Method for charging a superconductor magnet system, with a main superconductor bulk magnet and a shield superconductor bulk magnet

Abstract

Charging method for a superconductor magnet system with reduced stray field, weight and space includes: arranging the system within a charger magnet bore; with T.sub.main>T.sub.main.sup.crit and T.sub.shield>T.sub.shield.sup.crit, applying a current I.sub.charger to the charger magnet and increasing I.sub.charger to a first current I.sub.1>0; lowering a main superconductor bulk magnet temperature T.sub.main to an operation temperature T.sub.main.sup.op, with T.sub.main.sup.op<T.sub.main.sup.crit, while keeping T.sub.shield>T.sub.shield.sup.crit; lowering I.sub.charger to a second current I.sub.2<0, thereby inducing a persistent current IP.sub.main in the main magnet; lowering a shield magnet temperature T.sub.shield to an operation temperature T.sub.shield.sup.op, with T.sub.shield.sup.op<T.sub.shield.sup.crit; increasing I.sub.charger to zero, thereby inducing a persistent current IP.sub.shield in the shield magnet; removing the magnet system from the charger bore, and keeping T.sub.main≤T.sub.main.sup.op with T.sub.main.sup.op<T.sub.main.sup.crit and T.sub.shield≤T.sub.shield.sup.op with T.sub.shield.sup.op<T.sub.shield.sup.crit; where: T.sub.main.sup.crit: main magnet critical temperature and T.sub.shield.sup.crit: shield magnet critical temperature.

Claims

1. Method for charging a superconductor magnet system, the superconductor magnet system comprising a main superconductor bulk magnet defining a main bulk bore, a shield superconductor bulk magnet defining a shield bulk bore, and a cryostat system having a room temperature bore, wherein the cryostat system contains the main superconductor bulk magnet and the shield superconductor bulk magnet, wherein the shield superconductor bulk magnet radially surrounds the main superconductor bulk magnet such that the main bulk bore lies radially within the shield bulk bore, and wherein the main superconductor bulk magnet and the shield superconductor bulk magnet are arranged coaxially with the room temperature bore such that the room temperature bore lies radially within the main bulk bore, the method comprising: step a) arranging the superconductor magnet system at least partially within a charger bore of a charger magnet; step b) with T.sub.main>T.sub.main.sup.crit and T.sub.shield>T.sub.shield.sup.crit applying an electrical current I.sub.charger to the charger magnet and increasing I.sub.charger to a first current I.sub.1>0, step c) lowering T.sub.main to or below an operation temperature T.sub.main.sup.op of the main superconductor bulk magnet, with T.sub.main.sup.op<T.sub.main.sup.crit, while keeping T.sub.shield>T.sub.shield.sup.crit; step d) lowering I.sub.charger to a second current I.sub.2<0, wherein a main persistent current IP.sub.main is induced in the main superconductor bulk magnet, which stays below a critical current of the main superconductor bulk magnet at T.sub.main, with T.sub.main≤T.sub.main .sup.op; step e) lowering T.sub.shield to or below an operation temperature T.sub.shield.sup.op of the shield superconductor bulk magnet, with T.sub.shield.sup.op<T.sub.shield.sup.crit: step f) increasing I.sub.charger to zero, wherein a shield persistent current IP.sub.shield is induced in the shield superconductor bulk magnet, which stays below a critical current of the shield superconductor bulk magnet at T.sub.shield, with T.sub.shield≤T.sub.shield.sup.op; step g) removing the superconductor magnet system from the charger bore of the charger magnet, and keeping T.sub.main at or below T.sub.main .sup.op withT.sub.main.sup.op<T.sub.main.sup.crit as well as T.sub.shield at or below T.sub.shield.sup.op, with T.sub.shield.sup.op<T.sub.shield.sup.crit, with T.sub.main: temperature of the main superconductor bulk magnet; T.sub.main.sup.crit: critical temperature of the main superconductor bulk magnet; T.sub.shield: temperature of the shield superconductor bulk magnet; and T.sub.shield: critical temperature of the shield superconductor bulk magnet, wherein the temperatures T.sub.main T.sub.shield are set to different values during the steps c) and d), and wherein the main superconductor bulk magnet and the shield superconductor bulk magnet are, at least temporarily, substantially thermally decoupled during the charging method.

2. Method according to claim 1, wherein the charger bore is arranged coaxially with the room temperature bore of the cryostat system in the step a).

3. Method according to claim 1, wherein |I.sub.2/I.sub.1|≤0.33.

4. Method according to claim 1, wherein 0.7≤|I.sub.1/I.sub.2|*|R.sub.main.sup.2/R.sub.shield.sup.2|≤1.4, with R.sub.main: average radius of the main superconductor bulk magnet; and R.sub.shield: average radius of the shield superconductor bulk magnet.

5. Method according to claim 1, wherein T.sub.main.sup.op≤0.75*T.sub.main.sup.crit and T.sub.shield.sup.op≤0.75*T.sub.shield.sup.crit.

6. Method according to claim 1, wherein the cryostat system comprises a normal cryocooler and thermal connections of the normal cryocooler to a main bulk thermal stage connected to the main superconductor bulk magnet and to a shield bulk thermal stage connected to the shield superconductor bulk magnet, during the step c) and the step d), a shield thermal switch in the thermal connection to the shield bulk thermal stage is kept open, and during the step e) and after the step e), the shield thermal switch is kept closed.

7. Method according to claim 1, wherein at least during the steps c) through f), the cryostat system is equipped with an auxiliary cryocooler, during the steps c) through f), cooling power of the auxiliary cryocooler is directed to a main bulk thermal stage connected to the main superconductor bulk magnet, during the steps e) and f), cooling power of the auxiliary cryocooler is directed to a shield bulk thermal stage connected to the shield superconductor bulk magnet, after the step f), the auxiliary cryocooler is removed from the cryostat system, and the cryostat system comprises a normal cryocooler and thermal connections of the normal cryocooler to the main bulk thermal stage and to the shield bulk thermal stage, and after the step f), the normal cryocooler provides cooling power to the main bulk thermal stage and to the shield bulk thermal stage.

8. Method according to claim 7, wherein the normal cryocooler provides cooling power to the main bulk thermal stage and to the shield bulk thermal stage only after the step f).

9. Method according to claim 7, wherein the thermal connection between the normal cryocooler and the main bulk thermal stage and/or the thermal connection between the normal cryocooler and the shield bulk thermal stage include/includes a weak thermal link or a thermal switch, and the weak thermal link or the thermal switch slows down or blocks heat conduction between the normal cryocooler and the main bulk thermal stage and/or between the normal cryocooler and the shield bulk thermal stage during at least some of the step c), the step d), the step e) and/or the step f).

10. Method according to claim 1, wherein the cryostat system includes two normal cryocoolers, with a first of the normal cryocoolers having a thermal connection to a main bulk thermal stage connected to the main superconductor bulk magnet, and a second of the normal cryocoolers having a thermal connection to a shield bulk thermal stage connected to the shield superconductor bulk magnet, and the first normal cryocooler provides cooling power to the main bulk thermal stage beginning with the step c), and the second normal cryocooler provides cooling power to the shield bulk thermal stage beginning with the step e).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is shown through exemplary embodiments in the drawing.

(2) FIG. 1 shows a schematic longitudinal section of an inventive charger system, comprising a superconductor magnet system in a first embodiment with one normal cryocooler;

(3) FIG. 2 shows a schematic longitudinal section of an inventive charger system, comprising a superconductor magnet system in a second embodiment with one normal cryocooler and an auxiliary cryocooler;

(4) FIG. 3 shows a schematic longitudinal section of an inventive charger system, comprising a superconductor magnet system in a third embodiment with two normal cryocoolers;

(5) FIG. 4 shows a diagram illustrating bulk superconductor temperatures, charger current and magnetic flux densities during an exemplary variant of the inventive method for charging a superconductor magnet system.

DETAILED DESCRIPTION

(6) The invention is based on a hardware configuration of nested superconductor bulk rings/cylinders, which are charged by procedure which can be referred to as a “bipolar field-cooling”. The hardware configuration has to provide that two subdivisions of the nested superconductor bulks are kept at substantially different yet stable temperatures over a relatively long period of time, of the order of several hours.

(7) The first part of the inventive “bipolar field-cooling” cycle corresponds to a conventional field-cooling process. An outsert (“charger”) magnet is ramped up to peak field while the nested superconductor bulks all are above their critical temperature (typically with the nested superconductor bulks above 100 K for REBCO). Then the inner bulk or bulks (i.e. the main superconductor bulk magnet, also referred to as “direct bulk” here) are cooled down to well below their critical temperature (typically with the direct bulk at 40-60 K for REBCO), and the charger magnet is ramped to zero current (“first charging step”). In contrast to the conventional field cooling procedure, which stops at zero amps in the charger magnet, the invention suggests to continue the ramp-down in current to negative currents, providing an inverse polarization of the magnetic field of the charger magnet. Only then, the outer bulk or bulks (i.e. the shield superconductor bulk magnet, also referred to as “shield bulk” here) is cooled below their critical temperature (typically with the direct bulk at 40-60 K for REBCO), and becomes superconducting. Ramping “up” the current in the charger magnet to zero (“second charging step”) then leads to a trapped inverse polarization in the shield bulk. As the shield bulk has a much larger cross section (orthogonal to the magnetic axis) as compared to the direct bulk, the amplitude (or absolute final current strength in the shield bulk and corresponding magnetic field) of the second (“inverse”) charging step can be much smaller than the amplitude (or absolute final current strength in the direct bulk and corresponding magnetic field) of the first (“direct”) charging step in order to achieve comparable, opposing dipole moments, leading only to a loss in magnetic field that is small (e.g. 20% or less) as compared to the overall value. Note that the wall thickness of the shield bulk can accordingly be much smaller than the wall thickness of the direct bulk. In the example described below at FIG. 4, the direct charging step corresponds to 7.2 T, the inverse charging step corresponds to 1.2 T, and the net trapped magnetic flux is 6 T. This corresponds to a shield bulk diameter that is roughly twice the diameter of the direct bulk. Note that the magnetic flux is the magnetic flux density times area, and area scales as diameter squared.

EXPLANATION OF THE FIGURES

(8) It should be noted that the figures are schematic in nature, and some features may be shown in an exaggerated or understated way, in order to show particular features of the invention more clearly.

(9) FIG. 1 shows an inventive charging system 1, comprising a charger magnet 2 and an inventive superconductor magnet system 3, according to a first embodiment.

(10) The charger magnet 2 comprises here a solenoid type magnet coil 4 wound from a low temperature superconductor wire, such as a NbTi wire. The charger magnet 2 is arranged in a charger cryostat 5, which is cooled by a cooling device not shown in detail. The charger magnet 2 may be loaded with a charger current I.sub.charger via electrical contacts and a current supply, also not shown in detail. When charged, the charger magnet 2 generates a charger magnetic flux density B.sub.charger within a charger bore 6. The charger magnetic flux density B.sub.charger is basically parallel to a common axis A at the of the charger bore 6.

(11) The superconductor magnet system 3 comprises a cryostat system 7, a main superconductor bulk magnet 10 and a shield superconductor bulk magnet 11. In the embodiment shown, the cryostat system 7 comprises a common vacuum tank 8 encompassing both the main superconductor bulk magnet 10 and the shield superconductor bulk magnet 11. The cryostat system 7 or the common vacuum tank 8, respectively, has a room temperature bore 9 (compare wall 9a) arranged about the common axis A.

(12) In general, a main or shield superconductor bulk magnet 10, 11 is of a closed ring shape, made of a single superconductor ring structure or of a plurality of ring-shaped superconductor sub-structures such as discs or coatings on a substrate (such as a sheet metal or a foil); the ring-shaped sub-structures are arranged coaxially and stacked axially and/or radially, and may be combined into a so-called “composite bulk” by structurally connecting the sub-structures. All these variants constitute superconductor bulk magnets, in accordance with the invention.

(13) Structures or sub-structures for a superconductor bulk magnet may be grown from a melt; sub-structures to be combined into a “composite bulk” are typically made by coating a substrate. A superconductor bulk magnet, in accordance with the invention, allows a trapping of a magnetic field in its bore, wherein the superconductor bulk magnet in general does not possess any electric current supplies, but instead is designed for inductive charging only.

(14) The shield superconductor bulk magnet 11 shown in FIG. 1 is of generally cylinder jacket type shape, with a shield bulk bore 11a. Radially within the shield bulk bore 11a, there is arranged the main superconductor bulk magnet 10, which is also of generally cylinder jacket type shape, with a main bulk bore 10a. Radially within the main bulk bore 10a, there is arranged the room temperature bore 9 of the cryostat system 7. The main superconductor bulk magnet 10 comprises a high temperature superconductor material with a critical temperature T.sub.main.sup.crit, and the shield superconductor bulk magnet comprises a high temperature superconductor material with a critical temperature T.sub.shield.sup.crit. It should be noted that the superconductor materials of the two bulk magnets 10, 11 are preferably chosen to be identical, but may be chosen to be different from one another, if desired.

(15) The charger magnet 2 or its charger bore 6, respectively, further the shield superconductor bulk magnet 11 or its shield bulk bore 11a, respectively, further the main superconductor bulk magnet 10 or its main bulk bore 10a, and further the room temperature bore 9 of the charger system 7 are arranged coaxially with respect to the common axis A.

(16) The common axis A defines a z-axis of the superconductor magnet system 3. In a charged state, the superconductor magnet system 2 generates a basically homogeneous magnetic flux density (often called B.sub.0) basically in parallel to the z-axis in a sample volume 12 located about a magnetic center C within the room temperature bore 9. The magnetic flux density B.sub.0 can be used e.g. for NMR measurements on a sample in the sample volume 12. The magnetic flux density B.sub.0 results, above all, from a main persistent current IP.sub.main running circularly around the sample volume 12 (and around the z axis) in the main superconductor bulk magnet 10. At the same time, a shield persistent current IP.sub.shield runs circularly around the sample volume 12 (and around the z axis) in the shield superconductor bulk magnet 11, however with an opposite current direction (“inverse polarity”), what reduces the stray field of the superconductor magnet system 3, but also reduces somewhat the total magnetic flux density at the magnetic center C or the sample volume 12, respectively. The current strengths are preferably chosen such that 0.6≤|IP.sub.main/IP.sub.shield|*|R.sub.main.sup.2/R.sub.shield.sup.2|≤1.6 applies, preferably with |IP.sub.main/IP.sub.shield|*|R.sub.main.sup.2/R.sub.shield.sup.2|=1, with R.sub.main being the average radius of the main superconductor bulk magnet 10 and R.sub.shield being the average radius of the shield superconductor bulk magnet 11. In the example shown, about R.sub.shield=2*R.sub.main applies. In case of constant wall thickness of the bulk magnets 10, 11, as shown here, the average radius is simply the middle between the inner radius and the outer radius; else an averaging has to be done about the circumference and/or along the length of the respective bulk magnet 10, 11.

(17) In order to allow for the inventive charging procedure (an embodiment of which is described at FIG. 4 below) or the above described directions of persistent currents, respectively, the superconductor magnet system 3 is equipped with a cooling system 13. The cooling system 13 is designed to allow at least the following three states or temperature distributions of the temperature T.sub.main of the main superconductor magnet 10 and T.sub.shield of the shield superconductor bulk magnet 11: State a) T.sub.main>T.sub.main.sup.crit and concurrently T.sub.shield>T.sub.shield.sup.crit; State b) T.sub.main<T.sub.main.sup.crit and concurrently T.sub.shield>T.sub.shield.sup.crit; and State c) T.sub.main<T.sub.main.sup.crit and concurrently T.sub.shield<T.sub.shield.sup.crit.

(18) The cooling system 13 comprises a main bulk thermal stage 14 attached to the main superconductor bulk magnet 10, and a shield bulk thermal stage 15 attached to the shield superconductor bulk magnet 11. Each thermal stage 14, 15 comprises its own temperature sensor 14a, 15a and its own electrical heater 14b, 15b. In the example shown, the thermal stages 14, 15 are arranged on opposite axial sides of the sample volume 12 with respect to the common axis A.

(19) In the embodiment of FIG. 1, the cooling system 13 comprises a single normal cryocooler 16, which reaches into the cryostat system 7 or its vacuum tank 8, respectively, and in particular has its cold stage 16a inside the cryostat system 7. The normal cryocooler 16 or its cold stage 16a, respectively, is connected via a thermal connection 17 (e.g. a metal thermal conductor) to the main bulk thermal stage 14, and is connected via a thermal connection 18 (e.g. a metal thermal conductor) to the shield bulk thermal stage 15. The thermal connection 18 to the shield bulk thermal stage 18 here includes a shield thermal switch 18a. With the shield thermal switch 18a open, the normal cryocooler 16 provides, when operating, its cooling power basically only to the main bulk thermal stage 14, in particular in order to establish state b) mentioned above; further, a thermal short-circuit of the thermal stages 14, 15 via the cold stage 16a is prevented. With the shield thermal switch 18a closed, the normal cryocooler 16 provides, when operating, cooling power to both thermal stages 14, 15, in particular in order to establish state c) mentioned above. State a) mentioned above is typically established with the normal cryocooler 16 non-operating. Using the electrical heaters 14b, 15b, unwanted (remaining) cooling power input to the thermal stages 14, 15 may be opposed, in order to ensure a desired temperature above the critical temperature of the corresponding bulk magnet 10, 11.

(20) The normal cryocooler 16 is here used during both the charging procedure of the superconductor magnet system 3 and during steady state use of the superconductor magnet system 3 after charging, and is permanently installed in the superconductor magnet system 3.

(21) FIG. 2 shows another inventive charging system 1 with an inventive superconductor magnet system 3, similar to the charging system 1 shown in FIG. 1, so only the major differences are explained in detail.

(22) In the embodiment of FIG. 2, the cooling system 13 comprises the permanently installed normal cryocooler 16, and further an auxiliary cryocooler 19 which is installed only for the charging procedure, but is removed during later steady state use.

(23) The thermal connections 17, 18 from the normal cryocooler 16 to the thermal stages 14, here each comprise a thermal switch 17a, 18a, so the normal cryocooler 16 can be thermally decoupled from the thermal stages 14, 15 during the charging procedure, and also a thermal short-circuit between the thermal stages 14, 15 is prevented during charging. In particular, the normal cryocooler 16 can be non-operating during the charging procedure, which is useful if the normal cryocooler 16 cannot be operated as long as the stray field of the superconductor magnet system 3 is still high. The normal cryocooler 16 can be, for example, of pulse tube type.

(24) The auxiliary cooler 19 provides for cooling power during the charging procedure, and may direct cooling power independently to the thermal stages 14, 15. The auxiliary cryocooler 19 here comprises its own auxiliary cryostat 19b, and provides a fluid coolant (e.g. helium) at its cold stage 19a.

(25) In the embodiment shown, a forward line 20a for cold coolant leads to the main bulk thermal stage 14, and a return line 20b for warmed coolant leads from the main bulk thermal stage 14 back to the cold stage 19a. For controlling the cooling power at the main bulk thermal stage 14, a flow control device (adjustable valve) 20c is installed in one of the lines, here in the forward line 20a. Further, from the cold stage 19a of the auxiliary cryocooler 19 leads a forward line 21a for cold coolant to the shield bulk thermal stage 15, and a return line 21b for warmed coolant from the shield bulk thermal stage 15 back to the cold stage 19a. For controlling the cooling power at the shield bulk thermal stage 15, a flow control device (adjustable valve) 21c is installed in one of the lines, here in the forward line 21a.

(26) For establishing state a) mentioned above, typically the auxiliary cryocooler 19 is operating at a very low flow of coolant, with the flow control devices 20c, 21c almost shut, since only little cooling power is required. For establishing state b), the auxiliary cryocooler 19 is operating at a medium flow of coolant, typically with the flow control device 20c being wide open, whereas the flow control device 21c is still almost shut. For establishing state c), the auxiliary cryocooler 19 is initially operating at high flow of coolant, and typically both flow control devices 20c, 21c are wide open. Once both thermal stages 14, 15 have reached their respective desired low temperature (operation temperature) below the critical temperature of the respective bulk magnet 10, 11, cooling power input is shifted to the then turned-on normal cryocooler 16 (with the thermal switches 17a, 18a closed), and the auxiliary cryocooler 19 is turned off. Then the auxiliary cryocooler 19 typically together with parts of the lines 20a, 20b, 21a, 21b outside the cryostat system 7 and together with the auxiliary cryostat 19b are removed.

(27) FIG. 3 shows another inventive charging system 1 with a superconductor magnet system 3, similar to the charging system 1 shown in FIG. 1, so again only the major differences are explained in detail.

(28) In the embodiment of FIG. 3, the cooling system 13 comprises two permanently installed normal cryocoolers 16, 22. Both normal cryocoolers 16, 22 reach with their respective cold stage 16a, 22a into the cryostat system 7 or its vacuum tank 8, respectively.

(29) First normal cryocooler 16 has a thermal connection 23 from its cold stage 16a to the main bulk thermal stage 14. Second cryocooler 22 has a thermal connection 24 from its cold stage 22a to the shield bulk thermal stage 15. The two normal cryocoolers 16, 22 can be operated independently.

(30) For establishing the state a) mentioned above, typically both normal cryocoolers 16, 22 are operating at reduced cooling power. Further, in order to establish state b) mentioned above, typically first normal cryocooler 16 is operating at increased cooling power, and second normal cryocooler 22 is still operating at reduced cooling power. Finally, in order to establish state c) mentioned above, typically both normal cryocoolers 16, 22 are operating at increased cooling power.

(31) If desired, the thermal connection 24 from the cold stage 22a of second normal cryocooler 22 to the shield bulk thermal stage 15 may be equipped with a shield thermal switch (not shown), which is closed only for establishing state c).

(32) FIG. 4 illustrates in a schematic diagram the temporal sequence of steps and states during the course of an exemplary variant of the inventive method for charging a superconductor magnet system, for example as illustrated in one of the FIG. 1, 2 or 3. On the axis to the right, the time t is plotted, in minutes. On the axis to the top, there is illustrated the applied current I (top third) in Ampere, generated magnetic flux densities B in Tesla (middle third) and applied temperatures T in Kelvin (bottom third).

(33) The method begins in a state a), wherein the temperature T.sub.main of the main superconductor bulk magnet and the temperature T.sub.shield of the shield superconductor bulk magnet are above the respective critical temperatures T.sub.main.sup.crit (here about 95 K) and T.sub.shield.sup.crit (here about 100 K) at which the bulk magnets become superconducting. In the variant shown, T.sub.main and T.sub.shield are both at about 110 K during state a); this may be achieved by applying a low level of cooling power to the thermal stages. In this state a), as a first step a) taking place before time point A, the superconductor magnet system is arranged in the charger bore of the charger magnet, typically at the premises of the manufacturer of the superconductor magnet system.

(34) Still in state a), there is a step b), starting at time point A and taking until point of time B, wherein a charger current I.sub.charger is applied and increased from zero to a first (positive) current I.sub.1, here of about 120 A. This causes an increase of a magnetic flux density B.sub.charger generated by the charger magnet, here to about 6 T. Note that all indications of magnet flux relate to the only relevant component in z direction (in parallel to the common axis) and to the magnetic center (and its homogeneous vicinity, i.e. the sample volume) of the superconductor magnet system, for simplicity. Since both bulk magnets are in the non-superconducting state, any currents induced therein during step b) decay quickly, so no noticeable magnetic flux densities B.sub.main from the main superconductor bulk magnet or B.sub.shield from the shield superconductor bulk magnet are noted in the diagram. However, after step b), a short relaxation period from time point B to time point C is awaited.

(35) Then in a step c), between time points C and D, the temperature T.sub.main of the main superconductor bulk magnet is lowered to an operation temperature T.sub.main.sup.op of the main superconductor bulk magnet, with T.sub.main.sup.op<T.sub.main.sup.crit; note that T.sub.main.sup.op corresponds to a “maximum” for T.sub.main after step c). In the example shown, about T.sub.main.sup.op=50 K and about T.sub.main.sup.op=0.53*T.sub.main.sup.crit applies. Accordingly, the main superconductor bulk magnet becomes superconducting. After step c) is finished, the superconductor bulk magnet has reached state b), wherein T.sub.main=T.sub.main.sup.op with T.sub.main.sup.op<T.sub.main.sup.crit, but still with T.sub.shield>T.sub.shield.sup.crit. Again, after step c), a short relaxation time is awaited between time points D and E.

(36) In state b), in a subsequent step d), between time steps E and F, the charger current I.sub.charger is lowered from the first (positive) current I.sub.1 to a second (negative) current I.sub.2. This causes a corresponding decrease of the magnetic flux density B.sub.charger generated by the charger magnet, here from about 6 T to about −1.2 T. Concurrently, a circular (positive) current I.sub.main (not shown in the diagram) is induced in the main superconductor bulk magnet, which is in a superconducting state. This induced current I.sub.main comes along with a corresponding (proportional) magnetic flux density B.sub.main originating from the main superconductor bulk magnet, which increases from zero to here about 7.2 T. Together, the magnetic flux densities B.sub.charger and B.sub.main result during and at the end of step d) in a total magnetic flux density B.sub.total, which is constantly at 6 T, corresponding to the magnetic flux density of B.sub.charger at the end of step b) before, which is in accordance to the principle that the (total) magnetic flux density inside of a superconducting magnet, here inside of the main superconductor bulk magnet, stays constant. Since the shield superconductor bulk magnet is still non-superconducting, any current induced there decays quickly and is not relevant. Subsequent to step d), between time points F and G, a short relaxation time is again awaited.

(37) Next step e), between time points G and H, comprises lowering the temperature T.sub.shield of the shield superconductor bulk magnet from here about 110 K to an operation temperature T.sub.shield.sup.op of the shield superconductor bulk magnet of here about 50 K; note that T.sub.shield.sup.op corresponds to a “maximum” for T.sub.shield after step e). So then, in the example shown, T.sub.shield=T.sub.shield.sup.op with T.sub.shield.sup.op<T.sub.shield.sup.crit, here with about T.sub.shield.sup.op=0.5*T.sub.shield.sup.crit, and the shield superconductor bulk magnet becomes superconducting. At the end of step e), both T.sub.main=T.sub.main.sup.op with T.sub.main.sup.op<T.sub.main.sup.crit and T.sub.shield=T.sub.shield.sup.op with T.sub.shield.sup.op<T.sub.shield.sup.crit applies, so state c) has been reached.

(38) In the example shown, T.sub.main.sup.op and T.sub.shield.sup.op are equal at about 50 K, which can be established and maintained relatively easily in a common vacuum tank of a cryostat system and using a common (normal) cryocooler. However, it is also possible to choose T.sub.main.sup.op and T.sub.shield.sup.op at different temperatures, in particular using two separate (normal) cryocoolers, if desired. It should also be mentioned here that although shown as being practically constant in FIG. 4 after having been lowered in steps c) and e), T.sub.main and T.sub.shield may fluctuate somewhat in accordance with the invention, provided that T.sub.main stays at or below T.sub.main.sup.op from state b) on (i.e. after step c), respectively), and T.sub.shield stays at or below T.sub.shield.sup.op from state c) on (i.e. after step e), respectively), and any induced currents in the bulk magnets can be superconductively carried completely, i.e. without reaching the respective critical current carrying capacity of the bulk magnet.

(39) In the variant shown, again a short relaxation time is awaited between time points H and I.

(40) Then, in the state c), in step f) between time points I and J, the charger current I.sub.charger is increased from (negative) second current I.sub.2 to zero. This comes along with a corresponding increase of the magnetic flux density B.sub.charger generated by the charger magnet from here −1.2 T to zero. This induces a (negative) current I.sub.shield (not shown in the diagram) within the shield superconductor bulk magnet, which comes along with a corresponding (proportional) magnetic flux density B.sub.shield caused by the shield superconductor bulk magnet, here of about −1.2 T at the end of step f). In other words, the magnetic flux inside the shield superconductor bulk magnet formerly generated by the charger magnet is trapped by the shield superconductor bulk magnet inside of it, corresponding to a constant flux inside the superconducting shield superconductor bulk magnet. The total magnetic flux density B.sub.total during and after step f), i.e. the sum of B.sub.main, B.sub.shield and B.sub.charger, remains constant at 6 T, corresponding to a constant magnetic flux inside the superconducting main superconductor bulk magnet.

(41) At the end of step f) at time point J, the steady state of the superconductor magnet system has been reached. In a subsequent step g) taking place after time point J, the superconductor magnet system may now be removed from the charger magnet and transported to a site of application. As long as T.sub.main and T.sub.shield are kept at or below T.sub.main.sup.op and T.sub.shield.sup.op, hence well below T.sub.main.sup.crit and T.sub.shield.sup.crit, respectively, i.e. as long as state c) is maintained, the magnetic flux density B.sub.total=B.sub.0, with here B.sub.0=6 T, stays available and may be used for measurements, in particular NMR measurements of samples in the sample volume.

(42) The current I.sub.main of the main superconductor bulk magnet stays at a (positive) main persistent current IP.sub.main (not shown in detail in the diagram), which is proportional to the final B.sub.main value of here about 7.2 T, and the current I.sub.shield stays at a (negative) shield persistent current IP.sub.shield (also not shown in the diagram), which is proportional to the final B.sub.shield value of here about −1.2 T. It should be noted that typically, the absolute values of IP.sub.main and IP.sub.shield are of the order of tens to hundreds of kA (e.g. IP.sub.main=360 kA and IP.sub.shield=−60 kA), since the charger magnet coil in general has thousands of windings, whereas the main and shield superconductor bulk magnet typically have only one winding each. In the example shown, the absolute value of the ratio of the final B.sub.main and B.sub.shield values is about |7.2 T/1.2 T|=6, so about |IP.sub.main/IP.sub.shield|=6 applies here. With a typical ratio of R.sub.shield=2*R.sub.main, this results in a ratio |IP.sub.main/IP.sub.shield|*|R.sub.main.sup.2/R.sub.shield.sup.2| of about 1.5; note that values between 0.6 and 1.6 are preferred for this ratio, in order to have a good active shielding effect of the shield superconductor bulk magnet for the charged superconductor magnet system.

(43) In the variant illustrated in FIG. 4, the inventive charging procedure takes about 8 hours. However, the inventive procedure may in other variants take a shorter time, such as 3 hours or less, or may take a longer time, such as 24 hours or more, depending on the thermal coupling between the main and shield superconductor bulk magnets and the cooling system, which counteracts a warming e.g. caused by hysteresis effects during charging.

(44) In summary, an embodiment of the invention, as described above, relates to a method for charging a superconductor magnet system (3) comprising coaxially a main superconductor bulk magnet (10) and a shield superconductor bulk magnet (11), the method comprising the following steps: step a) arranging the superconductor magnet system (3) at least partially within a charger bore (6) of a charger magnet (2), step b) with T.sub.main>T.sub.main.sup.crit and T.sub.shield>T.sub.shield.sup.crit, applying an electrical current I.sub.charger to the charger magnet (2) and increasing I.sub.charger to a first current I.sub.1>0, step c) lowering T.sub.main to or below an operation temperature T.sub.main.sup.op of the main superconductor bulk magnet (10), with T.sub.main.sup.op<T.sub.main.sup.crit, while keeping T.sub.shield>T.sub.shield.sup.crit; step d) lowering I.sub.charger to a second current I.sub.2<0, wherein a main persistent current IP.sub.main is induced in the main superconductor bulk magnet (10); step e) lowering T.sub.shield to or below an operation temperature T.sub.shield of the shield superconductor bulk magnet (11), with T.sub.shield.sup.op<T.sub.shield.sup.crit; step f) increasing I.sub.charger to zero, wherein a shield persistent current IP.sub.shield is induced in the shield superconductor bulk magnet (11); step g) removing the superconductor magnet system (3) from the charger bore (6) of the charger magnet (2), and keeping T.sub.main at or below T.sub.main.sup.op with T.sub.main.sup.op<T.sub.main.sup.crit as well as T.sub.shield at or below T.sub.shield.sup.op with T.sub.shield.sup.op<T.sub.shield.sup.crit; with T.sub.main: temperature of the main superconductor bulk magnet (10); T.sub.main.sup.crit: critical temperature of the main superconductor bulk magnet (10); T.sub.shield: temperature of the shield superconductor bulk magnet (11); and T.sub.shield.sup.crit: critical temperature of the shield superconductor bulk magnet (11). The invention allows the stray field of the superconductor magnet system to be reduced, so that less weight and space are required for shielding purposes than were required conventionally.

LIST OF REFERENCE SIGNS

(45) 1 charging system 2 charger magnet 3 superconductor magnet system 4 magnet coil (charger magnet) 5 charger cryostat 6 charger bore 7 cryostat system 8 vacuum tank 9 room temperature bore 9a wall of the room temperature bore 10 main superconductor bulk magnet 10a main bulk bore 11 shield superconductor bulk magnet 11a shield bulk bore 12 sample volume 13 cooling system 14 main bulk thermal stage 14a temperature sensor (main bulk thermal stage) 14b electric heater (main bulk thermal stage) 15 shield bulk thermal stage 15a temperature sensor (shield bulk thermal stage) 15b electrical heater (shield bulk thermal stage) 16 (first) normal cryocooler 16a cold stage 17 thermal connection 17a thermal switch 18 thermal connection 18a (shield) thermal switch 19 auxiliary cryocooler 19a cold stage 20a forward line 20b return line 20c flow control device 21a forward line 21b return line 21c flow control device 22 (second) normal cryocooler 22a cold stage 23 thermal connection 24 thermal connection A common axis B.sub.charger magnetic flux density generated by charger magnet B.sub.main magnetic flux density generated by main superconductor bulk magnet B.sub.shield magnetic flux density generated by shield superconductor bulk magnet B.sub.total total magnetic flux density generated by charger magnet, main superconductor bulk magnet and shield superconductor bulk magnet C magnetic center I.sub.charger current of charger magnet IP.sub.main persistent current of main superconductor bulk magnet IP.sub.shield persistent current of shield superconductor bulk magnet I.sub.main induced current of main superconductor bulk magnet I.sub.shield induced current of shield superconductor bulk magnet I.sub.1 first current (charger magnet) I.sub.2 second current (charger magnet) R.sub.main average radius of main superconductor bulk magnet R.sub.shield average radius of shield superconductor bulk magnet T.sub.main temperature of main superconductor bulk magnet T.sub.main.sup.crit critical temperature of main superconductor bulk magnet T.sub.main.sup.op operation temperature of main superconductor bulk magnet T.sub.shield temperature of shield superconductor bulk magnet T.sub.shield.sup.crit critical temperature of shield superconductor bulk magnet T.sub.shield.sup.op operation temperature of shield superconductor bulk magnet