Superconductor magnetic shield

A large-area superconductor magnetic shield comprising a plurality of small superconductor unit layers stuck onto the surface of a large-area substrate, wherein the external peripheral sections of the adjacent unit layers are overlapped mutually or the end sections of the adjacent unit layers are abutted mutually such that the adjacent unit layers do not make contact with one another. Compared with conventional magnetic shields, the magnetic shielding amount at the overlapping sections of the magnetic shield of the present invention is substantially greater.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improvements of a magnetic shield for 
shielding intense magnetic fields, more particularly, improvements of a 
magnetic shield having a large area to prevent magnetic leakage in wide 
ranges. 
2. Prior Art 
To avoid adverse effects of magnetic fields generated from magnets or other 
substances, the art of magnetic shields which are made by using 
plate-shaped or sheet-shaped superconductors refrigerated below a critical 
temperature where superconductivity is generated is known as a 
conventional art for magnetically shielding a certain internal space. To 
achieve the object of shielding such magnetic fields, the second class 
superconductor which operates in the mixture region of the superconducting 
and normal conducting conditions is used more preferably than the first 
class superconductor, since the upper critical temperature of the second 
class superconductor is higher than that of the first class 
superconductor. 
The maximum magnetic field shielding intensity of a superconductor, that 
is, the shielding intensity of a superconductor having a shape of a plate, 
sheet, film or membrane (in the general explanations described below 
simply referred to as "layer") for completely shielding external magnetic 
fields is significantly dependent on the class, size and shape of the 
superconductor. As pointed out in the patent specifications cited below by 
the inventors of the present invention, the maximum magnetic field 
shielding intensity of a superconductor increases abruptly as its 
thickness increases in a limited range. If the thickness increases over a 
certain value, the increasing rate of the maximum magnetic field shielding 
intensity becomes gentle. This indicates that an inflexion point is 
present on a curve which represents the relationship between the thickness 
of the superconductor and its maximum magnetic field shielding intensity. 
Considering this phenomenon, the inventors of the present invention have 
proposed a magnetic shield, the maximum magnetic field shielding intensity 
of which is significantly enhanced by using superconductor layers, the 
thickness of which is made smaller than that corresponding to the 
reflection point of the magnetic field shielding intensity, by laminating 
a superconductor layer with a normal conductor layer such as aluminum foil 
and by increasing the number of laminated layers (Japanese Laid-open 
Patent Appln. 61-183979, U.S. Pat. No. 4,803,452, U.S. Pat. No. 4,797,646, 
Can. PAT. 1261050, EP Appln. 86 101613. 7-2208). 
To expand the range of a magnetic shielding space by enlarging the area of 
a magnetic shield, the size of a single superconductor layer is limited 
owing to the limitation in the production requirements of superconductors. 
In the case of producing larger superconductor layers exceeding the 
limitation, an art for enlarging such a magnetic shield by mutually 
overlapping the end sections of a plurality of oxide superconductor 
ceramic plates for example and by sticking the end sections together with 
conductive adhesive is known (Japanese Laid-open Patent Appln. 63-313897). 
The inventors of the present invention have also proposed a magnetic 
shield made by sticking a plurality of small superconductor pieces onto 
the external or internal surface of a cylinder with its one end closed 
(Japanese Laid-open Patent Appln. 1-302799). 
These days, superconductor magnets are made larger to generate more intense 
magnetic fields. Because of this enlargement, the space ranges affected by 
the intense magnetic fields are also expanded. To shield unnecessary 
permeation of intense magnetic fields, magnetic shields which can securely 
shield intense magnetic fields over large areas have been requested. Such 
magnetic shields are applied to superconductor motors and superconductor 
generators, as well as superconductor magnets themselves used in linear 
motor cars and electromagnetic propellent ships. 
The maximum magnetic field shielding intensity of a plate-shaped magnetic 
shield for completely shielding a magnetic field is apt to become lower at 
the external peripheral section than at the central section of the 
magnetic shield. In the case of a disc-shaped magnetic shield for example, 
the maximum magnetic field shielding intensity is lower at its peripheral 
section farther away from its central section in the radial direction. For 
this reason, although complete shielding is possible at the central 
section, a part of magnetism permeates the external peripheral section, 
causing magnetic leakage. Consequently, to completely shield a constant 
external magnetic field permeating a surface area, shielding is necessary 
at the central section of the surface area of the magnetic shielding plate 
by using a magnetic shield having the maximum magnetic shielding amount 
exceeding the external magnetic field intensity generated at the end of 
the surface area. As a result, the size of the magnetic shield must be 
considerably larger than the surface area. 
A serious problem in the magnetic shielding process is the generation of a 
flux jump phenomenon wherein magnetic flux permeating the external 
peripheral section of the magnetic shield flows abruptly to the central 
section of the magnetic shield. If this occurs, the magnetic shield is 
heated locally and its superconducting condition is converted into a 
normal conducting condition, thereby causing magnetic field leakage over 
the entire magnetic shield. If this flux jump occurs once, the magnetic 
shield cannot act as a superconductor and completely loses its magnetic 
shielding function. Since the amount of generated heat is greater as the 
transfer distance of magnetic flux is larger, it is difficult to stably 
maintain the superconducting condition in an intense magnetic field. 
In the magnetic shield of the above-mentioned prior art, which is made 
large by mutually laminating and sticking a plurality of superconductor 
ceramic plates, superconductive shielding current flows across every two 
ceramic plates. The structure of the magnetic shield is thus considered to 
be the same as that comprising a single superconductor, thereby being apt 
to cause the danger of losing the magnetic shielding effect due to the 
generation of the flux jump. 
In the case of a tubular magnetic shield formed by pressing and sticking 
superconductor pieces (comprising superconductive low-melting-point alloy 
powder) onto the surface of a tube, the structure of this magnetic shield 
is the same as that of a tube formed to have a large surface area made by 
using a single superconductor, since the adjacent superconductor pieces 
are joined by the superconductor alloy. Accordingly, the maximum magnetic 
shielding amount of the magnetic shield is lower at the upper peripheral 
section of the opening area of the tube and lowest at the sealed end 
section of the bottom of the tube, causing the danger of losing the 
magnetic shielding effect due to the generation of the flux jump at such 
sections. 
As a countermeasure for reducing the effect of the flux jump, a plurality 
of small through holes are provided in the surface area of a thin 
superconductor film such that the transfer range of the magnetic flux is 
limited to reduce heating and prevent a chain reaction of magnetic flux 
flow (Japanese Laid-open Patent Appln. 63-233577, U.S. Pat. No. 4,828,931, 
Can. Pat. 1296089, UK Pat. 2203909, German Pat. Appln. P38-09452. 5-34, FR 
8803822). Even when a magnetic shield is enlarged, however, it still has a 
problem of reduction in the maximum magnetic field shielding intensity at 
its peripheral section. 
SUMMARY OF THE INVENTION 
It is therefore the first object of the present invention to provide a 
magnetic shield capable of compensating for the reduction of the maximum 
magnetic shielding amount at the peripheral section of the magnetic shield 
and also capable of effectively shielding the intense magnetic field at a 
large surface area of the magnetic shield, and to prevent 
superconductivity from being lost owing to the generation of the flux jump 
which is relevant to the enlargement of the magnetic shield, thereby 
ensuring stability during magnetic shielding. 
The second object of the present invention is to provide a superconductor 
magnetic shield capable of achieving completely zero magnetic field 
intensity even in the presence of a locally intense magnetic field so that 
the magnetic shield can cope with the spatial change in the magnetic field 
intensity in an ununiformed magnetic field. 
To accomplish these objects, the superconductor magnetic shield of the 
present invention is made by arranging and sticking a plurality of small 
superconductor layers (each of which is hereafter referred to as "unit 
layer") onto the surface of a substrate (support member) having a desired 
shape, and the peripheral section of each unit layer is overlapped with 
the peripheral section of another unit layer in the normal direction in 
order to uniformize the maximum magnetic field shielding intensity of the 
magnetic field over the entire substrate or to locally strengthen the 
shielding intensity as a general feature.

DETAILED DESCRIPTION OF THE INVENTION 
The superconductor magnetic shield of the genus invention of the present 
invention comprises a substrate and a plurality of independent 
superconductor unit layers arranged adjacent to one another and stuck onto 
the surface of the substrate, wherein the peripheral section of each unit 
layer is overlapped with the peripheral section or the central section of 
another adjacent unit layer in a non-direct contact manner in the normal 
direction of the substrate so that the magnetic shielding intensity at 
these overlapping sections can be increased substantially. 
This genus invention can be generally classified into a first invention and 
a second invention. 
In the case of the superconductor magnetic shield of the first invention, 
the surface areal plane including the superconductor unit layers of the 
above-mentioned structure is a single layer in the normal direction and 
the above-mentioned peripheral sections are overlapped mutually within the 
single layer. This first invention corresponds to claim 2. 
In the case of the superconductor magnetic shield of the second invention, 
the surface areal planes including the superconductor unit layers of the 
above-mentioned structure are laminated to form a plurality of layers in a 
non-direct contact manner in the above-mentioned normal direction, the 
peripheral sections of the unit layers are in abutting contact with or 
proximate to the peripheral sections of the adjacent unit layers within 
the single layer, and the above-mentioned peripheral sections of the unit 
layers are overlapped mutually between every two layers in the normal 
direction. This second invention corresponds to claim 4. 
The first invention further includes a superconductor magnetic shield 
wherein a plurality of layers, each comprising a plurality of 
superconductor unit layers, are laminated (claim 3), and in the same way, 
the second invention includes a superconductor magnetic shield wherein 
three or more layers are laminated (claim 6) and used to cover the surface 
of the substrate. 
In addition, the present invention also includes a superconductor magnetic 
shield wherein the number of the layers comprising superconductor unit 
layers is increased at the magnetic flux concentration sections of the 
magnet shield (claim 7). 
First, the technical matters common to the first and second inventions are 
described below. 
The substrate is formed in accordance with the desired shape of the 
superconductor magnetic shield and is determined considering the shape of 
the superconductor magnet to be covered and the space range to be 
shielded. The substrate is mainly made of non-magnetic normal conductor 
material, such as copper, aluminum, stainless steel or low-temperature 
synthetic resin and is formed into a shape of a flat plate, curved plate 
or container. 
The superconductor unit layer includes films or plates made of 
superconductor material. Although it can be a single thin superconductor 
plate, it preferably comprises laminations of thin superconductor plates 
and thin non-magnetic metal plates. 
As superconductor material, Nb-Ti alloy, mixed crystal of NbN and TiN, 
Nb.sub.3 Al, Nb.sub.3 Ge, Nb.sub.3 (Al, Ge), Nb.sub.3 Sn, Y-Ba-Cu-O-based 
oxide, Bi-Sr-Pb-Ca-Cu-O-based oxide or Tl-Sr-Ca-Cu-O-based oxide are used. 
The superconductor unit layer can have a simple shape of a square, hexagon 
or circle. The appropriate size of the unit layer is 5 to 100 cm in 
diameter in the case of a circle. 
The unit layers are stuck together with low-temperature organic adhesive or 
normal conductor low-melting-point alloy onto the surface of the 
above-mentioned substrate. When sticking unit layers to provide only one 
layer, the peripheral sections of the unit layers are overlapped mutually 
to prevent formation of any gap which is not covered with unit layers as 
viewed in the normal direction to the surface of the substrate and to 
completely cover the substrate. In this case, it is important that the 
overlap is performed so that the peripheral section of each unit layer 
does not make contact with those of other adjacent unit layers. To 
accomplish this, the adhesion layer formed by the above-mentioned adhesive 
should be insulated, or organic insulation films should be intervened at 
the overlapping sections or the front and rear surfaces of the 
superconductor unit layers should be a lamination covered with normal 
conductor metal, such as copper film or aluminum film. 
To form two or more layers comprising the superconductor unit layers and to 
stick the layers onto the above-mentioned substrate, the unit layers are 
stuck sequentially onto the lower superconductor unit layer with the 
above-mentioned adhesive or normal conductor low-melting-point alloy. At 
the time of this process, although the thin superconductor films of the 
same layer can be abutted and arranged such that a gap generates between 
the films, the superconductor unit layers of the second layer must be 
arranged in the normal direction such that they cover the gaps generated 
in the first layer. 
In addition, an electric insulation layer or a normal conductor layer 
should be intervened in the same manner as described above such that the 
superconductor unit layers of the first and second layers do not make 
contact with each other. 
Generally, a superconductor sheet, for example, a disc-shaped 
superconductor does not have uniform magnetic shielding capability over 
its entire surface. The maximum magnetic field shielding intensity Hm of 
the superconductor at the time of complete shielding of an external 
magnetic field is highest at the center of the disc. The intensity lowers 
at the external peripheral section and becomes zero at the end section of 
the disc. The maximum magnetic field shielding intensity of the 
disc-shaped superconductor is shown as a skirt-shaped curve as shown in 
FIG. 1 (A) when a radius is taken as a variable. FIG. 1 (B) shows the 
relationship between an external magnetic field intensity and a magnetic 
shielding amount .sub..DELTA. H (a difference between the magnetic field 
intensity obtained when a magnetic shield is present at a measurement 
position and that obtained when the magnetic shield is absent) measured in 
the radial direction from the central section c to the end section a of 
the disc-shaped superconductor. According to the figure, the magnetic 
shielding amount at the central section c is equal to the external 
magnetic field intensity in response to the initial increase of the 
external magnetic field intensity and the magnetic field does not permeate 
the superconductor. However, if the external magnetic field intensity 
becomes greater than the maximum magnetic shielding amount .sub..DELTA. 
Hm, the magnetic shielding amount .sub..DELTA. H becomes smaller instead. 
In other words, if the external magnetic field intensity is greater than 
the maximum magnetic shielding amount .sub..DELTA. Hm, the magnetic flux 
of the magnetic field becomes to penetrate the superconductor. When the 
maximum magnetic shielding amount .sub..DELTA. Hm is assumed to be the 
maximum magnetic field shielding intensity Hm, the value of Hm becomes 
smaller at positions farther away from the central section of the disc and 
becomes zero at the end section, resulting in no magnetic shielding 
capability. 
The maximum magnetic field shielding intensity Hm at the central surface 
section of a sheet-shaped superconductor differs depending on the type, 
structure and thickness of the superconductor. When a disc-shaped 
superconductor is taken as an example, the intensity Hm generally 
increases abruptly as the disc diameter increases up to about 50 cm. The 
value still increases gradually as the diameter increases further. For 
this reason, the superconductor unit layer is limited to a small size of 5 
to 100 cm in diameter. 
The magnetic shield of the first invention is made by arranging and 
sticking such small superconductor unit layers onto the surface of the 
substrate while vertically overlapping the external peripheral sections of 
the superconductor unit layers and while using care so that the adjacent 
superconductors do not make contact with one another. With this structure, 
the maximum magnetic field shielding intensity at the overlapping sections 
is a sum of the magnetic field shielding intensity of the upper 
superconductor unit layer and that of the lower superconductor unit layer 
as shown in FIG. 2. By relatively expanding the width of the overlapping 
sections, the maximum magnetic field shielding intensity values at all 
surface area positions of the magnetic shield are uniformized to a value 
greater than a certain value. For this reason, when a large-area magnetic 
shield is formed by a single superconductor or by laminating a plurality 
of small superconductor pieces such that they make contact with one 
another, the maximum magnetic field shielding intensity of the magnetic 
shield reduces in the wide range of the peripheral sections of the 
superconductor as shown by the Hm' curve in FIG. 2. As a result, the 
external magnetic field which must be shielded cannot be shielded 
completely and magnetic field leakage occurs. To solve this problem, a 
magnetic shield with a surface area far wider than the surface area of the 
external magnetic field to be shielded is required. However, the magnetic 
shield of the present invention has superior magnetic shielding capability 
even at its peripheral sections except the very narrow ranges close to its 
end sections as high as the capability obtained at its central section, as 
indicated by the Hm curve shown in FIG. 2. The size of the surface area of 
the magnetic shield can thus be made approximately equal to that of the 
external magnetic field to be shielded. 
In the first invention, when the layer formed on the substrate by 
overlapping the peripheral sections of the superconductor unit layers is 
further laminated with another layer to form two layers, while the upper 
and lower superconductor unit layers are maintained in a non-contact 
manner and while the overlapping section of each lower unit layer is 
positioned at the central section of the corresponding upper unit layer, 
the distribution of the maximum magnetic field shielding intensity of the 
magnetic shield is further uniformized and flattened. To further flatten 
the distribution, three or more layers should be used. 
In the second invention, superconductor unit layers are laminated in two or 
more layers on the substrate. Adjacent layers are proximate to or abutted 
against each other. The peripheral section of each superconductor unit 
layer of the first layer is overlapped with one of the superconductor unit 
layers of the second layer. As a result, the maximum magnetic field 
shielding intensity of the superconductor unit layer located just above 
the peripheral section of another superconductor unit layer is added to 
the low maximum magnetic field shielding intensity of the peripheral 
section, thereby obtaining the maximum magnetic field shielding intensity 
that is larger than a certain level over the entire surface of the 
magnetic shield. 
In the magnetic shield of the second invention, there is no magnetic 
shielding capability at the abutted sections between the adjacent unit 
layers or the void portions proximate to the adjacent unit layers when 
only the first layer comprising superconductor unit layers is used. By 
laminating the second layer comprising the superconductor unit layers over 
the first layer, the magnetic field shielding capability at the 
above-mentioned abutted sections or the proximate void portions of the 
first and second layers can be compensated for. 
Since the magnetic shield of the present invention is formed by sticking a 
plurality of superconductor unit layers onto the surface of the substrate 
while maintaining the non-contact manner in the normal direction, the 
shielding current for preventing the intrusion of external magnetic fields 
flows only inside the corresponding unit layer, but the unit layer is 
isolated from other adjacent unit layers. With this structure, even if 
flux jump (the abrupt transfer of magnetic flux permeated at and 
transferred from the peripheral section to the central section of the unit 
layer) occurs, the mutual action between the magnetic flux and the 
shielding current is limited within the corresponding unit layer and the 
chain reaction of the flux jump is also limited within the corresponding 
unit layer, thereby causing no adverse effect to other adjacent 
superconductor unit layers. Since the size of the unit layer can be made 
small, the transfer distance of the magnetic flux can also be made small. 
Even when magnetic flux flow occurs frequently, the heating value of the 
unit layer is limited. Even if the temperature of the unit layer 
increases, the unit layer can be refrigerated by immersed refrigerant such 
as liquid helium. This minimizes the possibility of raising the 
temperature of the unit layer over its critical temperature, the 
possibility of converting the unit layer into a normal conductor and the 
possibility of losing the magnetic shielding function of the unit layer. 
For this reason, the present invention can form a large-area magnetic 
shield which rarely causes unstable conditions due to flux jump. 
In particular, when the superconductor unit layer is laminated with a 
normal conductor metal layer, the normal conductor metal layer shuts off 
the flow of the above-mentioned shielding current of the unit layer and 
isolates each superconductor layer. When aluminum, particularly copper or 
silver is used as metal, the superior heat conductivity of such metal is 
significantly effective in externally dispersing the heat generated by the 
flow of magnetic flux and thus also effective in refrigerating the layers, 
thereby effectively preventing instability due to the flow of the magnetic 
flux. 
When a single layer of the superconductor unit layer is used, it usually 
comprises rolled Nb-Ti alloy foil with a thickness of 100 .mu.m or less. 
As the thickness is larger, the maximum magnetic field shielding intensity 
of the superconductor layer becomes greater. However, if the thickness 
increases to 20 .mu.m or more, the increasing rate of the maximum magnetic 
field shielding intensity reduces. This increasing of the thickness is 
thus not advantageous. Rather than using the thicker foil, a lamination 
comprising Nb-Ti alloy foil of 20 .mu.m or less in thickness and normal 
conductor foil such as copper or aluminum foil should be used. Increasing 
the number of laminations is advantageous, since the maximum magnetic 
shielding amount can be made larger. When forming the lamination 
comprising the Nb-Ti alloy foil and the normal conductor foil, the 
sputtering method for the Nb-Ti alloy and copper or aluminum can be used. 
The sputtering method is also used when the unit layer is formed from the 
mixed crystal of NbN and TiN. More specifically, the above-mentioned 
sputtering method is conducted on one or two kinds of metal layers 
selected from the group consisting of copper, aluminum, nickel, stainless 
steel, titanium, niobium and niobium-titanium alloy to form the 
lamination. When forming the superconductor unit layer from Nb.sub.3 Al, 
Nb.sub.3 Ge, Nb.sub.3 (Al, Ge) and Nb.sub.3 Sn, the thin films formed by 
the sputtering method or the like can be used. In addition, the films 
which are heat-treated after rolling and lamination can be used. As the 
normal conductor layer, one or two kinds of metal layers selected from the 
group consisting of copper, aluminum, nickel, stainless steel and titanium 
can be used. 
For the magnetic shield of the present invention, a plate-shaped 
superconductor can be used as one of the superconductor unit layers. As 
the plate-shaped superconductor, a plate made of the Y-Ba-Cu-O-based 
oxide, Bi-Sr-Pb-Ca-Cu-O-based oxide or Tl-Sr-Ca-Cu-O-based oxide can be 
used. In addition, a sintered plate of 0.5 to 10 mm in thickness can also 
be used as the plate-shaped superconductor. 
When a plurality of small through holes are provided in the superconductor 
unit layer, the transfer of the magnetic flux at the peripheral section of 
the unit layer is limited in the range of the adjacent small holes and no 
flux jump occurs at the central section of the unit layer. The range of 
the flux jump can thus be restricted within the range approximate to the 
space between the holes, thereby eliminating the danger of making the 
superconductor unstable. If the diameter of the holes is made larger, 
however, the magnetic shielding amount is lowered. The opening area of the 
holes should therefore be 3 cm.sup.2 or less and preferably the diameter 
of the hole should be about 50 .mu.m. 
Since the present invention provides a large-area magnetic shield by 
sticking a plurality of small superconductor unit layers onto the 
substrate, the superconductor unit layer can have a small size of about 5 
to 100 cm. Such superconductor unit layers can thus be produced very 
easily. Furthermore, magnetic shields of larger sizes or with various 
curvatures can also be produced easily by sticking a plurality of unit 
layers onto the substrates made by welding and assembling. 
Moreover, since the magnetic characteristics of the unit layer can be 
measured accurately and easily by ordinary test methods, the 
characteristics of an assembled magnetic shield can be estimated fairly 
accurately by determining the sticking method according to the magnetic 
data of the unit layer. 
EXAMPLES 
An example of the first invention is first explained below referring to the 
drawings. FIG. 3 shows an example of a magnetic shield wherein square 
superconductor sheets 1a, 1b, . . . are laminated as unit layers on a 
rectangular flat substrate 2 so that the peripheral section of one sheet 
is overlapped with and stuck onto that of another sheet. As the 
superconductor sheets 1a, 1b, . . . , thin flexible alloy plates are 
appropriate, and particularly a single layer of Nb-Ti alloy foil of 100 
.mu.m or less in thickness or an alternate lamination of Nb-Ti alloy foil 
and aluminum foil of 20 .mu.m or less in thickness can be used. 
Low-temperature organic adhesive (not shown) is used for sticking. An 
adhesive layer is intervened between the overlapping sections of the 
superconductor sheets 1a and 1b such that the overlapping sections thereof 
do not make direct contact with each other. When using the lamination 
comprising the Nb-Ti alloy foil and aluminum foil, the front and rear 
surfaces of the lamination are covered with aluminum foil to prevent 
direct contact between the overlapping sections of the adjacent pieces of 
superconductor Nb-Ti alloy foil. This method is advantageous since the 
adhesive for preventing the direct contact is unnecessary. 
FIG. 4 shows an example of laminating and sticking disc-shaped 
superconductor sheets as superconductor unit layers. For the 
superconductor sheets shown in FIGS. 3 and 4, non-flexible thin oxide 
superconductor plates can also be used. Even in this case, an organic 
adhesive layer is intervened between the overlapping sections of the 
plates so that the superconductors do not make direct contact with each 
other at the overlapping sections, and the superconductor sheets are stuck 
onto the substrate 2. 
The substrate 2 of the magnetic shield shown in FIGS. 3 and 4 has a 
rectangular shape. By combining a plurality of such rectangular 
substrates, a container-shaped magnetic shield can be formed. Besides, 
since flexible superconductor sheets can be easily stuck onto a curved 
substrate, magnetic shields having desired shapes and large areas can be 
formed. 
By sticking and covering one or two more layers comprising superconductor 
unit layers of the same size over the layer comprising superconductor unit 
layers stuck as shown in FIGS. 3 and 4, the maximum magnetic field 
shielding intensity of the magnetic shield thus obtained can be enhanced 
and flattened. 
Examples of the second invention are then described below. 
FIG. 5(A) shows an example wherein square superconductor sheets used as 
unit layers are stuck with the end sections thereof abutted. This example 
is applied particularly to non-flexible rigid superconductor plates, for 
example the above-mentioned oxide superconductor plates. In this case, a 
small gap is provided at each end of the abutted section thereof to 
prevent the adjacent superconductors from making contact with one another 
and to allow an adhesive layer to intervene in the gap. In the case of 
this method of sticking the unit layers by abutting them, the maximum 
magnetic field shielding intensity at the abutting sections is 
approximately zero and magnetic leakage occurs. To solve this problem, the 
abutted and stuck superconductor plates must be laminated in two layers as 
shown in FIG. 5(B) and 5(C), and the abutting sections of the upper layer 
must be shifted in the surface direction so that the abutting sections of 
the upper layer do not coincide with those of the lower layer. To 
completely prevent magnetic shielding leakage and to uniformize the 
maximum magnetic field shielding intensity, the lamination structure 
should have three or more layers. 
FIG. 6(A) shows the arrangement of the first layer wherein discs 1a, 1b, . 
. . are abutted and stuck as unit layers. In this case, even when the 
discs are placed in the highest density, a substantial void portion 24 is 
formed among every three adjacent discs. 
As shown in FIG. 6(B), the superconductor discs 1'a, 1'b, . . . of the 
second layer are stuck and arranged to cover the void portion 24 enclosed 
by every three adjacent discs of the above-mentioned first layer. Even in 
this two-layer structure, magnetic leakage still occurs at each position 
where the void portion 24 of the lower layer coincides with the void 
portion 24 of the upper layer. To prevent this leakage, the superconductor 
discs 1"a, 1"b, . . . of the third layer are stuck to cover the void 
portions 24 located at such coincidence positions. By providing a 
lamination structure of three or more layers, the maximum magnetic field 
shielding intensity of this example can be uniformized and flattened. The 
method of sticking such discs is suited particularly for fragile 
superconductor ceramics, since disc-shaped sintered products can be made 
easily. 
FIG. 7 shows a vertical sectional view of an upper part of a 
container-shaped magnetic shield wherein a plurality of layers comprising 
plate-shaped superconductor ceramic layers used as unit layers are stuck 
inside. The three layers 11, 12 and 13 comprising the ceramic plates are 
stuck onto the inside surface of the container-shaped copper substrate 2. 
Furthermore, two layers 14 and 15 are additionally stuck onto the corner 
sections of the substrate to shield the high magnetic field intensity 
generated by the concentration of the magnetic lines of force at the 
corner sections. 
Next, the production method and test results of the magnetic shields of the 
present invention are described below. 
Superconductor unit layers were produced as follows. A lamination plate 
having a total of 25 layers was produced wherein each layer was made by 
laminating Nb-Ti alloy and copper which were alternately evaporated in a 
thickness of 0.4 .mu.m on a copper plate measuring a thickness of 18 .mu.m 
and a diameter of 30 mm by using a sputtering apparatus. The plate was 
then cut into square pieces having a side length of 50 mm to obtain 
superconductor sheets. 
The substrate was made of 5 mm thick copper plate. It was formed into a pan 
shape having a diameter of 400 mm and equipped with a flat bottom and 
curved external peripheral sections. Over the entire internal surface of 
the substrate, the 10 mm wide overlapping sections of the above-mentioned 
superconductor sheets were provided by the overlapping method described in 
the explanation of the example of the first invention shown in FIG. 3 and 
stuck with low-temperature epoxy adhesive to form a magnetic shield. Two 
superconductor sheet layers were provided at the central flat section of 
the magnetic shield and five layers were used at the curved peripheral 
sections and end sections thereof. 
As shown in FIG. 8, a superconductor coil 5 was provided at a lower section 
of an adiabatic vacuum container 4 having an internal diameter of 500 mm. 
Over the coil, a pan-shaped magnetic shield 3 was disposed with its 
internal surface side facing downwards. The interior of the adiabatic 
container was filled with liquid helium to refrigerate the superconductor 
coil 5 and the magnetic shield 3. The magnetic field inside the container 
was measured at four positions a, b, c and d over the magnetic shield 3 by 
using a magnetic field measurement sensor. 
The exciting current of the superconductor coil magnet 5 was first obtained 
so that the measured magnetic field intensity was 1000 G at the 
measurement points e and f without installing the magnetic shield 3. The 
magnetic shield 3 was then installed at the specified position, the same 
existing current was flowed in the superconductor coil 5, and the magnetic 
field intensity was measured. The measured magnetic field intensity values 
were 5 G or less at all measurement positions a, b, c and d. 
A magnetic shield made of an oxide represented by a chemical formula of 
Y.sub.1.0 Ba.sub.2.0 Cu.sub.3.0 O.sub.7-x used as an oxide superconductor 
was then tested. A mixture of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO was 
heated, melted and abruptly refrigerated, then pulverized. The power 
obtained in this way was press-molded into a shape of a disc and fired to 
produce a plate-shaped superconductor. To form thin films from the oxide, 
the oxide powder was applied to or printed on a metal plate made of copper 
for example, then fired. 
The fired superconductor discs measuring 50 mm in diameter and 5 mm in 
thickness were arranged in three layers as shown in FIG. 6 and stuck onto 
the internal surface of the pan-shaped substrate made of copper and 
measuring a thickness of 5 mm and an outer diameter of 400 mm as shown in 
FIG. 8. The magnetic shielding amount of this example was then measured 
using the above-mentioned testing apparatus. The maximum magnetic 
shielding amount of the example was 20000 G in the 70% surface area 
including the central section of the pan-shaped substrate. 
Consequently, the magnetic shield of the present invention can provide the 
following advantages. 
Since a plurality of small superconductor unit layers can be stuck onto a 
substrate while being overlapped at their peripheral sections without 
allowing the superconductors to make contact with one another and a 
large-area magnetic shield can be formed, the maximum magnetic field 
shielding intensity of the magnetic shield can be uniformized in the 
surface area of the magnetic shield without causing the reduction of the 
intensity at the external peripheral sections thereof except the ranges 
very close to the end sections. Besides, the reduction of the maximum 
magnetic field shielding intensity which occurs at the external peripheral 
sections of a large-area magnetic shield formed by a continuous 
superconductor can be compensated for and prevented by the magnetic shield 
of the present invention. The size of the magnetic shield can thus be made 
almost as large as the range of an external magnetic field to be shielded. 
The magnetic shield of the present invention can therefore be made 
relatively smaller than the magnetic shield formed by such a continuous 
superconductor. 
In addition, since two or more layers comprising stuck superconductor unit 
layers are used, the maximum magnetic field shielding intensity of the 
magnetic shield of the present invention can be enhanced, uniformized and 
flattened in the external peripheral direction. 
Furthermore, when a plurality of superconductor unit layers are abutted and 
stuck at their end sections and laminated to form two or more, 
particularly three or more layers, the magnetic leakage at the abutting 
sections can be prevented and the maximum magnetic field shielding 
intensity of the magnetic shield can be enhanced significantly. 
In particular, since the number of the laminations can be increased at the 
end sections of the magnetic shield, particularly in the limited ranges 
exposed to high magnetic field intensity, the magnetic shield of the 
present invention can cope with the change in the intensity of local 
external magnetic fields by compensation, thereby easily achieving 
complete magnetic shielding. 
Moreover, since the superconductors of adjacent superconductor unit layers 
are isolated so that they do not make contact with one another, the 
possibility of causing flux jump is eliminated over the entire magnetic 
shield. Even if flux jump occurs, it is limited in the range of a small 
superconductor and heat generation is minimal, thereby extremely reducing 
the danger of causing the superconductor to be converted into a normal 
conductor. In particular, when the superconductor unit layers have a 
lamination structure comprising superconductor and normal conductor films 
or when a plurality of small through holes are provided in the 
superconductor unit layers, the factors for causing instability due to 
flux jump can be eliminated substantially and the reliability of the 
magnetic shield of the present invention can be enhanced significantly. 
When the magnetic characteristics, size and shape as well as the sticking 
method and the number of laminations of the superconductor unit layers 
have been known, the magnetic shielding capability of a large-area 
magnetic shield made by sticking the unit layers can be estimated fairly 
accurately. In the case of a conventional magnetic shield, preliminary 
experimental testing of the shielding capability of a practically large 
magnetic shield is virtually impossible partly because larger testing 
apparatuses must be used. In the case of the magnetic shield of the 
present invention, it is possible to estimate the magnetic characteristics 
of the magnetic shield according to partial test results, since the 
unstable phenomena due to the above-mentioned flux jump do not occur.