Janis M. Niedra

SAIC, Inc., Cleveland, Ohio 

Steven M. Geng

Glenn Research Center, Cleveland, Ohio

External Magnetic Field Reduction Techniques for 

the Advanced Stirling Radioisotope Generator

NASA/TM—2013-216540

November 2013

NETS–2013–6813
	
	
	








	



	

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Janis M. Niedra

SAIC, Inc., Cleveland, Ohio 

Steven M. Geng

Glenn Research Center, Cleveland, Ohio

External Magnetic Field Reduction Techniques for 

the Advanced Stirling Radioisotope Generator

NASA/TM—2013-216540

November 2013

NETS–2013–6813


	



	


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NASA/TM—2013-216540 1 

External Magnetic Field Reduction Techniques for 

the Advanced Stirling Radioisotope Generator 

Janis M. Niedra 

SAIC, Inc. 

Cleveland, Ohio 44135 

Steven M. Geng 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Linear alternators coupled to high-efficiency Stirling engines are strong candidates for thermal-toelectric power conversion in space. However, the AC and DC magnetic field emissions of these 

permanent magnet excited alternators can interfere with sensitive instrumentation onboard a spacecraft. 

Effective methods to mitigate both the AC and DC electromagnetic interference (EMI) from solenoidaltype linear alternators (like that used in the Advanced Stirling Convertor) have been developed and 

demonstrated for potential use in the Advanced Stirling Radioisotope Generator. The methods developed 

avoid the complexity and extra mass inherent in data extraction from multiple sensors or the use of 

shielding. This paper discusses these methods and also provides analytical results for both the AC and DC 

techniques. Data obtained during breadboard testing of the AC external magnetic field technique are 

presented. Preliminary testing of the DC technique is also discussed. 

Nomenclature 

 polar angle (°) 

μo 	
"
	|7 Henries/m) 

AC alternating current 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

EMI electromagnetic interference 

FEA finite element analysis 

B magnetic flux density (T) 

Brem remanent magnetic flux density (T) 

DC direct current 

H magnetic field strength (kA/m) 

IAc Amperian current (A) 

M magnetization (T) 

Mbc magnetization of DC bucking magnet (T) 

Mmc magnetization of alternator magnet (T) 

MM 





			


2

) 

N number of turns, linear alternator 

Nb number of turns, AC bucking coil 

rb radius of AC bucking coil (mm) 

rbc radius of DC bucking magnet (mm) 

rmc radius of alternator magnet can (mm) 
NASA/TM—2013-216540 2 

ro radius of hoop equivalent to alternator coil (mm) 

tbc thickness of DC bucking magnet (mm) 

tmc thickness of alternator magnet (mm) 

Background 

Stirling engine-driven linear alternators are considered to be promising candidates for providing 

electrical power for deep space missions, such as to the outer planets, where the insolation is inadequate 

for practical use of solar energy. Typically these engines take heat from a radioisotope source and in turn 

convert their linear motion to electricity by means of a coupled linear alternator. The engines themselves 

are basically thermodynamic oscillators, having a free piston and operating in a Stirling cycle-type mode. 

Of particular interest to power instrumented planetary probes are Advanced Stirling Radioisotope 

Generators (ASRGs) in the 100-W class (Dudzinski (2011)). These spacecraft typically carry extremely 

sensitive instrumentation, such as magnetometers for field mapping, which can be adversely affected by 

the electromagnetic interference (EMI) fields emanating from nearby alternators. The magnetic field limit 

has been considered a potential roadblock to application of Stirling convertors to power certain 

instrumented missions. AC components are considered the most troublesome (Pope et al. (2009)) due to 

their amplitude, whereas the smaller DC components can presumably be cancelled locally at the sensor by 

a judicious placement of permanent magnets. 

This paper discusses methods of reducing both the AC and DC components of the B-field emissions 

over an extended region from the alternator. The methods basically amount to field cancellation by 

superposition close to their source. However, their implementation differs sufficiently to require separate 

discussions. 

Method of Suppression of AC Field Emissions 

B-Fields and Alternator Geometry 

An external B-field reduction technique was developed for use on an 80-W Advanced Stirling 

Convertor (ASC) intended for space power applications. The ASC is equipped with a permanent magnet 

excited linear alternator. The magnets (high-energy NdFeB-type) form a hollow can-like structure that is 

polarized radially outward and oscillates back and forth within a solenoidal pickup coil. This coil is 

surrounded by a shell of Hiperco-50 laminations and forms the stator of the alternator; the inner part of 

the stator is within the can and closes the magnetic circuit. 

Finite element computations using the Maxwell software showed that most of the alternating B-field 

at 1 m distance was due to the load current in the coil. The contribution due to the stator laminations, even 

though being a magnetic material, was considerably less. According to the plots in Figure 1, the field 

geometry is essentially close to dipolar, having maximum amplitude on the axis. In short, with a steady 

10 A in the coil, its far field essentially coincides with that of a point-like magnetic moment of strength of 

1.30 A m2

 and aligned with the axis of symmetry. These computations show that the stator laminations, 

under the influence of the field of the coil, contribute a B-field that roughly mimics the geometry of the 

dipole-like field of the coil, resulting in a total field of about 1.2 times the field of the coil alone. 
NASA/TM—2013-216540 3 

Figure 1.—B-field magnitude as a function of angle from the z axis. The dipole 

computation is according to classical formula. The magnet can is absent and coil current 

is 10 A. Note that the “coil only” and “simple dipole” curves coincide. 

Estimate of Bucking Coil From Classical Dipole Field and Experiment 

The external field due to the load current in the coil of the alternator can be reduced by winding a 

field bucking coil on the outside of the containment vessel and connecting it in series to carry the load 

current. 

The parameters of the alternator coil being known, it is quite simple to calculate its magnetic moment, 

or even to replace it by a single current loop of equivalent moment and computed radius ro. An order-ofmagnitude estimate of the bucking turns required can be based on equating to the moment of the 

alternator’s coil. This calculation gives a quick starting point for experiment, even though it might be 

expected to overestimate the required turns. For example, consider a single-layer bucking coil of radius rb, 

located just outside of the containment vessel of the alternator. Since the same current flows through both 

the alternator and bucking coils, the formula for the bucking turns is 

N= N









 (1) 

However, this overestimates the turns required in the present case by nearly a factor of two. The reason is 

that the bucking coil sees the whole stator as a highly permeable cylindrical stub through its center. A 

better, although more laborious, estimate can be obtained through finite element analysis (FEA). A twodimensional FEA model of the ASC linear alternator was created. The magnitude of the B-field at a 

distance of 1 m from the geometric center of the alternator was calculated for various bucking coil 

configurations and at various polar angles as shown in Figure 2. Comparing the coil and lams curves in 

Figure 1 with Figure 2, it is evident that about 18 turns should be an acceptable estimate. 

0.0 E+00

5.0 E-08

1.0 E-07

1.5 E-07

2.0 E-07

2.5 E-07

3.0 E-07

3.5 E-07

0 10 20 30 40 50 60 70 80 90 100

B, (T) 

Polar Angle , () 

Coil Only

Coil and Lams

Simple Dipole
NASA/TM—2013-216540 4 

Figure 2.—Magnitude of the B-field at 1 m distance produced by the bucking coil with the 

stator as a core. Plots are for the indicated number of turns carrying 10 A and rb = 3.4 cm. 

Experimental Verification of AC B-Field Reduction 

Two Stirling engine-driven linear alternators of above type were mounted in an opposed piston 

configuration and positioned as they would be in the ASRG application. Each alternator was equipped 

with a bucking coil wired in series with alternator output such that the turns count could be varied. 

Operation was electrically phase locked to minimize vibrations, which is the purpose of the configuration. 

Data taken are shown in Figure 3. As per custom, B-field values are shown in dBpT, which in the case of 

picoTesla is defined by 

dBpT = 20log 	


 ( 
)

 
  (2) 

Evidently, the use of 19-turn bucking coils reduced the axial B-field from 87.1 dBpT to 62.1 dBpT at the 

particular position of measurement, which amounts to a fractional reduction of 94 percent on a linear 

scale. At off-axis positions, a 20-turn bucking coil seemed to be preferable. This is a very significant 

reduction of the external B-field of an alternator, at least at a distance that is large compared to the size of 

the alternator, thus demonstrating the concept. Details of the mounting of a bucking coil are shown in 

Figure 4. 

Method of Suppression of DC Field Emissions 

In this case, partial cancellation of the external DC magnetic field of a solenoidal linear alternator can 

be achieved by superposition of a bucking field generated by a can-like configuration of external magnets, 

which mirrors as close as possible the moving magnet configuration doing the excitation. To be most 

effective, the bucking magnets clearly need to be placed as close as possible to the pressure vessel 

containing the alternator. Moreover, in the DC case, the use of permanent magnets to buck has two 

advantages. The use of equivalent external coils, or hoops, would require inconveniently large and steady 

currents and external DC field suppression by permanent magnets is always active, even if the alternator 

is inactive. Next follows the derivation of the heuristic approximations to define the details of the scheme. 

0 E+00

1 E-07

2 E-07

3 E-07

4 E-07

5 E-07

6 E-07

7 E-07

0 10 20 30 40 50 60 70 80 90 100

B, (T) 

Polar Angle , () 

36 Turns 

18 Turns 

9 Turns 
NASA/TM—2013-216540 5 

Figure 3.—B-field measured on the z-axis at 1 m from the center of the nearest alternator 

for selected number of bucking coil turns. Both alternators used the same number of 

bucking coil turns. The alternators are operating at 103 Hz, with a current of about 7-A 

root mean square (RMS), or 10 A peak. B-field values are RMS. 

Figure 4.—Bucking coil wound on the pressure vessel containing the 

linear alternator. 

Solenoidal alternators, such as the type used in the ASRG, have slat-like exciting magnets arranged in 

the shape of a hollow cylinder and are polarized radially relative to the longitudinal axis. Although the 

stator laminations efficiently direct the flux from the cylinder through the surrounding solenoidal coil, 

these laminations unfortunately cannot prevent a small part of the exciting field from escaping to the 

external space as a DC field, which wobbles a slight bit axially with the motion of the exciting magnet 

cylinder. Fortunately, a straight-forward magnetic moment analysis gives a useful estimate of field 

strength. 

Bucking Coil 
NASA/TM—2013-216540 6 

Slat Magnets Viewed as Amperian Current Loops 

A representative magnet slat that comprises the cylindrical arrangement has a lateral area, A, and a 

thickness, t, which is small compared to its lateral dimensions. From dimensional considerations of the 

form of a magnet constitutive equation 

B=H+M (3) 

it becomes clear that _o represents a magnetic moment per unit volume. Hence, the total moment of the 


	
"
'_o. This moment is also just I A, when the slat is replaced by its 

Amperian current (Furlani (2001)) flowing around its periphery. Thus, this cursory argument has shown 

that the peripheral Amperian current of a magnet slat is simply I Ac _o, from which follows the above 

remark that to simulate a magnet slat used here (t~1 mm, M~1T) requires an inconveniently large current 

(I~1000 A). Considering the assembly of all the magnet slats forming the cylinder, it is further evident 

that the longitudinal Amperian currents of neighboring slats cancel, leaving oppositely polarized current 

hoops at the ends of the cylinder. The magnetic moment of each hoop is simply 

MM = Ir (4) 

directed oppositely. The subscript “mc” simply refers to the moving magnet cylinder. These magnetic 

moments are considered as the source of the DC field that appears external to the alternator. 

A cylindrical arrangement of thin magnets external to the alternator will be effective to cancel the 

external DC field, if its end hoop moments are the same as those of the moving cylinder. Figure 5 is 

 an illustration of the arrangement of the external bucking magnets, showing also the hypothetical 

Amperian currents in two adjacent magnets. Here it is assumed that the magnetization M is uniform, that 

is,   M = 0. The Amperian currents in the volume of the magnets vanish and only the currents at the 

edges remain, which are proportional to n × M, where n denotes the unit normal vector to the magnet’s 

surface (Furlani (2001)). These are illustrated by the red loops in Figure 5. 

Figure 5.—Bucking magnets arranged on the containment vessel of the alternator. 

Amperian currents on adjacent edges of the magnets cancel, leaving current 

hoops on ends. 

DC magnetic field 

mitigation magnets 

Pressure vessel 

Amperian currents 
NASA/TM—2013-216540 7 

This should work well, except very close to the alternator, provided the external bucking magnet 

cylinder, denoted by the subscript “bc,” is as close as possible to, or even inside, the pressure vessel 

enclosing the alternator. Equivalency of magnetic moments gives a simple formula relating the design 

parameters of this bucking arrangement: 

















 



 = 1 (5) 

Numerical Simulation 

The concept was first verified by finite element computation, obtaining very promising results based 

on the geometry of an 80-W ASC. Without any field mitigation, computation shows the expected 

magnitudes of the external DC B-fields, plotted in Figure 6. 

Although small by local planetary standards, the fields displayed above are big enough to interfere 

with field mapping in interplanetary space. With the addition of a shell of carefully picked mitigating 

magnets, arranged as in Figure 5, the external DC B-fields can be reduced to extremely low values, as 

shown in Figure 7. 

Figure 6.—Unmitigated external B-fields computed for an 80-W solenoidal alternator. 

Figure 7.—External B-fields for the alternator of Figure 6 when equipped with a shell of 

bucking magnets having a Brem = 0.35 T. 

-8 E-08

-6 E-08

-4 E-08

-2 E-08

0 E+00

2 E-08

4 E-08

6 E-08

8 E-08

0 10 20 30 40 50 60 70 80 90 100

B (T)

Polar angle (°)

|B|

Bz

Br

-1.5 E-10

-1.0 E-10

-5.0 E-11

0.0 E+00

5.0 E-11

1.0 E-10

1.5 E-10

0 10 20 30 40 50 60 70 80 90 100

B (T)

Polar angle (°)

|B|

Bz

Br
NASA/TM—2013-216540 8 

These finite element computations for a single alternator show that the DC external magnetic field at 

a distance of 1 m can be reduced to about 1×1010 T or less, everywhere, by choosing the remanence 

(Brem) of the bucking magnets to be 0.35 T. A physical machine including the alternator has ancillary 

elements that modify the fields, but these computations give limits to what might be expected. To actually 

test DC B-field mitigation at such extremely low levels will require special facilities. 

Experimental Verification of DC B-Field Reduction 

A brassboard demonstration of the DC B-field reduction technique was performed on an ASC. For 

this demonstration, a series of arc magnets were positioned on the outside of the convertor pressure vessel 

as shown in Figure 8. The arc magnets were magnetized radially inward, and the flux density was 

adjusted to balance the external DC magnetic field emanating from the alternator. The concept was tested 

in the Glenn Research Center EMI test facility in September of 2012. The test results indicated that the 

external magnetic field emanating from the ASC linear alternator can be significantly reduced in specific 

zones. An 80 percent reduction in the DC magnetic field was measured in the radial direction with respect 

to the axis of the linear alternator at a distance of 1 meter. It is important to note that the purpose of the 

test was to demonstrate proof of concept and not optimal DC field mitigation. Further reductions in the 

ASC external DC field can be achieved. 

In the ASC alternator, the exciting magnets are slat-like and arranged in a can-like configuration. The 

magnets used in the external DC magnetic field bucking jacket are also slat-like and arranged in a similar 

can-like configuration. Ideally, all alternator magnets are alike and uniformly magnetized, and all DC 

magnetic field bucking magnets are alike and uniformly magnetized. Therefore, in the Amperian current 

loop model, currents along the adjacent edges of the magnets are equal and opposite, and cancel. This leaves 

only uniform Amperian loops on the ends of the cans. Unfortunately, the magnetizations of the DC 

magnetic field bucking magnets used in this test are known to vary from magnet to magnet, as shown by 

checking their remanence. This means that the Amperian currents along their adjacent edges will not cancel 

exactly, leaving spurious magnetic moments that have no bucking moments arising from the exciting 

magnet can. Such an effect might contribute to the less than optimal data observed in this experiment. 

The Glenn EMI test facility was not designed to measure the ultra-low DC magnetic fields 

encountered during this test. The ideal test facility would use large Helmholtz coils to cancel the Earth’s 

magnetic field, and would be remotely located. The Glenn EMI facility is located near other buildings and 

in a populated area. The movement of metal objects both inside (i.e., doors) and outside (i.e., 

automobiles) of the building affected the measurements. Therefore, none of the data recorded during the 

test are presented in this paper. 

Figure 8.—DC magnetic field bucking magnet jacket installed on Advanced 

Stirling Convertor (ASC). 
NASA/TM—2013-216540 9 

Conclusion 

In conclusion, the magnetic field emissions generated by the linear alternator of the Stirling power 

convertor-type used in the ASRG can be significantly reduced. Techniques for reducing both the AC and 

DC emissions of the ASC linear alternator have been developed and tested. Significant reductions in both 

fields have been verified experimentally. We have shown that modeling of the external B-field emissions 

from solenoidal-type linear alternators by dipole-like sources leads to successful suppression of these 

emissions by field superposition from “counter-dipole” sources, which are easy to implement. These 

techniques can be used on the ASC convertor to expand the potential set of missions for ASRG. ASRG 

can now be considered for use on missions that might have sensitive equipment onboard (such as fluxgate 

magnetometers). 

References 

Dudzinski, L.A., et al. (2011): NASA’s Radioisotope Power Systems and Technologies—A Forward 

Look. Proceedings of 9th Annual International Energy Conversion Engineering Conference, San 

Diego, CA. 

Pope, S.A., et al. (2009): Removal of Variable Spacecraft Field Effects in Scientific Magnetic Field 

Observations. ESA Workshop on Aerospace EMC, Florence, Italy. 

Furlani, E.P. (2001): Permanent Magnet and Electromechanical Devices: Materials, Analysis and 

Applications. Academic Press, San Diego, CA.