Operation of a superconducting magnet is considered. A method is described for; (1) obtaining a relatively high current in a superconducting magnet positioned in a bath of a gas refrigerant; (2) operating a superconducting magnet at a relatively high current level without training; and (3) operating a superconducting magnet containing a plurality of turns of a niobium zirconium wire at a relatively high current level without training

Boom, R. W.

English

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" I I L . Y 1 O
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https://ntrs.nasa.gov/search.jsp?R=19790013093 2020-03-20T18:33:58+00:00Z
March 21, 1967 R. w. BOOM 3,310,765
STABLE SUPERCONDUCTING MAGNET
Filed July 26. 1965 2 Sheets-Sheet 1
March 21, 1967
Filed July 26. 1965
R. w. BOOM
STABLE SUPERCONDUCTING MAGNET
3,310,765
2 Sheets-Sheet 2
3,310,765
Patented Mar. 21, 1967
3,310,765
STABLE SUPERCONDUCTING MAGNET
Roger W. Boom, Woodland Hills, Calif., assignor to
North American Aviation, Inc.
Filed July 26, 1965, Ser. No. 474,815
13 Claims. (CI. 335—216)
The present invention relates to a stable superconduct-
ing magnet, and more particularly to a method of operat-
ing a large superconducting solenoid without training.
Superconductors are conductive materials which show
an abrupt and total loss of electrical resistance (snd hence
no power dissipation) below a certain cryogenic tempera-
ture termed the critical temperature, T^, which is usually
less than 18° K. The factors affecting the supercon-
ductivity of such materials are the interrelation of mag-
netic field strength H, current density J, and temperature
T. The magnetic field strength, applied externally or
generated by a current in the superconductor, limits super-
conductivity to below certain critical temperatures and
current densities. Similarly, at a given current, an in-
crease in field strength above a critical value can ter-
minate superconductivity. The large current-carrying
capacity of superconductors provides the basis for very
compact, extremely powerful magnets which can be used
in numerous applications where strong magnetic fields
are required, for example, in lasers, masers, accelerators,
and bubble chambers. A number of superconducting ma-
terials are known to the art, particularly combinations of
elements in Groups IV and V of the Periodic Tabfe and,
of these, niobium-zirconium, titanium-niobium, and nio-
bium-tin are of particular practical importance.
In the operation of large superconducting magnets a
problem of some consequence results from the require-
ments of training the superconducting magnet Training
is a method by which superconducting magnets are
brought to operate at maximum current capacity at a
particular temperature, which is commonly 4.2° K, the
temperature of the liquid helium refrigerant in general
use. The maximum current-carrying capacity of a super-
conducting solenoid is not achieved initially, but must
be reached through a series of steps; in each step the
• current is progressively raised over tbe value in the preced-
ing step to the point that the superconductor is driven
normal. Training is thus featured by a series of current
increases interrupted by superconducting-normal transi-
tions. For example, a typical magnet might be trained
to achieve the following critical currents in sequence:
IS amps, 18 amps, 21 amps, 27 amps, 30 amps, and
35 amps. If maximum current and field strength are
desired then the magnet must be driven normal seven
limes in this example before 35 amps is reached; in
practice there are usually many normal-going transitions
at each current level. Further, when a magnet is re-
moved from a Dewar flask at the conclusion of a run
and then later returned for a new run, retraining is re-
quired, and retaining may also be required when the mag-
net is left in the refrigerant at the conclusion of a run
but de-energized.
Training is, therefore, a continuing matter in the op-
eration of a superconducting magnet. This creates seri-
ous problems in the operation of large superconducting
magnets, because the energy released in each supercon-
ducting-normal transition, ViLI2, heats the magnet and
consumes the costly liquid gas refrigerant. As a result
of such energy release and liquid gas consumption, it be-
comes physically difficult and economically unattractive
to train large magnets. Cne ordinarily accepts a current
level 'below that theoretically attainable with a given
superconducting composition and designs the magnet ac-
10
15
20
2*
cordingly. However, the disadvantage with this approach
is that a larger, and therefore more costly, magnet must
be built to achieve a given flux level.
The prfc-apai object of the present invention, there-
fore, is to provide a method of operating a superconduct-
ing solenoid at a relatively high current level without
training.
Another object is to provide a method of operating a
large superconducting magnet at a current level above
that which ca^ be practically realized without training,
although below the maximum theoretical Ic.
Another object is to provide such a method wherein the
superconducting magnet is not forcsd through a series
of superconducting-normal transitions, nor is a large
amount of liquid gas refrigerant lost by heat generation
in order to reach operating conditions.
Still another object is to provide a iarge, stable, super-
conducting magnet having means for carrying greater su-
percurrents than a similar magnet could carry without
training.
Still other objects and advantages of the present inven-
tion will become apparent from the following detailed
discussion and the appended claims.
In the drawings, FIG. 1 is a perspective view of a
superconducting solenoid embodying the present inven-
tion; '
FIG. 2 is an overall view, partiy in section, of a super-
conducting magnet apparatus; and
FIG. 3 is -a graph representing critical current as a
function of solenoid temperature.
In accordance with the present invention, higher prac-
tical current levels are reached in large magnets than can
otherwise be reached without training by adjusting the
temperature of at least a portion of the magnet windings
to above the temperature of the gas refrigerant in liquid
form but below the critical temperature (Tc) of the super-
conducting material. In this manner, a higher practical
current level is obtainable without a series of supercon-
ducting-normal transitions and, for a large magnet, the
limiting magnet current and field obtained is thus higher.
The fundamental basis for the present improved per-
formance is not entirely certain. It appears, however,
that a "window" is provided for the escape of magnetic
flux which is ordinarily trapped by flux pinning sites in
a superconducting wire. Such magnetic flux tends to
oppose electron travel by Lorenz forces, and hence di-
minishes transport current conduction. The heating of
a portion of the superconducting solenoid above 4.2° K.
(liquid helium temperature), but below Tw provides an
escape path for the expanding flux as a magnet is ener-
gized. It therefore appears that a material which is more
porous to flux motion and possesses weaker pinning sites,
both of which variables are a function of temperature,
provides a more stable magnet. At elevated temperatures
flux motion which is initiated by increasing solenoid cur-
rent can thus be accommodated without large energy re-
lease to drive the material normal. A theoretical basis
for the temperature effect of flux pinning which is utilized
in the present invention is given hy P.W. Anderson, "Phys-
ical Review Letters," 9, 309 (1962). While the theoreti-
cal Ig thus realizable at the higher temperature through
practice of the present invention is below the theoretical
!« at the lower temperature (Ic tends to decrease with in-
creasing temperature below Tc), the current level prac-
tically obtainable without training is actually higher.
Further explanation of the present invention may be
bad by reference to the graph in FIG. 3 in which the
maximum temperature for an epoxy potted solenoid is
plotted against the transport current observed at a first
superconducting-normal quench. The magnet was com-
posed of Nb-25% Zr wire, copper plated. It should be
3,310,765
understood that for other solenoid configurations and
sizes or other superconducting materials, the proper tem-
ferature of eperaSea wvu'.d be different and may be
readily deter.7iiaed by routine measurements. The cross-
hatched region of the graph designates the training region
where ft Is unpredictable where a first superconducting-
normal trnTisitioa win occur. Thus, it would be difficult
in this region to design z. magitet for a stable current level
of more than about iss 20-anp level, although it is seen
that with training higher currents, up lo about the 40-amp
level, might theoretically be resized. However, at some
higher temperature, fcr example, approximately 5.25° K.,
at least 30 amps may be assured without training. It is
therefore evident frcs, the graph that, while current de-
clines with the increasing temperature, a higher stable cur-
rent level cnn be achieved. Tn the given example, a mag-
net temperature of abcut 5-7° K. is preferred, for above
about 7° 1C, current level drop is not compensated by
any increased stabffizy. Interestingly, after such stable
current level is obtained, the magnet temperature may
then be reduced to that of the bath without going normal
or losing its current. This is consistent with the above-
mentioned eTT'anatira that flax motion drives supercon-
ductors-normal; ia th;< case temperature reduction causes
no flnx motion snd t?=erefore leaves the magnet current
stable si the preview fevel. While reducing the tempera-
ture to tbst of liquid helium after a stable current level
is reached is convenient and safer because one is then
further below the crftfcal value, economic advantages in
terms of refrigerant cost may be had by operating the
system (magnei aad bath) at the higher temperature.
Thus, in this einbodiiaeat of the invention helium could
be is the gaseous chase above 4.2° K.
In FIG- 1 ^ filusrrated a superconducting solenoid 2
comprising a plurality of layers of a fine superconducting
wire 4, such ss JO^s:! niobium-zirconium or niobium-
titanhtm, wound on s ncE-conductiug core or spool fl.
The spool may be cf E. cormal metal or an organic resin
such as : phereli-r .sfa. A norm:.; metal wire 8, such
as copper or afajnirxra, is positioned along the width of
each layer of sspsrccmJucting wirs 4 in the solenoid ex-
tending radiaiiy Inwrnf in the manner shown in the figure.
The current supplied *s> normal conductor 8 serves to raise
the temperature level cf at least a portion of the solenoid
to the desired temperature level which serves to effectively
limit current conduction in the entire solenoid to the level
set in the heated reccn.
FIG. 2 provides aa example «.f the use of the present
invention in a specific magnet device. Any number of
magnet ccnfigwaticEs may be devised by those skilled
in the art, and the Articular design wii! be determined
by the proposed use of the soperconducting device. The
solenoid 2 of FIG. t h positioned in a double-walled
Dewar flask IB having two vacuum regions 12 with
an intermediate repaa 14 containing liquid nitrogen.
Other thermally insulated containers may be used. The
vessel is filled with liquid helium 16 and separated from
the environment by suitable closure means. The space
18 between the liquid gas and the cover 20 is filled with
the refrigerant in gaseous form. Two separate electrical
circuits using aormaEy conductive wire are provided for
solenoid 2, wx 22 for energizing the superconducting
solenoid, and the oiier 24 for heating the wire which
adjusts the temperaiiire of the solenoid. In each cir-
cuit is provided a *x>«rer source of batteries 26 and 28,
an on-cff swfei 30 snd 32, acd an ammeter 34 and
34. In addition, so-'enoid heater circuit 24 has a volt-
meter 38 and a variable resistor 40. As there are no
power losses in the superconducting niagnet 2 once the
magnet is energized, no further power is required to sus- •
tain the magnetic field ard both external circuits 22 and
24 can be removed.
The fbOoTring example is offered to illustrate the
present inventioa in greater detail. , • •..-.,..
10
15
40
45
50
55
60
65
70
75
4
Example
A small solenoid in which a portion of the windings
could be heated, of the same design as shown in FIGS. 1
and 2, was constructed and tested. The solenoid con-
sisted of 1900 ft. of 0.010-in. diameter Nb-25% Zr copper
plated wire, wound on a 0.9-in. I.D. spool and rated at
0.343 kg./amp. A copper wire was wound back and
forth between successive layers of the superconducting
wire and parallel to the solenoid axis. The heated volume
was a 10 percent pie section (cylindrical coordinates) of
the solenoid. The critical current was plotted against
varying solenoid temperatures and the measurements
shown in FIG. 3 taken. From FIG. 3 it is seen that the
solenoid temperature limits the attained current. When
I max. was adjusted with temperature to less than about
35 amps, at temperatures above about 5° K., no training
was observed. For lower solenoid temperatures (below
about 5° K.) at current levels above about 20 amps
considerable training (i.e., superconducting-normal tran-
sitions) was observed.
The foregoing example is illustrative rather than re-
strictive of the present invention. It is understood that
different configurations of the present invention may be
devised by those skilled in the art within the scope of
the present invention, including the specific temperatures
employed, the composition of the superconducting mate-
rial, and the magnet power level. Accordingly, the
present invention should be understood to be limited
only as is indicated in the appended claims.
What is claimed is:
1. A method of obtaining a relatively high current
in a niagnet of a superconducting material, positioned in
a bath of a gas refrigerant, which comprises
(a) adjusting the temperature of at least a portion of
the superconducting material to above the tempera-
ture of said gas refrigerant in liquid phase but be-
lov/ the critical temperature for said superconducting
material, and
(b) passing a supercurrent through said magnet.
2. The method of claim 1 wherein the refrigerant is
in the vapor phase and the magnet is at the same tem-
perature below said critical temperature.
3. The method of cteim 1 wherein the bath is of
helium in the vapor phase and is at a temperature above
4.2° K. but below said critical temperature and wherein
the superconducting material is at the same temperature.
4. A method of operating a superconducting magnet at
a relatively high current level without training, which
comprises
(a) placing said magnet in a bath cf a liquid ga* re-
frigerant,
(b) increasing the temperature of a portion of the mag-
net above the temperature of said liquid gas refriger-
ant but below the critical temperature for the super-
conducting material, and
(c) then initiating the passage of supercurrent through
. said magnet.
5. The method of claim 4 wherein the liquid gas is
helium and the temperature of said magnet is increased to
between about 4.2° K. and about 18" K.
6. The method of claim 4 wherein the temperature of
the magnet is reduced to that of the liquid gas refrigerant
after passage of supercurrent through the magnet is ini-
tiated.
7. A method of operating a superconducting magnet
containing a plurality of turns of a niobium-zirconium
wire at a relatively high current level without training,
which comprises
(a) olacinig said magnet in a bath of liquid helium,
(b) heating said magnet to a temperature of about
5-7° K., and
(c) while said bath is at a temperature of about 42* K.
and said u.agnet at a temperature of about 5-7* 1L,
3,310,765
establishing the passage of a superconducting current
in said magnet
8. The method of claim 7 wherein said niobium-zir-
conium consists of Nfc-25 weight percent zirconium
alloy.
9. The method of claim 7 wherein the magnet tempera-
ture is reduced to about 4.2° K. alter establishment of a
superconducting current therein.
10. A method of operating a superconducting magnet
(c) electrical means connecting to said magnet for
initiating the passage of superconducting current
therethrough, and
(d) electrical means connecting to raid magnet for
heating a portion thereof to a temperature above the
temperature of the liquid gas refrigerant and below
said critical temperature.
12. The superconductive magnet of claim 11 wherein
said liquid gas in helium and said superconducting wire
containing a plurality of turns of a niobium-zirconium JQ consists of niobium-zirconium alloy,
wire at a relatively high current level without training, 13. The superconducting magnet of claim 11 wherein
which comprises
(a) placing said magnet in a bath of gaseous helium
which is at a temperature of about 5-7° 1C, and
(b) initiating the passage of a superconducting current 15
in said magnet
11. A superconducting magnet assembly comprising
said superconducting wire is further wound in a plurality
of layers on said core and wherein said electrical heating
wire is interleaved between said layers.
(a) a plurality of turns of a superconducting wire
wound on a magnet core,
(b) said magnet positioned in a container in a bath of 20
a liquid gas refrigerant at a temperature below the
critical icriperature of said superconducting ma-
terial.
References Cited by the Examiner .
UNITED STATES PATENTS ^
3,156,850 11/1964 Walters __________ 335— 216 X
3,253,193 5/1966 Lubell et al. ________ 335—216
BERNARD A. GILHEANY, Primary Examiner.
C. HARRIS, Assistant Examiner.


Stable superconducting magnet

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