Edmonson, N.

Gibbons, F. L.

Verwers, C. D.

English

NASA Technical Reports Server

Paper F-2
SHIELDING OF SPACE VEHICLES BY MAGNETIC FIELDS
N. Edmonson, C. D. Verwers and F. L. Gibbons
General Dynamics/Fort Worth
Abstract /.#-/S_ _
Protons emitted by solar flares represent a
significant radiation hazard to crew members of
an interplanetary space vehicle. Shielding the
vehicle from charged particles by the use of
magnetic fields is an obvious possibility.
Reduction of secondary radiation otherwise
produced in bulk shielding is an added incentive
to study magnetic shielding. The feasibility
of this type of shielding was reported by R. H.
Levy, who utilized the properties of new super-
conducting materials. A program has been
initiated at General Dynamics/Fort Worth to
study various aspects of magnetic shielding of
space vehicles. In one phase of the program, a
procedure has been formulated and coded for the
IBM-7090 computer for rapidly computing the
field of an optimized superconducting solenoid.
In another phase, samples of NbZr wire have
been irradiated with neutral particles from two
sources. Preliminary results are now available.
Irradiation with l0 ll neutrons per cm 2 from the
D-T reaction showed no change in the critical
current vers_2 magnetic field curve. Irradia-
tion with lO ±U neutrons (_2.9 Mev) per cm2
showed a slight downward shift in the critical
current. It is difficult to say whether this
shift was due to the irradiation or due to the
environment during the irradiation.
Introduction
A program for studying the magnetic shielding of space
vehicles against charged space radiations was initiated
approximately a year ago at the NARF facility of General
Dynamics/Fort Worth. Two assumptions are basic in this
program:
8O8
https://ntrs.nasa.gov/search.jsp?R=19630005279 2020-03-11T16:53:08+00:00Z
Io Magnetic shielding against charged space radiations
is feasible for space vehicles;
2. Superconductors even more effective than existing
superconductors will be developed.
The first assumption was based on a study by R. H. Levy 1.
The second assumption was made after a survey of the litera-
ture_,o and after conferences with many active research
workers in the field of superconductivity.
This program was broken down into the subdivisions:
I. Design of optimized superconducting magnets;
. Shielding effects of magnetic fields against charged
particles;
o Optimized solenoid configurations for shielding pre-
scribed volumes;
. Structural support design, refrigeration, and power
sources for a superconducting electromagnet system;
. Protection of the superconducting magnet system
against quenches;
. Experimental investigation of the effects of
different kinds of radiations, particularly neutrons
and protons on the properties of superconducting
materials.
Work is being actively carried out on phases l, 2, and 6. In
this paper, a discussion is given of the analytical and experi-
mental activities at GD/FW.
Io
o
.
R. H. Levy, Radiation Shieldin 5 of Space Vehicles by
Means of Superconducting Coils. Avco-Everett Research
Laboratory (April 1961, Contract AF04(647)-278.
J. E. Kunzler, "Superconductivity in High Fields. "
Rev. of M,d. Phys._ 33_ 4 (October 1961).
J. J. Haak, G. D. Cody, P. R. At,n, and H. E. Hitchcock,
(RCA Labs, Princeton, N. J. ), "Some Physical Properties
-n "of Deposited NbqS . High Magnetic Fields, MIT Press
(Cambridge) and_John Wiley and Sons, Inc., (New York),
1962.
8O9
Problems and Computational Techniques for Super-
conductin6 Ma6nets
For a given temperature in the temperature range for
which a given material is superconducting, there is for any
transverse magnetic-field strength within a definite range
of magnetic-field strengths a current Ic, the critical
current, so that for currents less than Ic the supercon-
ductor behaves as a superconductor and for currents greater
than Ic the superconductor behaves as a conventional or
normal conductor. The superconductor goes normal or Quenches
at I = Ic. This experimental fact is added to the classical
methods for computing the magnetic fields arising from
current-carrying circuits to develop methods for computing
the fields due to superconducting electromagnets. For
example, at a given location within the windings of a super-
conducting solenoid, the transverse magnetic field would be
the vector sum of the transverse external magnetic field
and the field generated by the solenoid. The current carries
by the superconducting winding at this point would be fixed
from above by this total transverse magnetlc-field strength.
Methods for computing the magnetic-fleld intensities
generated by superconducting solenoids have been developed
by a number of workers* -,P. These methods are discussed in
Reference 6.
To apply these methods, it is necessary to have proce-
dures for rapidly computing the field due to a superconducting
solenoid both within and without the solenoid structure.
A IBM-T090 computer FORTRAN code, MAGFI, based on
classical formulae for solenoids having a rectangular cross
section, has been prepared• This code, furnishes a very
fast means for mapping the field of a solenoid.
.
.
R. W. Boom, and R. S. Livingston, "Superconducting
Solenoids." 1961 Western Electronics Show and Conven-
tion, San Francisco, August 22-25, 1961.
W. F. Gauster and C. E. Parker, "Some Concepts for the
Design of Superconducting Solenoids." High Magnetic
Fields• pp. 3-13. MIT Press (Cambridge) and John Wiley
and Sons, Inc. (New York), 1962.
N. Edmonson, Magnetic Field Shieldin6 Against Char_ed
Space Radiations. GD/FW Report FZK-9-181 (to be
published).
810
If the magnetic field of a solenoid is mapped by MAGFI,
the solenoid requirements for superconducting material and
structural support for the superconductors can be reduced in
the following way. In general, the maximum field of a
rectangular solenoid occurs at the longitudinal midpoint of
the inner surface of the solenoid. The field decreases in
all directions from this point. Thus, if the solenoid were
wound in ring-shaped segments, the number of turns in each
segment could be reduced and the current raised in accordance
with the critical current-magnetic field relation of the
constant. This technique would lead to a reduction in
material and size for the solenoid for the same magnetic
field strength. An optimization procedure is currently
being developed for the IBM-7090.
Shieldin6 Effects of a Ma6netic Field
The shielding effect of a magnetic field is investigated
by use of St_rmer's concept of "forbidden regions" and by
computations of the orbits of individual charged particles
in a magnetic field.
The basic concepts of Stgrmer's theory are given in
References 7 and 8. Both of these references are concerned
with geomagnetic effects on charged particles. The geo-
magnetic field is approximated by a dipole. For the much
smaller space vehicle, the dipole approximation is not
sufficiently accurate. A more realistic approach is to
compute the vector potential of a loop current. The equiva-
lent solenoid may then be computed. Perhaps a more realistic
procedure is to compute a solenoid optimized as described
earlier and then to compute the vector potential of each one
of the ring-shaped segments of the solenoid. Then the total
vector potential of the solenoid is the result of a summation
of the elementary vector potentials. The computations of
the vector potential of a ring-shaped current is a classical
procedure to be found in any advanced treatise on electro-
magnetlsm9. Its application to the determination of regions
open to charged particles and closed (or forbidden) to
charged particles is discussed in References 1 and 6.
.
.
9.
M. S. Vallarta, "Theory of Geomagnetic Effects,
Handbuch der Ph_sik, Band XLVI/I, 88, Springer,
Gottingen, 1961.
C. Stormer, Polar Aurora, Clarendon Press, Oxford, 1955.
W. R. Smythe, "Static and Dynamic Electricity," 2nd Ed.,
New York; McGraw-Hill Book Co., (1950).
811
In applying the Stormer procedure to shielding problems,
a shut-off energy is selected, so that the total dose for all
charged particles having energies above this shut-off energy
is tolerable. For more exact information about the .effects
of charged particles in the neighborhood of this pre-selected
shut-off energy orbit, computations may be necessary. Such
computations are classical7-10.
The Experimental Program
A major goal of the present experimental program at
GD/FW is to investigate the effects of neutral particle
irradiation on a superconducting material. At present, the
principal interest in superconductors at GD/FW is for use in
producing a large volume magnetic field for shielding a space
vehicle. Clearly, magnetic shielding is not effective against
neutral particles; however, in the event that the space
vehicle is nuclear-powered, it is essential to determine
whether neutral radiation will affect the superconducting
properties of the material. The techniques developed during
the phase of neutral irradiation can be used during the later
phase of charged-particle irradiation.
The maximum current density of a superconducting material
or alloy is not only a function of the environmental condi-
tions during its use, but also its purity, crystal structure,
and the manufacturing process used to produce the material.
Work-hardenlng during the manufacturing process makes some
materials better superconductors. For example, the maximum
current density of extruded NbZr wire increases by 50%
between 20- and lO-mil wire. Since it is known that irradia-
ting a material can change the structural strength and crystal
structure, one may expect a change in maximum current density
after irradiation. Changes are particularly expected at low
temperatures, where lattice defects may remain frozen in the
material.
7. Ibid.
8. Ibid.
9. Ibid.
i0. L. Paige, Electrodynamics, New York, Van Nostrand.
812
Measurement of the critical current (Ic) as a function
of an applied external magnetic field is the method selected
to observe changes due to irradiation. The critical current
is described as the current necessary to cause a small resis-
tive voltage to appear across the sample. All measurements
are made in liquid helium at ambient pressure (approx. 4.2OK)
and in a magnetic field of 10 to 15 Kgauss.
•,,_ ._ _ _.,,_ _=_ _,,= experiment is 0. ^'"ua, inch in
diameter, hard drawn _99% reduction), and of a Nb 25 at. %
Zr alloy.
The Dewar used Is a conventional liquld-nltrogen-
Jacketed, glass, helium Dewar with a liquid helium capacity
of approximately one and one-half liters. Current leads are
brought into the dewar through liquid nitrogen to decrease
llquld-hellum boiloff. This setup allowed about 45 minutes
working time. (See Fig. 1.)
The circuit used to measure the critical current is
shown in Fig. 2. A Kelthly Model 149 millimicrovoltmeter is
used to measure the voltage across the sample. Its output
is attached to the y axis of a Sylvania Type B-281 x-y
recorder with the x axis attached to a precision resistor
through a D-C amplifier to record the current. The current
is supplied by two large storage batteries and controlled
through a transistorized series amplifier.
The sample holder (see Fig. 3) is cut so that it will
fit into the 5/8-in.-diam Dewar tip and hold the sample in
the center of the Dewar. The sample is mounted parallel
and coincident to the center plane midway between the poles
of the magnet. All samples are copper-plated, except for
three lengths between the four contact points. The contact
between the superconductor and current lead is made by winding
12 inches of wire around the current lead and then soft-
soldering with 60/40 lead-tin or indium. The contac_ resis-
tance with these Junctions was on the order of 5xl0- ohms.
Two pieces of NbZr wire were irradiated at ambient tem-
perature in the Ground Test Reactor and compared with control
samples. The total neutron flux was lO17 neutrons (_2.9 Mev)
per cm 2. X-ray diffraction and optical magnifications to
x 1000 showed no observable change in the crystal structure.
The micro-hardness test, using a Knoop micro-hardness tester
and converting the readings to the Rockwell "c" hardness
scale, showed a change of from 30.5 to 28.6 on the hardness
scale after irradiation. Another sample that was completely
annealed by heating changed from 30.8 to 25.1. Due to the
dependence of the current density on work-hardening these
813
measurements indicate a plausible explanation for any
possible change in the critical current due to irradiation.
Three different samples have been irradiated in the
Ground T_st Reactor at ambient temperature, with a total flux
of _I016 neutrons (_2.9 Mev) per cm 2. Preliminary results
of several measurements of the critical current before and
after irradiation are shown on the bottom curve of Fig. 4.
Effects of neutron irradiation are being studied by
irradiating samples with 14.2-Mev neutrons from the D-T
reaction using a Cockcroft-Walton type accelerator. The
critical current as a function of the magnetic field is
measured before, during, and after irradiation with the
sample temperature kept at 4.2°K during the experiment.
Preliminary results of this irradiation are shown by the
top curve of Fig. 4.
Summary of Experimental Results
The uDper curve of the accelerator irradiation shows
no significant change, but the lower curve with reactor
irradiation is suggestive of a downward shift in the critical
current curve. One other sample was irradiated at the accel-
erator and two other samples were irradiated with the reactor.
Those data show similar trends. However, the reproducibility
from one run to the next and between samples leaves something
to be desired. The spread in the points is comparable to the
apparent effects observed with the reactor irradiation. Note
that the accelerator exposure was only i0 II neutrons compared
to more than I0 ±O from the reactor.
The data then seem to suggest that there may be an effect
at the higher exposures, but our experimental techniques must
be improved to reach a firm conclusion. This will be the
next step in this current phase in the experimental program
at GD/FW. Beyond that, the next phase in the overall program
will include the construction and testing of optimized
magnets.
814
qFIGURE I. DEWAR AND MAGNET
815
CURRENT
SWEEP
FI GURE
CURRENT
CONTROL
SAAMPLESHUNT
MILLI-MICRO-
VOLTMETER
MPLE
RENT
SHUNT
D-C
AMPLIFIER
X-¥
RECORDER
I
I
L .........
2. BLOCK DIAGRAM OF ELECTRICAL CIRCUIT
816
LFIGURE 8.
SAHPL E HOL OER WITH NbZr
++ i_."
+-/,
+._+_:++y
L._ 0 i._
0
0
,,,i,,--I
o _ o_ o_ _o _ _ _
I-.
1.1.1
z
m
,....I
1.1..I
o.
r_
m
818


Shielding of Space Vehicles by Magnetic Fields

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