Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
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The present invention relates to a method and an apparatus for
providing extremely low-resistance connections between the end sections of
two superconductors which are to be cooled to cryogenic operating tempera-
tures wherein each superconductor contains at least one superconducting
conductor filament embedded in a matrix of normal-conducting stabilizing
material, and wherein the ends of these superconducting conductor filaments
are stripped at least partially of the stabilizing material, are placed in
contact and are welded to each other, and wherein an electrically highly
conducting bridge of normal-conducting material is developed between the
stabilizing material of the superconductors.
For superconducting magnet coil windings, so-called short
circuit operation may be required. Once the magnetic field of the magnet
winding is built up, the ends of the winding can be short-circuited by
means of an extremely low-resistance persistent-current switch for storing
the electric energy fed into the winding. The current then flows in -the
so-des:igned short circuit and the power supply required Eor exciting -the
magnet w:inding can thereEore be interrupted. The attenuation of the current,
i.c., its decreasc behavior, then depends essentially on the contact
resistances between the individual superconducting parts of the circuit.
Such contact resistances occur not only at the continuous-current switch
but are found also in connections of the end sections of individual super-
conductor parts, of which the magnet winding is generally comprised.
In nuclear magnetic resonance apparatus, great constancy of
the field generated by the field magnets of the apparatus over time is
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generally required. High-resolution systems with superconducting field
coils are therefore available which operate as a short circuit. Corres-
ponding systems of nuclear spin tomography mustJ for instance, have a
constancy of the field in time of ~ B/B clo 7, where B is the magnetic
field induction. The total resistance in the circuit of the magnet wind-
ing, which is composed of the resistance contribution of the conductor
connections within the winding and the contact resistance of the persist-
ent current switch must therefore be smaller than 2 X 10-9 ohm for an
inductance of 74 H. Since the individual magnet windings of such a system
are composed, for instance, of 17 individual conductor sections, a contact
resistance of approximately 10 11 ohm is desired for a single connection
between the end sections of two superconductors.
Connecting devices with contact resistances of this small
magnitude can generally be produced by superconducting contacts. The con-
ductor wires of multifilament conductors are, for instance, joined directly
together without the interposition of normal-conducting materials.
SUCh 1 joining technique is indicated, for :instance in German
OS 1 939 224. According to this technique, the ends of these conductor wires
are stripped of the stabilizing material at the end sections of two super-
conductors which are to be joined together. As explained, the superconduc-
tors generally contain several superconducting conductors embedded in normal-
conducting stabilizing material. Subsequently, the conductors are directly
placed together and finally joined together with low resistance, for instance7
by welding or soldering. So as not to interrupt the stabilization in the
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1 ~ vicinity of the connecting device provided in thi3 matter~ a
2 ¦ special bridge of the stabilizing material is further provided
3 between the stabilizing material.
4 In order to obtain extremely low contact resistances
1l of the order of magnitude mentioned above~ the superconducting
6 1I conductor filaments can be brought into contact directly by
7 mechanical pressure. Such a method i5 described, for
8 1l in3tance, in U.S. Patent 3,422,529. According to thi3 known
g Imethod, two superconducting conductor filaments are twisted
o lltogether~ then taken to an appropriate pressing mold and
11 finally joined together by cold deformation.
12 ,~ Since, as is well known, superconducting alloy~ such
13 as NbTi or Nb~r have a strong affinity to oxygen, their
¦ surfaces are always coated with an oxide layer which, in
)l addition, is normal-conducting. So that a low-resistance
16 ~Isuperconducting contact can be made with these superconducting
17 ~I materials, the interfering oxide layer mu~t therefore be
18 1l removed at the contact point. In the above-mentioned method,
19 1l a special cleaning treatment of the superconducting oonductor
20 ¦I filament~ and the mold by means of a chemical cleaning
21 1 solution is provided before the individual parts of the
22 ¦I connecting devlce are connected together. Due to the pressure
23 ' action, direct contact or welding at individual contact points
24 ~1 is thus made possible. Nevertheless, it has been found that
I such connecting devices have only a relatively small current-
2~ I carrying capacity and, in addition, exhibit a distinct
27 li decrease of the critical current with the magnetic field
28 1l applied, vis-a-vis the superconducting original conducting
29 1I material.
30 ¦I SUMMARY OF THE INYENTION
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It is an object of the present invention to provide a
method and apparatus for connecting the end sections of two
superconductors in such a manner that their contact resistance
is extremely lowO
It is a further object to provide such a connecting
apparatus and method which meets the mentioned requirements for
magnet windings of nuclear spin (nuclear magnetic resonance)
tomography systems.
The above and other objects of the present invention
are achieved in a superconducting connecting device and method
wherein the superconducting conductor filaments are joined
together by means of ultrasonic metal welding.
Thus, in accordance with a broad aspect of the
invention, there is provided a connecting device for joining end
pieces of two superconductors which are to be cooled down to
cryogenic operating temperature and which each contain
superconductive conductor filaments embedded in a matrix of
normaLly conducting tabilising material, where the enas of the
~a superconducting conductor filaments which are to be connected
are stripped of the stabilising material, brought into direct
contact with one another, and welded to one another, and where
an electrically highly conductive bridge of normally conductive
material is provided bewteen the stabilising material of the
superconductors, characterised in that the ends of the
superconducting conductor filaments of the two superconductors
to be connected and which have been etched out of the matrix
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material are inserted into one another, placed intermingled in a
welding mould, and by means of ultrasonic metal welding are
connected to one another and at least in part also to the
welding mould.
In accordance with another broad aspect of the
invention there is provided a connecting device between the end
pieces of two superconductors which are to be cooled down to
cryogenic operating temperature and which each contain
superconductive conductor filaments embedded in a matrix of
normally conductive stabilising material, where the
superconductive conductor filaments which are to be connected
are partially stripped of the stabilising material, brought into
direct contact with one another, and welded to one another, and
where an electrically highly conductive bridge of normally
conducting material is provided between the stabilising material
of the superconductors, characterised in that the
superconductors have a layer made of a plurality of
superconductive conductor filaments twisted together; that at
the ends of the superconductors on one side in each case, a
window-like zone of the superconductive conductor filaments is
exposed by etching away the stabilising material, the extension
of these window-like zones in the longitudinal direction of the
conductors being a multiple of the twist length of the
individual superconductive conductor filaments and that the
superconductive conductor filaments are connected to one another
within the window-like zones by ultrasonic metal welding.
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Ultrasonic metal welding is generally known per se.
See, for instance, "Der Elektriker", 1978, nos. 7/8, pages 190
and 191, or "Werkstatt and Betrieb", vol. 114, 1981, no. 7,
pages 441 to 443 or "DVS-Berichte", vol. 70, 1981, pages 128 to
135. This welding technique is also used for joining
superconducting and normal-conducting material in the production
of stabilized superconductors. See e.g., German
Patent 16 65 790. It has now been discovered that with this
weldlng technique, joints can also be made advantageously
between two superconducting conductor wires in a relatively
simple manner and with extremely low resistance. By using
ultrasonic metal welding, the oxide coatings on the individual
superconductin~ conductor wires can be broken up and removed as
planned, so that an intimate weld with a large area can be
achieved at the mutual contact place. Special pretreatments of
the parts to be welded together can therefore be dispensed with.
Other objects, features and advantages oE the present
invention will be apparent from a reading of the
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1 ¦ detailed description which follows.
2 1I BRIEF DESCRIPTION OF THE DRAWINGS
3 1I The invention will be explained in detail in the
4 1~ following detailed description, with reference to the drawings
~l in which:
6 ¦~ FIG. 1 shows one embodiment of a joining device
7 1 according to the present invention in perspective view;
8 1l FIG 2 is a cross sectional view of the joining
9 ~Idevice shown in FIGS. 1;
i FIGS. 3 and 4 show, respectively, a top and side
11 I view of a superconducting end section of a further connecting
12 l~device according to the present invention which can be made
with the machine indicated in FIG. 5; and
FIG. 5 is a side view of the machine for making the
I connecting device shown in FIGS. 3 and 4.
16 I DETAILED DESCRIPTION
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17 With reference now to the drawing~, FIG. 1 shows
18 I schematically the essential parts of a connecting device
19 according to the invention. Thi3 connecting device is to be
I developed between the end section~ 2 and 3 of two
21 I multifilament superconductor3 of a superconducting magnet
22 I winding, such as must be provided particularly for a System
23 ¦I for nuclear spin tomography. Each superconductor contains a
24 1I multiplicity of superconducting conductor filaments 4 which
1', are embedded in a matrix material 5 of normal-conducting
26 1I stabilizing material ~uch as copper or aluminum. In the
27 ¦' vicinity of the ~oint to be made, the matrix material 5 is
28 1; etched off the superconducting conductor filament~ 4 and the
29 11 ends 4' of the conductor filament~ exposed in this manner are
I pushed or twi~ted together to form a bundle. In this bundle,
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1 !I conductor filament ends 4' are arranged mixed advantageously
2 ~ as well as possible, 50 that conductor filaments of the one
3 I superconductor end section 2 and of the other end section 3
4 1 meet in an alternating fashion, as well as possible. The
1I conductor filament ends 4' are placed in a metal profile piece
6 'l~ 6 of U-shaped cross section which serves as the welding form
7 1l and is covered with a mating metal plate. The U-shaped piece
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8 1 and the cover 7 may comprise an electrically and thermally
g ll highly conducting material such as copper. Also harder
I materials such as CuSn, ~u~e, and NbTi are particularly well
11 , suited because these materials can couple the ultrasonic
12 1 energy efficiently into the filament zone of the arrangement.
13 1 In addition, also material pairings such as Cu U-profiles with
14 1l inserted NbTi bottom and cover can also be used.
l According to the invention, the superconducting
16 1I conductor filament ends 4~ which are in~erted into the welding
17 ll form and are covered by the cover 7 are joined together by
18 I means of ultrasonic metal weldine. The ultrasonic energy is
19 I coupled-in, for ln3tance, via an appropriately profiled
20 ¦¦ ~onotrode which generates longitudinal vibrations. The
21 ¦1 sonotrode is placed on the cover 7 with a predetermined
22 pressure. Parameters in ultrasonic metal welding are the
23 ll welding time and the amplitude of the energy, besides the
24 !! contact pressure~ Typical welding times are between 0.7 to 3
25 ¦l ~ec ~or amplitudes of about 20 um, while the contact
2S 1¦ pres~ure is variable between 70 and 120 No 20 kHz, for
27 ~ instance, can be chosen as the ultrasonic frequency. However,
2~ I it is also possible to work with frequencies in the sound
29 ~I range in addition to frequencies in the ultrasonic range.
3~ ¦ If welding forms 6 and covers 7 are made of copper,
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1 l¦ thls material can be coupled by ~oldering to additional
2 I stabilizing ~aterial which, after the ultrasonic metal
3 l welding, subsequently takes up the entire joint. For this
4 purpose, a copper tube or sleeve 8, for instance, can be
provided. As is indicated in the figure by dashed lines, this
6 l sleeve is closed in the circumferential direction. It is
7 Islipped over the welded joint and is soldered to it with a
8 l, normal-conducting s~lder, for instance, PbSn or AgSn solder,
9 ' or in particular with a superconducting solder such as PbBi.
j' This measure can lead to an increase of the current-carrying
11 capacity by a factor of about 1.5.
12 I FIG. 2 shows a cross section of the joint made
accordingly in the region of the welding die 6. Parts
coinciding with FIG. 1 are provided here with the same
I referenoe symbols. In particular, the superconducting soft
16 I solder 9 dispersed within the stabilizing sleeve 8, is
17 Il illustrated, which surrounds the conductor wire end~ 4' in the
18 ll welding form 6 welded to each other and optionally also to the
19 1 weldin~ form.
1 A~ ls further indlcated in FIG. 1, the two end
21 1l sections 2 and 3 of the superconductors to be joined are
~2 I placed against each other with their flat sides adjacent and
23 il are soldered together over a large area. This purpose is
24 ll served by a normal-conducting soft solder 11, which provides
25 ll an electrically highly conducting bridge between the
26 !I stabilizing material 5 of the two superconductors in the
27 1 region of the connecting device.
28 , For the further stabilization of the welded joint,
29 1 additional material can also be inaorporated into the U-shaped
30 ll welding form 6 prior to the welding operation. For instance,
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1 I copper wires or chips can be placed into the inside region of
2 , the filament zone. An internally stabilized conductor
3 ~ arrangement thereby is obtained after the welding. In a
4 ~ similar manner, inserting additional superconducting filaments
ll or chips of suitable dimensions leads to an increased packing
6 l density and an increased number of contact points between the
7 superconductlng conductor filament ends. In particular, voids
8 within the junction can thereby be eliminated.
9 According to the embodiment shown in FIGS. 1 and 2,
it was assumed that the end sections of superconducting
11 multifilament conductors are to be joined together. The
12 ll connecting device according to the invention is suited equally
13 I well, however, for joining ~tabilized superconductors with
14 only a ~ingle conductor wire. Also conductors with very thin
15 1I superconductor filaments and single-layer filament zones can
16 l~ be joined together in accordance with the invention. An
17 1 embodiment of the end section of a conductor of the last-
18 ¦¦ mentioned type i~ schematically illustrated in FIG. 3 in a top
19 I view and in FIG. 4 in a side view. In each conductor end
~ection 13, a window 14 is etched on one side out of the
21 enclosing stabilizing matrix 15 so that a corresponding zone
22 ~ 16 of the conductor wire is exposed on one side. The axial
23 ll extent of the window 14 is advantageously provided o~ a length
24 1I which is a multiple of the twist lay of the individual
25 ll conductor wire, whereby all exposed conductor wires
26 ¦I participate uniformly in the current transfer. The welding
27 ll can be accomplished advantageously in a rolled-seam welding
2~ 1¦ machine such as is indicated schematically, for instance, in
29 ~, FIG. 5 in a side view onto the end face of the welded end
30 ll sections. Parts identical with FIGS. 3 and 4 are indicated by
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1 ,the ~ame reference qymbols.
2 ll The machine indicated in FIG. 5 contains an upper
3 1Iguide roller 17 which is coupled to an ultrasonic welding
4 ldevice, as well as a corresponding lower counter roller 18.
The regions to be welded of the sections 13 and 13' of the
6 ll Superconductors are conducted between the two rollers, whereby
7 !la predetermined contact pressure on the exposed zones of the
8 ' superconducting conductor filaments devoid of the stabilizing
9 llmatrix material 15 and 15'1 respectively, is provided. These
I zones, welded together, are designated by reference numeral
11 19 in FIG. 5. In order to guide the end sections, guiding
12 ' slots 20 and 21 matching the dimensions of the respective end
13 sections are provided in the rollers for guiding the end
14 ll sections in the circumferential direction. The welded joint
I~made in this manner can then be provided further from the
16 1, outside with additional stabilizing material, for instance, in
17 ¦¦ accordance with the embodiment described in connection with
18 1~ FIGS. 1 and 2.
19 11 In the foregoing specification, the invention has
20 ¦I been de~cribed with reference to specific exemplary
21 ll embodiments thereoE. It will, however, be evident that
22 ll various modifications and changes may be made thereunto
23 without departing from the broader spirit and scope of the
24 ~ invention as set forth in the appended claims. The
I specification and drawings are, accordingly, to be regarded in
26 1 an illustrative rather than a re~trietive ~ense.
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