Quench detection based on strain or on magnetic field

Abstract

A method for detecting a pre - quench condition in a superconducting magnet including an HTS magnetic field coil. The magnetic field coil includes: a plurality of turns, the plurality of turns including an HTS material and a metallic stabilizer; and a conductive material connecting the turns such that current can be shared radially between the turns via the conductive material. Monitor the strain of the HTS magnetic field coil and / or the support structure of the HTS magnetic field coil. Compare the monitored strain with the expected strain during normal operation of the magnet. In response to the comparison, determine whether the magnetic field coil is in a pre - quench condition. A similar method is provided, wherein the magnetic field of the HTS magnetic field coil is monitored to detect a pre - quench condition instead of the strain.

Description

Technical Field

[0001] The present invention relates to quench detection in a high temperature superconducting magnet system, and more particularly, to a method of quench detection and a magnet system configured to implement the method. Background Art

[0002] The challenges of producing fusion energy are very complex. In addition to tokamak devices, many alternative devices have been proposed, but none have been able to produce any results comparable to those of the best currently operating tokamak devices such as JET.

[0003] World fusion research has entered a new phase since the construction of ITER began. ITER is the largest and most expensive (about 15 billion euros) tokamak device ever built. The successful route to a commercial fusion reactor requires long pulses, stable operation, and high efficiency required to make power production economical. These three conditions are particularly difficult to achieve simultaneously, and the planned projects will require many years of experimental research and theoretical and technical research on ITER and other fusion facilities. It is generally expected that a commercial fusion reactor developed through this route will not be built until 2050.

[0004] To obtain a fusion reactor required for economic power generation (i.e., output power much greater than input power), conventional tokamak devices must be huge (as exemplified by ITER) so that the energy confinement time (which is roughly proportional to the plasma volume) can be large enough so that the plasma can be hot enough to undergo thermonuclear fusion.

[0005] WO 2013 / 030554 describes an alternative method, including using a compact spherical tokamak device as a neutron source or an energy source. The low aspect ratio plasma shape in the spherical tokamak device improves the 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. High temperature superconductor (HTS) magnetic field coils are a promising technology for such magnets.

[0006] Superconducting materials are generally classified into "high-temperature superconductors" (HTS) and "low-temperature superconductors" (LTS). LTS materials such as Nb and NbTi are metals or metal alloys whose superconductivity can be described by the BCS theory. The critical temperature of all low-temperature superconductors (above which the material cannot superconduct even in zero magnetic field) is below about 30 K. The BCS theory does not describe the behavior of HTS materials, but the critical temperature of such materials can be above about 30 K (although it should be noted that it is the physical differences in superconducting operation and composition rather than the critical temperature that define HTS and LTS 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, usually Y or Gd). Other HTS materials include iron pnictides (such as FeAs and FeSe) and magnesium diborate (MgB 2 ).

[0007] ReBCO is usually fabricated as a tape, the structure of which is as Figure 1 shown. Such a tape 100 is typically about 100 microns thick and includes a substrate 101 (usually electropolished Hastelloy about 50 microns thick), a series of buffer layers deposited on the substrate 101 by IBAD, magnetron sputtering, or other suitable techniques, the series of buffer layers being referred to as a buffer stack 102 and having a thickness of about 0.2 microns. An epitaxial ReBCO-HTS layer 103 (deposited by MOCVD or other suitable techniques) covers the buffer stack and is typically 1 micron thick. A silver layer 104 of 1 to 2 microns is deposited on the HTS layer by sputtering or other suitable techniques, and a copper stabilizer layer 105 is deposited on the tape by electroplating or other suitable techniques, the copper stabilizer layer 105 typically completely encapsulating the tape.

[0008] The substrate 101 provides a mechanical backbone that can be fed through a production line and allows the growth of subsequent layers. The buffer stack 102 is required to provide a biaxially textured crystal template on which the HTS layer grows and to prevent the chemical diffusion of elements from the substrate into the HTS, which would impair its superconducting properties. The silver layer 104 is required to provide a low-resistance interface from the ReBCO to the stabilizer layer, and the stabilizer layer 105 provides an alternative current path in the event that any part of the ReBCO stops superconducting (enters the "normal" state).

[0009] In addition, "stripped" HTS tapes can be fabricated that lack the substrate and buffer stack and have silver layers on both sides of the HTS layer. Tapes with a substrate will be referred to as "substrate" HTS tapes (tape).

[0010] HTS tapes can be arranged in HTS cables. The HTS cable includes one or more HTS tapes, which are connected along their lengths via a conductive material, typically copper. The HTS tapes can be stacked (i.e., arranged such that the HTS layers are parallel), or they can have some other tape arrangement that can vary along the length of the cable. Notable special cases of HTS cables are single HTS tapes and HTS pairs. An HTS pair includes: a pair of HTS tapes arranged such that the HTS layers are parallel. Where a substrate tape is used, the HTS pair can be of type 0 (HTS layers facing each other), type 1 (HTS layer of one tape facing the substrate of the other tape), or type 2 (substrates facing each other). Cables including more than two tapes can arrange some or all of the tapes as HTS pairs. Stacked HTS tapes can include various arrangements of HTS pairs, most commonly stacks of type 1 pairs or stacks of type 0 pairs (and / or, equivalently, type 2 pairs). HTS cables can include a mixture of substrate tapes and peel tapes.

[0011] A common type of HTS coil is the "flat coil", where the HTS cable 201 is wound in a manner similar to a tape reel to form a flat coil. The flat coil can be made to have an inner perimeter of any two-dimensional shape. Typically, the flat coil is provided as a "double flat coil", which includes two flat coils wound in opposite directions, with insulation between the flat coils and the internal terminals connected together. This means that voltage only needs to be supplied to the external terminals, which are usually more easily accessible, to drive current through the turns of the coil and generate a magnetic field.

[0012] HTS coils can be "insulated" - having an electrically insulating material between the turns of the coil, or "non-insulated", where the turns of the coil are electrically connected radially and along the cable (e.g., by welding or by direct contact to the copper stabilizing layer of the cable). Additionally, the coil can be "partially insulated" - i.e., having a layer between the coils that has an intermediate resistance between the insulator used to insulate the coils or the metal of the cable that joins the non-insulated coils. For example, the partial insulation can be a relatively thick layer of a metal or semiconductor with a relatively high resistance, or a composite layer configured to provide a relatively high resistance. Alternatively, a partially insulated coil can be formed by providing other radial current paths made of a conductive material - e.g., on one side of the magnetic field coil. Summary of the Invention

[0013] According to a first aspect, a method for detecting a pre - quench condition in a superconducting magnet including an HTS magnetic field coil is provided. The magnetic field coil includes: a plurality of turns including HTS material and a metallic stabilizer; and a conductive material connecting the turns such that current can be shared radially between the turns via the conductive material. Strain of the HTS magnetic field coil and / or the support structure of the HTS magnetic field coil is monitored. The monitored strain is compared with the expected strain during normal operation of the magnet. In response to the comparison, it is determined whether the magnetic field coil is in a pre - quench condition.

[0014] According to a second aspect, a method for detecting a pre - quench condition in a superconducting magnet including an HTS magnetic field coil is provided. The magnetic field coil includes: a plurality of turns including HTS material and a metallic stabilizer; and a conductive material connecting the turns such that current can be shared radially between the turns via the conductive material. The magnetic field of the HTS magnetic field coil is monitored. The monitored magnetic field is compared with the expected magnetic field during normal operation of the magnet. In response to the comparison, it is determined whether the magnetic field coil is in a pre - quench condition.

[0015] According to a third aspect, an HTS magnet system including a high - temperature superconducting (HTS) magnetic field coil is provided. The HTS magnetic field coil includes: a plurality of turns including HTS material and a metallic stabilizer; and a conductive material connecting the turns such that current can be shared between the turns via the conductive material. The HTS magnet system further includes a quench protection system and one or more strain sensors located on the HTS magnetic field coil or on the structural support of the HTS magnetic field coil; the quench protection system is configured to monitor the strain measurements of the one or more strain sensors; compare the strain measurements with the expected strain during normal operation of the magnet; and in response to the comparison, determine whether the magnetic field coil is in a pre - quench condition.

[0016] According to a fourth aspect, a high - temperature superconducting (HTS) magnet system including an HTS magnetic field coil is provided. The HTS magnetic field coil includes: a plurality of turns including HTS material and a metallic stabilizer; and a conductive material connecting the turns such that current can be shared between the turns via the conductive material. The HTS magnet system further includes a quench protection system and one or more magnetic field sensors; the quench protection system is configured to monitor the magnetic field measurements of the one or more magnetic field sensors; compare the magnetic field measurements with the expected magnetic field during normal operation of the magnet; and in response to the comparison, determine whether the magnetic field coil is in a pre - quench condition.

[0017] According to a fifth aspect, a tokamak device is provided, including the HTS magnet system according to the third and fourth aspects, wherein the toroidal magnetic field coil or the poloidal magnetic field coil of the tokamak device includes an HTS magnetic field coil. Description of the Drawings

[0018] Figure 1 is a schematic diagram of an HTS tape;

[0019] Figure 2 is a schematic diagram of a superconducting magnet system;

[0020] Figure 3 is a schematic diagram of a superconducting magnet system including several coils. Detailed implementation mode

[0021] Partially insulated and non-insulated coils, i.e., coils having (non-superconducting) conductive paths between turns that allow current to flow radially between turns, generally resist quenching (the magnet becoming non-superconducting during operation) and resist damage during quenching (since this part is caused by arc discharges between turns in the insulated coils). However, it has been found that due to large changes in the magnetic field and the resulting strain generated by currents transferred from a helical path (i.e., in the HTS of the coil) to a radial path (i.e., directly through metal connections or partial insulation), quenching in partially insulated and non-insulated coils can cause significant damage. This is particularly significant in magnet systems having multiple coil sections - for example, the toroidal field (TF) coil sets used in tokamak devices. If one "branch" of a TF coil quenches, then due to large unbalanced forces, the resulting magnetic imbalance can cause significant damage to the entire TF coil set.

[0022] Although the large damaging changes in strain and magnetic field are clearly problems, the following description presents using the smaller strain and magnetic field changes that occur during the onset of quenching due to current sharing between turns to detect an initial quench and provide sufficient warning to safely ramp down the magnet and reduce or prevent damage caused by quenching. Generally, quench detection involves detecting "pre-quench conditions", i.e., conditions that are prone to causing a quench, or signs indicating that a quench may occur soon, such as current sharing between coils or hot spots within a coil.

[0023] Quench detection can be performed by monitoring either or both of the strain in each coil (and / or nearby structural components) in the set or the magnetic field near each coil in the set. In a broad example, when there is any deviation from the expected measurement value (e.g., greater than the measurement accuracy of the strain gauges used) during magnet operation, a pre-quench condition can be signaled. Alternatively, when any such deviation is greater than a threshold (e.g., 1% greater than the expected measurement value), a pre-quench condition can be signaled. This would be suitable for systems where the potential cost of a large number of unnecessary downtimes is worth bearing to save the potential cost of an uncontrolled quench.

[0024] Alternatively, the quench protection system can be configured to respond only to certain measurements of strain and / or magnetic field sensors, e.g., in response to a magnetic field perpendicular to the coil magnetic field during normal operation ("off-axis" magnetic field), or in response to strain generated in an unexpected component or in an unexpected direction (where "unexpected" means "unexpected during normal operation" - i.e., expected in a quench or pre-quench condition).

[0025] In a multi-coil system, detection of a pre-quench condition in one coil can be based on changes in strain in and around another coil of the system - because a change in the magnetic field of the first coil will cause a change in the balance of forces on the other coils in the system. This applies whether the multiple coils are part of the same magnet (e.g., individual branches of a TF coil set).

[0026] The determination of "strain / magnetic field during normal operation" can be based on the power currently being supplied to the coils - e.g., the quench protection system can: receive details of the current supplied to each coil as an input, determine a strain and / or magnetic field model based on these currents (e.g., by referring to a look-up table or by calculation in a simple model), and compare the readings of the strain and / or magnetic field sensors with the strain and / or magnetic field model. As described above, for any significant deviation from the model, or for certain types of deviation, e.g., perpendicular to the expected magnetic field / strain, a pre-quench condition (and quench prevention procedures, e.g., reducing magnet excitation) can be signaled.

[0027] In a balanced multi-coil system, i.e., a system in which the strain / magnetic field pattern of each coil should be the same during normal operation, the expected strain / magnetic field during normal operation for comparison can be based on the measured strain / magnetic field of the other coils - i.e., the expected strain pattern is that the strain on each coil is the same within the instrument accuracy. A particular pattern of strain deviation can indicate a pre-quench condition - e.g., where there are equal and opposite deviations on either side of the coils of two coils, and reduced equal and opposite deviations on the next nearest adjacent coils.

[0028] Similar considerations apply to systems that are not completely balanced but have symmetry - e.g., when a multi-coil system has two sets of coils with reflection symmetry with respect to each other, the expected strain / magnetic field can be based on the measured strain / magnetic field of each coil, and it can be expected that the strain / magnetic field pattern should also have reflection symmetry.

[0029] In a typical TF coil of a small spherical tokamak device (plasma major radius of about 1.5 m), the expected strain can reach 0.25% (2500 microstrain), and the sensitivity of the strain sensor can be better than 0.01 microstrain. Thus, the strain on the magnet can be determined very precisely with high resolution.

[0030] Figure 2 An exemplary superconducting magnet system is shown in schematic form. The magnet system includes:

[0031] An HTS magnetic field coil 201 having a support structure 202;

[0032] A plurality of strain sensors 203 located on the HTS magnetic field coil 201 and the support structure 202;

[0033] A plurality of magnetic field sensors 204 positioned to monitor the magnetic field generated by the HTS magnetic field coil 201;

[0034] A quench protection system 205 configured to:

[0035] Monitor the measurements of the strain sensors and the magnetic field sensors;

[0036] Compare the monitored measurements with an expected strain distribution during normal operation and an expected magnetic field distribution during normal operation;

[0037] Determine whether the magnetic field coil is in a pre - quench condition based on the comparison.

[0038] Figure 3 A multi - coil magnet system is shown, which includes a plurality of coils 201 (having associated support structures 202 and sensors 203, 204) as shown. The quench protection system 305 is configured to: Figure 2 Monitor the measurements of the strain sensors and the magnetic field sensors;

[0039] Compare the monitored measurements with an expected strain distribution during normal operation and an expected magnetic field distribution during normal operation;

[0040] Use both sensors on each magnetic field coil and sensors on other magnetic field coils to determine whether each magnetic field coil is in a pre - quench condition based on the comparison.

[0041] As explained in the more detailed example above, the magnet system can also be constructed to have only strain sensors or only magnetic field sensors, and a quench protection system configured to consider only strain or magnetic field, as appropriate.

[0042] The quench protection system can also be configured to activate some form of quench prevention or mitigation after determining that the magnetic field coil is in a pre - quench condition - for example, triggering the dumping of the magnet current into the cold mass, such as by switching to a resistive load or intentionally quenching most of the magnet.

[0043] The quench protection system can also be configured to activate some form of quench prevention or mitigation after determining that the magnetic field coil is in a pre - quench condition - for example, triggering the dumping of the magnet current into the cold mass, such as by switching to a resistive load or intentionally quenching most of the magnet.

Claims

1. A method for detecting a condition prone to quench in a superconducting magnet including a plurality of HTS magnetic field coils, each magnetic field coil comprising: a plurality of turns including HTS material and a metallic stabilizer; and a conductive material connecting the turns to enable radial sharing of current between the turns via the conductive material; the method comprising: monitoring the strain and / or magnetic field of each HTS magnetic field coil; comparing the monitored strain and / or magnetic field of each HTS magnetic field coil with the monitored strain and / or magnetic field of at least one other HTS magnetic field coil among the plurality of HTS magnetic field coils; responsive to the comparison, determining whether one or more of the HTS magnetic field coils are prone to quench.

2. The method according to claim 1, wherein determining whether one or more of the HTS magnetic field coils are prone to quench includes one or more of the following: determining that one of the HTS magnetic field coils is prone to quench if the monitored strain or magnetic field is different from the strain or magnetic field of at least one other HTS magnetic field coil by more than a threshold; determining that one of the HTS magnetic field coils is prone to quench if the monitored strain or magnetic field has a component perpendicular to and of greater magnitude than the strain or magnetic field of at least one other HTS magnetic field coil.

3. The method according to claim 2, wherein the threshold or each threshold is a predetermined proportion of the strain or magnetic field of the at least one other HTS magnetic field coil.

4. An HTS magnet system including a plurality of high temperature superconducting HTS magnetic field coils, each HTS magnetic field coil comprising: a plurality of turns including HTS material and a metallic stabilizer; a conductive material connecting the turns to enable sharing of current between the turns; the HTS magnet system further including a quench protection system and a plurality of sensors, the plurality of sensors including: one or more strain sensors located on each HTS magnetic field coil or on the structural support of each HTS magnetic field coil, and / or one or more magnetic field sensors configured to monitor the magnetic field of each HTS magnetic field coil; wherein the quench protection system is configured to: use the plurality of sensors to monitor the strain and / or magnetic field measurements of each HTS coil; compare the strain and / or magnetic field measurements of each HTS coil with the strain and / or magnetic field measurements of at least one other HTS magnetic field coil among the plurality of HTS magnetic field coils; responsive to the comparison, determine that one or more of the magnetic field coils are prone to quench.

5. The HTS magnet system according to claim 4, wherein the conductive material is contained within a partial insulating layer, the partial insulating layer including any one of the following: a discontinuous insulating layer; a semiconductor; a metal strip having discontinuous insulating layers on each side; and a metal-insulator transition material.

6. A tokamak device including the HTS magnet system according to claim 4 or 5, wherein the plurality of HTS magnetic field coils are toroidal field coils of the tokamak device.

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