EP1544872B1 - Superconducting magnet system with continuously working flux pump and and related methods of operation - Google Patents

Description

The invention relates to a magnet arrangement with a superconducting magnet coil system, which in operation has an ohmic resistance greater than or equal to zero, and with a flux pump, which comprises at least one superconducting switch and at least two superconducting secondary coils.

Describe such a magnet arrangement with a superconducting magnetic coil system TP Bernart et al., Rev. Sci. Instrum., Vol. 46, no. 5, May 1975, pages 582-585 ,

The superconducting magnet coil system comprises one or more series-connected magnetic coils which form a closed superconducting circuit. The superconducting magnet coil system is typically arranged in a cryostat. It may have an ohmic resistance greater than zero in the operating state if the superconductors used are loaded to just below the critical current or if they have no sharp transition from superconducting to normal conducting. The principle of a flux pump is to compensate by inductive coupling of energy resistive losses of the solenoid or to charge or discharge the coil without large currents must be performed in the cryostat. The invention relates in particular to superconducting magnet coil systems with a flux pump, which comprises at least one superconducting switch and at least two superconducting secondary coils in which a voltage can be inductively established. In order for this voltage can be fed to the superconducting magnet coil system to compensate for resisitive losses or for charging or discharging, the secondary coils must be connected in series with the superconducting magnet coil system in series, which can be done for example by closing a superconducting switch.

A magnet arrangement with a flux pump, which comprises at least two superconducting secondary coils, is known from TP Bernat et. al., Rev. Sci. Instrum., Vol. 46, No 5, May 1975 , and from LJM van de Klundert et. al., Cryogenics, May 1981 , This flux pump is based on the fact that the superconducting magnet coil system is bridged with two current paths, each comprising a switch and a superconducting secondary coil. In a primary coil whose inductive coupling with the secondary coils is the same opposite in size, power is cyclically switched on and off again. If in the same cycle connected in series with the secondary coils superconducting switch alternately opened and closed, arises over the solenoid system over the entire cycle constant voltage, apart from voltage peaks when opening the switch.

The typical application of flow pumps is the charging and discharging of superconducting magnet coil systems. The advantage over directly feeding the operating current into the coils is that the currents for operating the flux pump are much weaker than the typical magnetic currents. Thus, the power supply lines can be made smaller and the heat input in the cryostat can be reduced.

However, the field of application of superconducting magnets also encompasses fields of application in which the magnet coils remain on the field for years after the charging process and should have the lowest possible field drift. These include in particular superconducting magnet coil systems for magnetic resonance methods. In such magnet systems, the use of a flux pump is less of interest for charging the magnet system, but for stabilizing the magnetic field in the operating state. An efficient flow pump would bring several advantages in this regard. For example, magnets could be built with partial coils of high-temperature superconductors, which according to the current state of the art do not meet the drift specifications for magnetic resonance applications without additional measures. This would allow the construction of magnets with stronger than the usual fields today. Further, by using a flux pump for field stabilization, the superconductors in the magnet could be charged higher, which would allow the construction of more compact and less expensive magnets.

For use for precise field stabilization over long periods, the known flow pumps are not suitable. On the one hand, voltage peaks occur across the magnetic coil system when opening superconducting switches, which is not tolerable for sensitive applications such as magnetic resonance methods. On the other hand, at least one must be in each phase of the pumping cycle Open superconducting switch so that the voltage induced in the secondary coil voltage can be fed into the magnetic coil system. In the case of the conventional switches, an amount of heat is produced which leads to large losses of cooling liquid in the cryostat. For the stability of the field, the thermal stability in the cryostat is very important, that is, in sensitive applications such as magnetic resonance methods, the heat input in the cryostat must be minimized.

Object of the present invention is to improve a flow pump according to the prior art so that in addition to the charging and discharging of a superconducting magnet coil system and a good stabilization of the magnetic field of the magnet coil system in the operating state for a long time is possible, especially if the magnetic coil system easily is resistant and the requirements for the field stability are very high. In particular, it is an object of the present invention that the improved flow pump arrangement permits an operating method with which a constant voltage across all the cycles of the flow pump can be applied across the magnetic coil system.

According to the invention, this object is achieved in a magnet arrangement of the type described above in that at least one superconducting current path is present, in which the superconducting magnet coil system or parts thereof is connected in series with at least two secondary coils and in which at least one secondary coil by closing a superconducting switch superconducting can be bridged, and that at least two primary coils are present, which can be fed independently with a respective current and which are each inductively coupled to at least one of the secondary coils.

Briefly, the invention provides that there is a superconducting current path in which the superconducting magnet coil system or parts thereof are connected in series with at least two secondary coils and in which at least one secondary coil can be bridged by closing a superconducting switch. In particular, it can be provided that the secondary coils are inductively coupled, each with its own primary coil.

This arrangement makes possible an operating method of the flow pump in which, in a first step, a first primary coil, which is coupled to a first, non-superconducting bridged secondary coil, is charged until a maximum final current is reached in the primary coil. As a result, a voltage can be built up across the superconducting magnet coil system which, for example, exactly corresponds to the resistive voltage to be compensated in the magnet coil system. In a second step, the first primary coil must be recharged to its initial current. During this phase, the superconducting switch is opened via a second, previously superconducting bridged with a closed switch secondary coil and in that primary coil, which inductively couples with this secondary coil, the current is raised, whereby in this secondary coil, a voltage is induced. The current ramp in the second primary coil is selected such that the voltage induced in the second secondary coil compensates for both the voltage induced by the discharge of the first primary coil in the first secondary coil and the resistive voltage across the superconducting magnet coil system. After the first primary coil has reached its initial current, the switch over the second secondary coil is closed again and the second primary coil is - with the switch closed - returned to its initial current. The cycle can now start over.

The advantage of an arrangement according to the invention is therefore that, owing to a plurality of primary coils supplied independently with power, different voltages can be induced in different secondary coils, which voltages are added to a total voltage thanks to the series connection of these secondary coils. The series connection of the secondary coils with the superconducting Magnetic coil system allows the supply of this total voltage in the superconducting magnet coil system. The great flexibility of the arrangement allows a desired voltage across the superconducting magnet coil system to be maintained through appropriate process steps in each phase of the flow pump cycle.

It turns out that in the above-described operation method of the flow pump at any time during the entire cycle must be a superconducting short circuit on the first secondary coil. This means that according to the invention of n ≥ 2 secondary coils at most n-1 secondary coils must be bridged with a switch. In the simplest case of n = 2 , therefore, only a single switch is required, which in addition only has to be open during the short time during which the current in the first primary coil is reset. As a result, the heat output is significantly reduced by the switch heater over a prior art flow pump. This embodiment of the invention is therefore particularly advantageous.

Also preferred is an embodiment of the inventive arrangement in which a superconductive switch bridges a secondary coil together with a resistor connected in series with this secondary coil, the resistor having a value, measured in ohms, between 0 and the value of the inductance of this secondary coil measured in Henry. The advantage of this arrangement is that when charging and discharging a primary coil, which is inductively coupled to this secondary coil, with the superconducting switch closed no uncontrolled high currents can be induced in the secondary coil.

A particularly preferred embodiment of the arrangement according to the invention is characterized in that instead of the resistor used in the above embodiment, a further superconducting switch is used. This embodiment thus provides that a superconducting switch, a secondary coil together with another superconducting Bridged switch, which is connected in series with said secondary coil; see also FIG. 3 , As a result, the current in the secondary coil can be controlled in a controlled manner by suitable charging and discharging of the associated primary coil as well as by opening and closing of the further switch. In particular, it can thus be prevented that a current flows through this switch at a certain point of the pumping cycle before opening the first switch. This voltage pulses are prevented over the superconducting magnet coil system, which is essential especially for sensitive applications such as nuclear magnetic resonance method. In addition, no heat is generated by the reduction of electricity in the first switch, which allows a further saving of cooling liquid. This arrangement allows the use of an operating method of the flow pump, which guarantees an undisturbed, continuous pumping power with a minimum of heat input in the cryostat.

In two further advantageous embodiments of the arrangement according to the invention secondary coils are each inductively coupled with exactly one primary coil, or secondary coils are inductively decoupled from each other. As a result, the induced voltages in the secondary coils when charging or discharging the primary coils can be better controlled and the methods for operating the flow pump are simplified.

Embodiments of the arrangement according to the invention in which primary or secondary coils are inductively largely decoupled from the superconducting magnet coil system or produce substantially no field in the working volume of the superconducting magnet coil system are also particularly advantageous. Thus, disturbances of the magnetic field in the working volume are prevented during operation of the flow pump.

A further advantageous embodiment of the arrangement according to the invention is characterized in that at least one primary coil is superconducting. In contrast, a current flowing in a superconducting primary coil generates to normal conducting primary coils no heat. If the primary coils are in the cryostat, then the coolant losses can be reduced.

A further improvement in terms of reducing coolant losses is achieved if the leads to the coils in the cryostat or to the switches are at least partially superconducting.

Another embodiment provides that at least one of the superconducting switches can be actuated by a heater whose supply lines are at least partially superconducting.

An advantageous embodiment of the arrangement according to the invention is characterized in that at least a portion of the superconducting magnet coil system is bridged superconducting or with a resistor. This arrangement can be used to dampen the effect of small voltage fluctuations, such as when opening switches of the flow pump, on the overall field of the superconducting magnet system. For the damping to be effective, the resistance (in ohms) must not exceed the magnitude of the inductance (in Henry) of the bridged section.

The arrangement according to the invention is particularly advantageous if it is part of an apparatus for nuclear magnetic resonance. In such magnet arrangements, an apparatus for active field stabilization, in which the flow pump according to the invention is preferably used in this field of application, places particularly high demands with regard to stability of the stabilization voltage and minimization of the heat input in the cryostat. Exactly these criteria are better fulfilled in the above-mentioned embodiments of the flow pump according to the invention than with flow pumps according to the prior art.

An advantageous embodiment of the arrangement according to the invention comprises a superconducting magnet coil system, in which one or more coils are wound with high temperature superconductors. The potentially higher drift when using high-temperature superconductors can be compensated with the flow pump according to the invention, while maintaining the field stability of the superconducting magnet coil system.

The advantages of the arrangement according to the invention can be fully exploited only by using suitable methods for operating the flow pump. A first method is characterized by a particularly simple cycle of charging and discharging the primary coils and opening and closing of the switches. In this method of operating a device having at least first and second superconducting secondary coils and a first superconducting switch, the first superconductive switch bridging the second secondary coil is periodically opened and closed. When the first switch is closed, the current in a first primary coil, which inductively couples with the first secondary coil, is driven from an initial value to an end value. When the first switch is open, the current in this primary coil is largely restored to the initial value. At the same time, when the first switch is open, the current in a second primary coil, which couples to the second secondary coil, is driven from an initial value to an end value and largely reset to the initial value when the first switch is closed.

An improved method using the further, second superconducting switch is characterized in that, when the first switch is closed, a second superconductive switch, which is connected in series with the second secondary coil and is bridged together with the latter by the first superconductive switch, is opened at least temporarily , The advantage of this method is that the second secondary coil does not charge uncontrollably when resetting the current in the second primary coil.

It is particularly advantageous to reduce the current in the second primary coil to zero again in order to generate less heat in the supply lines and - in the case of a normally-conductive second primary coil - in the coil itself.

This process variant can be further improved by setting the current in this coil to an amount of I * L / K before reaching the final current of zero amperes in the second primary coil and that the second superconducting switch is opened at the latest after reaching this current, and that then, while resetting the current in the second primary coil to the final current of zero amperes and until the first opening of the first superconducting switch, the second superconducting switch remains superconducting closed, the current in the superconducting magnet coil system, L the self-inductance of the second secondary coil and K denoting the inductive coupling in Henry between the second secondary coil and the second primary coil. This process is described in more detail in the example below. Its particular advantage is that no current flows over it before opening the first superconducting switch. Thus, voltage peaks are prevented over the superconducting magnet coil system, which is an important criterion for the use of the flow pump according to the invention for field stabilization in sensitive applications.

In two further advantageous variants of the method, the steps of the described methods are repeated cyclically in order to either charge or discharge the superconducting magnet coil system or to stabilize the current in the magnet coil system to an operating value precisely.

The arrangement according to the invention also makes it possible to use a method variant which is particularly advantageous with regard to reducing the heat input in the cryostat, in which that phase of the pumping cycle during which no superconductive switch is open lasts longer than the phases with open, ie heated, superconducting switches. In contrast, must in flow pumps according to the prior art, permanent switches are heated.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, according to the invention, the above-mentioned features and those which are still further developed can each be used individually for themselves or for a plurality of combinations of any kind. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Fig. 1

ein Verdrahtungsschema einer erfindungsgemäßen Magnetanordnung mit einem supraleitenden Magnetspulensystem und einer Flusspumpe;

Fig. 2

ein Verdrahtungsschema einer erfindungsgemäßen Magnetanordnung mit einem supraleitenden Magnetspulensystem und einer Flusspumpe mit einem zusätzlichen Widerstand im Strompfad der Flusspumpe;

Fig. 3

ein Verdrahtungsschema einer erfindungsgemäßen Magnetanordnung mit einem supraleitenden Magnetspulensystem und einer Flusspumpe mit einem zusätzlichen supraleitenden Schalter im Strompfad der Flusspumpe;

Fig. 4

ein Verdrahtungsschema einer erfindungsgemäßen Magnetanordnung mit einem supraleitenden Magnetspulensystem und einer Flusspumpe und einem zusätzlichen Widerstand, welcher einen Abschnitt des supraleitenden Magnetspulensystems überbrückt;

Fig. 5

die Ströme und Schalterzustände der Flusspumpe sowie die über dem supraleitenden Magnetspulensystem aufgebaute Spannung während mehrerer Pumpzyklen für ein besonders vorteilhaftes Verfahren zum Betrieb einer erfindungsgemäßen Flusspumpe.

The invention is illustrated in the drawing and is explained in more detail with reference to an embodiment. Show it:

Fig. 1

a wiring diagram of a magnet assembly according to the invention with a superconducting magnet coil system and a flow pump;

Fig. 2

a wiring diagram of a magnet assembly according to the invention with a superconducting magnet coil system and a flow pump with an additional resistance in the current path of the flow pump;

Fig. 3

a wiring diagram of a magnet assembly according to the invention with a superconducting magnet coil system and a flow pump with an additional superconducting switch in the current path of the flow pump;

Fig. 4

a wiring diagram of a magnet assembly according to the invention with a superconducting magnet coil system and a flux pump and an additional resistor which bridges a portion of the superconducting magnetic coil system;

Fig. 5

the currents and switch states of the flow pump and the voltage built up across the superconducting magnet coil system during several pump cycles for a particularly advantageous method for operating a flow pump according to the invention.

Based on FIG. 1 schematically an inventive arrangement is shown, which comprises a superconducting magnet coil system M and a flow pump P. The magnet coil system M may have a resistance of the size R. With the magnet coil system M , two further superconducting coils M1 and M2 are connected in series, which serve as secondary coils in the flow pump P. In these coils can be induced by changing the current I1 or I2 in the primary coils C1 and C2 of the flow pump P by inductive coupling a voltage. One of the secondary coils, namely M2, is bridged by a superconductive switch S1 .

FIG. 2 schematically shows an inventive arrangement in which the secondary coil M2, which is bridged with the superconducting switch S1 , is connected in series with a resistor R2 , such that the switch S1 bridges both the coil M2 and the resistor R2 .

FIG. 3 shows an inventive arrangement as in FIG. 2 , with the difference that instead of the resistor R2, a second superconductive switch S2 is used.

FIG. 4 shows an inventive arrangement as in FIG. 1 in which in addition a portion of the superconducting magnet coil system M is bridged with a resistor r .

FIG. 5 shows for an operating method of the flow pump according to the invention FIG. 3 the currents I1 and I2 in the primary coils C1 and C2 of the flow pump P and the switching states of the superconducting switches S1 and S2 , the current IS1 in the switch S1 and the voltage VMagnet established by the flux pump P across the superconducting magnet coil system M. To the right, time t is plotted. The method is optimized to keep the voltage VMagnet constant over any number of pump cycles and to produce no voltage spikes. In addition, the duration during which the superconductive switches are opened is minimized.

• LM1 = LM2 = 10^-6H (Induktivität der Sekundärspulen M1 und M2),
• KM1C1 = KM2C2 = 10^-4H (induktive Kopplung zwischen der Sekundärspule M1 und der Primärspule C1 beziehungsweise zwischen M2 und C2),
• IM = 100A (Betriebsstrom des supraleitenden Magnetspulensystems M).
• Alle anderen Kopplungen sind null.

In the following, the invention will be explained with reference to an example. The embodiment of the arrangement according to the invention on which the example is based is that of FIG. 3 , The method used to operate the flow pump P is that FIG. 5 , The goal is to maintain a constant voltage VMagnet of 25 μV across a superconducting magnet coil system M. The components of the flow pump are designed as follows:

• LM1 = LM2 = 10 ^-6 H (inductance of the secondary coils M1 and M2 ),
• KM1C1 = KM2C2 = 10 ^-4 H (inductive coupling between the secondary coil M1 and the primary coil C1 or between M2 and C2 ),
• IM = 100A (operating current of the superconducting magnet coil system M ).
• All other couplings are null.

At the beginning and during the first phase of the cycle of the flow pump P from t = 0 to t1 = 8s (see FIG. 5 ), the two switches S1 and S2 are superconducting closed and the operating current IM of the superconducting magnet coil system M flows through the current path M-M1-M2-S2. The current I2 in the second primary coil C2 is zero and the current I1 in the first primary coil C1 is treated with a continuous ramp of 0.25 A / s during 8s of - loaded 1A + 1A. As a result, a voltage of 25 μV is induced in the secondary coil M1 . Because the secondary coil M1 is superconductively connected to the magnet coil system M , the condition VMagnet = 25 μV is thus already fulfilled in this first phase. At time t1 , the current I1 in the primary coil C1 has reached the maximum value of + 1A and should return to the initial value by the time t2 = 10s be discharged from -1A . The voltage induced in M1 is -100μV in this phase . In order to keep the voltage VMagnet constant at 25μV during this phase, the switch S1 is opened and the current in the second primary coil C2 is driven from zero to 2.5A. Thereby, a voltage of 125 microvolts is induced in the second secondary coil M2. Because the switch S1 is open, the voltages induced in M1 and M2 in the current path M-M1-M2-S2 add up to 25μV, which also satisfies the condition VMagnet = 25μV during this phase. At the time t2 = 10s , the switch S1 is closed again and the charging cycle of the primary coil C1 starts again.

However, the system is not yet back to its initial state because the current I2 in the second primary coil C2 is not zero. When resetting I2 to zero, it must also be ensured that the operating current IM at the end again flows through the secondary coil M2 and not via the closed switch S1 , that is, IS1 should be zero. If this condition is not met, the renewed opening of the switch S1 in the following cycle of the flow pump P generates an undesired voltage pulse across the superconducting magnet coil system M.

The goal of bringing both I2 and IS1 to zero is achieved by driving I2 to the value - IM * KM2C2 / LM2 between t2 and t3 , in example -1A. In this case, the switch S2 is opened, whereby the current in M2 is kept at zero. Between t2 and t3, therefore, the magnet current IM flowing through the closed switch S1, ie IS1 = IM = 100A. At time t3 , the switch S2 is closed again and then, until time t4, the current I2 in the second primary coil C2 is reduced to zero. As a result, a current of the magnitude IM in the direction of the operating current of the superconducting magnet coil system M is induced in the second secondary coil M2 , so that, starting at the instant t4, the entire operating current IM flows again via the current path M-M1-M2-S2 . Thus, the second primary coil C2 and the current path M2-S1-S2 from the time t4 in the initial state.

It is to be noted that the operations during the resetting of the second primary coil C2 and the current path M2-S1-S2 to the initial state have no influence on the voltage VMagnet applied across the superconducting magnet coil system M. The reason for this is that during this phase, the switch S1 is always superconducting, so that no voltage can arise over the connection points of S1 to the current path M-M1-M2-S2 . Thus, during this phase, the voltage VMagnet across the superconducting magnet coil system M is given solely by the voltage induced in the secondary coil M1 , which is set by the current ramp in the primary coil C1 to the desired value of 25 μV .

With reference to the method shown in this example for operating a flow pump P according to the invention, the advantages of this arrangement become clear. First, the voltage over the entire cycle of the flow pump P can be kept constant and there are no voltage spikes when opening superconducting switches. Second, the switches are open only during a fraction of the operating cycle of the flow pump P , whereby the heat input in the cryostat through the switches is minimal.

Compared to a prior art flow pump with only one primary coil, at least two primary coils C1 and C2 must be supplied with current in an arrangement according to the invention. As a result, the heat input in the cryostat is increased by the power supply lines of the primary coils. However, in the example shown, this disadvantage only has a slight effect because the second primary coil C2 carries current only during a fraction of the operating cycle of the flow pump P , as a result of which the heat development in the supply lines is kept small.

If a superconducting magnet coil system is to be used for the magnetic nuclear magnetic resonance, the requirements for the temporal stability of the magnetic field are particularly high. Typically, the The total resistivity of the magnet coil system is on the order of 0.1 * 10 ^-9 ohms at most , so that the field drift is acceptable. With a flow pump according to the invention of the above example, however, the field can still be stabilized even if the resistivity of the superconducting magnet coil system in the order of VMagnet / IM = 25μV / 100A = 250 * 10 ^-9 ohms . The resistivity of the magnet coil system may therefore be over a thousand times greater than in an arrangement without the inventive flow pump.

A magnet arrangement according to the invention comprises a superconducting magnet coil system M and at least two superconducting secondary coils M1, M2, which are connected in series with the magnet coil system, and a first superconducting switch S1 , which can superconductingly bridge the second of the secondary coils M2 . Particularly advantageously, the magnet arrangement has a second superconducting switch S2 , which is connected in series with the second secondary coil M2 , wherein the first superconducting switch S1 can bridge the entirety of the second secondary coil M2 and the second superconducting switch S2 . By inductive coupling can be generated by means of at least two independent primary coils C1 , C2 a predetermined voltage in each of the secondary coils M1, M2 , regardless of the other secondary coil. The system of secondary coils, primary coils and superconducting switches forms a flux pump P for the magnet coil system. This flux pump can be used well to stabilize the magnetic field of the magnetic coil system in the operating state for a long time, that is for drift compensation in the magnetic coil system.

Claims (11)

1. Magnet arrangement comprising
   a superconducting magnet coil system (M) which has an ohmic resistance (R) of zero or more during operation, and
   a flux pump (P) which comprises at least one superconducting switch (S1) and at least two superconducting secondary coils (M1,M2), characterized in that at least one superconducting current path is provided, in which the superconducting magnet coil system (M) or parts thereof is/are connected in series with at least two secondary coils (M1, M2), and in which at least one secondary coil (M2) can be superconductingly bridged through closing a superconducting switch (S1),
   and in that at least two primary coils (C1, C2) are provided each of which can be fed independently of each other with a current (I1, 12) and each of which is inductively coupled with at least one of the secondary coils (M1, M2).
2. Arrangement according to claim 1, characterized in that at least one superconducting current path is provided, in which the superconducting magnet coil system (M) or parts thereof is/are connected in series with n≥2 secondary coils (M1, M2), and in which at least one, but at most n-1 secondary coil(s) (M2) can be superconductingly bridged through closing one or more superconducting switch(es) (S1).
3. Arrangement according to any one of the preceding claims, characterized in that a superconducting switch (S1) bridges a secondary coil (M2) with a resistance (R2) which is connected in series with said secondary coil (M2), wherein this resistance (R2) has a value, measured in ohms, of between 0 and the value of the inductance of the said secondary coil (M2), measured in Henrys.
4. Arrangement according to any one of the preceding claims, characterized in that a superconducting switch (S1) bridges a secondary coil (M2) together with a further superconducting switch (S2) which is connected in series with said secondary coil (M2).
5. Arrangement according to any one of the preceding claims, characterized in that at least one of the secondary coils (M1) is inductively coupled with exactly one of the primary coils (C1) and at least one further secondary coil (M2) is inductively coupled with exactly one further primary coil (C2).
6. Arrangement according to any one of the preceding claims, characterized in that at least two of the secondary coils (M1, M2) are largely inductively decoupled.
7. Arrangement according to any one of the preceding claims, characterized in that at least one section of the superconducting magnet coil system (M) is bridged by a resistance (r), wherein this resistance has a value, measured in ohms, of between 0 and the value of the inductance of the bridged section, measured in Henrys.
8. Method for operating a device in accordance with one of the preceding claims, comprising at least one first (M1) and one second (M2) superconducting secondary coil and a first superconducting switch (S1), characterized in that the first superconducting switch (S1), which bridges the second secondary coil (M2), is periodically opened and closed, wherein, when the first switch (S1) is closed, the current (I1) in a first primary coil (C1) is brought from an initial value to a final value and, when the first switch (S1) is opened, is reset largely to the initial value and, when the first switch (S1) is opened, the current (12) in a second primary coil (C2) is brought from an initial value to a final value and, when the first switch (S1) is closed, is largely reset to the initial value.
9. Method for operating a device according to claim 4 with at least one first (M1) and one second (M2) superconducting secondary coil, and a first (S1) and a second (S2) superconducting switch, characterized in that the first superconducting switch (S1), which bridges the second secondary coil (M2), is periodically opened and closed, wherein when the first switch (S1) is closed, the current (I1) in a first primary coil (C1) is brought from an initial value to a final value and, when the first switch (S1) is opened, is largely reset to the initial value and, when the first switch (S1) is opened, the current (12) in a second primary coil (C2) is brought from an initial value to a final value and, when the first switch (S1) is closed, is largely reset to the initial value and, when the first switch (S1) is closed, a second superconducting switch (S2), which is connected in series with the second secondary coil (M2) and is bridged together therewith by the first superconducting switch (S1), is opened at least temporarily.
10. Method according to claim 9, characterized in that the final value of the current in the second primary coil (C2) is substantially 0 ampere.
11. Method according to claim 10, characterized in that, prior to reaching the final current of 0 ampere in the second primary coil (C2), the current in this coil is set to a value I*L/K and, at the latest when this current has been reached, the second superconducting switch (S2) is opened, and the second superconducting switch (S2) remains superconductingly closed during resetting of the current (I2) in the second primary coil (C2) to the final value of 0 ampere, until the first superconducting switch (S1) is opened again, wherein I designates the current in the superconducting magnet coil system (M), L designates the self inductance of the second secondary coil (M2), and K designates the inductive coupling in Henry between the second secondary coil (M2) and the second primary coil (C2).

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