Device for generating a magnetic field

Abstract

Device (20) for generating a magnetic field, comprising a coil former (1) around which windings of a conductor coil are arranged, the coil former (1) having a first (21) and a second end (22); a thermal contact surface (14) arranged at the first end (21) of the coil body (1) for thermal contact with a cooling device; wherein the conductor coil (1) has contacting areas (10, 11) at the first (21) or second end (22) of the coil body (1); and wherein the conductor coil (1) is connected to at least one protective circuit (4, 5, 6, 7) which is arranged at the opposite end (22, 21) of the coil body (1).

Description

The present invention relates to a device for generating a magnetic field.Superconducting magnets are commonly known for their ability to generate strong magnetic fields without energy loss. This knowledge returns until 1955 when George Yntema (Cornell) first reached a magnetic field of 0.7 T at 4.2 K with a niobium wire, at a current density of 888 A / mm 2. This discovery caused many researchers to work worldwide on this subject, so that further breakthroughs followed impact-to-impact: 8.8 T magnetic field with Nb 3 Sn as the wire material was already achieved in December 1960 by Erniie Buehler, at a current density of 1500 A / mm 2. The first international conference on high magnetic fields already took place in November 1961, and the first company dealing with the subject of superconducting magnets was made the same year (Oxford Instruments Ltd). At the same time, the demand for drawn wires of different niobium alloys such as NbNb and NbTi has increased greatly, whereupon the first company which approached this topic was devised (Supercon Inc).Since the 1960s, commercialization of superconducting magnets has advanced at a fast pace, particularly driven by the need for magnets for nuclear magnetic resonance imaging. This is the largest market today for superconducting magnets.At the same time, various superconducting magnets for large research purposes are produced which are intended to achieve particularly updated goals from a technological point of view, in particular in synchrotrons, particle accelerators and fusion reactors. This form of magnet is highly complex and very expensive, but irrelevant to the broad mass of the scientist.Furthermore, in the laboratory sector there is a significantly low availability of high-quality, but also low-cost superconducting magnets for applications in basic research, such as, for example, in material sciences. These magnets are typically referred to as 1-D solenoids or 2-D / 3-D vector magnets. For example, field strengths of 1 T to 18 T in the range of solenoids and, for example, 9 / 1 / 1 T to 9 / 5 / 5 T in the range of vector magnets can be achieved.It is the object of the present invention to provide a device for generating a magnetic field which offers improvements in the field of design and construction compared to the prior art, with particular focus on scalability, reduction of waste and increased quality and safety on the part of the user.This object is achieved according to the invention by a device having the features of claim 1.According to this, the object is achieved by a device for generating a magnetic field, comprising a coil body around which windings of a conductor coil are arranged, wherein the coil body has a first and a second end. Furthermore, a thermal contact surface arranged at the first end of the coil body is provided for thermal contact with a cooling device. In this case, the conductor coil has contact regions at the first or second end of the coil body. In addition, the conductor coil is connected to at least one protective circuit which is arranged at the opposite end of the coil body.The device is designed in particular for applications in helium-free environments.The device can be designed in particular as a 1-D solenoid. An extension to a 2-D or 3-D vector magnet can be obtained here by transferring the present design to a plurality of spatial axes.In superconducting magnets, an abrupt transition of the wire material into the normally conducting state can occur, in particular in the case of temperature fluctuations, such as, for example, in the case of local heating of the material. The sudden increase in resistance results in a rapid decrease in current flow in the coil, which in turn induces high voltage spikes. One speaks of a "quench". The high voltages occurring in this case can damage the magnet.At least one protective circuit is provided in the device. In a partitioned magnet, protection circuits may be provided for the individual partitions.The protective circuit comprises, for example, protective diodes, by means of which a rapid decay of the high induction voltage is achieved in the quench case. In particular, a protective circuit for the coil body or for a partition of the coil body comprises two protective diodes with opposite forward directions. The protective diodes or the corresponding protective circuits therefore serve for protecting the magnet in the quench case.In one embodiment, the thermal contact surface is formed in the case of a flange arranged at the first end of the coil body.In particular, the flange can be formed from copper.The coil body can be formed in a manner known per se. The windings of the conductor coil are formed of a wire material wound around the bobbin.In a further embodiment, the coil body is designed as a hollow cylinder, in particular with an inner sample space.The device can have, for example, a sample tube with a free inner diameter between 1" and 5".A sample holder can be arranged in the sample tube, for example, and / or devices for manipulating a sample can be provided.The bobbin has first and second ends opposite to each other. The ends of the coil body are defined in particular along its longitudinal axis.The coil body is in particular constructed in such a way that the radial compression, which is caused by a high tensile stress of the wire and a large number of windings, does not lead to any deformation of the coil body or that it is at most in the micrometer range.For cooling the magnet, in particular in a helium-free environment, a thermal contact surface is provided at the first end of the coil body. The cooling can then take place by thermal contact with a cooling device known per se.A cryostat, pulse tube or GM cooler can be used as the cooling device, for example.In a development, the coil body has at least one slot formed along a longitudinal axis, which slot is formed in particular to reduce induction of eddy currents in the coil body. The slot can furthermore be formed obliquely to the longitudinal axis. Furthermore, a plurality of slots can be provided, which can be distributed, for example, regularly around the circumference of the coil body.The conductor coil is formed of a superconducting wire material, in particular.In one embodiment, the coil body is formed from a material having high thermal conductivity at low-temperature temperatures, in particular from copper or aluminum. The coil body has a thermal conductivity of, for example, 400 to 20,000 W / m*K at 4 K, in particular in the case of copper.In a further embodiment, the device further comprises at least one thermalization element which is connected in a thermally conductive manner to the thermal contact surface or to the flange and which bears against the conductor coil from the outside. In particular, an insulation layer is formed between the thermalization element and the conductor coil. The thermalizing element may be directly attached to the flange. The thermalization element can be configured, for example, in the form of a strip, a net, as a woven fabric or as a braid. The thermalization element can be formed from a material with good thermal conductivity, for example from copper.For example, a copper braid can be provided as thermalization element, which is in thermal contact with the thermal contact surface at the first end of the coil body or with the flange arranged there or can be attached directly thereto.The thermalization element is configured in particular to bring a radially outer region of the conductor coil into improved thermal contact with the thermal contact surface and to improve the cooling in this outer region.In one development, the conductor coil has at least two partitions.In particular, the partitions of the conductor coil can be connected in series. This can be advantageous due to the high inductance of the conductor coil. Typically, the segments can be formed with inductances of 10 H to 100 H.In one embodiment, the partitions of the conductor coil are arranged radially or axially with respect to one another.In a further embodiment, the conductor coil is formed with a wire made of a superconducting material or high-temperature superconducting material, for example, NbTi and / or Nb 3 Sn.It can be provided that a field strength of 1 T to 9 Tesla is generated. NbTi, for example, can be used as the material for the conductor coil.Furthermore, it can be provided that a field strength of 1 T to 18 T is obtained. Nb 3 Sn, for example, can be provided as the material for the conductor coil.The contacting portions of the conductor coil are portions in which the superconducting wire material can be connected to a power supply. In a contact-making region, the superconducting wire can be passed through so-called "bobbins" which mechanically protect the contact region.The bobbins are used in particular to establish an electrical connection between the superconducting material of the conductor coil and one of the connected cables leading to a power source, in particular a copper cable. In addition, the bobbins can ensure that the superconducting wire material is cooled as well as possible up to the transition to the copper conductor, since otherwise a quench could occur at this critical point, in particular during operation close to the critical current density.In the device, the conductor coil or a partition of the conductor coil is connected to the protective circuit from the contact region. For this purpose, a line is provided, in particular a copper line. Since the protection circuit is arranged at the other end of the coil body, this copper line is routed along the length of the coil body.In the device, the contact regions can be arranged at the second end of the coil body and the protection circuit at the first end. In this case, the protective circuit is located close to the thermal contact surface of the coil body, so that improved cooling is ensured and, in the quenching case, the electrical energy converted into heat in the protective circuit can be dissipated particularly quickly and efficiently.By arranging the protective circuit and in particular the protective diodes used therein as close as possible to a cold reservoir, in particular with a connection as directly as possible to a cooling head, a cooling which is as quick and good as possible is achieved. If the protective circuit is arranged at a greater distance therefrom, high temperature gradients can occur during a quench because of the limited thermal conductivity of the materials used, which are undesirable.The bobbins can be thermally contacted, for example, with copper braids or similar devices for improved heat conduction in order to optimally thermally couple them to the cooling system. A heat input, which can be carried out, for example, via the connected feed lines, can thus be sufficiently removed, so that only a small temperature gradient occurs.In the case of a quench, the protective circuit can dissipate an energy in the range of significantly more than 100 kJ in a few seconds. The heat generated in this case can be dissipated in an optimum manner by arranging the protective diodes on the side of the cold head.Conversely, the contact regions can be arranged at the first end of the coil body and the protection circuit at the second end of the coil body. In this case, it is advantageously ensured that the contact regions are cooled particularly well and that, for example, a transition from copper to superconducting wire material is not heated too strongly. If bobbins are provided in the contacting region, these too can be cooled particularly efficiently and quickly.In one development, the conductor coil is produced in a wet winding method. In this case, the wire layers of the conductor coil are optionally arranged in a hexagonal arrangement or a circular packing.The wire can be wound and cast under high tension. In particular, the superconducting wire material is arranged hexagonally due to the high radial pressure, so that the wires each have the largest possible thermal contact area with the adjacent wire. This ensures a high thermal conductivity within the magnet.In various embodiments, one of the two ends of the coil body can be made metallic or electrically insulating. Electrical insulation facilitates the installation of the complex thermal-electrical interconnection of the above-mentioned partitions of the conductor coil.In one embodiment, so-called "bobbins" are provided in the contact regions of the conductor coil. The bobbins can comprise, for example, G10, Micarta (G3), Torlon and / or PEEK as material or be formed from one of these materials. Such bobbins are formed in particular from an insulating material.The superconducting wires of the conductor coil can in particular be thermally coupled completely within the magnet. They are led out of the magnet or terminated at the bobbins. The bobbins themselves are in turn thermally coupled to the thermal contact surface of the device, so that the entire superconducting wire is in thermal equilibrium with the coil body itself over its entire length, for example 5 km to 15 km wire length.Further details and advantages of the invention will now be explained in more detail with reference to an exemplary embodiment shown in the drawings.The following are shown: FIG. 1 shows an exemplary embodiment of the device; FIG. 2 is a schematic sectional view of the embodiment of the device; FIG. 3 shows a top view of a first end of the exemplary embodiment of the device; FIG. 4 is a side view of the embodiment of the device; FIG. 5 is a plan view of a second end of the embodiment of the device; and FIG. 6 shows an embodiment of a protection circuit for the device.Referring to Figs. 1 to 5, an embodiment of the apparatus will be explained.The device 20 has a first end 21 and a second end 22. The second end 22 is opposite the first end 21 along a longitudinal axis A.At the first end 21, a flange 3 is arranged, the upper surface of which is formed as a thermal contact surface 14.In the embodiment, a flange is also provided at the second end 22.In the exemplary embodiment, the flange 3 is formed from copper.In the top view of FIG. 3, the flange 3 is shown in detail.The thermal contact pad 14 is configured to be thermally coupled to an external cooling device. A cryostat, pulse tube or GM cooler can be provided as cooling device, for example.The device 20 further comprises a coil body 1, which in the exemplary embodiment is formed from aluminum. In the example, the upper flange 3 is connected to the bobbin 1.The coil body 1 is hollow-cylindrical and encloses a sample tube 23.Around the bobbin 1, a superconducting wire conductor coil having partitions 15, 16, 17, 18 is wound.In the case shown, the magnet is partitioned or the conductor coil of the magnet is formed by the four partitions 15, 16, 17, 18, each of which has 4 H inductance in the example.In the example, the partitions 15, 16, 17, 18 are arranged one above the other in the radial direction, that is to say they are wound surrounding one another around the coil former 1.In further embodiments, other arrangements of the partitions 15, 16, 17, 18 can be provided, for example in order to adapt the distribution of the magnetic field, in order to improve the homogeneity of the magnetic field, in order to wind different wire diameters or in order to wind, for example, a compensation partition which reduces the leakage field outwards.In the exemplary embodiment, NbTi and / or Nb 3 Sn is provided as material.In the exemplary embodiment, it is provided that the conductor coil is wound from a total of approximately 14 km of wire.The wire is wound by a high tension wet winding method in the example.The device 20 has so-called "bobbins" 10, 11 in which the superconducting wires of the partitions 15, 16, 17, 18 of the conductor coil are converted into copper wires. Contact areas 10, 11 are therefore located here, in which the electrical contact is established between the wire material and the externally used copper wire.The bobbins 10, 11 are formed from PEEK in the example. Other materials may be G10, micarta (G3) or torlon.The bobbins 10, 11 are arranged at the second end 22 of the device 20 so as to be thermally coupled to the rest of the device 20. The material of the bobbins 10, 11 is also chosen to be electrically insulating in order to insulate them from the magnet.Furthermore, housing sections 12, 13 are provided at the second end 22, which are designed such that they provide external mechanical protection of the bobbins. These can be seen in the view of the device 1 from below in FIG. 5.The coil body 1 is designed such that deformation does not occur due to the windings of the conductor coil, or that a supply of at most a few micrometers is produced.In the example, the coil body 1 has slots 24 for reducing eddy currents, which slots are formed along the longitudinal axis A. The slots 24 can be formed as elongate recesses and can be arranged distributed in particular uniformly around the circumference of the coil body 1. The number, length and width of the slots 24 is selected in particular such that the stability of the coil body 1 is not impaired.The device 20 further comprises thermalizing elements 8, which serve for thermalizing the magnet or, in particular, for improved themic contacting of the outer windings of the conductor coil.In the example, the thermalizing elements 8 are designed as strips of copper, namely as copper bending parts, which can be seen particularly well in the side view of FIG. 4. Other materials may also be used, with high thermal conductivity being sought.In further examples, the thermalization element 8 can have a different shape and can be configured, for example, in the form of a net, as a woven fabric or as a braid.The thermalising elements 8 extend from the flange 3 at the upper end 21 to the flange at the lower end 22, which are therefore in thermal contact with the thermal contact surface 3, via which the device 20 is connected to the cooling device.In the example, the thermalizing elements 8 are screwed to the upper flange 3 and the lower flange, which allows simple technical implementation.In further examples, other types of attachment may be provided, such as by welding, and more particularly laser welding. The type of fastening enables the thermal contact with the upper flange 3 to be improved, wherein it is to be taken into account during welding that the heat input into the coil body 1 has to be limited in order not to damage it.The thermalizing elements 8 allow a heat flow from the outside of the conductor coil to the cooling device to which the thermal contact surface 3 is connected.In the case of a so-called "quench", in which the superconducting material of the conductor coil heats up and therefore suddenly loses its superconducting properties, protective circuits 4, 5, 6, 7 are provided. In these, the energy, which originates, for example, from inductive voltage peaks, can be emitted. In this way, they prevent damage to further elements of the device 20 and therefore serve for so-called "quench protection".The protection circuits 4, 5, 6, 7 comprise power diodes. They are explained in more detail below with reference to FIG. 6.The protection circuits 4, 5, 6, 7 are arranged at the end of the device 1 opposite to the location of the bobbins 10, 11.In the exemplary embodiment, the contact regions 10, 11 with the bobbins 10, 11 are located at the second end 22 of the device 1. on the other hand, the protective circuits 4, 5, 6, 7 are arranged at the first end 21 on the copper flange 3 of the magnet in direct proximity to the thermal contact surface of the device 20.From this arrangement, it is understood that in the device 20, the protection circuit 4, 5, 6, 7 having the power diodes in FIG. 2 are arranged at the top, i.e., in the vicinity of the cooling, while the bobbins 10, 11 in which the superconducting wire of the conductor coil is connected to the copper wire are arranged at the bottom. This has the effect that, on the one hand, the power diodes of the protective circuit 4, 5, 6, 7 are well cooled and can efficiently emit the energy in the case of a quench, and that, in addition, the bobbins 10, 11 are not exposed to the heat and are damaged in the case of a quench. This means that the copper wire must be led from below, that is to say from the region of the bobbins 10, 11, upwards to the protective circuit 4, 5, 6, 7.In a further exemplary embodiment (not shown), the reverse arrangement of the bobbins 10, 11 relative to the protection circuit 4, 6, 7 can be provided.In the exemplary embodiment, the device 20 is designed as a 1-D solenoid. By supplementing further coils, the example can be extended to 2-D or 3-D magnets.With reference to FIG. 6, an exemplary embodiment of the circuit 30 for quench protection provided in the device is explained.In the quench case, in particular when the superconducting wire material is heated above a critical value, a sudden increase in the resistance occurs. The current intensity i decreases very rapidly and a very high voltage U ind according to the formula is induced in the coil with inductance L. The voltage peaks occurring in this case can lie in the range of several kilovolts and correspondingly high induction currents occur.In the device 20, these induction currents are intercepted in the quench case via protective circuits 32, 33, 34, 35 and released as heat. In this case, the internal resistance of diodes D 1 to D 6 is used, which diodes are provided in the protective circuits 32, 33, 34, 35 for both possible directions of the current flow.The circuit 30 comprises a current source 31 connected in series with four inductances L 1, L 2, L 3, L 4. The inductances L 1, L 2, L 3, L 4 correspond to the partitions 15, 16, 17, 18 of the conductor coil in the exemplary embodiment described here.The protective circuits 32, 33, 34, 35 are each assigned to the individual inductances L 1, L 2, L 3, L 4 and are each connected in parallel with these inductances.The protective circuits 32, 33, 34, 35 each comprise a first diode D1, D3, D5 with a first forward direction and a second diode D2, D4, D6 connected in parallel thereto with a reverse forward direction.In particular, silicon power diodes with high current and voltage tolerance are used.This means that in parallel with the coils L 1, L 2, L 3, L 4 of the partitions 15, 16, 17, 18, a diode D 1 to D 6 is provided in each case for the two current directions with which the magnet can be operated. These diodes D1 to D6 absorb the energy when, as a result of heating, the wire becomes normally conductive and receives a resistance, the current intensity drops and, on the other hand, a high voltage U ind is induced.The features of the device explained in the present case allow standardized and scalable production of superconducting magnets, in particular for laboratory applications. In this case, better reproducibility during production is advantageously achieved. In addition, an improved thermal coupling is advantageously made possible for use in helium-free surroundings. In addition, improved protection of the magnet in the case of a quench is provided.List of reference characters1 Coil former 3 flange 4 protection circuit 5 protection circuit 6 protection circuit 7 protection circuit 8 thermalization element (copper bending parts) 10 bobbine; contact region 11 bobbine; contact region 12 housing section 13 housing section 14 thermal contact surface 15 partition of the conductor coil 16 partition of the conductor coil 17 partition of the conductor coil 18 partition of the conductor coil 20 device 21 first end 22 second end 23 sample tube; sample space 24 slot 30 circuit 31 current source 32 protection circuit 33 protection circuit 34 protection circuit 35 protection circuit A longitudinal axis D 1, D 3, D 5 diode (first forward direction) D 2, D 4, D 6 diode (second forward direction) L 1, L 2, L 3, L 4 partition (of the conductor coil)

Claims

Device (20) for generating a magnetic field, comprising a coil body (1) around which windings of a conductor coil are arranged, wherein the coil body (1) has a first (21) and a second end (22); a thermal contact surface (14) arranged at the first end (21) of the coil body (1) for thermal contact with a cooling device; wherein the conductor coil (1) has contact regions (10, 11) at the first (21) or second end (22) of the coil body (1); and wherein the conductor coil (1) is connected to at least one protection circuit (4, 5, 6, 7) arranged at the opposite end (22, 21) of the coil body (1).Device (20) according to Claim 1, characterized in that the thermal contact surface (14) is formed in the case of a flange (3) arranged at the first end (21) of the coil former (1).Device (20) according to claim 2, characterised in that the flange (3) is made of copper.Device (20) according to one of the preceding claims, characterized in that the coil body (1) is designed as a hollow cylinder, in particular with an inner sample space (23).Device (20) according to one of the preceding claims, characterized in that the coil body (1) has at least one slot (24) which is formed along a longitudinal axis (A) and is in particular formed to reduce induction of eddy currents in the coil body (1).Device (20) according to one of the preceding claims, characterized in that the coil body (1) is formed from copper or aluminum; wherein the coil body (1) has a thermal conductivity of, for example, 400 to 20,000 W / m*K at 4 K.Device (20) according to one of the preceding claims, characterized byat least one thermalization element (8) which is connected in a thermally conductive manner to the thermal contact surface (14) or to the flange (3) and which bears against the conductor coil from the outside; wherein optionally the thermalization element (8) is formed in the form of a strip, a network, as a woven fabric or as a braid; wherein optionally the thermalization element (8) is formed from copper.Device (20) according to one of the preceding claims, characterized in that the conductor coil has at least two partitions (15, 16, 17, 18).Device (20) according to Claim 8, characterized in that the partitions (15, 16, 17, 18) of the conductor coil are arranged radially or axially with respect to one another.Device (20) according to one of the preceding claims, characterized in that the conductor coil is formed with a wire made of a superconducting material or high-temperature superconducting material, for example NbTi and / or Nb 3 Sn.The device (20) according to any one of the preceding claims, characterized in that the conductor coil is manufactured in a wet winding process; optionally wherein the wire layers of the conductor coil are arranged in a hexagonal arrangement or a circular packing.Device (20) according to one of the preceding claims, characterized in that bobbins (10, 11) are provided in the contact regions (10, 11) of the conductor coil; wherein the bobbins (10, 11) optionally comprise, as material, for example G10, micarta (G3), torion or PEEK or are formed from one of these materials.

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