialtotheoperationoftheonboaxdpropulsion/powersystem.Therefore,theauthorsrecommendthat

furtherdevelopmentstepsbeundertakento advancethetechnology.Theauthorsalsostronglyrecommend

thatvigorousfollow-onresearcheffortsbeinitiatedto developcontinuouslyoperatingflight-weightmagnets

basedonlow-temperaturebinarymetalalloysuperconductors(e.g.,Nb3SnandNbTi)andhigh-temperature

ceramicoxidesupercounductors(e.g.,BiSrCaCuOandYBaCuO).Theseenablingtechnologieswill be

absolutelyessentialto thefuturedevelopmentof plasma-basedpropulsionsystems,anda diligentlongterm appliedresearcheffortis needednowto mitigateoneof themajortechnicalrisksassociatedwith

advancedpropulsionsystems.

22
REFERENCES

1. Thome, R.J.; and PTarrh, J.M.: MHD and Fusion Magnets: Field Force and Design Concepts, John

Wiley & Sons, New York, 1982.

2. Wilson, M.N.: Superconducting Magnets, Oxford University Press, London, 1983.

3. Stekly, Z.J.J.; Zax, J.L.; and Hoppie, L.: "Design of Superconducting Magnet Systems," Vols. 1 and 2,

APL-TR-66-126, U.S. Air Force, 1967.

4. Stekly, Z.J.J.; and Thome, R.J.: "Light Weight Superconducting MHD Magnets; Vol. I: Saddle Magnet

Design, Construction, and Preliminary Test Results; Vol. II: 10 MW Level Magnet System Design

and Projections for Future Development," AFAPL-TR-72-32, U.S. Air Force, 1972.

5. Reed, W.A.: "Experimental Methods of Measuring High-Field Magnetoresistance in Metals," in

Methods of Experimental Physics, Vol. 2, Solid State Physics Series, Academic Press, New York, 1974.

23
REPORT DOCUMENTATION PAGE Form Approved

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1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

January 2002 Technical Publication

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

High-Purity Aluminum Magnet Technology for Advanced Space

Transportation Systems

6. AUTHORS

R.G. Goodrich,* B. Pullam,** D. Rickle,*** R.J. Litchford,

G.A. Robertson, and D.D. Schmidt

7. PERFORMING ORGANIZATION NAMES(S) AND ADDRESS(ES)

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546_0001

8. PERFORMING ORGANIZATION

REPORT NUMBER

M-1037

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA/TP--2002-211464

11. SUPPLEMENTARY NOTES

Prepared by the Advanced Space Transportation Program, Space Transportation Directorate

*Department of Physics and Astronomy, Louisiana State University, **National High Magnetic Field Laboratory, Florida State University,

***National High Magnetic Field Laboratory, Los Alamos National Laboratory

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 20

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12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Basic research on advanced plasma-based propulsion systems is routinely focused on plasmadynamics, performance, and

efficiency aspects while relegating the development of critical enabling technologies, such as flight-weight magnets, to follow-on

development work. Unfortunately, the low technology readiness levels (TRLs) associated with critical enabling technologies tend

to be perceived as an indicator of high technical risk, and this, in turn, hampers the acceptance of advaaaced system architectures

for flight development. Consequently, there is growing recognition that applied research on the critical enabling technologies

needs to be conducted hand in hand with basic research activities. The development of flight-weight magnet technology, for

example, is one area of applied research having broad crosscutting applications to a number of advaaaced propulsion system

architectures. Therefore, NASA Marshall Space Flight Center, Louisiana State University (LSU), and the National High Magnetic

Field Laboratory (NHMFL) have initiated an applied research project aimed at advaaacing the TRL of flight-weight magnets. This

Technical Publication reports on the group's initial effort to demonstrate the feasibility of cryogenic high-purity aluminum

magnet technology and describes the design, construction, and testing of a 6-in-diameter by 12-in-long aluminum solenoid

magnet. The coil was constructed in the machine shop of the Department of Physics and Astronomy at LSU and testing was

conducted in NHMFL facilities at Florida State University and at Los Alamos National Laboratory. The solenoid magnet was first

wound, reinforced, potted in high thermal conductivity epoxy, and bench tested in the LSU laboratories. A cryogenic container

for operation at 77 K was also constructed and mated to the solenoid. The coil was then taken to NHMFL facilities in Tallahassee,

FL, where its magneto resistance was measured in a 77 K environment under steady magnetic fields as high as 10 T. In addition,

the temperature dependence of the coil's resistance was measured from 77 to 300 K. Following this series of tests, the coil was

transported to NHMFL facilities in Los Alamos, NM, and pulsed to 2 T using an existing capacitor bank pulse generator. The coil

was completely successful in producing the desired field without damage to the windings.

14. SUBJECT TERMS

magnet, high-purity aluminum, magnetoresistance, flightweight

15. NUMBER OF PAGES

32

16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

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