HTS magnet quench starting system

Abstract

An apparatus comprising a high temperature superconductor (HTS) circuit; wherein the HTS circuit comprises: a quenchable portion comprising HTS material and connected in series to other elements of the HTS circuit, the HTS material comprising a stack of HTS tapes, the stack of HTS tapes comprising at least one HTS tape; the apparatus further comprising: a quench system configured to quench the HTS material in the quenchable portion; a quench protection system configured to detect a temperature rise in the HTS circuit and, in response to detecting the temperature rise, cause the quench system to quench the superconducting material in the quenchable portion to transfer stored magnetic energy from the HTS circuit to the quenchable portion; wherein the HTS circuit is configured, in use, such that a magnetic field on the or each HTS tape is substantially parallel to an a-b plane of the HTS tape, and the quench system is configured to quench the HTS material by generating an additional magnetic field along the length of the or each HTS tape in the quenchable portion such that the additional magnetic field has a component perpendicular to the a-b plane of the HTS tape.

Description

Technical Field

[0001] The present invention relates to superconducting devices and more particularly to methods and apparatus for quench protection in such devices, in particular to methods and apparatus for quench protection in magnets used in nuclear fusion reactors. Background Art

[0002] A superconducting magnet is an electromagnet made from a coil of superconducting material. Because the electromagnetic coil has zero resistance, a superconducting magnet can carry large currents with zero losses (although there will be some losses from non-superconducting components), and thus can achieve high fields with lower losses than conventional electromagnets.

[0003] Superconductivity only occurs in certain materials and only at low temperatures. In the region defined by the critical temperature of the superconductor (the maximum temperature at which the material is a superconductor when no magnetic field is applied) and the critical field of the superconductor (the maximum magnetic field at which the material is a superconductor at 0K), the superconducting material will behave as a superconductor. The temperature of the superconductor and the magnetic field present limit the current that can be carried by the superconductor when the superconductor does not become resistive (or "normal", used in this article to mean "not 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 allows magnetic flux above the lower critical magnetic field in the local normal region called the magnetic flux vortex to penetrate the superconductor. These two superconducting materials are no longer superconducting at the upper critical magnetic field. These characteristics enable them to be used as wires for constructing superconducting magnets. Important efforts have been made to pin the position of the magnetic flux vortex to the atomic lattice, which increases 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 below about 30K (even in zero magnetic field, above which the material cannot be superconducting). The performance of HTS materials is not described by the BCS theory, and such materials may have a critical temperature above about 30K (but it should be noted that it is the physical differences in superconducting operation and composition that define HTS materials rather than the critical temperature). The most commonly used HTS are "cuprate superconductors" - ceramics based on cuprates (compounds containing copper oxide groups), for example, BSCCO or ReBCO (where Re is a rare earth element, typically Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diboride (MgB2).

[0005] ReBCO is usually manufactured as a ribbon with Figure 1The structure 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 called 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 to 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, the copper stabilizer layer 505 typically completely encapsulating 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 the HTS layers are grown, and to prevent chemical diffusion of elements from the substrate to the HTS that would damage 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 portion of the ReBCO ceases to superconduct (enters a "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. This can occur due to fluctuations in temperature or magnetic field, physical damage or defects in the superconductor (for example, cracks or degradation caused by neutron irradiation if the magnet is used in a fusion reactor). Because of the large currents in the magnet, when even a small portion of the superconductor becomes resistive, it heats up rapidly. All superconducting wires are equipped with some copper stabilizer for quench protection. The copper provides an alternative path for the current if the superconductor becomes resistive. The more copper there is, the slower the temperature rises in the hot spot that forms around the quenched superconductor area.

[0008] In an LTS magnet, when a quench occurs, the "normal region" will propagate rapidly - on the order of several metres per second. This is due to the low heat capacity of all materials at low temperatures, and due to the fact that LTS materials are typically operated much closer to their critical temperature. This means that the quench will propagate rapidly in an LTS magnet, and the stored magnetic field energy that was dissipated during the quench will spread throughout the magnet, causing it to heat up.

[0009] HTS materials operating at high temperatures have a higher specific heat capacity, so the energy required to bring a portion of the wire to normal is much greater. This means that HTS magnets are much less likely to quench than LTS magnets. However, this also means that the normal region propagates much more slowly - on the order of a few centimeters per second compared to meters per second for LTS magnets. Since the quench will initially only affect a very small volume of the magnet, only this area will be resistive - so the energy dissipated during the quench will be transferred to this small volume (or more specifically, to the copper that draws the current from the normal region). 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 usually cooled indirectly, rather than being immersed in a container of liquid coolant - so the local cooling capacity is reduced compared to LTS magnets.

[0010] The energy stored in the magnetic field is given by:

[0011]

[0012] That is, the greater the flux density and the larger the volume, the greater the stored energy of the magnet. The energy released by a large magnet can be of the order of a stick of dynamite. For an LTS magnet, this energy can be dissipated in heating the entire magnet. For an HTS magnet without quench protection, this energy may be dissipated in a small portion of the magnet volume. Typically, large HTS magnets will require an active quench protection system, including a detection phase in which the quench is detected before significant heating occurs, followed by a dissipation phase in which the magnet current is rapidly reduced before the hot spot temperature rises too high.

[0013] To date, most HTS magnets (using BSCCO and ReBCO coated conductors) are not actually quench protected. As mentioned earlier, this is because they are mostly small, low-cost prototypes with little stored energy, and also because a well-designed HTS magnet should have an extremely low probability of quenching. Therefore, the decision whether to quench protect an HTS magnet is actually an economic decision: in the rare case that a small prototype magnet quenches, it can be repaired relatively easily. As a result, quench protection technology for HTS magnets remains immature.

[0014] One application of HTS magnets is in Tokamak fusion reactors. Tokamaks are characterized by a combination of strong toroidal magnetic fields, high plasma currents, and generally large plasma volumes and significant auxiliary heating to provide a thermally stable plasma that enables fusion to occur. Auxiliary heating (e.g., via high-energy neutral beam injection of tens of megawatts of H, D, or T) is required to increase the temperature to sufficiently high values ​​for nuclear fusion to occur, and / or to maintain the plasma current.

[0015] The problem is that they are expensive to build and operate because of the large size, large magnetic fields, and high plasma currents typically required, and the engineering design must be robust to cope with the large stored energies present both in the magnet system and in the plasma (which has a habit of "disrupting" and, in extremely unstable situations, megaamperes of current can drop to zero in a few thousandths of a second).

[0016] This situation can be improved by shrinking the doughnut-shaped torus of a conventional tokamak to its limit, giving it the appearance of a cored apple - a "spherical" tokamak (ST). This concept, first implemented in the START tokamak at Culham, demonstrated a huge improvement in efficiency - the magnetic field required to contain the hot plasma can be reduced by a factor of 10. In addition, plasma stability is improved and construction costs are reduced.

[0017] To obtain the fusion reactions required for economical power generation (i.e., power output much greater than power input), conventional tokamaks must be very large so that the energy confinement time (which is roughly proportional to the plasma volume) is large enough that the plasma is hot enough for thermal fusion to occur.

[0018] WO2013 / 030554 describes an alternative approach involving the use of a compact spherical tokamak for use as a neutron source or energy source. The low annulus ratio plasma shape in a spherical tokamak improves particle confinement time and allows net power generation in a much smaller machine. However, a small diameter central column is required, which poses a challenge to the design of the plasma confinement magnet.

[0019] The main attraction of HTS to tokamaks is the ability of HTS to carry large currents in strong magnetic fields. This is particularly important in compact spherical tokamaks (STs), in which the flux density on the surface of the central column will exceed 20 T. A second advantage is the ability of HTS to carry large currents in strong magnetic fields at higher temperatures (e.g., about 20 K) relative to LTS. This enables the use of thinner neutron shielding, resulting in higher neutron heating of the central column, which will exclude operation using liquid helium (i.e., at or below 4.2 K). This also enables the design of a spherical tokamak with a main plasma radius of less than about 2 m (e.g., about 1.4 m would be considered); such a device would circulate a percentage of its power output for cryogenic refrigeration.

[0020] However, such magnets are far larger than those previously designed using HTS materials. Even a toroidal field (TF) magnet for a relatively small tokamak would be the largest HTS magnet ever built, and would represent a huge magnet with very high stored energy even by LTS standards. A carefully developed quench protection system that can cope with critical current decay in the conductor is essential. For a spherical tokamak operating at a major radius of 60 cm, the stored energy of a TF magnet (about 4.5 T) would be 150-200 MJ, and for a 140 cm tokamak, a TF magnet (about 3 T) would be over 1200 MJ.

[0021] The role of the quench protection system is to detect a local quench or "hot spot" as soon as possible after initiation to minimize damage, or before initiation by detecting the transfer of current from the superconductor to the copper stabilizer; and to open a circuit breaker to transfer the stored energy of the magnet to a resistive load. The energy transfer can be achieved by passing a current through a resistor external to the magnet's cryostat at room temperature, or by heating the "cold volume" of the magnet to make it resistive, optionally using the magnet's own stored energy to artificially cause the quench to propagate more rapidly across the superconducting coils (so that the energy is dissipated through the entire magnet, which heats the entire magnet gradually, rather than causing a sharp increase in the temperature of the hot spot). Artificial propagation is difficult in HTS magnets because the amount of heat required to quench the entire magnet is much greater than in LTS, and is difficult to achieve in practice.

[0022] In order for the quench protection system to prevent severe damage to the magnet, the temperature of the hot spot should be kept below about 200K. The time it takes for the hot spot temperature to rise from the operating temperature of the HTS magnet of about 20K to 200K depends on the amount of copper stabilizer provided in the conductor. In a compact spherical tokamak carrying 35MA in a central column of about 35cm radius (i.e.: engineering design current density of about 90A / mm2), the time between the onset of the quench and the activation of the energy transfer should be less than 0.5s to limit the hot spot temperature to less than 200K. There are two aspects for achieving this: increasing the speed of quench detection (even detecting conditions that may cause a quench, so energy can be transferred before the actual quench occurs); and increasing the speed of energy transfer.

[0023] Previous work has proposed using heaters to intentionally quench portions of an HTS magnet, or to quench the LTS portion of a combined HTS / LTS magnet. For example, in a toroidal field coil in a tokamak, space in the center column is limited, but there is plenty of space in the return branches, so heaters can be added in the return branches to quench the superconducting material there, transferring the magnetic energy to a large portion of the return branch rather than the small region where the quench originally occurred. However, it takes a small but significant amount of time for the heat to diffuse from the heater to the superconducting material and heat it to the point where a controlled quench occurs. A faster method is needed to induce an intentional quench in a superconducting circuit. Summary of the invention

[0024] According to a first aspect of the present invention, there is provided a device comprising a high temperature superconductor HTS circuit.

[0025] The HTS circuit includes:

[0026] a quenchable portion comprising an HTS material and connected in series to other elements of the HTS circuit, the HTS material comprising a stack of HTS tapes, the stack of HTS tapes comprising at least one HTS tape;

[0027] The device also includes:

[0028] a quench system configured to quench the HTS material in the quenchable portion;

[0029] a quench protection system configured to detect a temperature rise in the HTS circuit and, in response to detecting the temperature rise, cause the quench system to quench the superconducting material in the quenchable portion to transfer the stored magnetic energy from the HTS circuit to the quenchable portion;

[0030] wherein the HTS circuit is arranged, in use, such that the magnetic field on the or each HTS tape is substantially parallel to the ab plane of the HTS tape, and the quench system is arranged to quench the HTS material by generating an additional magnetic field along the length of the or each HTS tape in the quenchable portion such that the additional magnetic field has a component perpendicular to the ab plane of the HTS tape.

[0031] According to a second aspect of the invention, there is provided a quench system for intentionally quenching an HTS tape in a magnetic field substantially parallel to an ab plane of the HTS tape, the quench system comprising means for generating a second magnetic field passing through the HTS tape and having a component perpendicular to the ab plane. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a graph of the critical current of the HTS strip versus the magnetic field angle;

[0033] Figure 2 is a schematic diagram of an exemplary quench system;

[0034] Figure 3 is a schematic diagram of an exemplary HTS cable;

[0035] Figure 4 yes Figure 2 A plot of critical current versus time for the HTS strip in FIG.

[0036] Figure 5 is a graph of heat capacity versus temperature for stainless steel and copper;

[0037] Figure 6 is a graph of the resistivity of stainless steel and copper versus temperature. DETAILED DESCRIPTION

[0038] As described above, when the current in the material approaches and exceeds the critical current, the superconducting material will become normal (i.e., behave as a resistor). For high temperature superconductors (HTS), the critical current depends on the temperature of the superconducting material, the strength of the magnetic field, and the direction of the magnetic field to which the HTS is exposed. The critical current can be reduced by higher temperatures, stronger magnetic fields, and larger angles between the magnetic field and the ab plane of the superconductor. For most commercially available ReBCO HTS tapes, the ab plane is roughly aligned with the plane of the HTS tape, but it may deviate by as much as 35 degrees (in fact, there may be greater deviations - 35 degrees is just the maximum angle of the HTS tapes currently available). For simplicity, the remainder of this disclosure will assume that the ab plane is aligned with the plane of the HTS tape, and those skilled in the art will understand that for HTS tapes that are not the case, appropriate modifications can be made.

[0039] As an alternative or in addition to the scheme of inducing quench by heating the magnets, a system is proposed below to induce quench by changing the angle of the magnetic field to which the HTS strip is subjected. Since the main uses of HTS strips are magnets and other high current applications, such as fault current limiters, the external magnetic field on each HTS strip is usually already very large - either designed for magnets or due to the existence of other high current HTS strips for other applications. However, as Figure 1 As shown, the critical current is very strongly dependent on the direction of the applied magnetic field - for some commercially available HTS tapes, the critical current is reduced by half for a change in magnetic field angle of less than 10 degrees. The intentionally quenchable portion of the HTS coil can be designed to operate with an I / I close to 1. C The system is run in proportion to the number of HTS units used (usually as this is the case as reducing the number of HTS units used will result in significant savings).

[0040] The system performs best when the coil portion of the magnet containing the HTS tape that is to be quenched is arranged so that the magnetic field is substantially aligned with the ab plane of the HTSReBCO layer. This ensures that the critical current of the HTS tape is at or close to the maximum possible value at that flux density and temperature. If the magnetic field is then rotated in the correct direction, the critical current will drop sharply until it is below the transmission current. The affected coil portion will then quench, generating heat that heats the quenched portion to ensure that the critical current cannot be restored after the magnetic field pulse ends.

[0041] Such a quench initiation system should be designed to generate a magnetic field perpendicular to the ab plane in some or all of the HTS strips in the coil, and at least in enough of the HTS strips in the coil, so that the total critical current of the HTS strips exposed to the additional perpendicular magnetic field will be less than the current in the HTS strips (which may be all of the HTS strips in the coil). The most effective method is to generate a magnetic field with a large component perpendicular to the ab plane of the HTS strips (i.e., parallel to the c-axis of the HTS), which, when combined with the existing external magnetic field, will produce a magnetic field at an angle to the ab plane sufficient to quench the HTS strips. However, it is also possible to generate the magnetic field at any angle that is not parallel to the ab plane, although this will require a much stronger magnetic field.

[0042] like Figure 2As shown, a simple method for generating such a magnetic field is to pass a current through a wire 201 extending parallel to the length of the HTS tape 202 and located in the ab plane of the HTS tape. The magnetic field 203 around such a wire will intersect the HTS tape parallel to the c-axis of the HTS tape (i.e., perpendicular to the HTS tape). The magnetic field can be enhanced by having a similar wire 204 extending on the opposite side of the HTS tape and carrying current in the opposite direction. In order to quench the HTS tape 202, a current pulse is sent through the wires 201, 204, causing the wires to generate a magnetic field perpendicular to the HTS tape, which, when combined with the existing magnetic field, has the effect of momentarily rotating the magnetic field encountered by the HTS tape. Once the HTS tape 202 becomes normal due to the influence of the magnetic field, the heating caused thereby will generally lead to a complete quench. The length of the current pulse is a balance between the requirement to ensure a complete quench (which will be more certain with a longer pulse) and the requirement to avoid damage to the wires 201, 204 (caused by sending a large current flowing through the resistive wire).

[0043] The magnetic field pulse will necessarily be of short duration, since the pulsed field generating coil will heat up rapidly, and the voltage required to maintain the large DC current through it will increase. A large transient voltage (typically several kilovolts) will be required to drive the large current through the pulse generating coil, so it should be electrically isolated from the electromagnetic coil. However, the conductors of the pulse generating coil should be positioned between and close to the conductors of the portion of the electromagnetic coil to be quenched, in order to generate the maximum pulsed field possible. Furthermore, the pulsed field and electromagnetic conductors should be maintained in as good thermal contact as possible (consistent with adequate voltage isolation) to allow the heat pulse from the pulse generating coil to diffuse into the electromagnetic coil, thereby further propagating the quench.

[0044] An arrangement that alternates the direction of the current in adjacent pulse generator coil turns will result in a low inductance winding. This has the further advantage of reducing the induced voltage generated by the rapidly changing current during the rising and falling edges of the pulses.

[0045] An additional benefit of using a pair of conductors 201, 204 is that the additional electric field away from the pair is small because at a distance long compared to the distance between them the magnetic fields from each conductor will tend to cancel each other.

[0046] Figure 3 A configuration of a quenchable HTS cable is shown for use in situations where the main magnetic field on each HTS tape comes from other tapes in the cable. Such HTS tapes can be used in DC power transmission applications or in magnets with low magnetic field portions (e.g., outside of the return branch of a toroidal field coil). Similar principles can be applied to HTS cables used in other situations, but the arrangement of the HTS tapes in the cable will be different.

[0047] The HTS cable 300 includes an HTS tape or a stack of HTS tapes 301 and conductors 302, 303. The HTS tapes are arranged so that the ab plane of the ReBCO HTS layer in the HTS tape 301 is perpendicular to the radius of the cable 300. The pulse generating conductors 302, 303 are arranged so that there is one conductor between each of the HTS tape stacks 301. The conductor 302 carries current in one direction ( Figure 3 "outward from the page"), wire 303 carries current in the opposite direction ( Figure 3 Each HTS ribbon stack 301 is adjacent to a wire 302 carrying current in one direction, and a wire 303 carrying current in the opposite direction. The effect of this arrangement is that when a pulsed current flows through the wires 302 and 303, each HTS ribbon 301 is subjected to an additional pulsed magnetic field applied parallel to the c-axis of the HTS ribbon 301, which will cause the critical current of the HTS ribbon stack to be reduced.

[0048] Current can be easily supplied to this arrangement of wires by alternately connecting the top and bottom of each adjacent pair and passing current through the resulting series circuit. However, other arrangements with parallel connection schemes are also possible, which allow the current and voltage conditions of a practical pulse current generator to be obtained.

[0049] By providing a controllable pulsed magnetic field via current flowing through a resistive component close to the HTS strip, the resistive component will also generate heat which will further reduce the critical current of the HTS strip (after an additional delay as the heat will take time to diffuse). Figure 4 Show according to Figure 2200 . An exemplary graph of critical current versus time for a system of FIG. 201 . At time T0, a quench occurs elsewhere in the HTS circuit, and the critical current of the quenchable portion is at its normal value 401. After a short delay, at time T1, the quench is detected and a current pulse is sent to the conductors 201, 204, causing the critical current of the HTS tape to drop sharply 402 to a value lower than the current in the HTS tape 400, causing the HTS tape (along the entire length subjected to the angular displacement of the magnetic field) to quench. The drop in critical current 402 occurs at time T2 shortly after the current pulse, because the current pulse will induce eddy currents in the metal parts of the HTS cable that act to resist the change in magnetic flux. After the current pulse, the critical current of the HTS tape may rise again (because the external magnetic field is again substantially aligned with the main plane), but it reaches a lower value 403 than before due to the heating caused by the quench in the HTS tape. Shortly thereafter, at time T3, the critical current will drop again 404 as heat from the current flowing through the conductors 201, 204 diffuses through the intermediate structure to the HTS. If the critical current after the current pulse is insufficient to keep the HTS strip quenched, a further drop in the critical current will ensure that a quench occurs.

[0050] Depending on the length of the current pulse, the magnetic and electrical properties of the HTS cable, and the degree of thermal contact between the wires 201, 204 and the HTS ribbon 202, the heat from the current pulse may arrive during or after the pulsed magnetic field caused by the current pulse itself.

[0051] A quenching system for quenching a quenchable portion of an HTS circuit has been described above. To provide a complete quench protection system for the HTS circuit, the quenchable portion is arranged in series with other components of the HTS circuit, and a quench detection system is configured to detect a quench in the HTS circuit and, in response, cause the quenching system to quench the quenchable portion. The quenchable portion should have a heat capacity sufficient to maintain the temperature of the quenchable portion below a first predetermined temperature (e.g., 700K to prevent melting of stabilizers in the quenchable portion; or 200K to maintain acceptable strains in the quenchable portion due to differential thermal expansion; or 100K or 50K to ensure stability of the cooling system), and a resistivity sufficient to cause the decay of the current of the magnet to be rapid enough to maintain the temperature of the HTS ("hot spot") that initially quenched below a second predetermined temperature (e.g., 700K, 300K, 100K or 50K). This may be achieved by providing the quenchable portion with a non-superconducting stabilizer comprising a metal having a resistivity to volumetric heat capacity ratio greater than that of copper, for example, stainless steel.

[0052] In order to efficiently transfer energy from an HTS circuit with minimal risk of damage to the circuit, the quenchable portion may have a resistance high enough to rapidly reduce the current in the HTS circuit when not superconducting ("normal resistance"), have a heat capacity high enough to absorb the energy stored by the magnet, certainly not melt, and preferably not rise to a temperature much above room temperature. The hot spot temperature at the time of an HTS quench will depend in part on the normal resistance of the quenchable portion (which depends in part on the resistivity of the material selected), while the maximum temperature of the superconductor in the quenchable portion will depend primarily on the heat capacity of the quenchable portion. These are conflicting requirements in HTS circuits where the length of the quenchable portion is limited, for example when the quenchable portion is arranged in the return leg of a toroidal field coil. The heat capacity can be increased by increasing the cross-section of the quenchable portion (for example, by increasing the cross-section of the non-superconducting stabilizer in the quenchable portion), but this will also reduce the normal resistance. Using materials other than copper as non-superconducting stabilizers can provide higher heat capacity without making the normal resistance too small. For example, if Figure 5 and Figure 6 As shown, the heat capacity of stainless steel (501) is similar to the heat capacity of copper (502), but the resistivity of stainless steel (601) is higher than the resistivity of copper (602). Generally, metals with a resistivity to volumetric heat capacity ratio greater than the resistivity to volumetric heat capacity ratio of copper will be suitable for this application.

[0053] In addition to the copper stabilizer, non-copper stabilizers may be included - for example, by providing a commercially available copper-stabilized superconductor in a stainless steel matrix. The ratio of copper to stainless steel should be adjusted so that the temperature of the superconductor is limited to a specified value, such as 300 K, when all the energy of the coil is transferred to the quenchable portion. The rate at which the transmission current decays when transferring to the magnet will depend on the inductance and temperature-dependent resistance of the quenchable portion.

[0054] In the case where the HTS circuit is a coil generating a magnetic field, the quenchable parts can be arranged to contribute to the magnetic field, or they can alternatively be arranged to be non-inductive (i.e. not contribute significantly to the magnetic field). The former arrangement is particularly useful for magnetic field coils such as toroidal magnetic field coils, which have parts (i.e. return branches) that do not contribute much to the magnetic field in the region of interest (the plasma radius close to the central column of a spherical tokamak in the case of a TF coil). This allows the quench system to be easily placed in the HTS cable of the return branch. The corresponding dilution of the current density in the return branch does not significantly impair the performance of the TF magnet.

Claims

1. A device comprising a high temperature superconductor HTS circuit; in, The HTS circuit comprises: a quenchable portion comprising an HTS material comprising a stack of HTS tapes, the stack of HTS tapes comprising at least one HTS tape and connected in series to other elements of the HTS circuit; The device also includes: a quench system configured to quench the HTS material in the quenchable portion; a quench protection system configured to detect a temperature rise in the HTS circuit and, in response to detecting the temperature rise, cause the quench system to quench superconducting material in the quenchable portion to transfer stored magnetic energy from the HTS circuit to the quenchable portion; wherein the HTS circuit is arranged, in use, such that the magnetic field on the or each HTS tape is substantially parallel to the ab plane of the HTS tape, and the quenching system is arranged to quench the HTS material by generating an additional magnetic field along the length of the or each HTS tape in the quenchable portion such that the additional magnetic field has a component perpendicular to the ab plane of the HTS tape.

2. The device according to claim 1, wherein Each quenchable portion has a heat capacity sufficient to cause the temperature of the quenchable portion to remain below a first predetermined temperature when energy is transferred from the HTS circuit to the quenchable portion, and a resistivity sufficient to cause the current of the HTS circuit to decay rapidly enough to cause the temperature of the portion of the HTS circuit where the temperature rise is detected to remain below a second predetermined temperature.

3. The device according to claim 1, wherein: The quench system is configured to quench the HTS material by generating an additional magnetic field perpendicular to the ab plane of the HTS tape.

4. The device according to claim 1, wherein: The quench system comprises a current source and a first conductor substantially parallel to the length of the respective HTS tape and substantially in the ab plane of the respective HTS tape, wherein the quench system provides the additional magnetic field by passing a current through the first conductor.

5. The device according to claim 4, wherein: The quench system comprises a second conductor on the other side of the respective HTS tape relative to the first conductor, the second conductor being substantially parallel to the length of the respective HTS tape and substantially in the ab plane of the respective HTS tape, wherein the quench system provides the additional magnetic field by causing current to flow through the first conductor in one direction and through the second conductor in an opposite direction.

6. The device according to claim 5, wherein: The quenchable portion comprises a plurality of HTS tape stacks arranged such that ab planes of the plurality of HTS tape stacks are parallel to a local magnetic field, and wherein the quench system comprises wires between each pair of adjacent HTS tape stacks, wherein the quench system is configured to provide the additional magnetic field by passing current through the wires such that one of the two wires adjacent to each HTS tape stack carries current in one direction and the other carries current in an opposite direction.

7. The device according to claim 6, wherein: The wires are connected in series.

8. The device according to claim 1, wherein: The first predetermined temperature is any one of the following: about 700K or about 300K.

9. The device according to claim 1, wherein: The second predetermined temperature is any one of: about 300K, about 200K, or about 100K.

10. The device according to claim 1, wherein: Each quenchable portion also includes a non-superconducting stabilizer.

11. The device according to claim 10, wherein: The non-superconducting stabilizer includes a metal having a ratio of resistivity to volumetric heat capacity greater than that of copper.

12. The device according to claim 11, wherein The non-superconducting stabilizer includes stainless steel.

13. The apparatus according to claim 1, wherein: The HTS circuit is a magnetic field generating coil, and the quenchable portion contributes to the magnetic field of the magnetic field generating coil.

14. The apparatus according to claim 1, wherein: The HTS circuit is a toroidal magnetic field coil of a tokamak, and the quenchable portion is a part of a return branch of the toroidal magnetic field coil.

15. The device according to claim 14, wherein: Each return branch of the toroidal field coil includes a quenchable portion.

16. The apparatus according to claim 1, wherein: The HTS circuit is the polar magnetic field coil of the tokamak.

17. The apparatus according to claim 1, wherein: The quench system comprises a resistive component arranged to provide the additional magnetic field when a current passes through the resistive component.

18. The apparatus according to claim 1, wherein: The quench system comprises a conductor arranged along the length of the respective HTS tape to provide the additional magnetic field when current is passed through the conductor.

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