Monitoring equipment for cryogenic systems

Abstract

A monitoring device for use in a cryogenic system. The monitoring device comprises a first conductive element (201), a second conductive element (202) and a current detector (205). The first conductive element comprises a high temperature superconducting HTS material and is configured to be connected to a current source (203) and inserted into the cryogenic system. The second conductive element comprises an HTS material and is connected in parallel to the first conductive element through a first joint and a second joint (204). The current detector is configured to detect a current in the second conductive element. When the HTS material in each of the first conductive element and the second conductive element is in a superconducting state, the resistance RT of the first conductive element between the first joint and the second joint is less than the sum of the resistance of the second conductive element between the first joint and the second joint and the resistance of the first joint and the second joint RB, RT<RB.

Description

Technical Field

[0001] The present invention relates to a monitoring device for cryogenic systems. In particular, the present invention relates to a system for monitoring the critical current of superconducting materials in ultra-cryogenic systems (e.g., superconducting magnets). This information can be used to detect quenching in superconducting systems. Background Art

[0002] A superconducting magnet is an electromagnet formed from a coil of superconducting material. Since the resistance of the electromagnetic coil is zero, a superconducting magnet can carry high currents with zero losses (although some losses are caused by the non-superconducting components), and can therefore achieve high magnetic fields with lower losses than conventional electromagnets.

[0003] Superconductivity occurs only in certain materials, and only at low temperatures. In the region defined by the critical temperature of the superconductor (the highest temperature at which the material is a superconductor under zero applied magnetic field) and the critical field of the superconductor (the highest magnetic field at which the material is a superconductor at 0K), the superconducting material will act as a superconductor. The temperature of the superconductor and the magnetic field present limit the current that the superconductor can carry without making the superconductor resistive (or "normal", in this article, "non-superconducting"). There are two types of superconducting materials: Type I superconductors completely exclude magnetic flux penetration and have a low critical field, and Type II superconductors allow magnetic flux to penetrate the superconductor above the low critical field in a local ordinary region called a magnetic flux vortex. They are no longer superconducting at the upper critical field. This feature enables them to be used in the wires that construct superconducting magnets. Great efforts have been made to fix the magnetic flux vortex sites to the atomic lattice, which improves the critical current at higher magnetic fields and temperatures.

[0004] Superconducting materials are generally divided into "high temperature superconductors" (HTS) and "low temperature superconductors" (LTS). LTS materials (e.g., Nb and NbTi) are metals or metal alloys whose superconductivity can be described by the BCS theory. All low temperature superconductors have a critical temperature (the temperature above which the material does not superconduct, even in zero magnetic field) below about 30K. The behavior of HTS materials is not described by the BCS theory, and such materials can have a critical temperature above about 30K (although it should be noted that it is the physical differences in superconducting operation and composition, not the critical temperature, that define HTS materials). The most commonly used HTS are "cuprate superconductors" - ceramics based on cuprates (compounds containing copper oxide groups), such as BSCCO or ReBCO (where Re is a rare earth element, typically Y or Gd). Other HTS materials include iron phosphides (e.g., FeAs and FeSe) and magnesium diborate (MgB 2 ).

[0005] ReBCO is usually manufactured as a tape with a structure such as Figure 1As shown. Such a ribbon 500 is typically about 100 microns thick and comprises a substrate 501 (typically electropolished Hastelloy about 50 microns thick) on which is deposited a series of buffer layers, referred to as a buffer stack 502, about 0.2 microns thick, by IBAD, magnetron sputtering or another suitable technique. An epitaxial ReBCO-HTS layer 503 (deposited by MOCVD or another suitable technique) overlies the buffer stack and is typically 1 micron thick. A 1-2 micron silver layer 504 is deposited on the HTS layer by sputtering or another suitable technique, and a copper stabilizer layer 505 is deposited on the ribbon by electroplating or another suitable technique, which typically completely encapsulates the ribbon.

[0006] The substrate 501 provides a mechanical backbone that can be transported through the production line and allows the growth of subsequent layers. The buffer stack 502 is needed to provide a biaxially textured crystal template on which to grow the HTS layer and to prevent chemical diffusion of elements from the substrate into the HTS, which destroys the superconducting properties of the HTS. The silver layer 504 is needed to provide a low resistance interface from the ReBCO to the stabilizer layer, and the stabilizer layer 505 provides an alternative current path in the event that any part of the ReBCO stops superconducting (enters the "normal" state).

[0007] One problem that can occur in superconducting magnets is a quench. A quench occurs when a portion of a superconducting wire or coil goes into a resistive state (sometimes referred to as "going 'normal'). This can be due to fluctuations in temperature or the magnetic field, or physical damage or defects in the superconductor (for example, caused by neutron irradiation if the magnets are used in a fusion reactor). Due to the high currents in the magnet, if even a small portion of the superconductor becomes resistive, it will heat up rapidly. All superconducting wires are provided with some copper stabilizer for quench protection. The copper provides an alternative path for the current if the superconductor goes normal. The more copper there is, the slower the temperature rises in the hot spot that forms around the quenched superconductor region.

[0008] In an LTS magnet, when a quench occurs, the "normal section" will propagate rapidly - on the order of several meters per second. This phenomenon occurs due to the low heat capacity of all materials at low temperatures, and the fact that LTS materials are usually operated closer to their critical temperature. This means that the quench propagates rapidly in an LTS magnet, and the stored magnetic field energy dissipated in the quench will spread throughout the magnet, heating it evenly.

[0009] The electric field inside the HTS strip (i.e., the voltage per unit length) depends on the transport current I in a highly nonlinear manner:

[0010]

[0011] Among them, E 0=1μV / cm is the critical current standard defined, I C is the critical current of the band, and n is an experimental parameter that models the sharpness of the superconducting to normal transition. For ReBCO, n is typically in the range of 20-50. Depending on the value of n, for a=I / I less than about 0.8 C The critical current will decrease due to temperature, external magnetic field, strain on the ribbon and other factors.

[0012] HTS materials operate at high temperatures and have a higher specific heat capacity, so the energy required to bring a portion of the wire to a normal state is much greater. This means that the likelihood of a quench occurring in a properly designed HTS magnet is much smaller than in an LTS magnet. However, this also means that the propagation speed of the normal section is much slower, on the order of a few millimeters per second compared to meters per second in an LTS magnet. Since the quench initially affects only a small volume of the magnet, only that area will be resistive, and therefore the energy dissipated during the quench will be dumped into that small volume (or more specifically, dumped into the copper where the current from the normal section is diverted). This concentration of energy can cause permanent damage to the HTS tape, such as melting, arcing, etc. This is further complicated by the fact that HTS magnets are typically cooled indirectly, rather than being immersed in a liquid coolant bath, so the "spot" cooling capabilities are reduced compared to LTS magnets.

[0013] The energy stored in the magnetic field is given by:

[0014]

[0015] That is, the greater the flux density, and the larger the radius, the greater the stored energy of the magnet. The energy released by a large magnet can be similar to many kilograms of TNT. For an LTS magnet, this energy can be dissipated to heat the entire magnet. For a similarly sized HTS magnet without quench protection, this energy may be dissipated in a small fraction of the magnet volume. Typically, large HTS magnets will require an active quench protection system that includes a detection phase during which the quench is detected before significant heating occurs, followed by a dissipation phase during which the magnet current is rapidly reduced before the hot spot temperature rises too high.

[0016] To date, most HTS magnets (using BSCCO and ReBCO coated conductors) are not actually quench protected. As mentioned earlier, this is due to the fact that they are mostly small, low-cost prototypes with little stored energy, and because the probability of a quench occurring in a well-designed HTS magnet should be extremely low. Therefore, the decision whether to quench an HTS magnet is essentially a question of economics: in the rare event that it quenches, a small prototype magnet can be repaired relatively easily. As a result, quench protection technology for HTS magnets remains immature.

[0017] One application for HTS magnets is in Tokamak fusion reactors. An operating Tokamak fusion reactor is a very "noisy" electromagnetic environment in which to make sensitive electrical measurements. This presents a problem for HTS quench detection systems, as the electromagnetic noise can overwhelm and mask the small voltages that would normally indicate a quench has occurred, and preventive action may not be taken until it is too late.

[0018] A quench detection system for a tokamak has been described in WO2016 / 052712. A conductive element in the form of a length of HTS tape, known as a "canary tape", is inserted into the magnet of the tokamak and thermally connected to other conductive elements (tapes) in the magnet while remaining electrically isolated from them. The canary tape is connected to a separate power supply and operated at a higher portion of its critical current than the other tapes in the magnetic conductor, so that it stops superconducting at a lower temperature (or lower magnetic field / strain / etc.) than the other tapes. When the temperature of the canary tape exceeds this temperature, a voltage will be generated, providing a warning that a hot spot has been created and a quench in the magnet is about to occur or is occurring. Such a system can also be used to detect temperature and / or magnetic field changes in other cryogenic or superconducting systems.

[0019] However, the voltage across the canary strip can be small and difficult to detect, especially in a system such as a tokamak, which inherently operates with significant electromagnetic noise. If a quench in the canary strip is not detected quickly enough, the canary strip may be permanently damaged.

[0020] Another potential use for HTS magnets is in proton beam therapy devices. Proton beam therapy (PBT, also known as proton therapy) is a type of particle therapy used to treat cancer (and other diseases that respond to radiation therapy). In PBT, a proton beam is directed toward the treatment site (e.g., a tumor).

[0021] Another similar therapy is proton boron capture therapy (PBCT), in which boron-11 is introduced to the target site and a proton beam is used to initiate p+ 11B→3α reaction. The same device can be used to deliver proton beams for PBT or PBCT.

[0022] The proton beams used for PBT and PBCT are generated by particle accelerators such as cyclotrons or linear accelerators. Accelerators commonly used for PBT and PBCT typically produce protons with energies in the range of 60 to 250 MeV, while the most powerful facilities currently in operation have a maximum energy of 400 MeV.

[0023] Broadly speaking, there are two types of designs for PBT devices that allow for changing the beam angle. In the first type of design, e.g. Figure 8 As shown, the accelerator 3001 is mounted on a gantry 3002, which allows the gantry 3002 to rotate (generally around a horizontal axis) around a patient 3003. The patient is placed on a movable bed 3004, which provides further degrees of freedom (e.g., translational movement and rotation around a vertical axis).

[0024] The second type of design is Fig. 9 The accelerator 4001 is stationary, and the beam is directed to the patient via steering magnets 4002 (typically including both quadrupole and dipole magnets), at least some of which are located on a gantry 4003 so that the beam can rotate (e.g., around a horizontal axis) around the patient 4004. The patient is placed on a movable bed 4005.

[0025] Both designs require a gantry to hold electromagnets capable of manipulating protons at beam energies, which can be as high as 400 MeV. This requires very high magnetic fields, so using HTS magnets can greatly reduce the mass and size of the electromagnets and the gantry required to move them. HTS magnets can be used for quadrupole magnets in accelerators, steering magnets, or dipole magnets in steering magnets.

[0026] Quench detection is particularly important in PBT equipment because movement of the gantry can interfere with the cooling system and make hot spots more likely to form. In addition, the location of the HTS magnets relatively close to the patient means that dumping the energy in a controlled and safe manner is critical. Summary of the invention

[0027] According to a first aspect of the present invention, a monitoring device for use in a cryogenic system is provided. The monitoring device includes a first conductive element, a second conductive element, and a current detector. The first conductive element includes a high-temperature superconducting HTS material, and is configured to be connected to a current source and inserted into a cryogenic system. The second conductive element includes an HTS material, and is connected in parallel with the first conductive element through a first joint and a second joint. The current detector is configured to detect the current in the second conductive element. When the HTS material in each of the first conductive element and the second conductive element is in a superconducting state, the resistance RT of the first conductive element between the first joint and the second joint is less than the sum of the resistance of the second conductive element between the first joint and the second joint and the resistance of the first joint and the second joint RB, RT<RB.

[0028] According to a second aspect of the present invention, there is provided a monitoring system comprising a plurality of monitoring devices according to the first aspect and a controller configured to monitor a current detected by each monitoring device.

[0029] According to a third aspect of the present invention, there is provided a superconducting magnet comprising an HTS excitation coil and a monitoring device according to the first aspect or a monitoring system according to the second aspect, wherein the first conductive element or each first conductive element is located adjacent to the HTS excitation coil.

[0030] According to a fourth aspect of the present invention, there is provided a quench protection system for use in a superconducting magnet, the quench protection system comprising a monitoring system according to the first aspect, and a system for dumping energy from a superconducting magnet. A controller of the monitoring system is configured to identify a reduction in a critical current of a first conductive element of a monitoring device based on a current detected in a second conductive element of each monitoring device. The controller is configured to identify a condition that may cause a quench based on a rapid reduction in the critical current identified for one or more of the monitoring devices. The controller of the monitoring system is configured to trigger the system for dumping energy from a superconducting magnet based on the identification of a condition that may cause a quench. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a schematic diagram of the HTS band;

[0032] Figure 2 is a schematic diagram of the monitoring equipment;

[0033] Figure 3 yes Figure 2 Equivalent circuit of the device;

[0034] Figure 4A is a graph showing current applied to a monitoring device;

[0035] FIG. 4B to FIG. 4Dis a graph showing current flowing through a bypass strap of a monitoring device;

[0036] Figure 5 It is a schematic diagram of the canary band collection;

[0037] Fig. 6A and Figure 6B is a schematic diagram of the striped canary band; and

[0038] Figure 7 It is a schematic diagram of a pair of monitoring devices;

[0039] Figure 8 is a schematic diagram of a proton beam therapy device;

[0040] Fig. 9 is a schematic diagram of another proton beam therapy device. DETAILED DESCRIPTION

[0041] As discussed above, the use of conductive elements in the form of canary straps is hampered by the low signal-to-noise ratio of the voltage produced, particularly in environments with significant electrical interference (e.g., tokamaks and other high power magnets, which are among the primary uses of HTS materials, and which would therefore be the most useful systems to be protected with canary straps). While this can be partially mitigated by using higher currents in the canary straps to induce higher voltages when the canary straps become common, producing a measurable voltage in the canary straps still requires that a sufficient portion of the canary straps become common, which is likely to damage the canary straps. While the use of a canary strap can prevent damage to the magnet (as it will quench before the main magnet coils), the need to balance the low signal-to-noise ratio with the need to replace the canary straps greatly reduces their practicality (even at high currents, the signal-to-noise ratio may be insufficient). It would therefore be desirable to provide a canary strap with an improved signal-to-noise ratio.

[0042] Figure 2 A monitoring device comprising a canary strip is shown in FIG. 1 , which allows an indirect measurement of voltage. Figure 2The monitoring device comprises: a first conductive element in the form of a canary strap 201 for insertion into a cryogenic system (e.g., contained in a coil structure of a superconducting magnet), and a second conductive element in the form of a bypass strap 202. Both conductive elements comprise HTS material (in the form of a strap in this example) and are arranged to have minimal inductance (e.g., doubled over on itself to minimize the area of ​​the loop formed). When superconducting, the HTS material will not have resistance, but other parts of the canary strap 201 or the bypass strap 202 may be resistive (e.g., joints to allow the strap to "turn"). The canary strap 201 is connected to a current source 203. The bypass strap 202 is connected in parallel with the canary strap via a joint 204, which may be resistive. At least some resistive (non-superconducting) material should be included in the current path, which should include the bypass strap and the joint. Such resistive material may be present in the joint 204 and / or at other locations along the bypass strap 202. A current detector 205 is provided to detect the current in the bypass strap.

[0043] Figure 3 Shows Figure 2 The combined resistance R of the junction 204 and any resistive non-HTS portion of the bypass strap 202 is B Greater than the resistance R of the non-HTS portion (if any) of the canary strap 201 between the connectors T When the current through the HTS is significantly less than the critical current (i.e., when both bands are superconducting), the resistance of the HTS portion of the bypass band and the canary band is B and HTS T Therefore, when current is provided by current source 203, current will flow in canary strap 201, but only a negligible amount will flow in bypass strap 202, so current detector 205 will detect little or no current.

[0044] If the input current I is the critical current I of the canary belt 201 C A large part (e.g., I>0.8I C ), then the resistance HTS of the HTS material in the canary strip is T will become significant and a voltage will be developed across the canary strip.

[0045]

[0046] where L is the length of the common section. This voltage will cause a current to flow through the junction 204 and the bypass strap 202. This current is detected by the current detector 205. In practice, this will cause the current IT through the canary strap to decrease, thereby reducing the voltage, but when I approaches I C The principle that the measurable current will flow through the bypass strap still holds true - the actual current I BThe calculation of is relatively complex but can be determined as the solution to a set of simultaneous equations (e.g., Kirchhoff’s current and voltage laws, and V given above). HTS However, for basic functionality of the monitoring device (e.g. for simple quench detection) such an accurate calibration is not necessary, since any detected current in the bypass band above the background noise indicates that the current in the canary tape is close to the critical current.

[0047] Since a full quench is not required to provide a measurable voltage across the canary strip, the monitoring device according to the present disclosure may be considered a "critical surface detector" or a "critical current detector" rather than just a "quench detector", i.e., the system of the present disclosure monitors the critical current of the canary strip or detects changes in the critical current rather than just detecting a quench.

[0048] An advantage of the above monitoring device is that while the canary strip is placed in the superconducting field coil to detect conditions that result in a reduction in critical current, the bypass strip can be placed in an electromagnetically quiet location away from the magnet so that the current can be measured with less noise.

[0049] The primary example used in this document will be a superconducting magnet and the use of a canary tape to detect conditions that may cause such a magnet to quench. However, it should be understood that the monitoring devices disclosed herein may be used in a variety of cryogenic or superconducting systems to detect changes in temperature, magnetic field, neutron or ionizing radiation flux, strain, or other conditions that may alter the critical current of the HTS.

[0050] A general advantage of the device is that it can be used as a distributed sensor. Very long canary strips allow the user to monitor large volumes of space. There is virtually no limit to the size, since there are no ohmic losses or voltage drops inside a superconductor in its superconducting state. For example, work is underway on the use of superconductors for many kilometers of electric current transmission. In theory, it would be possible to monitor a cable used for such transmission along its entire length using a single canary strip, potentially replacing hundreds or even thousands of individual local sensors.

[0051] Various techniques can be used to measure the current in the shunt strap, such as:

[0052] The magnetic field generated by the bypass strip can be measured by a Hall probe, or the change in the magnetic field can be measured by induction in a loop having high mutual inductance with the bypass strip;

[0053] Strain gauges can be connected to two parallel sections of the shunt strip and can measure the force between the two sections (which depends on the current and can be enhanced by providing an additional background field, e.g. using a permanent magnet);

[0054] Other current measurement techniques are known in the art.

[0055] Current measurements may be made in a low magnetic field region away from the magnet and / or in a "magnetic shield" (eg, a superconducting bulk magnet) to shield out any background fields.

[0056] Various modifications can be made to the canary strap to obtain more information about the state of the magnet by monitoring the current in the bypass strap.

[0057] For example, a modulated or AC current may be supplied to the canary strip whose peak value is close to the expected critical current of the canary strip during ordinary operation of the magnet. This may be a pure AC current, or a combination of AC and DC current (i.e., a modulated current) such that the peak combined current is close to the critical current. The use of pure AC current allows the effects of induced currents in the canary strip to be neglected (as they will have opposite effects on the positive and negative canary strip currents), while the use of a combination of AC current and DC current may allow the sensitivity of the detection to be increased (as the current spends more cycles close to the critical current of the strip), Figure 4A A diagram of an exemplary modulation current waveform is shown in FIG. FIG. 4B to FIG. 4D shows the current measured in the bypass band, where the critical current has the following characteristics: Figure 4A Values ​​B to D shown. In each case, current flows in the bypass band only when the current in the canary band is above the critical current. The example is simplified by marking any current in the bypass band as "1". In practice, the current in the bypass band will increase as the current in the canary band approaches the critical current, but the transition will be relatively sharp. It can be seen that the signal will be measured over a portion of the critical current cycle, which depends on the critical current of the canary band. Since the signal will be synchronized with the provided modulated current, phase-sensitive ("synchronous") detection techniques known in the art can be used to increase the sensitivity of the current measurement (e.g., a lock-in amplifier). When the critical current is between the minimum input current and the maximum input current, the lower the critical current in the canary band, the higher the duty cycle of the output will be (i.e., the longer the time that a current above the background noise is detected in each cycle).

[0058] The use of AC current or modulated input current allows the critical current of the canary strips to be continuously monitored. Various magnet properties can be inferred from the changes in critical current in a single strip and from the different critical currents of different strips. For example, a drop in critical current for a single canary strip or a small group of closely spaced canary strips is likely due to a rise in temperature. A transient or sudden drop in a more widely spaced group of canary strips (e.g., all the strips in a single return branch) is likely due to strain in the magnet or a change in the magnetic field, especially when the drop in critical current is related to the orientation of the canary strips. For a large number of canary strips, a gradual decrease in critical current over time may be due to neutron damage to the strip, where the strip is used in a nuclear fusion reactor. Since the canary strap is located in close proximity to the HTS field coil of the magnet, the reduction in the critical current of the canary strap can be used as a proxy to monitor the "health" or stability of the magnet, and if the critical current of the canary strap drops below a threshold, or the measured critical current shows other undesirable properties, a quench protection system can be initiated, in which power is dumped from the magnet (e.g., to a resistive load).

[0059] Alternatively or additionally, multiple bypass circuits may be attached to a single canary strap, each successive bypass circuit having a higher resistance, and wherein the total critical current of all bypass straps except the one with the highest resistance is less than the current supplied to the canary strap (and therefore less than the peak critical current of the canary strap, i.e., the critical current at low temperatures and in the absence of an external magnetic field). For example, where the current supplied to the canary strap is 100A, a critical current value of 50A and a resistance R1 may be provided for bypass circuit B1, and a critical current value greater than 50A and a resistance R2 greater than R1 may be provided for bypass circuit B2. When the canary strap becomes ordinary, if the resistance of the canary strap is approximately equal to R1, a current of approximately 50A will flow into B1, and a small amount (depending on the ratio R2 / R1) will flow into B2. When the resistance of the canary strap is approximately equal to R2, a current of 50A still flows into B1, and a large portion of the remaining 50A current will flow into B2. When the resistance of the canary strip is significantly greater than R2, a current of 50A will flow in B1 and approximately 50A will flow in B2. Thus, by measuring and calibrating the currents flowing in B1 and B2, the resistance of the canary strip (and therefore the severity of any critical current reduction in the canary strip) can be determined to a higher accuracy than a single bypass strip. Such a sensor will still protect itself if the sum of the critical currents of all bypass strips is greater than the current supplied to the canary strip.

[0060] In a similar embodiment, a variable shunt resistance may be provided, for example by using a potentiometer, by intentionally oscillating portions of the shunt strap in a controlled manner, or by heating a tap intentionally placed in the shunt circuit. By varying the shunt resistance, the resistance of the canary strap may be determined, since the dependence of the shunt current on the shunt resistance will be determined by the resistance of the canary strap and the (known) input current. For example, in the case where the current in the shunt strap is half the input current, the resistance of the canary strap equals the resistance of the shunt strap.

[0061] When installed in a magnet, a canary tape may pass through regions with different properties, for example, higher or lower temperatures or magnetic fields, which will cause the critical current to vary along the tape during ordinary operation of the magnet. To counteract this effect, and ensure that the tape is uniformly sensitive along its length, the width of the superconducting tape in the canary tape may be modulated along its length to provide a substantially constant critical current value during ordinary operation. Alternatively or additionally, a wider length of canary tape may be constructed by welding several narrow tapes together.

[0062] For a canary strip of constant width, or a canary strip with a modulated width as described above, it is not possible to determine where along the strip the reduction in critical current occurs. This can be achieved by providing a plurality of canary strips in parallel, each having a "measured width" (i.e., a width that provides a reduced critical current) along a portion of its length, which can be modulated as described above to provide a substantially constant I C ), and is greater than the measured width over its remaining length (thus providing a higher I C By arranging such traces so that the “measurement width” portions only partially overlap, it is possible to determine I C Reduced positions - that is, only in those positions where I C I can only be detected on the tape with a "measurement width" at a reduced position C Reduce. Figure 5 An exemplary arrangement is shown where 6 strips 5001, 5002, 5003, 5004, 5005, 5006 are provided with a "measurement width" 5010 portion and a "non-sensitive width" 5011 portion to form a binary linear encoder - for example any position can be specified by a binary number (resolution equal to the length of the "measurement width" portion of strip 5006), where strip 5001 provides the leftmost bit and strip 5006 provides the rightmost bit, and the other strips are arranged accordingly. For example, a critical current reduction event at each of lines 5021 and 5022 will give a corresponding binary output of 001110 and 101011 (where "1" represents the current detected on the bypass strip connected to that canary strip).

[0063] To reduce the current required for the canary strap, and thus reduce the need for high current leads and their associated cooling, the canary strap can be configured to have a relatively low critical current (e.g., by reducing the width of the strap, or by intentionally degrading the strap (e.g., by deoxidation) so that a smaller probe current will still result in sensitive detection. The bypass strap can be set with a higher critical current than the canary strap, which will cause the canary strap to "self-protect" because currents high enough to damage the canary strap will be redirected completely into the bypass strap during the period of severely reduced critical current.

[0064] All of the width reductions mentioned above can be achieved by cutting through the HTS layer of the canary tape (e.g., using a laser or scribing tool), or by locally heating the canary tape using a laser to the extent that the HTS layer of the canary tape degrades without cutting the other layers of the tape. Similar techniques can be used to provide "striped canary tape," wherein canary tape loops are provided in a single tape by cutting an odd number of lines extending from one end of the tape to substantially the other end of the tape, the lines alternating at the ends to which they extend. Fig. 6A and Figure 6B Example strips with 1 cut and 3 cuts are shown respectively. The more lines that are cut, the more parallel HTS channels are provided in the strip and the more sensitive the canary strip will be. This allows a canary strip to be provided without any resistive sections in the canary strip, which means that in practice the ideal situation can be achieved, i.e. the resistance of the canary strip between the joints is zero.

[0065] In this case, it is advantageous if the canary strap is calibrated against the total resistance R of the bypass strap and the junction. B The sensitivity is as close to zero as possible while still being non-zero. However, this low resistance can, under the influence of an electric field or a changing background field, allow parasitic induced currents to flow in the loop created by the bypass and canary strips, which will cause noise in the measurement. A higher resistance would cause these induced currents to dissipate more quickly. In practice, given the conditions in which the canary strip is used, the resistance of the bypass strip can be chosen to balance these two factors to achieve an acceptable signal-to-noise ratio. For calibration purposes, the critical current of the canary strip can be intentionally reduced in a non-permanent manner (e.g. by heating or applying a magnetic field) while keeping the bypass strip superconducting, and applying a current to the canary strip to measure the resistance in the bypass loop.

[0066] In order to reduce the influence of the inductive current on the critical current detection, the canary straps and the bypass straps may be provided in pairs, wherein each set of canary straps and bypass straps are oppositely connected together, such as Figure 7As shown. The first canary strip 710 is connected to the first bypass strip 720 so that the upper branch 721 of the bypass strip is connected to the left branch 711 of the canary strip, and the lower branch 722 of the bypass strip is connected to the right branch 712 of the canary strip. The second canary strip 730 is connected to the second bypass strip 740 so that the upper branch 741 of the bypass strip is connected to the right branch 732 of the canary strip, and the lower branch 742 of the bypass strip is connected to the left branch 731 of the canary strip. The two canary strips are supplied with current in the same direction (e.g., from the left branch 711, 731 to the right branch 712, 732). The canary strips 710, 730 are located on top of each other, and the bypass strips 720, 740 are located on top of each other. Therefore, any current induced in the loop formed by the canary straps 710, 730 and the bypass straps 720, 740 will be induced in the opposite direction, and therefore combining the measurements of the current in the bypass straps 720, 740 will cause the induced current to be removed.

[0067] The canary straps and / or bypass straps need not be made of the same HTS superconductor as used in the magnet coils, or of HTS superconductors that are identical to each other. Indeed, although the terms "canary straps" and "bypass straps" have been used herein, non-strap HTS superconductors may also be used. For example, the canary straps and / or bypass straps may be made of Bi-2212 round multifilament wires, the shape and mechanical properties of which may make them more easily embedded in advantageous locations within the magnet. Typically, the canary straps are a first conductive element comprising an HTS material, and the bypass straps are a second conductive element comprising an HTS material and connected in parallel with the first conductive element.

[0068] The monitoring device can be assembled into a monitoring system, wherein several such devices are connected to a single controller that monitors the current detected in each bypass band to determine when the critical current of the canary band has been reduced. As described above, the controller can identify the cause of the reduction based on the pattern of the critical current reduction. The controller can be connected to the magnet's quench protection system and triggered to dump energy from the magnet (via an energy dumping system such as a superconducting switch and an external resistor, or other energy dumping known in the art) based on the identified critical current reduction. For example, if the critical current of a single canary band drops below a threshold value (either as an absolute value or as a percentage of the expected critical current of the band), the energy dumping system can be triggered.

[0069] The monitoring devices in such a monitoring system may be connected in series to a single current source, which will result in the same current flowing through each canary strip. Alternatively, the monitoring devices may be connected in parallel to a single current source, with each canary strip connected via a resistor having a much larger value than the resistance in the monitoring device (ideally, at least an order of magnitude larger, i.e., at least 10 times larger or at least 100 times larger). This will result in the current being divided between the monitoring devices based on these resistors. As another alternative, the monitoring system may include a collection of monitoring devices connected in series, which are themselves connected in parallel via resistors. As another alternative, separate power supplies may be provided for the monitoring devices and / or collections of monitoring devices connected in series or in parallel.

[0070] The above disclosure can be applied to various HTS magnet systems. In addition to the tokamak fusion reactors mentioned above as examples, it can also be used for HTS magnets in nuclear magnetic resonance imaging (NMR / MRI) equipment, manipulating magnetic devices in non-magnetic media via magnetic fields (for example, robotic magnetic navigation systems for manipulating medical devices in patients), and magnets for electric motors, such as for electronic vehicles. As a further example, the present disclosure can be applied to a proton beam therapy device including an HTS magnet system, the HTS magnet system including the disclosed features, wherein the HTS magnet system is used for an accelerator of a PBT device, a quadrupole steering magnet or a dipole steering magnet of a PBT device, or any other magnet of a PBT device.

Claims

1. A superconducting magnet system, comprising a high temperature superconducting HTS excitation coil and a monitoring device, wherein the monitoring device comprises: a first conductive element, comprising HTS material and configured for connection to a current source, located within and electrically insulated from the HTS field coil; a second conductive element comprising an HTS material and connected in parallel to the first conductive element via a first connector and a second connector; as well as a current detector configured to detect a current in the second conductive element; Wherein, when the HTS material in each of the first conductive element and the second conductive element is in a superconducting state, the resistance R of the first conductive element between the first joint and the second joint is T is less than the sum of the resistance of the second conductive element between the first joint and the second joint and the resistance of the first joint and the second joint R B , that is, R T <R B .

2. The system according to claim 1, wherein: The first conductive element and / or the second conductive element are arranged as one or more pairs of parallel elements, the elements in each pair being adjacent to each other and configured to carry current in opposite directions.

3. The system according to claim 1, wherein: The current detector includes one or more of the following: a conductive loop around the second conductive element; Magnetic field detectors; Hall probe; as well as A strain gauge is coupled to two portions of the second conductive element, the two portions carrying current in different directions.

4. The system according to claim 1, wherein: The monitoring device includes a current source configured to provide a current to the first conductive element.

5. The system according to claim 4, wherein: The current source is modulated to provide a periodic, time-varying current.

6. The system according to claim 5, wherein: The current detector includes a phase sensitive detector.

7. The system according to claim 5, wherein: The current detector is configured to measure a duty cycle of a current in the second conductive element.

8. The system according to claim 4, wherein: The current source is configured to provide a peak current of at least 80% of a critical current of a portion of an HTS material of the first conductive element during normal operation of the system.

9. The system according to claim 1, wherein: The first conductive element and / or the second conductive element comprises a HTS tape having a HTS layer.

10. The system according to claim 9, wherein: The width of the HTS layer varies within the portion of the first conductive element between the tabs.

11. The system according to claim 9, wherein: The HTS layer is divided into a plurality of strips connected in series.

12. The system of claim 1, wherein: The current sensor is enclosed in a magnetic shield.

13. The system according to claim 12, wherein: The magnetic shield includes a bulk superconductor.

14. The system of claim 1, wherein: The monitoring device includes a third conductive element, the third conductive element includes HTS material and is connected in parallel with the first conductive element through a third joint and a fourth joint, wherein when the HTS material in each of the first conductive element, the second conductive element and the third conductive element is in a superconducting state, the resistance R of the first conductive element between the first joint and the second joint T is less than the sum of the resistance of the third conductive element between the third joint and the fourth joint and the resistance of the third joint and the fourth joint R B2 , and the sum of the resistance of the third conductive element between the third joint and the fourth joint and the resistance of the third joint and the fourth joint R B2 is less than the sum of the resistance of the second conductive element between the first joint and the second joint and the resistance of the first joint and the second joint R B , that is, R T <R B2 <R B , and wherein the peak critical current of the HTS material of the third conductive element is less than the peak critical current of the HTS material of the first conductive element, and wherein the current detector is additionally configured to detect current in the third conductive element.

15. The system of claim 1, wherein: The monitoring device includes a plurality of additional conductive elements, which include HTS material and are connected in parallel with the first conductive element.

16. The system of claim 1, wherein: The second conductive element includes a variable resistor.

17. The system of claim 16, wherein: The variable resistor is one of the following: Potentiometer; a system for controllably returning the HTS material in the second conductive element to normal; and A joint or part of a generally conducting material having a resistance that depends on temperature and a device for controlling its temperature.

18. The system of claim 1, further comprising a plurality of the monitoring devices and a controller configured to monitor the current detected by each monitoring device.

19. The system of claim 18, wherein: The controller is configured to identify a decrease in a critical current of the first conductive element of the corresponding monitoring device based on a current detected in the second conductive element of the corresponding monitoring device.

20. The system of claim 19, wherein: The controller is configured to identify a cause of a decrease in critical current based on a decrease pattern of the critical current identified for the monitoring device.

21. The system of claim 18, wherein: The controller is configured to identify a condition that may lead to a quench based on a rapid decrease in critical current identified for one or more of the monitoring devices.

22. The system of claim 18, wherein: The monitoring devices are arranged in one or more pairs, each pair comprising a first corresponding monitoring device and a second corresponding monitoring device, wherein: The first and second conductive elements of the first respective monitoring device are arranged adjacent to respective first and second conductive elements of the second respective monitoring device; and The first respective monitoring devices and the second respective monitoring devices are arranged such that current in the first conductive element of each monitoring device flows in the same direction and current in the second conductive element of each monitoring device flows in opposite directions.

23. The system of claim 18, wherein: The monitoring devices are arranged in one set or in a plurality of sets, each set comprising a monitoring device having a first conductive element, the monitoring device having different patterns of measurement areas and non-measurement areas along the first conductive element; The measurement region has a reduced critical current compared to the non-measurement region; as well as The controller is configured to identify the location of a condition causing a reduction in critical current based on current detection at a subset of the monitoring devices in a collection.

24. The system of claim 18, wherein: The monitoring devices are connected in series.

25. The system of claim 18, wherein: The monitoring devices are connected in parallel, with each monitoring device connected in series with a respective resistor having a resistance at least 10 times greater than the total resistance of the monitoring devices.

26. The system of claim 18, wherein: The monitoring devices are arranged in one or more groups, the monitoring devices in each group are connected in series with respective resistors having a resistance at least 10 times the total resistance of the monitoring devices of the group, and the groups are connected in parallel.

27. The system of claim 21, further comprising a quench protection system for dumping energy from the HTS field coil, wherein: The controller is configured to trigger the quench protection system for dumping energy from the HTS field coil upon identification of a condition that may cause a quench.

28. A tokamak fusion reactor comprising the system of claim 1, wherein: The HTS excitation coil is one of a toroidal excitation coil or a polar excitation coil.

29. A proton beam therapy (PBT) device comprising the system according to claim 1, wherein: The HTS field coil is one of the following: An excitation coil of an accelerator of the PBT device; A dipole magnet or a quadrupole magnet of a proton beam steering system of the PBT device.

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