Low Magnetic Field Technology for Space Exploration E.J

Low magnetic field technology for space exploration E.J. Iufer

To cite this version:

E.J. Iufer. Low magnetic field technology for space exploration. Revue de Physique Appliquée, Société française de physique / EDP, 1970, 5 (1), pp.169-174. 10.1051/rphysap:0197000501016900. jpa-00243354

HAL Id: jpa-00243354 https://hal.archives-ouvertes.fr/jpa-00243354 Submitted on 1 Jan 1970

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. REVUE DE PHYSIQUE APPLIQUÉE TOME 5, FÉVRIER 1970, PAGE 169.

LOW MAGNETIC FIELD TECHNOLOGY FOR SPACE EXPLORATION

By E. J. IUFER (1), National Aeronautics and Space Administration (Nasa), Ames Research Center, Moffet Field, California (U.S.A.).

Résumé. 2014 Une observation définitive de la morphologie du champ magnétique inter- planétaire ne dépend pas seulement de la reproductibilité et de l’intégrité spectrale des magnéto- mètres envoyés dans l’espace, mais aussi de l’existence de véhicules ayant des champs perturbateurs négligeables. Un étalonnage convenable des magnétomètres et la possibilité de faire des mesures magné- tiques dans l’espace sont essentiels pour obtenir un résultat. Étant donné le petit nombre de publications portant directement sur la technique de réalisation, bien des recherches concernant la mise au point de matériel pour des observations magnétiques au cours de missions spatiales n’utilisent qu’une petite partie des connaissances disponibles. En conséquence, il a fallu un temps inutilement long pour faire accepter de nouvelles réalisations dans ce domaine ; elles ont en outre, en ce qui concerne les caractéristiques magnétiques et les essais, été considérées avec réticence, sans nécessité par certains. Le but de cet article est de présenter une revue critique de l’état actuel des connaissances dans les domaines des mesures du vecteur champ magnétique à basse fréquence, de la standar- disation des magnétomètres et de la réalisation de véhicules spatiaux à faible champ magnétique. On décrit les caractéristiques des performances de l’appareillage et les techniques instru- mentales utilisées pour la mesure du vecteur champ magnétique standardisé dans le domaine de 0,05 à 60 000 nT (1 nT = 1 gamma). On établit une comparaison entre des modèles pouvant estimer les propriétés magnétiques des matériaux et montages pour véhicules spatiaux à l’aide de nombreux résultats de laboratoire. On discute de façon détaillée les critères généraux et les considérations permettant de comparer les techniques de blindage magnétique, de compensation active du champ, et d’emploi de constituants non magnétiques. Une revue des principaux points de l’étude qui aboutit à la mesure du champ magnétique de 0,25 nT à partir des véhicules du type Pioneer VI est indiquée ; on présente les caractéristiques des véhicules et de l’appareillage pour les missions interplanétaires futures.

Abstract. 2014 Definitive observation of interplanetary magnetic fields depends not only upon the development of high sensitivity spaceflight magnetometers but also on the availability of spacecraft having negligible disturbance fields. Test results obtained in the Pioneer VI-IX program have demonstrated that through careful design, spacecraft magnetic fields can be reduced by a factor of 25 so that spacecraft field levels of 10-6 gauss or less can be realized. Since the literature contains little information on the magnetic properties of spacecraft parts and materials, low-magnetism design principles and low-field magnetic testing techniques, those recently concerned with low-magnetism design may be working with only a small fraction of the information currently known. A critical review of the current state-of-the-art for low- magnetism spacecraft design and test is presented.

Introduction. - Analysis of data from magnetometers used in space exploration has changed the concept of the Earth’s magnetic field from that approximated by a dipole in free space to that of the magnetosphere. The boundary of the Earth’s magnetic field is produced by the solar plasma flow which compresses the Earth’s field to about 15 Earth-radii (15 Re) on the day-time side and extends it to greater than 80 Re on the night- time side. Immediately beyond the magnetosphere, one encounters the interplanetary medium where the ambient magnetic field has a quiescent value of about 5 X 10-5 gauss due predominantly to the Sun. At greater solar distances, as illustrated in figure 1, the interplanetary field becomes weaker and at Jupiter FiG. 1. - Magnetic Fields in the Solar System. its value is thought to be about 5 X 10-6 gauss. The ambient field may not decrease significantly at greater (1) Research Scientist, National Aeronautics and Space ranges because of the galactic magnetic field. Administration, Ames Research Center, Moffett Field, Observation of the and Califomia 94035, Presented at Measurement of Low steady-state, temporal spatial Magnetic Fields of Spatial and Geophysical Interest fluctuations ofsuch weak magnetic fields can be comple- Conference, Paris, France, May 19-25, 1969. tely obscured by the spacecraft’s own magnetic field

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0197000501016900 170

FLUXGATE MAGNETOMETERS where H is the net magnetizing force acting on thé rod, ELEKTRON 2, 4 Ho is the gross external magnetizing force, N is the rod EXPLORER VI, X, XI, XIV, XV, XVIII, XXI demagnetization factor which depends upon geo- XXVI, XXVIII, XXXIII, XXXIV, XXXV metry, and I is the intensity of magnetization. OGO 1- T7 Because the ratio of H0/H for spacecraft parts is OV 2-6 usually large, one finds that spacecraft induction and residual induction can be considered linear for external SP UT NIK 3 field exposures of 15 gauss or more. The ability of PIONEER VI-IX a magnetized rod to produce an external magnetic field is characterized by its magnetic moment, M. PROTON PRECESSION MAGNETOMETERS For magnetizations along the axis of the rod, the VANGUARD I, III magnetic moment is the product of the volume, V, of the rod and its intensity of magnetization, I. The ALKALI VAPOR MAGNETOMETERS intensity of magnetization is related to the induction, B, the EXPLORER X, XVIII, XXI, XX TlIII, XXXIV demagnetization factor, N, and the applied field, Hn, by : OGO I h 0 v 1-10

HELIUM MAGNETOMETERS This expression is approximate because both the inten- of and are functions MARINER IV, V sity magnetization permeability offlux density which is non-uniform in a rod. the above one calculate the SEARCH COIL MAGNETOMETERS Using expression, may induced moment of a rod from : EXPLORER VI OG0 I h Calculations using this approximate method provide FIG. 2. - Having Magnetic Spacecraft results accurate to an order of magnitude or better. Field Experiments. Although several ways are known to analyse the magnetic fields of bodies in two dimensions, it is only unless strict attention is given to the magnetic design recently that an analysis has been performed which and test of the spacecraft. The design of low-magne- treats magnetic fields in three dimension and accommo- tism spacecraft for magnetic exploration require the dates a field-dependent permeability. This analysis and in application of principles techniques which, by A. Halacsy subdivides a magnetic body into a many cases, may be new to structural and electronic finite number of elemental boxes, each having its designers of space flight hardware. magnetic moment concentrated at its center. Partial As showns in figure 2, a large number of scientific differential equations are written for the scalar magne- spacecraft have flown magnetometers. Of the space- tic potential and the permeability of each box. The to the craft measurements reported date, spacecraft partial differential equations are linearized and solved having the lowest magnetic field is the Pioneer with by matrix inversion. This analysis does not require 2.5 X 10-l gauss at the magnetometer sensor located boundary conditions because by avoiding the use of 2 meters from the spacecraft center. vector potential, no integration is necessary. The In the following sections, the nature of spacecraft accuracy of this method increases as the number of will fields, their reduction and measurement be dis- elemental boxes increase. With this method, one cussed. Facility requirements and high sensitivity requires only the hysteresis curve and geometrical shape will be instrument calibration methods described. of a body to calculate its magnetic field. A FORTRAN computer program which solves of Fields. - The The Calculation Spacecraft mag- these equations has been written and successfully netic field of a is a of the indi- spacecraft composite run on an IBM fields fields of all ofits which contain 360/50 computer. Computed vidual static parts alloys for rods, cubes and spheres have been checked against nickel or cobalt and the active fields of iron, produced laboratory measurement by the author and found to electrical current loops. by have greater accuracy than fields calculated by pre- arise from a of Static fields heterogenous assembly vious methods, figure 3. small bodies which can usually be charac- magnetized The calculation of total spacecraft fields can be terized as rods of small geometrically ferromagnetic linear of the vector ratio. The reluctance accomplished by superposition length to diameter high of space fields of subassemblies. Before these analytical tools to that of the ferromagnetic material causes compared were available, the design oflow-magnetism spacecraft the net force inside the rods to be less than magnetizing was based on frequent and detailed measurements of the field. Classically, this field diminution applied parts in a low field test facility and a to free at the ends of the rods. As spacecraft by is attributed poles general policy of avoiding all magnetic material a the curve for the rod result, hysteresis specimens whenever are this clockwise when with possible. If sample parts available, appears to be sheared compared is still considered the best choice. of closed The net policy Analytical the hysteresis loop rings. field, methods should be used in predicting the fields of to a rod is given by : acting magnetize parts which have not been built or which are otherwise not available for test. 171

the spacecraft. Demagnetizing fields in the order of 50 gauss are commonly used. After this process, the soft remanence is in its lowest value. Exposures due to handling, shipping and launching may expose the spacecraft to fields of 5 gauss or greater. These exposures will increase the spacecraft soft remanence fields. If the exposure field was fairly uniform over the volume of the spacecraft, the resulting magnetizations of the assemblies will be approximately parallel and the

FIG. 3. - Laboratory Measurements of Field of Cube Kovar Sample Compared to Calculated Values.

Magnetic Cleanliness. - For interplanetary missions, the elimination of even milligrams of magnetic material in a part is valuable if the number of times this part is used is large. The magnetic field of a transistor in a TO-5 nickel case with 1.4 mm Kovar leads can be reduced from 60 X 10-5 gauss to 3 X 10-5 gauss at 7.5 cm by placing it in a smaller TO-46 case made of nickel-silver. By strict avoidance of unnecessary magnetic material, the properly designed low-magnetism spacecraft can have 1/25 or less of the magnetic field produced by a conventionally designed counter- part. This leads to the first principle of design-avoid magnetic materials where possible. The second principle is that the acceptable magnetic moment of a body increases as a cube of distance. For example, a material used to encapsulate a magnetometer sensor can be unsuitable due to suspended particles too small to be seen with the unaided eye. If these particles produce a field of say 1 X 10-6 gauss FIG. 4. at 10 mm, then to obtain the same level of interference Typical Results of Spacecraft Field Measurements. at a distance of two meters (according to the inverse cube law) the effective moment of this material would will in have to be larger by a factor of 8 X 106. Rapid field resulting field be approximately dipolar nature decay with increasing distance must be considered in and will obey the inverse cube law of attenuation. dealing with spacecraft fields allowances because it is Because the soft remanent fields are produced by ran- possible to safely relax the magnetic moment specifi- dom exposures and are somewhat unstable with time, cation of parts and materials as magnetometer boom very low limits are set on the amount of magnetic lengths increase. material permitted in spacecraft construction. Space- craft specifications are frequently set such that random The Nature of Spacecraft Fields. - It has become exposures of 5 gauss or less produce magnetic fields convenient to arbitrarily subdivide the sources of space- which are at or below the threshold of the spacecraft craft fields into four categories : soft remanence, hard magnetometer. The soft remanence of the magneto- remanence, current-loop and induction. Soft rema- meter instrument flown on Pioneer IX represents the nence refers to that portion of total residual induction state-of-the-art for low-magnetism design. This ins- which has sufficiently low coercive force as to be altered trument weights 2.5 kg, contains a power supply and by ambient momentary magnetic field exposures of over 1 000 integrated circuits. The change in moment 25 gauss or less. Hard remanence refers to the balance for this instrument due to a 5 gauss exposure was of total residual induction which cannot, or in the case 8 X 10-6 gauss at one meter. The progress in low- of certain necessary magnets, must not be removed by magnetism spacecraft design is shown in figure 4. demagnetization treatment. Current-loop stray fields HARD REMANENCE. - Hard remanence fields are are defined as those due to uncompensated leakage from wave tube fields resulting from electrical current flowing in ca- produced relays, solenoids, traveling bling, solar-array assemblies and electronic assemblies. and certain magnets used in experiments. Traveling wave tubes use of and The term induction refers here to magnetic induction magnets high energy product If the orientation of these stable of materials while they are exposed to a magnetizing great stability. highly field sources is fixed relative to force, H. the flight magnetometer, the external field can be reduced by 1) installing mat- SOFT REMANENCE. - It is customary to demagnetize ched pairs to effect mutual cancellation; 2) adding the spacecraft just before launch in a coil facility. This equally stable compensating magnets; or 3) by precisely process is performed to remove all soft remanence of measuring the field and removing its contribution from 172 the scientific data analytically. In practice, all three stray fields produced by this array was 2 X 10-7 gauss techniques are used. Permanent magnet fields can be when measured at a distance of two meters from the reduced by 20-to-1 and made to be stable during geometric center of the cylinder. environmental exposures. Figure 6 illustrated the state-of-the-art for existing interplanetary spacecraft magnetic fields and contrasts CURRENT-Loops. - fields due to the flow Magnetic these values with those required for a Jupiter mission. of currents can be minimized very effectively by redu- By increasing the boom length, the level of magnetic the effective area encircled current. This is cing by cleanliness is only slightly stricter for individual assem- current in coaxial or twisted accomplished by carrying blies on the Jupiter mission. pair cables. Because of the high conductivity materials used, care must be taken to avoid current flow in SHIELDING. - Reducing the magnetic fields of space- spacecraft structures. For individual electronic assem- craft parts by ferromagnetic shielding is not permitted blies consuming 2-4 watts of power, stray fields on the as a general rule. Shielding to reduce external effects order of 5 X 10-7 gauss have been routinely achieved of permanent magnets or electromagnets should be at 1 meter. considered only in cases where the field of the magnet The solar array used on the Pioneer VI series consis- is relatively unstable. Often, the shield itself would ted of 10,368 solar cells mounted on a cylindrical be more unstable than the magnet it encloses and hence surface having diameter of 95 cm and length of 89 cm. causes a larger rather than a smaller field uncertainty A cell temperature of 277 OK is maintained by a rota- at the spacecraft magnetometer. Effective and light- tion of approximately one per second about an axis weight shields require careful design and the high normal to the sun line. This array provides a nominal nickel content alloys employed are significantly degra- current 1.7 ampere at 31.5 volts when illuminated with ded by cold working or by stressing after final annea- one sun. Compensation was achieved by back wiring ling. Shielding, however, continues to be suggested each series string of cells as illustrated in figure 5. The as a panacea for reducing the fields of motors, relays and similar parts by those not yet familiar with the principles of low-magnetism spacecraft design. Small shields have been used in isolated cases to prevent unwanted coupling of signals between circuits. The shielding used in these instances should be nearly spherical in shape and be as small as possible.

Magnetic Test Facilities. - A single axis set of air core coils may be used to produce a magnetic field matching the magnitude and uniformity of Earth’s field but directed antiparallel to the ambient field. However, it is more common to use a triaxial coil system that cancels the orthogonal components of the ambient field. Coil design has its origins in the work of Ampere dating back nearly 150 year. Ampere’ss single circular current loop had a very small volume of field uniformity. Some 30 years later, Helmholtz and possibly others discovered that placing two identical circular coils in parallel planes, on a common axis, spaced at a distance equal to one-half their diameters, would produce a highly uniform field. The region of high uniformity is characterized by a minimum gradient in the axial component of the field produced by the coil.

AMES RESEARCH CENTER FACILITY. - The small coil an FIG. 5. - Pioneer Solar Array Panel Wiring Showing facility at NASA Ames Research Center is adapta- Compensating Positive Lead. tion of a design originated by Rubens which consisted

FIG. 6. - Properties of Low Magnetism Spacecraft. 173 of five equally spaced square coils forming a cube (the ment of magnetic bodies near the test area. For coil turns were in the ratio 19 :4 :10 :4 :19) and produced example, moving automobiles can produce 1-gamma a volume of ::1::: 0.1 % uniformity having the approxi- signals at a range of 100 yards, moving steel doors and mate shape of a cylinder oflength 0.45 d and diameter push carts can produce gamma fields at 50 feet, and of 0.20 d (where d is the length of one side of the coil). the hardware on one’s person can produce gamma The method of Rubens and a digital computer were fields at ranges to 10 feet (1 gamma =10-5 gauss). used to a new of radial develop design improved OTHER FACILITIES. - The NASA Ames is a different ratio of coil turns and Facility uniformity having for magnetometer calibration and coil The Ames cubic coil uses three designed primarily unequal spacing. Much coil sets nested which annul subsystem magnetic properties testing. larger together independently coil facilities for exists at Nasa God- the vertical, North-South and East-West spacecraft testing components near D.C. and near Los of the Earth’s field. the North-South axis dard, Washington, Angeles, Alining California. The Goddard facilities include triaxial to the local meridian reduces the parallel magnetic 12 meter and 6 meter Braunbek coils. required output of the East-West axis sufficiently to high-uniformity The California facility has a 6 meter Fanselau coil provide proper uniformity with only a Helmholtz pair. system. It is on this basis that the Ames cubic coil system uses 12 coil loops instead of 15. It has been found that the FACILITY CALIBRATION. - To perform a proper measured transfer function of the coil system at it d-c calibration of coil output, magnetic field intensity center agrees with the theoretical transfer function to and electrical current standards are required. The within 0.016 %. Although the theoretical improve- most accurate reference standards for magnetic field ments predicted for this coil design were partially offset measurements use the gyromagnetic ratio of the proton. by the practical limitations imposed by fabrication, These instruments can measure magnetic fields to these degradations did not reduce the volume of uni- absolute accuracies of 0.001 %. A practical limitation formity to less than theoretically predicted for the of this type of instrument is that the accuracy is degra- original Rubens’ design. ded if the field level is less than 0.2 gauss. By means The values of theoretical performance were compu- of the proton reference standards, reference coils and ted on an IBM 7094 digital computer using a For- conventional d-c bridge measurement systems, calibra- tran IV adaptation of the MAFCO computer pro- tions with accuracies of + 0.2 gamma + 0.002 % of gram developed at the University of California Law- reading are performed at Ames and are traceable to rence Radiation Laboratories. the National Bureau of Standards. This equipment The facility is housed in a 15 by 60 foot nonmagnetic has been used to calibrate flight magnetometers, sole- building located approximately 0.6 mile North of noids and other portable coil systems used as transfer Ames Research Center at Moffett Field, California, as standards to other laboratories. shown in figure 7. This site was selected for its low Measurement Philosophy. - ASSEMBLY MEASURE- level of magnetic noise. In most cases, the noise at MENT. - the the power frequencies can be eliminated by output filters, Usually, purpose ofmapping magnetic saturation of the field of a piece of hardware is to provide a basis for provided magnetometer preamplifier the field contributed that hardware when does not occur. A significant source of noise is move- predicting by mounted aboard the spacecraft with respect to the magnetometer. Normally, this represents a separation distance that is much larger than the dimensions of the assembly. From a data reduction standpoint, it is desirable to measure under conditions simulating the position of the spacecraft magnetometer. This normally is not practical, since the field contributed by a single assembly must be only a fraction of the total field permitted for an integrated spacecraft and would, therefore, have a magnitude below the threshold of available magnetometers. For this reason, measurements are commonly taken at closer ranges, and estimates of the contributed fields are made using inverse cube rules for a dipolar source and spherical harmonic analysis for higher order multipolar sources. The general rule is to measure at a range where the field of the assembly falls off as the inverse cube of distance. This range may be from three to six times the largest linear dimension of the assembly. Acceptance testing of spacecraft assemblies is accomplished as follows : a) Measuring (mapping the remanent fields of the assembly prior to magnetic treatment). b) Mapping after the assembly has been dema- gnetized. c) Mapping after the assembly has been exposed to a standard d-c exposure of 25 gauss. FIG. 7. - Nasa Ames Low-Field Magnetic Test Facility d) Mapping the field due to energized circuits in 12 Foot Cubic Coil Assembly. the assembly. 174

In acceptance testing the experimental or measurement uncertainty is usually kept to 0.2 gamma RMS or less in the 0.01 to 20 hertz band. Radial field intensity is plotted as a function of azimuthal angle plots in the X-Y, Y-Z and Z-X planes of the specimen. SPACECRAFT MEASUREMENT. - Both the average value and the fluctuations in the geomagnetic field must be considered during spacecraft magnetic field measurements. For interplanetary spacecraft, only the remanent and current-loop fields are significant since the interplanetary field is too weak to induce a measurable field. To accurately measure the remanent fields it is customary to place the spacecraft in a low-field magnetic coil facility where the geomagnetic FIG. 8. field has been reduced. Measurements indicate that Spacecraft Magnetic Measurement Scanning Modes. the interplanetary medium can be adequately simula- ted by attenuating the geomagnetic field 40 dB or more. The static value of the net uncompensated rement. The author has managed to improve on this geomagnetic field can be removed from the output of method by eliminating the need for continually remo- the facility magnetometers by biasing solenoids. ving the spacecraft from the coils. Figure 8 illustrates The stability of the net field represents a more strin- this new technique. Three separate and consecutive gent requirement that the level of ambient field reduc- orientations of the spacecraft are used. If one assumes tion. In order to measure spacecraft fields accurately the designated location of the spacecraft magnetometer to 10-6 gauss, with a signal-to-noise ratio of 1:1, it is to be out the spacecraft X axis with axes X l, Yl and Z, necessary to have a noise level of about 114 dB below parallel to the spacecraft X, Y, Z axes, then in the first the magnitude of the geomagnetic field. Conse- orientation, the field contribution due to the spacecraft quently, the electrical and mechanical stability of moments along Y and Z will produce outputs in the the coil system is critical. For example, tempera- Y-Z moment magnetometers periodic with each space- ture changes of 0.1 OK can change coil output by craft rotation and symmetrical about zero. 1 X 10-s gauss. Coil dimensions are controlled by this test using the three stabilizing temperature and coil current is controlled By performing spacecraft orientations shown, and the magne- by constant current by labelling facility power supplies having regulation tometer with the instantaneous azimuthal of 0.002 or better. The outputs % facility magnetometers it is to the three sensitivities to 100 full-scale are spacecraft position, possible separate having up microgauss of the field at the equiped with low-pass filters. geometrical components spacecraft magnetometers from the background. The facility SPECIAL TECHNIQUES. - In addition to these tech- magnetometers are positioned at 1) the range of the niques, systems which sample and compensate the spacecraft sensor; 2) at a range where the spacecraft diurnal variation (250-500 microgauss) are employed field should be doubled; and 3) at a range where the to stabilize the field during spacecraft measurements. field should be quadrupled. The customary radial- Even when all these methods are used, one frequently map magnetometers produce data which can be degra- finds that facility magnetometers will drift at the micro- ded by large concentrations of magnetic materials that gauss level in a matter of seconds. The effect of drift change their range to the sensor with each rotation. in magnetometer output has been minimized by remo- This new method, using moment magnetometers avoids ving the spacecraft from the coil system, zeroing the this problem since all magnetic sources remain at magnetometer, quickly placing the spacecraft in the constant range with respect to the moment mag- coils and then rotating the spacecraft during the measu- netometers.

REFERENCES

ANON, Magnetic Fields. Earth and Extraterrestrial, IUFER (E. J.) and DROLL (P. W.), Space Magnetic Envi- Nasa SP-8017, march 1969. ronment Simulation for Spacecraft Testing. Paper presented at ASTM/IES/AIAA Second Space Simu- ANON, Spacecraft Magnetic Torques, NASA SP-8018, lation Conference, Am. Soc. Testing Mats., 1967. march 1969. RUBENS (S. M.), Cube-Surface Coil for Producing Uni- form Magnetic Field, Review of Scientific Instruments, HALACZY (A.), Study to Develop Methods Predicting Sept. 1945. Spacecraft Magnetic Fields, Nasa CR-73256, 1969. PERKINS (W. A.) and BROWN (J. C.), MAFCO. A Magnetic Field Code for Handling General Current Elements in IUFER (E. J.) ed. Proceedings of Symposium on Space Three Dimensions, CRL-7744, University of California, Magnetic Exploration and Technology, Reno, Nevada, 1964, 3. 1967, 28-30. CORLISS (W. R.), Scientific Satellites, NASA SP-133,1967.