High di/dt Superconductive Joint Technical Field

The invention provides a superconductive joint for use in a superconductive switch, a method of producing a superconduc ^¬ tive joint and a method of producing a superconductive mag- net.

Technical Background Superconductive magnets are used in a variety of applica ^¬ tions, for example as magnetic field generators in Magnetic Resonance Imaging (MRI) equipment. Coils of superconductive wire are held at cryogenic temperatures, typically at about 4 Kelvin, the boiling temperature of helium.

Since MRI devices are commonly run in persistent mode, there is no power supply attached to the superconductive magnet. When the magnet is commissioned, the installation engineer energises the magnet by means of an external power supply. After reaching the desired operating current, a superconduc ^¬ tive switch connected in parallel to the magnet coils of the superconductive magnet is closed and the current circulates within the magnet. The superconductive switch has a resis ^¬ tance of several ohms in normal state (also referred to as "quenched state") . When the magnet is energised, the super ^¬ conductive switch is heated thus causing its resistance to appear. The current from the external power supply goes straight to the magnet coils which are in a superconducting state and thus show no resistance. After reaching the desired operating current, the heater of the switch is turned off and the switch returns to the superconducting state. Given the internal resistance of the power supply, the current prefers to flow through the superconductive switch. The external power supply can now be disconnected as the current is trapped in the superconductive magnet. Superconductive switches are made using several tens of me ^¬ tres of superconductive filaments embedded in a resistive ma ^¬ trix which provides the desired resistance when the supercon ^¬ ductive switch is in the normally conducting state. The wire used for the coils of a superconductive magnet, on the other hand, uses superconductive filaments embedded in a copper ma ^¬ trix .

The resistive matrix renders the wire extremely unstable and prone to accidental quenching due to the low minimum quench energy. One way around these instability problems is to al ^¬ ways use a plurality of superconductive switches which are thermally isolated from one another and wired in parallel so that they can share current and provide protection should one switch quench. However, when a switch quenches, the current previously conducted by this switch must be taken up by the remaining switches causing a change in current in them

("di/dt") . This di/dt can cause local heating by self- induction proportional to the amount of change of current. The heat caused by the self-induction can lead to another quenching switch and thus to another change in current that is of even greater magnitude since the same amount of current has been carried by an already decimated number of switches.

Obviously the di/dt caused by a quenching switch can be re- duced by connecting a large number of superconductive

switches in parallel because the share of the total current that is carried by one individual switch drops as the number of switches increases. However, this increases manufacturing cost and circuit complexity.

Accordingly it is an object of the invention to provide an enhanced superconductive joint for use in a superconductive switch, a method of producing such a superconductive joint and a method of producing an enhanced superconductive magnet.

Summary of the Invention

For these reasons the invention provides a superconductive joint for use in a superconductive switch. The superconduc ^¬ tive joint comprises superconductive filaments extending along an entire length of the superconductive joint and a ma ^¬ trix surrounding the superconductive filaments. According to the invention the matrix comprises a resistive matrix in a middle section of the superconductive joint and a copper ma ^¬ trix in a remainder of the superconductive joint. For exam- pie, the remainder may comprise both ends of the joint with the middle section extending between the ends.

If the superconductive joint is used for producing a super ^¬ conductive switch, the superconductive switch may further comprise a heater arrangement adapted to heat the supercon ^¬ ductive joint in response to a quench signal. The heat trans ^¬ ferred from the heater arrangement to the superconductive filaments causes the superconductive switch or a part thereof to leave the superconducting state which represents the

Λορθη' (barring) state of the superconductive switch. How ^¬ ever, the superconductive joint of the invention may also be used for other applications such as superconductive current limiters which may also require high di/dt capability. Such superconductive current limiters do not comprise heaters as they self-quench when the current through the superconductive current limiter exceeds a threshold specified for the super ^¬ conductive current limiter.

The superconductive joint of the invention can withstand high di/dt' s with a largely reduced risk of accidental quenching thereby allowing for reducing the number of superconductive joints or switches connected in parallel in any superconduc ^¬ tive magnet application such as MRI .

The invention is based on the understanding and encloses this understanding that whilst the wire of superconductive

switches is inherently capable of tolerating high di/dt, the superconductive joints used to connect multiple switches to ^¬ gether and to the superconducting magnet arrangement are fun ^¬ damentally limited.

As shown in Figure 1, superconductive joints used to join su ^¬ perconductive wires together consist of two parts, a super ^¬ conductive filamentary joint 6 where superconductive fila ^¬ ments 5 intertwine and a long length (usually several metres) of the respective matrix wires soldered together which is re ^¬ ferred to as a shunt 7. When wires with a copper matrix 8 (magnet wire) are soldered together, the resultant shunt 7 forms a very low resistance parallel path for the current 9 to flow in. This low resistance enables the joint to carry very high rates of change of current in the order of millions of Ampere per second without the filamentary joint quenching. The filamentary joint cannot take high di/dt' s since the self-inductance of the filaments causes sufficient local heating to quench the joint. It is consequently the length of the soldered shunt that protects the filamentary joint from high di/dt.

When this jointing technique is used to join wires where one or more of them are made with a resistive matrix, the di/dt rating of the joint is drastically reduced and in fact lim ^¬ ited. This is due to the heating caused by changing currents flowing in the resistive matrix. If the shunt is too short, the filamentary joint quenches due to the high di/dt. If the shunt is very long, substantial di/dt heating occurs in the first few tens of centimetres since the self-inductance of the shunt concentrates the di/dt here and the shunt itself quenches. For this reason conventional superconductive joints do not tolerate high di/dt. The invention overcomes this ma ^¬ jor boundary to switch performance by providing a superconductive joint that allows for superconductive switches that can withstand high di/dt' s.

The invention solves the problem by replacing the resistive matrix of the switch wire with copper at the ends of the su ^¬ perconductive switch. In this way a resistive matrix in the middle section of the superconductive switch and a copper ma- trix at the ends of the superconductive switch are combined without any intervening joint. These ends can now be soldered or electroplated together to form a purely copper matrix shunt. A standard filamentary joint is produced thus creating a joint that has the same characteristics as a standard su- perconductive magnet wire joint and thus can withstand high di/dt' s .

The superconductive filaments of the inventive superconduc ^¬ tive switch may comprise or consist of Niobium-titanium

(NbTi) and/or Niobium-tin (Nb ₃ Sn) . NbTi and Nb ₃ Sn are relatively inexpensive materials with good superconductive prop ^¬ erties that can be easily worked into filaments.

The resistive matrix may comprise or consist of Copper- Nickel. CuNi is a suitable material for the resistive matrix due to its superior ductility. However, other materials such as Al, NiCr and stainless steels could theoretically be used.

A second aspect of the invention provides a method of produc- ing a superconductive joint. The method includes:

- providing superconductive filaments;

- embedding a middle section of the superconductive filaments in a resistive matrix; and

- embedding a remainder of the superconductive filaments in a copper matrix. Preferably the step of embedding the middle section of the superconductive filaments in the resistive matrix includes embedding the remainder of the superconductive filaments in the resistive matrix and subsequently removing the resistive matrix in the remainder of the superconductive filaments be ^¬ fore embedding the remainder of the superconductive filaments in the copper matrix. In this way an inventive superconduc ^¬ tive joint may be produced easily from standard switch wire. The resistive matrix may be removed from the remainder of the superconductive filaments by etching. For example, nitric acid may be used for removing the resistive matrix. Alterna ^¬ tively the resistive matrix may be removed from the remainder of the superconductive filaments by dissolving the resistive matrix. For example, the resistive matrix may be dissolved using molten tin.

A third aspect of the invention provides a method of produc ^¬ ing a superconductive magnet. The method includes steps of: - providing at least one superconductive switch according to the first inventive aspect;

- providing a superconductive magnet coil arrangement;

- soldering a first end of the remainder of the at least one superconductive switch to a first end section of a magnet wire of the superconductive magnet coil arrangement thereby forming a first shunt;

- soldering a second end of the remainder of the at least one superconductive switch to a second end section of the magnet wire of the superconductive magnet coil arrangement thereby forming a second shunt; and

- forming first and second filamentary joints from the superconductive filaments of the at least one superconductive switch and respective superconductive filaments of the magnet wire of the superconductive magnet coil arrangement at outer ends of the first and second shunts, respectively. The superconductive magnet produced in this way shows a bet ^¬ ter robustness against accidental quenching of one or more superconductive switches connected in parallel.

Brief Description of the Drawings

The invention will be better understood from the following drawings in which preferred embodiments of the invention will be illustrated by way of example. Throughout the drawings the same reference numerals refer to the same or similar items. In the drawings:

Figure 1 shows a conventional joint between two superconduc- tive wires having a copper matrix;

Figure 2 shows an embodiment of a superconductive magnet ac ^¬ cording to the invention; Figure 3 shows an embodiment of a superconductive switch ac ^¬ cording to the invention;

Figure 4 shows a conventional joint between two superconduc ^¬ tive wires having a resistive matrix and a copper matrix, re- spectively;

Figure 5 shows an embodiment of a joint according to the in ^¬ vention; and

Figures 6 through 9 illustrate an embodiment of a method producing a superconductive switch.

Detailed Description of the Drawings

Figure 2 shows an embodiment of a superconductive magnet 1 according to the invention. The superconductive magnet 1 in- eludes one or more superconductive coil (s) 4 that are con ^¬ nected between two terminals 3. As already explained above, upon commission of the superconductive magnet 1 an external power supply (not shown) may be connected to the terminals 3. The superconductive magnet 1 further comprises at least one superconductive switch 2 that may be used for connecting the superconductive coil (s) 4 in a circular fashion.

Figure 3 shows an embodiment of a superconductive switch 2 according to the invention. The superconductive switch 2 comprises superconductive filaments 5 embedded in a matrix. In a middle section 11 of the superconductive switch 2 the matrix is a resistive matrix 10 which may show the desired resis ^¬ tance when the switch 2 is quenched on purpose. The middle section 11 may have a length of several tens of metres. The remainder of the superconductive switch 2 has a copper matrix 8. In the example of Figure 2 the remainder comprises end sections 12 arranged at the ends of the superconductive switch. The end sections 12 may have a length of several me- tres. The advantages entailed by the superconductive switch 2 of the invention will be explained in more detail referring to Figures 4 and 5.

Figure 4 shows a conventional joint between two superconduc- tive wires having a resistive matrix 10 and a copper matrix 8, respectively. The wire having the resistive matrix 10 be ^¬ longs to a superconductive switch which needs to be jointed to a superconductive coil having a copper matrix in order to produce a superconductive magnet. The inventor found that no matter how long the shunt 7, changing currents 9 must always pass through the resistive matrix 10 which develops much more heat than the copper matrix 8 used for both sides of a standard joint as shown in Figure 1. This heat may cause the con ^¬ ventional joint to quench which is why the conventional joint between a magnet wire having a copper matrix 8 and a switch wire having a resistive matrix 10 has a fundamentally limited di/dt rating. Figure 5 shows an embodiment of a joint according to the in ^¬ vention. For the superconductive switch a part of the resis ^¬ tive matrix 10 has been replaced by a copper matrix 8. This may be done only in a part of the shunt 7, throughout the en ^¬ tire length of the shunt 7, or for the entire length of the shunt 7 and a part of the remaining switch wire. In order to achieve this, the resistive matrix 10 may be removed, e.g. by etching or dissolving, and the exposed filaments 5 may be electroplated with copper to form a copper matrix 8. This now enables a joint construction including the usual filamentary joint 6 and the shunt 7. In this enhanced joint current 9 can now flow in the copper matrix 8. This results in a joint construction having an inherently high di/dt rating allowing for a much reduced risk of accidental quenches.

Figures 6 through 9 illustrate an embodiment of a method of producing a superconductive switch as shown in Figure 3.

Firstly, as shown in Figure 6, superconductive filaments 5 are produced, e.g. from Niobium-titanium (NbTi) . Next, the filaments 5 are embedded in a resistive matrix 10 thereby forming switch wire (Figure 7) . In a subsequent step the re ^¬ sistive matrix 10 is removed from end sections 12 leaving the resistive matrix 10 only in a middle section 11. The resis- tive matrix 10 may be removed by etching (e.g. in nitric acid) or dissolving (e.g. in molten tin) as explained above (Figure 8) . Finally, the end sections 12 are embedded in a copper matrix 8 which may be done by electroplating. The resultant superconductive switch 2 may be jointed together with superconductive coils 4 at both ends in order to form a su ^¬ perconductive magnet 1. Furthermore, a plurality of such su ^¬ perconductive switches 2 may be connected in parallel for the reasons explained in the introduction. However, the number of superconductive switches 2 required to achieve a specific di/dt rating may be greatly reduced. Although the invention has been shown and described with respect to exemplary embodiments thereof, various other chang ^¬ es, omissions, and additions in form and detail thereof may be made therein without departing from the spirit and scope of the invention.

While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifi ^¬ cations, and equivalents as may be included within the scope of the invention as defined by the appended claims.