Quench Protection Device

Abstract

A quench protection apparatus for a superconducting magnet is disclosed, the apparatus including: a superconducting magnet including a plurality of magnet sections; a plurality of varistors, each varistor electrically connected in parallel across a respective one of the plurality of magnet sections; and a heater device electrically connected to the plurality of varistors and configured to apply heat to each of the plurality of magnet sections in response to a change in voltage of any one or more of the plurality of varistors. A method of protecting the superconducting magnet is also disclosed.

Description

[Technical field]

[0001] The present invention relates to a quench protection apparatus, and more particularly to a quench protection apparatus for a superconducting magnet including a plurality of varistors connected to respective portions of the superconducting magnet. [Background technology]

[0002] A quench in a superconductor, particularly a superconducting magnet, is a sudden loss of internal superconductivity that occurs when a portion of the magnet coil returns to its normal resistive state. A quench can occur due to a variety of conditions, typically an excessive magnetic field or rate of change within the magnet, or a physical defect. As a result, a large current continues to flow through the portion of the coil that is no longer superconducting, causing rapid localized Joule heating at that location, raising the temperature of the surrounding regions. These regions then also return to their normal resistive state, thus setting up a heating chain reaction that raises the temperature of the entire magnet above the superconducting threshold and returning the magnet to its normal state.

[0003] When a quench occurs, the sudden reduction in current can cause induced voltage spikes and arcing. These high voltage and heating effects can damage magnets and other components. Also, in "wet" magnets, such as liquid helium bath magnets, which are maintained at a low operating temperature by a cryogenic fluid, the large amount of energy stored in the magnet while operating as a superconductor can be rapidly converted to heat, causing the cryogenic fluid to suddenly and explosively boil. Furthermore, in "wet" magnets, the boiling of this fluid can pose an asphyxiation hazard.

[0004] When a quench occurs, the resistance of the coil increases, causing a reduction in the current flow. However, the current usually decreases slowly, as the quench may take several seconds to develop depending on the size of the coil. Also, a magnet section usually operates in series with one or more other magnet sections, and in such a case, if only one magnet section quenches, the inductance of the other magnet sections generates a voltage aimed at maintaining the current, which may cause the energy stored in not only the quenched magnet section but also these connected magnet sections to be released in the quenched magnet section.

[0005] It is known to provide safety devices in superconducting magnets to avoid the concentration of unwanted and potentially harmful thermal energy in the quenched magnet by stopping or limiting the current as soon as possible after a quench occurs and diffusing or dissipating the generated heat as much as possible throughout the magnet part and other magnet parts connected to it. A known technique for achieving this diffusion effect is to initiate a quench in other coils by intentionally heating them when a quench is detected in a magnet coil.

[0006] The use of resistors and diodes in inductive superconducting magnet sections is a known approach for quench protection. Typically the resistors are constructed of NiCr, stainless steel, or another resistive material that has a linear voltage-current characteristic at a given temperature. In such existing protection systems, quenches propagate entirely passively through the coil sets by mutual inductance and current circulation in the resistors. This approach can be referred to as a "passive" protection approach.

[0007] Initiation of the quench can be achieved by the rate of change of current or reaching a critical current in each section. In some systems, a heater network is used to initiate and accelerate the quenching of adjacent coils. This approach can be called a "passive heater" approach. The heater is driven by a voltage generated across a resistor in the section where the quench occurred and / or in one of the other protective sections of the magnet.

[0008] Alternatively, the quench may be detected externally and in response cause a capacitive or battery discharge which is used to trigger or drive a heater within the magnet. This type of protection may be referred to as "active". [Prior art documents] [Patent documents]

[0009] [Patent Document 1] European Patent No. 3014634 [Patent Document 2] British Patent No. 2514372 Summary of the Invention [Problem to be solved by the invention]

[0010] Existing quench protection solutions have several drawbacks. For example, the use of conventional resistors in protection devices with high-field magnets can cause these resistors to melt and open circuit if excessive current flows through them, resulting in uncontrolled electrical arcing between ground and magnet parts. In such a scenario, damage to the magnets can occur. Additionally, the voltage rise time on the magnet parts can be long (typically around 100 ms), limiting the responsiveness of protection provided by passive and heater-based passive configurations.

[0011] For example, there is a need for improved methods of protecting superconducting magnets against quenches by more effectively driving the heaters in "passive heater" systems, further increasing and enhancing energy dissipation, and improving protection especially for small, high field and high current density magnets. [Means for solving the problem]

[0012] According to a first aspect of the present invention, there is provided a quench protection apparatus for a superconducting magnet comprising: a superconducting magnet including a plurality of magnet portions; a plurality of varistors, each varistor connected electrically in parallel across a respective magnet portion of the plurality of magnet portions; and a heater device electrically connected to the plurality of varistors and configured to apply heat to each of the plurality of magnet portions in response to a change in voltage of any one or more of the plurality of varistors.

[0013] The inventors have realised that varistors can be advantageously employed in cryogenic environments, and in particular can be used to improve protection against magnet quenches. A varistor is an electronic component that changes electrical resistance in response to an applied voltage, generally with increasing current resulting in a decrease in resistance. Figure 1A shows a comparison between the current-voltage relationship of a varistor and that of a fixed or linear resistor. Thus, a varistor can be understood as a voltage-dependent, non-linear electrical component. A varistor can generally be characterised as having a high electrical resistance at low voltages, which decreases with increasing voltage as discussed above. This response can generally be understood as following a sigmoidal curve. These components are commonly used as voltage surge limiters in other technical fields. It has been found that varistors can be advantageously used in protection circuits having low temperature superconductor (LTS) or high temperature superconductor (HTS) magnet parts. Such a device can produce high voltages and currents through the varistor at the onset of a magnet quench. This effect can be used to drive quench heaters in coils adjacent to the quench coil or otherwise positioned to provide heat to facilitate quenching, or to vary the current distribution between magnet portions to facilitate quenching throughout the magnet. In this manner, the device can be used to achieve propagation of the quench throughout the magnet more quickly than conventional resistor-based devices. The device can therefore distribute the quench more reliably and responsively throughout the magnet, thereby reducing the deleterious effects of highly localized heating. The rapid dissipation of energy throughout the magnet facilitated by the varistor device can advantageously reduce coil hot spots that arise during the occurrence of a quench. The term "voltage" as used in this disclosure can be understood to mean a potential difference. That is, the voltage of a varistor can be understood as the potential difference of the varistor.

[0014] As mentioned above, an important point about using varistors is that, generally, at low currents, the voltage across the varistor is relatively high, while at maximum current it remains within certain limits. The performance of a varistor with respect to current is given by V=CI β The relationship can be approximated by: V = V β / C β , where V is the voltage of the varistor, β is the nonlinearity parameter, and C is the resistance value when a current of 1 A flows through the varistor. Typically, these parameters are different at different operating temperatures for any given material. As a guideline, at room temperature, a varistor comprising a silicon carbide (SiC) material may typically have a nonlinearity parameter value of β 0.5, and a varistor comprising a zinc oxide (ZnO) material may typically have a nonlinearity parameter value of β 0.05. One or more of the multiple varistors are preferably selected based on their nonlinearity and resistance characteristics, typically quantified by their β and C parameter values.

[0015] By way of example, Fig. 1B and Fig. 2 show characteristic voltage-current graphs for a varistor with β=0.5, C=15 compared to a 1 Ω linear resistor. In Fig. 1B the response is plotted over the entire range of interest. If we assume a maximum allowable voltage of 250V for the protected magnet part, this corresponds to a current of 250A when using a 1 Ω linear resistor. At this current the voltage of the selected varistor falls within the voltage limit. In Fig. 2 we show an expanded part of the curve at a lower current. In this region the voltage of the varistor is higher than that of a resistor carrying the same current. It can be seen that a "passive" quench heater can require 100V to operate reliably, and while the varistor reaches this voltage with a current of less than 50A, a 1 Ω resistor needs a current of 100A to achieve this.

[0016] In various embodiments and configurations, the change in voltage that results in the application of heat to the magnet can be a change in voltage value in a positive or negative direction. Typically, the device is adapted to apply heat based on a voltage change that satisfies a predetermined condition, where the magnitude of the voltage change, i.e., the absolute value of the difference between the value before and after the voltage change, preferably meets or exceeds a set or predetermined threshold difference value.

[0017] In some preferred embodiments, the heater device is configured to apply heat in response to the voltage of any one or more of the multiple varistors reaching a threshold value. Thus, the change may be such that the voltage of the portion equals or exceeds the threshold value. As noted above, these values ​​may be positive or negative, so typically the change involves a voltage magnitude at or above the threshold value.

[0018] The device described in this disclosure can advantageously rely on a particular aspect in which the voltage of the varistor changes when a quench occurs in the magnet part to which it is connected, to heat the magnet or some other part of the magnet, in order to mitigate a quench occurring in the first part. This aspect can in principle be achieved by one heater. However, in preferred embodiments, the heating device comprises multiple heaters. More preferably, the heater or heaters are arranged to maximize the distribution of heat around the entire superconducting magnet or at least a part thereof that is to be heated to induce a quench. Thus, in some embodiments, the heater device comprises multiple heaters, each heater of the multiple heaters being arranged or configured in use to apply heat to a respective magnet part of the multiple magnet parts. Providing each magnet part with a respective heater configured to be activated when a quench occurs in any part allows the energy in the magnet to be dissipated more quickly and effectively.

[0019] Some preferred embodiments can be understood to be similar to the "passive heater" protection scheme described above. Thus, in some embodiments, a heater device is configured to generate heat to be applied to each of the multiple magnet portions by Joule heating as a result of a voltage change in said one or more of the multiple varistors. Passively driving the heater via a current generated or increased through one or more heaters connected to a protection circuit including a varistor also achieves a dramatic improvement in the responsiveness of the device to a quench of any part of the magnet, while maintaining the advantages of the passive scheme described above.

[0020] Typically, the varistors connected to each magnet section are provided as a single varistor component. However, in preferred embodiments, one or more or all of the multiple varistors each include two or more varistor components. That is, each varistor component in such embodiments is an individual varistor, and in these applications, these components are configured to operate in groups that function as varistors across a given magnet section. In various embodiments, each of these composite varistors that include multiple varistor components can include two, three, four, five, six or more, or any number of such components. The number of varistor components in each varistor can be the same or different for any two of the multiple varistors in the device, for example depending on the structure of the associated superconducting magnet coil.

[0021] When using varistors that include multiple varistor components grouped together, it may be advantageous to provide the grouped components in the form of a stack. Preferred embodiments therefore include these varistor components having a stackable shape, for example a tile, plate or disk. In some preferred embodiments, each varistor component is formed in the shape of a disk, with one or more of said varistor components each including two or more varistor components, the varistor components being arranged in a stack.

[0022] Additionally or alternatively, to utilize a varistor to activate a passive heater protection circuit, an active protection circuit can also benefit from the voltage response to a varistor quench. Specifically, the use of a varistor and its ability to provide a rapid voltage rise upon the occurrence of a quench is useful for active schemes, since the voltage change can be used as a signal or trigger to activate a protection device or system that is externally driven, powered or configured rather than driven by a current resulting from the voltage change.

[0023] Thus, in some embodiments, the apparatus further includes an active quench protection system including a quench detector module configured to detect a quench occurring in one or more magnets of the plurality of magnet portions based on the voltage of one or more varistors of the plurality of varistors connected across one or more magnet portions of the plurality of magnet portions reaching a predetermined quench threshold voltage value, which may be the same as or different from the threshold voltage value mentioned above. The active quench protection system may further include a quench inducer system configured to induce a quench condition in one or more magnet portions of the plurality of magnet portions in response to the quench detection module detecting the occurrence of a quench. This apparatus is advantageous in that by employing varistors, the threshold voltage can be reached more quickly, i.e., these active protection schemes, which have different methods of inducing a quench throughout the magnets as further described below, can be triggered more quickly.

[0024] A particular advantage of the rapid rise in varistor voltage is that if a quench occurs at a magnet current that is low relative to the magnet's operating current, the voltage of one or more varistors in the protection circuit rises above the active quench trigger level, whereas in the case of conventional resistor protection, the voltage falls below this level and the heater circuit does not trigger. A "low" current in this context is understood to correspond to a current that is typically less than half the magnet's operating current. Thus, in some embodiments employing an active protection system, the device can be configured such that a quench occurring in one or more of the magnet parts causes the voltage of one or more of the varistors to reach a predetermined quench threshold voltage value while the magnitude of the current through the magnet part is less than 50% of a predetermined current value corresponding to the magnet's operating current. In this way, such a device can extend quench protection compared to conventional approaches. This extension can be achieved by configuring the device, and in particular the varistors, such that the voltage condition for triggering active protection is met at a current level lower than the current level at which a resistor-based protection device operates. In some preferred embodiments, the device may be configured such that the voltage reaches a predetermined quench threshold voltage value when the magnitude of the current is less than 40%, 30%, 20% or 10% of the operating current.

[0025] The devices described in this disclosure can be used to protect various types of superconducting magnets. In addition to low temperature superconducting (LTS) magnets, hybrid magnets that also include high temperature superconducting (HTS) sections along with LTS sections generally require separate or additional devices to effectively protect the HTS sections. Some embodiments may employ the same methods of quench protection for both types of superconducting coils, but typically quench protection methods that apply to LTS are not necessarily applicable to, or particularly effective at, protecting, the HTS magnet sections.

[0026] The "stability margin" of HTS vs. LTS, usually understood as the minimum quench energy density, is three orders of magnitude larger for HTS coils than for LTS coils. This is explained for example in section 6.2.6 of Iwasa, "Case Studies in Superconducting Magnets", Springer (2nd ed., 2009), with particular reference to table 6.4. That is, magnets wound from HTS conductors are more stable against disturbances and less likely to quench than LTS windings.

[0027] Although the HTS windings may not quench, this does not necessarily mean that the HTS magnet parts will not be damaged, especially in magnets that are configured so that the HTS parts are inserted into an LTS magnet or housed in use. Typically in such configurations, as the current in the LTS winding decays, the current in the HTS winding increases due to inductive coupling via Lenz's law. One possible damage mechanism is that the resulting additional stresses overstress the magnet windings.

[0028] The high stability also means that it will be significantly more difficult to induce a quench to protect the magnet using devices or schemes typically applied to LTS magnets. Also, if a quench is induced locally in the HTS coil, it is less likely to propagate through the conductor windings for the same reason. Thus, if applied improperly, local heaters can have the undesirable effect of creating hot spots, which in principle could lead to overheating damage to the HTS.

[0029] To accommodate these differences, different devices have been used to protect both the HTS magnets and the HTS windings contained in hybrid magnets. There are many examples of different HTS protection schemes that are triggered based on voltage signal or change detection or other indicators that the magnet has quenched. One example is to open a switch from outside the magnet to dissipate energy from the coil. Another is to actively apply alternating current to induce heating in the bulk of the windings rather than locally at the surface of the coil. A device according to the latter example is described in EP 3014634, where it is called "Coupling Loss Induced Quench".

[0030] In some embodiments where an LTS / HTS hybrid superconducting magnet is to be protected, existing devices for protecting HTS magnets and magnet components can be employed. In such embodiments, the superconducting magnet portion includes at least one low temperature superconductor (LTS) magnet portion and at least one high temperature superconductor (HTS) magnet portion, and the quench protection device includes an HTS quench protection system adapted to apply quench protection to each of the at least one HTS magnet portion in response to the voltage of any one or more of the multiple varistors reaching a threshold voltage value. As mentioned above, typically the HTS portion is configured to be used as an insert in the LTS magnet portion. Various configurations, which may also be referred to as "outserts", can be used to combine the HTS insert and LTS portions of the magnet. For example, the LTS and HTS portions can be excited separately using two different respective power supplies that can operate with different currents. In some embodiments, the two portions can be connected in series and plugged into one power supply and operated with the same current. Other configurations are also possible where each portion can be excited with inductive coupling. Typically, HTS systems use a voltage condition as a trigger to activate techniques that specifically protect the HTS portion in the event of a quench occurring in any of the magnet portions.

[0031] As described above, the HTS quench protection system is adapted to apply quench protection by any one or more of: dissipating energy from at least one HTS magnet portion to a body external to the magnet; and applying an alternating current having a predetermined frequency and magnitude configured to cause Joule heating in a coil of the at least one HTS magnet portion. Energy can be dissipated by opening a switch to conduct energy from the coil to an external connection.

[0032] Various embodiments may use different types of varistors connected across the magnet portions, including those described above in this disclosure. It is preferred that the device includes a silicon carbide (SiC) varistor due to its properties. More preferably, one or more or a subset of the plurality of varistors includes silicon carbide, and even more preferably, each of the plurality of varistors includes silicon carbide.

[0033] In some preferred embodiments, the varistors included in the device are high energy silicon carbide varistors. Such components typically comprise silicon carbide, generally in granular form, dispersed or otherwise disposed within a matrix, typically formed from one or more binders and / or additives. Typically, the matrix is ​​formed from a clay material. Typically, each varistor has the shape of a disk comprising the material(s) and typically has electrical contacts attached thereto.

[0034] Typically, varistors are selected according to the particular characteristics desired for a given application, including any of the voltage, current, and energy characteristics of the varistor. Varistors may also be selected based on the degree of nonlinearity of the component's resistance, which is typically quantified in terms of the varistor's beta value.

[0035] The varistor may also approach its component failure rating due to the failure mode of a localized short circuit at the disk of a typical varistor caused by the present protection device. The robust material composition of the varistor according to the exemplary embodiment is beneficially retained in cryogenic environments, which typically allows magnet testing to be completed without arcing to ground and subsequent damage to the magnet coils.

[0036] A variety of SiC varistors are suitable for use in the present device. The inventors have surprisingly realised that varistors traditionally used in room temperature applications can be used in cryogenic environments to provide a quench protection device that offers many advantages over existing protection systems. Typically, the varistor and other components of the arrangement, such as the protection circuitry and / or heater circuitry, are configured to operate in a cryogenic environment.

[0037] This device has the advantage that it can facilitate the incorporation of protection circuitry that takes advantage of the inherent properties of the varistor to facilitate quenching in a number of ways.

[0038] This arrangement can be beneficial because it maximises the current density in the varistor (which is usually more stable when compared to a conventional resistive circuit) whilst reducing the risk of damage to the magnet by having high voltage at low current and low voltage at high current. Usually high voltage at low current initially, followed by a drop in resistance, means that the varistor can be configured to avoid high current build-up in the magnet area.

[0039] Also, in some embodiments, the voltage from the varistor can be used to drive the heater more efficiently, particularly in terms of the speed at which a quench can be initiated in one or more other coils, which can also make the heater more reliable in different quench scenarios since the heater voltage is typically self-limiting at least to some extent.

[0040] The varistors are preferably configured to provide voltage clamping, thereby maintaining a safe voltage at which the magnet coils are protected despite excess current. The size of the SiC grains in the varistor(s) can be selected to optimize the clamping voltage and / or current density.

[0041] Additionally, an "active" protection circuit preferably has a large initial partial voltage, allowing for faster triggering (positive and negative) at set voltage thresholds.

[0042] In some embodiments, a combination of both active protection circuits, each of which is varistor-based as described above, and "heater passive" devices is beneficial. For example, such a combination is particularly beneficial in magnets that include both LTS and HTS components, since two different protection schemes can be applied to the most suitable components. This is described in more detail below.

[0043] In some preferred embodiments, a varistor is connected in series with a back-to-back diode across the magnet portion to be protected. The device can include a protection circuit designed to draw more current and burn out the heater, especially in a "passive with heater" scenario. In these embodiments, the discovered advantageous properties of the varistor are beneficially utilized.

[0044] The device may also provide improved passive quench protection, especially under cryogenic conditions such as T=4.2K, thus providing fail-safe operation.

[0045] As discussed above in this disclosure, an important advantage that can be achieved by the described apparatus is that a quench occurring in a given magnet portion can propagate more quickly to other magnet portions. This advantage is achieved by providing as part of the apparatus a protection circuit with one or more heaters connected to rapidly apply heat across the magnet. The heater device is preferably associated with a number of varistors such that when a quench occurs in one of the magnet portions, the resulting voltage across the varistor causes the heater device to provide heat to at least one further magnet portion of the magnet portions.

[0046] According to a second aspect of the present invention, there is provided a method of protecting a superconducting magnet including a plurality of magnet portions, the method comprising the steps of: each of a plurality of varistors electrically connected in parallel across a respective magnet portion of the plurality of magnet portions; a heater arrangement electrically connected to the plurality of varistors; and applying heat to each of the plurality of magnet portions in response to a change in voltage of any one or more of the plurality of varistors.

[0047] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. [Brief description of the drawings]

[0048] [Figure 1A] 1 is a line graph illustrating the characteristics of a typical varistor compared to a typical linear resistor. [Figure 1B] 1 is a line graph illustrating the characteristics of an example varistor compared to the characteristics of a resistor over a range of interest. [Diagram 2] 1 is a line graph comparing the characteristics of a varistor and a linear resistor at low currents. [Diagram 3] FIG. 1 is a schematic diagram of a cross section of a magnet test coil used to model quench behavior using varistors instead of resistors in a magnet quench protection device. [Figure 4]1 is a line graph showing a comparison of modeled coil voltages for a two-part magnet having a quench section protected by a varistor and a quench section protected by a fixed resistor. [Diagram 5] 1 is a line graph plotting modeled coil current through a magnet with both coils quenched, showing the current when the magnet is protected by a varistor and when it is protected by a linear resistor. [Figure 6] FIG. 1 is a schematic diagram of an example protection device showing a protection circuit associated with a superconducting magnet circuit; the magnet shown is a simplified example having only two coil sections. [Figure 7] FIG. 2 is a schematic diagram including a comparison of the protection circuit of the protection device according to the invention across one protected coil section with an equivalent circuit according to the prior art; [Figure 8] FIG. 1 is a schematic diagram showing an example apparatus and protection circuit according to the present invention that combines an internally actuated quench heater on an LTS magnet with a trigger for externally actuated quench protection of an HTS insert within the magnet. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Quench modeling was performed to demonstrate the capabilities of varistors in a quench protection device. For simplicity, and given the tendency of varistor effects to be obscured or introduce systematic modeling errors in more complex modeling scenarios, a simple two coil design was used to test the concept. This testing is for illustrative purposes only, and it is assumed that the quench protection device can be implemented with any number of magnet coils. Computer software was used to model the coil configuration and quench modeling.

[0050] Figure 3 shows a cross-section of one half of the cylindrical coils modeled in this general example. The coils are shown as cylindrical blocks centered at R=0 (corresponding to the y-axis of the diagram) and extending in the z-direction (parallel to the x-axis of the diagram). Coil 1 and Coil 2 are both made of the same size NbTi wire, chosen to have similar resistance and inductance for simplicity and for comparison of the quench modeling results with approximate theoretically derived analytical calculations. The varistor parameters used were derived from experimental measurements of voltage-current characteristics at cryogenic temperatures. The superconducting wire and other material properties, as well as other internal model parameters, have long been established and used reliably in quench model computer programs during magnet design and for analysis of magnet test results.

[0051] The effect of varistor-based quench protection on the derived voltage of the quench section was demonstrated by modeling a quench starting at coil 2. It was found that when a varistor was used to protect the magnet instead of a resistor, the voltage rise was significantly faster compared to when the quench section was protected with a conventional resistor. Based on the data visualized in Figure 4, it can be seen that the quench heater can be initiated approximately 50 ms earlier by using a suitable varistor in the protection circuit instead of a resistor.

[0052] To demonstrate the effect of using a varistor on the maximum overcurrent induced in a coil during a quench, a model was run with both modeled coils quenched. Thus, the results show the case where the quench heaters are initiated across all coils. This also approximates the case where the effect from the quenched coil is used to induce a quench in other coils shortly after the first coil begins to quench.

[0053] The model showed that protecting the magnets with varistors significantly reduced the peak current during the slowest quench compared to when they were protected with conventional resistors. Based on the data visualized in Figure 5, the peak overcurrent could be reduced by more than half by using an appropriate varistor in the protection circuit instead of a conventional resistor.

[0054] 4 and 5 show exemplary outputs of quench modeling.

[0055] An example of a quench protection device that employs the principles demonstrated by quench modeling will now be described.

[0056] In some instances it may be preferred that a varistor be connected in series with a "back to back" diode across the magnet section to be protected and the derived voltage used to operate a heater in the "passive with heater" scenario described above. A protection scheme such as that described in GB2514372 is suitable for implementation as a means of protecting the magnet coils in response to varistor voltage changes.

[0057] 6 is a schematic representation of a first apparatus example showing a magnet having only two coil sections for simplicity. It is envisioned that applications of this arrangement will include significantly more shielded magnet coil sections than this simplified example.

[0058] FIG. 7 is a schematic representation of an example protection circuit including a varistor on the right side of the figure, and a prior art circuit including a linear resistor on the left side of the figure.

[0059] In a further example particularly suited for protecting magnets including LTS magnets with HTS inserts, different protection schemes are used for the LTS coil and the HTS coil. These two magnet parts can be operated in parallel using separate power supplies or in series. This example employs a "passive heater" scheme for protecting the LTS part, similar to the device example above. The device is also configured to detect voltage changes across the LTS magnet protection elements more quickly than was possible with prior art devices, in order to activate a quench protection scheme external to the HTS magnet. Such combinations of protection schemes for hybrid magnets have been described above. Additional elements included in this example are shown diagrammatically in FIG. 8.

[0060] Next, an example of a SiC varistor and a superconducting magnet quench protection device including the same will be described in detail.

[0061] As mentioned above, the characteristics of a varistor are V=CI β The "beta" value β, which determines the degree of nonlinearity of the varistor, affects the rate of discharge and can be modified by dopants and process adjustments in the varistor material.

[0062] Some varistor examples are semiconductor devices made from SiC particles in a clay matrix to produce discs or tiles.

[0063] Failure mode testing of the example device was performed in liquid nitrogen at energies more than five times higher than when the device was at room temperature. The varistor disk was found to develop non-propagating hot spots that continued to carry current even when it failed by short circuit, a property that is particularly useful for protecting magnets. The SiC material of this example varistor maintained its integrity even in cryogenic conditions, and with material optimization, it is envisioned that further examples could become effective replacements for linear damping resistors.

Claims

1. 1. A quench protection apparatus for a superconducting magnet, comprising: a superconducting magnet including a plurality of magnet portions; a plurality of varistors each connected electrically in parallel across a respective one of the plurality of magnet portions; a heater device electrically connected to the plurality of varistors and configured to apply heat to each of the plurality of magnet portions in response to a change in voltage of any one or more of the plurality of varistors; An apparatus comprising:

2. the heater device is configured to apply heat in response to a voltage of any one or more of the plurality of varistors reaching a threshold voltage value.

2. The apparatus of claim 1.

3. the heater device includes a plurality of heaters, each heater of the plurality of heaters being positioned to apply heat to a respective magnet portion of the plurality of magnet portions; 3. Apparatus according to claim 1 or 2.

4. the heater device is configured to generate heat by Joule heating to be applied to each of the plurality of magnet portions as a result of the voltage change of the one or more varistors of the plurality of varistors.

4. Apparatus according to any one of claims 1 to 3.

5. one or more varistors of the plurality of varistors each include two or more varistor components; 5. Apparatus according to any one of claims 1 to 4.

6. Each varistor component is formed in a disk shape, and in each of the one or more varistors, the varistor components include two or more varistor components, the varistor components being arranged in a stack.

6. The apparatus of claim 5.

7. a quench detector module configured to detect a quench occurring in one or more magnets of the plurality of magnet portions based on a voltage of one or more varistors of the plurality of varistors reaching a predetermined quench threshold voltage value; a quench inducer system configured to induce a quench condition in one or more magnet portions of the plurality of magnet portions in response to the quench detection module detecting an occurrence of a quench; and and an active quench protection system including:

7. Apparatus according to any one of claims 1 to 6.

8. configured such that a quench occurring in one or more of the plurality of magnet portions while a magnitude of a current through the magnet portion is less than 50% of a predetermined current value corresponding to an operating current of the magnet causes the voltage of one or more of the plurality of varistors to reach the predetermined quench threshold voltage value.

8. The apparatus of claim 7.

9. the superconducting magnet portions include at least one low temperature superconductor (LTS) magnet portion and at least one high temperature superconductor (HTS) magnet portion, and the quench protection device includes an HTS quench protection system adapted to apply quench protection to each of the at least one HTS magnet portion in response to a voltage of any one or more of the plurality of varistors reaching a threshold voltage value; 9. Apparatus according to any one of claims 1 to 8.

10. each of the plurality of varistors comprises silicon carbide; 10. Apparatus according to any one of claims 1 to 9.

11. the heater device is associated with the plurality of varistors such that, upon a quench occurring in one of the plurality of magnet portions, a resulting voltage across the varistor causes the heater device to supply heat to at least one further magnet portion of the plurality of magnet portions; 11. Apparatus according to any one of claims 1 to 10.

12. 1. A method of protecting a superconducting magnet including a plurality of magnet portions, each of a plurality of varistors being electrically connected in parallel across each of the plurality of magnet portions, a heater device being electrically connected to the plurality of varistors, the method comprising applying heat to each of the plurality of magnet portions in response to a change in voltage of any one or more of the plurality of varistors. A method comprising:

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