DE102015216882A1 - Coil device with continuous current switch - Google Patents

Abstract

It is a coil device with at least one electric coil winding with superconducting conductor material specified, wherein the coil winding is part of a self-contained circuit for forming a continuous current. The closed circuit has a persistent current switch with a switchable conductor section which is switchable between a superconducting state and a normal conducting state. The switchable conductor section has a high-temperature superconducting conductor and has a resistance of at least 300 ohms in the normal conducting state of the conductor. The switchable conductor section is a Abklingwiderstand of at least 50 ohms and / or a load connected in parallel with an effective resistance of at least 50 ohms. Furthermore, a continuous current switch for such a coil device and a method for loading or unloading a continuous current is specified.

Description

The present invention relates to a coil device having at least one electrical coil winding with superconducting conductor material, wherein the coil winding is part of a self-contained circuit for forming a continuous current, and wherein the closed circuit has a continuous current switch with a switchable conductor section which between a superconducting state and a normal conducting state is switchable. Furthermore, the invention relates to a continuous current switch for such a coil device and a method for loading or unloading a continuous current in a coil winding of such a coil device.

Various applications are known in the prior art, in which superconducting coils are used to maintain a continuous current in a closed superconducting circuit almost lossless. This can be used, for example, to generate magnetic fields and / or to store energy. In magnetic resonance imaging and magnetic resonance spectroscopy, such superconducting magnet coils are used to generate the strong and temporally constant static magnetic fields required here. In rotating electrical machines, shorted superconducting coils can be used to provide a permanent field magnetic field. Such a short-circuited coil can be arranged in particular on the rotor of such a machine. Another application of shorted superconducting magnetic coils is in the field of superconducting magnetic energy storage (SMES), in which the loading and unloading of such a short-circuited coil is used to temporarily store electromagnetic energy in the magnetic field generated by the coil and then back in an external circuit To make available. The described applications have in common that typically a direct current flows permanently through a short-circuited coil and thereby generates a stationary magnetic field. The present invention addresses the specific problems of loading and unloading the current flowing in such coils.

In order to charge such a coil with a current, a superconducting permanent current switch is typically integrated in the closed circuit. Such a continuous current switch has a superconducting conductor section, which can be put into an ohmic conducting state, for example, by heating. If the coil is simultaneously connected to a power source in an external circuit, it can be charged with an open permanent-current switch.

Discharging the current flowing in the closed circuit through the coil winding current can be triggered intentionally or unintentionally. Intentional discharge may be triggered by opening the persistent current switch, and the coil may be connected to an external circuit in which the current previously flowing through the closed circuit may at least partially discharge. This can be used, for example, to retrieve the energy stored in an SMES. On the other hand, if the coil is not connected to an external circuit during discharge, the electromagnetic energy stored in the coil is completely dissipated as heat in the area of the continuous current switch. This heat could destroy the switch in large magnet systems. Such an opening of the switch without an external circuit is therefore to be avoided there.

Discharging the coil may also be triggered in other ways than by controlled opening of the persistent current switch: Thus, a local temperature increase in the coil winding may cause the superconducting properties of the entire coil winding to collapse starting from this region. As a result of this heat development, the superconducting properties of the entire coil winding can collapse, which is referred to as "quenching". Such a quench can spread rapidly due to the thermal coupling of the different regions. With an arrangement of the superconducting coil winding in a liquid helium-cooled cryostat, such a quench typically vaporizes all of the helium, resulting in relatively high costs and failure in the useful life of the system. To avoid damage to the superconducting conductor elements by local overheating of the conductors, the rapid propagation of such a quench may even be promoted. Local overheating can be detected, and other areas of the coil winding can then also deliberately be quenched, so that the current flow collapses as quickly as possible and no damage to the first locally heated conductor area occurs. This procedure is commonly used with magnetic resonance coils to protect the conductors from permanent overheating.

It may also be expedient to trigger such a quench via an emergency stop switch in order to enable a rapid shutdown of the magnetic field generated by the coil in the event of a fault of the coil device. About such an emergency stop, for example, one of the coils can be intentionally heated to a subsequent in the episode Collapse of the superconductivity in the entire coil winding to trigger.

A disadvantage of the described prior art is that in the discharge and quenching processes, a high part of the energy stored in the coil winding is dissipated as heat in the region of the coil winding. This can lead to unnecessary loss of expensive coolant, loss of energy and / or thermal stress on the sensitive superconducting conductor materials. Particularly in the case of magnetic resonance coils, the loss of the liquid helium in such quenching processes is a high cost factor. In the case of SMES storage coils, the loss of stored energy is particularly disadvantageous if some of it is lost as a result of quenching or controlled discharge as heat in the coil winding. Another disadvantage of the known persistent current switch is that the switching time for the opening of the switch is relatively slow and thus discharging the energy into an external circuit can be done only slowly. This, too, can lead to higher losses and higher heat development in the coil winding than would be the case with a faster discharge.

Object of the present invention is therefore to provide a coil device which avoids the disadvantages mentioned. In particular, a continuous current switch is to be specified with which a reduction in the dissipation of heat in the region of the electrical coil winding can be achieved. Another object is to provide such a continuous current switch for such a coil device and a method for loading or unloading a continuous current.

These objects are achieved by the coil device described in claim 1, the persistent current switch described in claim 14 and the method described in claim 15.

The coil device according to the invention comprises at least one electrical coil winding with superconducting conductor material, wherein the coil winding is part of a self-contained circuit for forming a continuous current. The closed circuit has a persistent current switch with a switchable conductor section which is switchable between a superconducting state and a normal conducting state. The switchable conductor section has a high-temperature superconducting conductor and has a resistance of at least 300 ohms in the normal conducting state of the conductor. The switchable conductor section is a Abklingwiderstand of at least 50 ohms and / or a load connected in parallel with an effective resistance of at least 50 ohms.

Under a high-temperature superconductor or high-T _c superconductor (HTS) should be understood here in general a superconducting material having a critical temperature above 25 K. In some classes of materials, such as the cuprate superconductors, the critical temperature of these superconductors is even above 77 K, so that the operating temperature can be achieved by cooling with other cryogenic materials than liquid helium. An advantage of high-temperature superconductors over many metallic low-temperature superconductors for use in said persistent current switch is that they often have a relatively high resistance in the normally conductive state and thus that the continuous current switch in the open state can also be made relatively impact resistant.

In the coil device according to the invention, the continuous-current switch has a decay resistance of at least 50 ohms or a corresponding load connected in parallel with it. An essential advantage of this parallel current path is that, when the permanent current switch is opened, a large part of the electrical energy is diverted into this parallel current path and thus this part is not dissipated as heat in the coil winding or in the permanent current switch. The Abklingwiderstand or the effective resistance of the load is large enough with at least 50 ohms to avoid flowing out of the continuous current in this parallel-connected current path in the closed state of the persistent current switch, because then the current in the almost resistance-free closed superconducting current path of the coil winding and the persistent current switch flows.

The resistance of the constant current switch of at least 300 ohms in the normally conducting, ie open, state causes the current flows in the open state of the persistent current switch to a greater extent on the Abklingwiderstand or the consumer and only to a lesser extent on the normal conducting conductor portion of the persistent current switch.

This advantageously achieves that when discharging the coil winding, a smaller part of the energy previously stored in the coil winding is dissipated as heat in the coil winding and a larger part is either dissipated via the Abklingwiderstand or implemented in the consumer. Thus, an energy loss within the closed circuit of the coil winding and the persistent current switch is advantageously reduced. Very particularly advantageous, the dissipation of heat energy in the coil winding can be reduced so far that the superconducting properties can be maintained in the coil winding, so the coil winding does not quench. at a cooling of the superconducting components with helium is then simultaneously avoided or at least reduced a loss of liquid helium, since the coolant surrounding the superconductor is not completely evaporated. In this way, the duration can be advantageously reduced to a possible re-loading of the coil winding.

The persistent current switch according to the invention is designed for a coil device according to the invention. It has a switchable conductor section, wherein the latter comprises a high-temperature superconducting conductor and has a resistance of at least 300 ohms in the normal-conducting state of the conductor. The advantages of the continuous current switch according to the invention result in interaction with the other elements of the coil device and correspond to the advantages described above in connection with the coil device.

The inventive method is used for loading or unloading a continuous current in a coil device according to the invention. In this case, the switchable conductor portion of the persistent current switch is switched from a superconducting state to a normal conducting state. The advantages of this method are analogous to the advantages of the coil device described above.

Advantageous embodiments and further developments of the invention will become apparent from the dependent claims of claim 1 and the following description. In this case, the described embodiments of the coil device, the continuous current switch and the method can be generally combined with each other in an advantageous manner.

The coil means may comprise a cryostat for cooling the superconducting conductor material, wherein the superconducting coil winding and the switchable conductor portion are arranged inside the cryostat. The Abklingwiderstand and / or the consumer is then advantageously arranged outside the cryostat. With the help of the cryostat, the superconducting conductor materials of the coil winding and the switchable conductor section can be cooled to a temperature below the transition temperature of the respective superconductor, so that a continuous superconducting closed circuit can be obtained. The cryostat can be, for example, an open bath cryostat from which the coolant used can optionally evaporate. Alternatively, it may also be a cryostat with a closed coolant circuit in which a coolant is either liquefied or can change between a liquid and a gaseous (or even supercritical) state. The arrangement of the Abklingwiderstands or consumer outside of the cryostat advantageously causes that at a discharge of the coil winding in the Abklingwiderstand or the consumer transmitted energy does not contribute to a heating of the interior of the cryostat. A maintenance of the cooling and the superconducting operating state for the components arranged within the cryostat is thereby also made possible during the unloading of the coil.

The switchable conductor section can advantageously have a resistance in the normal conducting state of the conductor, which resistance is greater than the decay resistance and / or the consumer. The main advantage of this embodiment is that after opening the persistent current switch, a larger part of the energy previously stored in the coil winding is dissipated via the decay resistance and / or the load, and only a comparatively smaller part is dissipated in the area of the persistent current switch. Even a majority of the energy in the area of decay resistance and / or consumers is particularly advantageously dissipated. For this purpose, in particular the resistance of the switchable conductor section in its normally conducting state can be greater than a total resistance of the parallel current path having the decay resistance and / or the load. The resistance of the switchable conductor section may, for example, be higher by at least a factor of 5, in particular by at least a factor of 20, than the total resistance of the parallel current path in the normally conducting state.

The switchable conductor section may have a resistance of at least 1000 ohms, in particular at least 5000 ohms, in the normal conducting state of the conductor. Such high resistances are particularly advantageous for implementing during discharging a particularly high, in part a majority, of the energy previously stored in the coil winding in the region of the decay resistance and / or of the consumer and only a small dissipation of energy in the region of the coil winding and / or the coil Permanent contactor to cause. The higher resistance values of at least 5000 ohms are particularly advantageous in connection with the application in SMES systems, since there only a particularly small proportion is to be dissipated in the region of the coil winding. The essential part of the energy stored in an SMES system should rather be able to reach a load connected in parallel to the constant-current switch when discharging. But even for other systems, a particularly high resistance of the open circuit breaker is generally advantageous: With a resistance of at least 1 Mohm in the open state, for example, when applied during the decay time, a voltage of about 20 kV, a leakage current of only about 0.02 A. come about. This will be during of the decay in persistent current switch a short-term power of only 400 W dissipated, which can withstand the continuous current switch during the duration of the switching advantageous.

Thus, the switchable conductor section in the normal conducting state of the conductor can generally advantageously have a dielectric strength for voltages of at least 20 kV. This makes it possible to ensure that there is no voltage flashover even during the switching, that is to say in particular during the transition from the superconducting to the normal conducting state of the switchable conductor section, and that the short-circuited continuous current can be switched off in a controlled manner. A dielectric strength of at least 20 kV is particularly useful for magnetic coils for magnetic resonance imaging, as can be caused by the very high magnetic fields and very high continuous currents when switching very high local voltages on the persistent current switch. To avoid burnout of the switch, for example, the resistance of the switchable conductor portion in the normal conducting state and its thermal coupling to the other components can be chosen so that the remaining leakage current, the continuous current switch at the maximum voltage occurring at a temperature in the range below 320 K. heated. The distance of the switchable conductor section should generally be long enough to achieve the required resistance in the normal conducting state. On the other hand, the cross section of the conductor section should be large enough to reach the current carrying capacity required for the respective continuous current.

The switchable conductor section may comprise a ceramic high-temperature superconductor. With a ceramic material can be achieved particularly well a high dielectric strength. In particular, in comparison to metallic low-temperature superconductors with such a ceramic material also a high resistance in the open state of the switch can be easily achieved. In order to achieve a high resistance and / or a high dielectric strength of the entire switchable conductor section in the normally conducting state, it is particularly advantageous if this conductor section does not have any continuous metallically conductive elements. In embodiments in which the ceramic superconducting material is deposited as a layer on a support, it is therefore advantageous if this support is not metallic, but is formed from poorly conductive or non-conductive materials such as glass, ceramic and / or composite materials. Alternatively, however, the conductor section can also have a ceramic superconducting conductor element present in bulk, that is to say in a solid form. If the superconducting ceramic has a resistance that is too high or too undefined for the requirements of the continuous current switch, then a relatively poorly conducting parallel resistor can be connected in parallel. Such a resistance may, for example, have values in the range between 0.3 kOhm and 1000 kOhm.

Particularly advantageously, the switchable conductor section may comprise a material of the BaKBiO type. By this type is meant a material which is present as a compound of the said four elements, in particular as a compound of the composition Ba _x K _y BiO ₃ , where, for example, x may be about 0.6 and y about 0.4. This is a ceramic superconducting material, which is characterized by a for a HTS material comparatively low transition temperature in the range of about 30 K and by a comparatively low upper critical magnetic field of about 60 mT.

Generally, the upper critical magnetic field strength B _c2 for the superconducting material of the switchable conductor section may be below or at 100 mT. Such a low critical magnetic field strength B _c2 causes the switchable conductor section can be easily switched by a change of a locally acting magnetic field. For example, the upper critical magnetic field strength B _{c2 may be} in the range between 40 mT and 80 mT.

Advantageously, the transition temperature for the superconducting material of the switchable conductor section is at most 50 K, for example between 25 K and 35 K. Such a relatively low transition temperature for a high-temperature superconductor causes the switchable conductor section to be easily switched by local heating. As an alternative to a real high-temperature superconductor, a superconducting material with a transition temperature below 25 K can in principle also be used analogously for the switchable conductor region, provided that this material is a ceramic material which is thus not metallically conductive.

An operating temperature for the superconducting state of the switchable conductor section can advantageously be between 4 K and 40 K. With such an operating temperature can be achieved that the conductor section can be brought by a relatively small increase in temperature and / or a relatively small increase in a locally acting magnetic field in the normal conducting state. Particularly advantageous is the operating temperature for the superconducting state of the switchable conductor section between 10 K and 25 K.

The switching time for the switching of the switchable conductor section from the superconducting to the normal conducting state can be advantageous in at most 0.5 s, in particular at most 0.1 s. Such a short opening time has the advantage that during a discharge of the superconducting coil winding, a high proportion of the electrical energy stored in the coil is transmitted to the Abklingwiderstand or the consumer and thus is not dissipated in the persistent current switch or the coil winding. This is especially true for an accident in which the unloading of the coil winding should be performed as quickly as possible. By such a rapid unloading can be achieved in particular that it does not necessarily come to a complete quench. In other words, the electrical coil winding can be discharged and still remain substantially in the superconducting state, for example, after a local heating the opening of the persistent current switch takes place fast enough that only a small proportion of the stored energy is dissipated in the coil winding and thus the remaining areas the coil winding remain superconducting. Also, the locally heated area can quickly reach its operating temperature after such a rapid unloading. For example, after such a local heating and a subsequent discharge of the coil, a magnetic coil of a magnetic resonance apparatus can very quickly return to its original operating state if there is no quenching of the magnetic coil and thus no complete evaporation of the surrounding liquid helium. This reduces downtime and coolant consumption. In particular, after such an incident, the system can be remotely restarted, and after a complete quench usually required visit of a service technician to refill the helium can be omitted.

By such a rapid switching the risk of local overheating of the conductor material of the coil can be further reduced at a (full or local) quench: Especially with coil windings with ceramic high temperature superconductors or magnesium diboride, the risk of local overheating is particularly high, since these materials have a low thermal conductivity and such overheated zones spread accordingly slowly. Rapid opening of the persistent current switch in the detection of a local quench can therefore effectively prevent material damage to these poorly thermally conductive superconductors.

By contrast, in embodiments in which the persistent current switch need not be designed for sudden discharge upon local heating of the coil winding, it is sufficient if the switch opens quickly enough to permit controlled discharge of the current to the decay resistance and / or to the load cause. So if in a magnetic resonance coil no sudden quench must be prevented, but only a controlled discharge or shutdown of the system is to be made possible, a switch opening time of up to 10 s may be sufficient. This also applies, for example, to SMES systems or to exciter coils of electrical machines, which typically do not have to expect such sudden quenching processes and do not have to dissipate as much energy when discharging as in magnetic resonance systems.

The coil device may have a further magnetic coil for generating a local magnetic field B _ext for switching the switchable conductor section. For example, opening of the persistent current switch can be effected by switching on such a magnetic coil by causing the magnetic field B _{ext to} exceed the upper critical magnetic field strength B _{c2 of} the superconducting material of the persistent current switch. The opening of the persistent current switch can in principle be done solely by a change in the local magnetic field or by a combination of magnetic and thermal switching. The magnetic field of the further magnetic coil can also be superimposed on an existing magnetic field in order to generate a magnetic field B _ext , with which the magnetic field strength B _c2 can be exceeded.

The coil device can have a shielding coil arranged around the switchable conductor section and having a short-circuited current path, wherein the short-circuited current path can be opened by a switchable conductor section of the shielding coil. In other words, the closed current path of the shielding coil can be opened via this further switchable conductor section, which considerably reduces the magnetically shielding effect. In this way, a magnetic background field, which was previously largely shielded by the shielding coil, act on the switchable conductor region of the persistent current switch and bring this by exceeding its upper critical magnetic field strength B _c2 in its normal conducting state, so open the persistent current switch.

As already described in the introductory part, the coil device can be, for example, a magnetic coil device for generating a magnetic field in a device for magnetic resonance imaging or magnetic resonance spectroscopy. Alternatively, it can be an excitation coil device for generating an electromagnetic exciter field in a rotating electrical machine. Or it can, for example, a magnetic coil device for storing energy in a superconducting magnetic energy storage (SMES) act. In principle, the basic idea of the present invention is suitable for all systems in which a continuous current flows in a short-circuited superconducting coil system.

The superconducting conductor material of the coil winding may in principle be a different material than the superconducting material of the switchable conductor section. In particular, the material of the coil winding can also have magnesium diboride or it can be a metallic superconductor, that is, for example, a material based on NbTi or Nb3Sn. Alternatively, the superconducting material of the coil winding and the persistent current switch may also be identical or at least have the same components. The coil winding can also comprise a ceramic HTS material other than the continuous current switch, for example a REBa ₂ Cu ₃ O _x (REBCO) material for short, where RE stands for a rare earth element or a mixture of such elements.

In cases where different superconducting materials for coil winding and persistent current switch are used, these materials for the closed circuit superconducting or must be connected to each other with the lowest possible connection resistance. Superconducting connections between a ceramic superconductor and an NbTi-based conductor are already known from the prior art. Due to the large coherence length of NbTi, such superconducting compounds are relatively easy to produce. For example, a solid ceramic superconductor can be provided with holes that are filled for contacting with NbTi filaments and superconducting solder. Alternatively, a contact can be created via a press sintering.

In the method for loading or unloading a continuous current, a magnetic field B _ext acting at the location of the switchable conductor section can be changed for switching over. This change can be effected, for example, by switching a magnetic coil on or off or by breaking or joining a magnetic shielding coil. In this case causes a higher local magnetic field B _ext opening of the switch and a lower local magnetic field closing the switch, provided that the relevant for the respective temperature upper critical magnetic field B _{c2 is} exceeded or _fallen below. Alternatively or additionally, opening of the switch can be effected by heating the switchable region, or closure of the switch can be effected by removal of the heating. In other words, the persistent current switch can be switched either magnetically or thermally or by a combination of both methods. In this case, the continuous current switch is generally opened for loading or unloading the coil winding and closed for operation of the coil winding in the continuous current mode.

The discharging of the continuous current can for example be triggered in a targeted manner if a local heating is detected in the coil winding. Rapid and, in particular, automatic discharge in such an event can save the coil winding from complete breakdown of the superconducting properties and the system from loss of the coolant.

Alternatively, the unloading may also be initiated manually or otherwise controlled by a control unit, for example, a controlled shutdown of the system, removal of the energy stored in an SMES system, or even the actuation of an emergency stop switch.

In the following, the invention will be described by means of some preferred embodiments with reference to the appended drawings, in which:

1 shows a schematic equivalent circuit diagram of a coil device according to the prior art,

2 shows a schematic equivalent circuit diagram of a coil device according to a first embodiment,

3 shows a schematic equivalent circuit diagram of a coil device according to a second embodiment,

4 shows a schematic representation of the dependence of upper critical magnetic field strength of the temperature for a persistent current switch according to a third embodiment,

5 shows a persistent current switch according to a fourth embodiment,

6 shows a persistent current switch according to a fifth embodiment and

7 shows a persistent current switch according to a sixth embodiment.

In 1 is a schematic equivalent circuit diagram of a coil device 1 shown in the prior art. Shown is a superconducting coil winding 3 that together with a persistent power switch 6 inside a cryostat 11 is arranged. By the cryostat, the superconducting conductor elements of the coil winding 3 and of Continuous current switch 6 be cooled to a temperature below their respective transition temperature. About the coil winding 3 and the continuous current switch 6 is a closed circuit 5 formed in which a continuous current I ₁ can flow almost lossless due to the superconducting properties and thus does not decay or only extremely slowly over time. In the coil device 1 it may, for example, be a magnetic coil device of a magnetic resonance device.

To the continuous current I ₁ in the coil winding 3 charging for the first time, the previously described part of the closed circuit with an external power source 13 be connected in such a way that this power source 13 the continuous current switch 6 is connected in parallel. The power source 13 is outside the cryostat 11 arranged. It can be over a switch 10 be electrically connected to the other components. In an operation of the coil device 1 can she either like in 1 shown remain connected via a line with the other components, or it can also after a loading process from the coil means 1 be separated. For loading, the continuous current switch is opened, that is, it is set from its superconducting state to a normal conducting state, whereby its line resistance increases greatly. In the open state, therefore, only a small leakage current flows through the persistent current switch 6 , and most of it from the power source 13 generated charging current I ₂ is as current in the coil winding 3 fed. Now, if the persistent current switch closed by a change in its superconducting state, and then shut down the charging current I ₂ , the current I ₁ remains as a ring-shaped continuous current in the coil winding. The switching of the persistent current switch is typically effected by turning on and off a heating element in the prior art.

A discharge of in 1 shown coil device 1 can be triggered, for example, by a conductor section of the coil winding 3 is heated locally and thus becomes normally conductive. According to the state of the art, such a region spreads rapidly and leads to a quenching of the entire coil, the coil winding 3 is heated strongly. Controlled unloading into the parallel loading circuit located outside the cryostat, similar to loading, would theoretically also be possible, but the known persistent current switches are 6 generally too slow to prevent complete quenching after initial local heating of a portion of the superconducting conductor.

2 shows a similar schematic equivalent circuit diagram of a coil device 1 according to a first embodiment of the invention. Shown is one too 1 analog arrangement of coil winding 3 , Continuous current switch 6 , Cryostat 11 and external power source 13 , In addition to these components is the persistent power switch 6 one outside the cryostat 11 arranged decay resistance 9a connected in parallel. This decay resistance 9a has a resistance of at least 50 ohms. The loading of the coil device 1 with the help of the power source 13 takes place analogously as for 1 described. Again, the power source after loading can either be disconnected or remain connected to the rest of the system. When charging only an extremely small proportion of the charging current I _{2 flows} through the Abklingwiderstand 9a because this resistance is significantly higher than the total resistance of the coil winding 3 in the superconducting state.

When unloading the in the coil winding 3 flowing continuous current I ₁ can in the embodiment of the coil device according to the invention by opening the persistent current switch 6 a flow of a significant portion of the current through the parallel Abklingwiderstand 9a be effected. When opening the continuous current switch 6 becomes a switchable conductor section of the closed circuit 5 from a superconducting to a normal conducting state, and the resistance increases correspondingly strong. The resistance of the conductor section in the normal conducting state should here amount to at least 300 ohms, so that during discharging only the smallest possible part of the current as leakage current I ₄ via this open permanent current switch 6 flows. The ratio between the over the decay resistance 9a flowing discharge current I ₃ and the over the open switch 6 flowing leakage current I ₄ is about the ratio of the resistances of Abklingwiderstands 9a and the open switch specified. Therefore, the highest possible resistance of the open persistent current switch 6 advantageous in order to discharge the highest possible proportion of the current on the Abklingwiderstand 9a to let flow and so in particular to avoid unwanted heating of the arranged in the cryostat elements.

A controlled discharge can be done by switching the persistent current switch 6 be triggered in its normal conducting state. At a favorable resistance ratio may depend on the circuit 5 applied voltage, current and stored energy and from the operating points of the superconducting conductor elements advantageously a quenching of the superconductivity in the coil winding 3 be avoided. In this case, the operating point should be understood as meaning, in particular, the operating temperature and the magnetic field acting locally in the region of the conductors.

An uncontrolled, so spontaneously triggered discharge, as described by a local heating of a conductor region of the coil winding 3 to be triggered. Advantageously, such a local quench can be detected by a sensor, and it can then open the persistent current switch 6 which, as described above, subsequently causes a discharge of a large part of the current as discharge current I ₃ via the decay resistance. Again, with a sufficiently high resistance ratio and again depending on voltage, current, energy and operating points due to the fast discharge, a quenching of the entire coil winding 3 be avoided advantageous. Advantageous for a successful avoidance of such a complete quench is a sufficiently fast opening time of the persistent current switch 6 which may advantageously be less than 0.5 s. Such a fast switching can be made possible in particular if the switching between the superconducting state and the normal conducting state is at least partially caused by the change of a locally acting magnetic field, since such a magnetic field can typically be switched much faster than the temperature of the switchable conductor section can. Due to the heat capacities of the elements typically thermally coupled to such a conductor section, purely thermal switching is normally slower than switching based at least in part on the change of electromagnetic fields. Fast switching is thus advantageous, above all, for avoiding quenching in the event of spontaneous discharge. On the other hand, such a spontaneous event is due to the choice of operating conditions of the coil winding 3 largely exclude and only a controlled discharge must be possible, then the continuous current switch 6 also be switched slower, so for example by purely thermally triggered switching.

3 shows a schematic equivalent circuit diagram of a coil device 1 according to a second embodiment of the invention. The coil device 1 is similar to the example of the 2 , In contrast, instead of the decay resistance 9a a consumer 9b parallel to the continuous circuit 5 arranged. In the example of 3 it may in particular be an SMES system, in which excess electrical energy of a parent circuit in the coil winding 3 can be cached. To give this energy to the consumer 9b In turn, a controlled discharge process can be exploited by opening the permanent-current switch 6 to be triggered. Analogously as described above then flows when the switch is open 6 a discharge current I ₅ over the consumer 9b , so that the electrical energy previously stored in the coil winding from the consumer 9b can be used. To ensure a high efficiency of use for this energy, this should be the ratio between the resistance of the open permanent-current switch 6 and the effective resistance of the consumer 9b be as high as possible, for example at least 10: 1, in particular at least 100: 1. Thus, a current flowing through the open permanent current switch leakage current I _{4 are kept} as low as possible and the associated energy consumption is reduced. An additional, optional switch 10a can in the example of 3 between consumer and coil winding 3 be arranged to connect the consumer only when needed with the energy storage system.

4 shows a plot of the in the range of a continuous current switch 6 locally acting magnetic field B _ext against the temperature T. The dependence of the upper critical magnetic field strength B _{c2 of} the superconducting material of the switchable conductor section on the operating temperature T, ie B _c2 (T), is shown as a curve in this coordinate system. This curve B _c2 (T) is a relatively flat decreasing function of the temperature in the low temperature range and decreases more steeply towards higher temperatures. T _c indicates the critical temperature of the material for a local magnetic field B _ext going to zero. Above this temperature no superconducting state is possible. Shown is a schematic exemplary course for a ceramic superconductor material, wherein the exact values are highly dependent on material. For operating points P - ie value pairs of operating temperature T _P and local magnetic field B _P - which are left below the curve shown, the switchable conductor material is superconducting, for operating points right above the curve, it is normally conducting. By a shift of the shown, initially superconducting operating point P on the curve B _c2 (T) addition, thus opening the persistent current switch is possible.

As in 4 In principle, such an opening of the persistent current switch can be effected in different ways, since either the temperature dependency of the superconducting properties or the magnetic field dependency or both can be utilized simultaneously. The one with the reference number 14a marked arrow shows a change of the operating point P, in which only the temperature is changed. Such purely thermal switching into the normal conducting operating range can therefore be triggered by a local heating of the switchable conductor section. In contrast, the one with the reference number 14b marked arrow a change of the operating point P, in which only the locally acting magnetic field B _{ext is} changed. In such a purely magnetic switching can thus be effected for example by switching on a magnetic coil or by interrupting a magnetic shield collapse of the superconductivity. Finally, the reference number indicates 14c Arrow marked a combined thermal-magnetic switching, in which by simultaneous change of temperature and magnetic field, the material is brought into the normal conducting state. The proportion of change of these two operating parameters can also be chosen differently. It is essential that such a change can be effected particularly effectively by a combination of both changes. For the switching variants 14b and 14c in which the opening is effected at least in part by the change in the magnetic field B _ext , the switching can generally generally be faster than in the purely thermal switching 14a , Especially with ceramic high-temperature superconductors, which are preferred for the present invention due to their poor conductivity after breakdown of the superconductivity, purely thermal switching by the poor thermal conductivity is typically slower than at least magnetically assisted switching.

To enable the fastest possible switching, it is advantageous if the operating point P as in the 4 indicated schematically, is more near the flat portion of the curve B _c2 (T). Then even a slight change of the local magnetic field B _{ext can} trigger an opening of the switch, while the risk of unintentional opening due to thermal fluctuations is low.

Advantageously, a distance of the operating point P from the curve B _c2 (T) with respect to the magnetic field coordinate is at most ΔB = 100 mT, in particular between 10 and 50 mT. The distance with respect to the temperature coordinate can advantageously be at least 3 K, in particular between 10 K and 40 K.

For example, the switchable conductor section in the persistent current switch of the third embodiment may be based on a BaKBiO type material. This switchable conductor section can then be operated advantageously at an operating point between 10 and 25 K and between 0 and 50 mT. As a result, a fast switching, which is at least partially triggered magnetically, facilitated. When using a low-temperature superconducting material or magnesium diboride for the coil winding 3 Thus, the operating temperature of the persistent current switch is also relatively close to the operating temperature of the coil winding, so that there is no excessively high temperature gradient within the cryostat 11 or via the conductor loop of the circuit 5 must be maintained.

5 shows a persistent current switch 6 according to a fourth embodiment of the invention. Shown is a superconducting conductor 23 , The further course not shown here with a coil winding 3 to a closed circuit 5 is connected, similar in the 2 and 3 , The superconducting conductor 23 has a switchable conductor section 7 which in this example is inside a shielding coil 15 is arranged. This shielding coil 15 serves to an existing external magnetic field in the region of the switchable conductor section 7 shielded, with the magnetic field without shielding above the given at the operating temperature B _c2 (T). This existing external magnetic field may be at least partially that of the coil winding 3 acted field act. In particular, it may be the comparatively strong background field of a magnetic resonance apparatus. The arranged around the switchable conductor section shielding coil 15 significantly reduces this external magnetic field in its interior, leaving the section 7 is largely shielded from the magnetic field in normal operation. Thus, according to the figure, an operating point P can be set below the B _c2 (T) curve. The shielding effect of the coil 15 can in turn be canceled or at least reduced by switching the coil properties. For this purpose, a connection within the shielding coil 15 be opened.

For example, the shielding coil 15 as in 5 be indicated a cylindrical coil. It may in turn also comprise a superconducting material, in particular a cylindrical superconducting element. To open the switch, the superconductivity is interrupted at least in a portion of the cylinder, whereby the shielding effect is largely eliminated. Thereupon a local magnetic field above B _c2 (T) acts also in the interior of the shielding coil, and the switchable conductor section 7 is placed in a normal conducting state, the continuous current switch 6 will be opened. As a shielding coil, for example, a hollow cylinder made of bulk (bulk) superconductor material can be used. Alternatively, a hollow cylinder coated with a cylindrical superconducting layer may be used. Suitable superconducting materials for the shielding coil 15 For example, magnesium diboride or a REBCO type material. To break the shield, a circumferential segment may be used 15a of the cylinder with a heating element 17 be heated up so far that the superconductivity in this section 15a collapses.

This in 5 shown magnetic switching in which a superconducting shielding coil 15 turn is switched to thermal, can be clear be realized faster than if the switchable conductor section would be switched directly thermally.

A major reason for this is that the shielding coil has to be dimensioned as a second switch for significantly lower voltage strengths and / or current carrying capacities than the continuous current switching 6 in the main circuit 5 , In particular, the conductor cross-section and / or the conductor ground may be smaller. Thus, the shielding coil can be designed so that even with purely thermal switching a rapid change in their superconducting properties is possible. This for switching the shielding coil 15 used heating element 15a may alternatively to the in 5 Arrangement shown also be arranged so that the inner switchable conductor section 7 is heated with. And / or it can be arranged inside an additional heating element to the conductor section 7 to heat separately. In this way, a combined thermal-magnetic switching according to arrow 14c of the 4 will be realized.

6 shows a schematic representation of another continuous current switch according to a fifth embodiment of the invention. Shown again is a switchable conductor section 7 around which a shielding coil 15 is arranged. In contrast to the example of 5 However, this is not a flat cylindrical conductor, but a coil winding with multiple windings w _i from a superconducting conductor material. The conductors of the outermost turns are in this case via a superconducting contact 19 connected to a closed shield circuit. A switchable ladder section 15a this shielding circuit is in turn thermally with a heating element 17 coupled, over this ladder section 15a can be brought into a normal conducting state. Again, therefore, there is a secondary thermal switch over which a superconducting shield can be turned off, so that a switchable conductor section 7 of the continuous current switch 6 is switched indirectly magnetically in a normal conducting state. Again, the thermal switching of the shielding coil 15 faster than with a purely thermal switching of the conductor section 7 possible would be because the head of the coil 15 not for the high voltage strengths and current carrying capacities of the circuit 5 the coil winding 3 must be dimensioned. In this case too, switching of the switchable conductor section can additionally be effected by a further optional heating element (not shown here) in the region of the conductor section 7 be supported, or the heating element shown 17 may alternatively be arranged so that this conductor section 7 is heated with.

The conductor of the shielding coil 15 may for example be formed of similar superconducting wire as the coil winding 3 , In particular, a low-temperature superconductor or a magnesium diboride-based conductor can also be used here. For such materials, technologies are known to provide the necessary superconducting contact 19 manufacture. Advantageously, the thermal conductivity of the superconducting material of the shield winding 15 be higher than the superconducting material of the switchable conductor section 7 , since it is not necessary to take into account the highest possible electrical resistance in the normally conducting state. When choosing a ceramic material for the switchable conductor section 7 and a metallic conductor material for the shield winding 15 So can in the secondary switching range 15a Even with thermal switching, a higher switching speed can be achieved than with purely thermal switching of the conductor section 7 it is possible. Magnetically assisted switching with a thermally switched secondary switch 15a can thus the opening switching time of the persistent current switch 6 be shortened advantageous.

In order to achieve an even shorter opening time of the persistent current switch, alternatively, the coil winding 15 be formed with a fast magnetically switchable conductor material such as BaKBiO. This can be switched, for example, with a magnetically coupled thereto magnetic coil. In other words, such rapid switching can be enabled via a cascade of two superconducting magnetically switchable conductor regions.

In all variants in which a magnetic shielding is canceled for switching, it is advantageous if the shielded volume is comparatively small. In particular, in magnetic resonance applications, the shielding should be chosen small in order to avoid magnetic field distortions in the imaged or measured volume. Also in the other variants of the magnetic, thermal or thermal-magnetic switching, it is advantageous to choose the smallest possible volume for the persistent current switch, since then the switching times can be kept very small. Advantageously, the volume of the continuous current switch is generally not more than 100 cm ³ .

In the embodiments in which a magnetic shield is released for switching, it is advantageous if in series with the switchable conductor section 7 a second, not shown in the figures, is provided with a heating element thermally switchable conductor section. Such a serial second switch can be used advantageously to the coil winding 3 With to charge a charging current. For this charging process, the second switch should be opened, because with an initial charging of the coil system 1 that is from the coil winding 3 generated background field not yet large enough to allow a magnetic switching by canceling a shield. The same applies to a field winding which, as described below, interacts with a background field to exceed the B _c2 (T) curve. In such cases, an additional thermal switch is useful to allow for initial charging and possibly complete discharge without the nominal background field. In this case, however, a complete discharge is less critical, since a current flowing in the continuous current switch leakage current I ₄ generally during discharge to a thermal load of the switchable conductor section 7 and thus leads to a longer lasting leaving the superconducting work area.

7 shows a schematic representation of another continuous current switch according to a sixth embodiment of the invention. In this embodiment, simplified two different variants of the switching are shown in a figure, which can either be used separately or optionally can be combined with each other.

Shown again is a switchable conductor section 7 a continuous current switch 6 which in this example has two superconducting contacts 19 with a superconducting conductor 23 is connected, which together with a coil winding, not shown here 3 a closed circuit 5 forms, similar to that in the 2 and 3 , In this example, therefore, is the switchable conductor area 7 made of a different superconducting material than the rest of the superconducting conductor 23 , Such a choice of another material and the introduction of additional superconducting contacts 19 Optionally also in the embodiments of the 5 and 6 be used, so for each of the described switching variants. Alternatively, however, in principle also a uniform superconducting material for switchable conductor section for the respective variants 7 and the rest of the ladder 23 as well as the coil winding 3 be used.

In the example of 7 is a magnetic field winding 21 around the switchable ladder section 7 or at least part of it. This magnetic field winding 21 is vorteilhat small compared to the coil winding 3 However, it should be sufficient to be in the area of the ladder section 7 to achieve a local magnetic field B _ext above the respective B _c2 (T). It advantageously has a comparatively low inductance so that the magnetic field can be switched on quickly in order to quickly open the persistent current switch 6 to enable. The magnetic field winding 21 may have a superconducting conductor material, but it may in principle also be normally conductive.

Advantageous is the persistent current switch 6 in this switching method in an area within the cryostat 11 arranged in which the magnetic field of the coil winding 3 is comparatively low. This is particularly important for magnetic resonance systems with high background magnetic fields, so that B _c2 (T) is not already exceeded by the background field, but only when switching on the magnetic field winding 21 , The magnetic field of this magnetic field winding 21 can then advantageously cooperate with the background _{magnetic field} to exceed B _c2 (T), so that by the additional winding 21 generated magnetic field does not have to be very large.

Particularly advantageous in the embodiment of the 7 for the switchable line section 7 a material with anisotropic, ie directional B _c2 (T) can be used. For example, a crystalline material with a high B _c2 (T) in the crystallographic ab direction and a low B _c2 (T) in the crystallographic c direction can be used. The persistent current switch can then be aligned so that the background field is oriented in the ab direction of the material and thus the superconducting region of the curve according to FIG 4 will not leave. Only by switching on the magnetic field winding 21 In the c-direction, a sufficiently high magnetic field B _{ext is} generated in order to leave the superconducting region below the B _c2 (T) curve.

In a further advantageous variant of the sixth exemplary embodiment, the magnetic field strength required for the magnetic switching can be reduced by adding, for the switchable conductor region 7 a ladder geometry is chosen that promotes the formation of local field peaks. For example, a flat, band-shaped conductor can be used and the magnetic field of the field winding 21 can impinge substantially perpendicular to such a strip conductor. Shielding currents in the conductor then cause a field increase in the area of the edges. As soon as these become normally conducting by exceeding the B _c2 (T) curve, the field increase penetrates further into the interior, until the entire switchable conductor section 7 becomes normally conductive. Thus, with a relatively small additional magnetic field field winding 21 the ladder section 7 be switched effectively and quickly.

The described embodiments with a magnetic switching according to arrow 14b of the 4 represent complete solutions for which the in 7 shown optional heating element 17 is not needed. Such a heating element 17 However, it may additionally be present in order to support and / or accelerate the switching operation in the manner of a thermal-magnetic switching according to the arrow 14c of the 4 , In 7 For reasons of clarity, the heating element is opposite the field winding 21 axially offset. However, it may also be advantageous on the same section of the conductor 7 be disposed within the field winding and / or surrounding, so that the thermal and magnetic effects on the superconducting properties of the conductor material mutually reinforce each other on a very short section.

Alternatively to the in 7 illustrated embodiment, the field winding 21 also omitted, and the switchable ladder section 7 can in principle only by the heating element 17 according to variant 14a of the 4 be switched between its superconducting state and its normal conducting state. Compared to the purely thermal switching of metallic superconductors, the thermal switching of ceramic superconducting conductor sections 7 take place faster because a lower mass of ceramic conductor material is needed for the purpose to be achieved resistors.

Claims (15)

Coil device ( 1 ) with at least one electric coil winding ( 3 ) with superconducting conductor material, - wherein the coil winding ( 3 ) Part of a self-contained circuit ( 5 ) to form a continuous current (I ₁ ), - wherein the closed circuit ( 5 ) a persistent power switch ( 6 ) with a switchable conductor section ( 7 ), which is switchable between a superconducting state and a normal conducting state, - wherein the switchable conductor section ( 7 ) has a high-temperature superconducting conductor, - wherein the switchable conductor section ( 7 ) in the normal conducting state of the conductor has a resistance of at least 300 ohms - and wherein the switchable conductor section ( 7 ) a decay resistance ( 9a ) of at least 50 ohms and / or a consumer ( 9b ) is connected in parallel with an effective resistance of at least 50 ohms. Coil device ( 1 ) according to claim 1, which contains a cryostat ( 11 ) for cooling the superconducting conductor material, - wherein the superconducting coil winding ( 3 ) and the switchable conductor section ( 7 ) are arranged within the cryostat - and wherein the Abklingwiderstand ( 9a ) and / or the consumer ( 9b ) is arranged outside the cryostat. Coil device ( 1 ) according to one of claims 1 or 2, wherein the switchable conductor section ( 7 ) in the normal conducting state of the conductor has a resistance that is greater than the Abklingwiderstand ( 9a ) and / or the effective resistance of the consumer ( 9b ). Coil device ( 1 ) according to one of the preceding claims, in which the switchable conductor section ( 7 ) in the normal conducting state of the conductor has a resistance of at least 1000 ohms, in particular at least 5000 ohms. Coil device ( 1 ) according to one of the preceding claims, in which the switchable conductor section ( 7 ) in the normal conducting state of the conductor has a dielectric strength for voltages of at least 20 kV. Coil device ( 1 ) according to one of the preceding claims, in which the switchable conductor section ( 7 ) has a ceramic high temperature superconductor. Coil device ( 1 ) according to claim 6, wherein the switchable conductor section ( 7 ) has a material of the type BaKBiO. Coil device ( 1 ) according to one of the preceding claims, in which the upper critical magnetic field strength (B _c2 ) for the superconducting material of the switchable conductor section ( 7 ) is at most 100 mT. Coil device ( 1 ) according to one of the preceding claims, in which an operating temperature for the superconducting state of the switchable conductor section ( 7 ) is between 10K and 40K. Coil device ( 1 ) according to one of the preceding claims, in which the switching time for switching the switchable conductor section ( 7 ) is from the superconducting to the normal conducting state at most 0.5 seconds. Coil device ( 1 ) according to one of the preceding claims, which comprises a further magnet coil ( 15 ) for generating a magnetic field (B _ext ) for switching the switchable conductor section ( 7 ) having. Coil device ( 1 ) according to one of the preceding claims, which comprises a switchable conductor section ( 7 ) arranged shielding coil ( 15 ) with a short-circuited current path ( 16 ), wherein the short-circuited current path ( 16 ) by a switchable conductor section ( 15a ) of the shielding coil ( 15 ) can be opened. Coil device ( 1 ) according to one of the preceding claims, which - is designed as a magnetic coil device for generating a magnetic field in a device for magnetic resonance imaging or magnetic resonance spectroscopy or - is designed as excitation coil means for generating an electromagnetic exciter field in a rotating electric machine or - as a magnetic coil means for storing energy in one Superconducting magnetic energy storage (SMES) is formed. Continuous current switch ( 6 ) for a coil device ( 1 ) according to one of the preceding claims with a switchable conductor section ( 7 ), - wherein the switchable conductor section ( 7 ) has a high-temperature superconducting conductor - and wherein the switchable conductor section ( 7 ) has a resistance of at least 300 ohms in the normal conducting state of the conductor. Method for loading or unloading a continuous current (I ₁ ) in a coil winding ( 3 ) a coil device ( 1 ) according to one of claims 1 to 13, in which the switchable conductor section ( 7 ) of the continuous current switch ( 6 ) is switched from a superconducting state to a normal conducting state.

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