RU2799587C2 - Quick reset of a partially insulated superconducting magnet - Google Patents

Abstract

FIELD: high-temperature superconductors.

SUBSTANCE: invention relates to a magnetic system using high-temperature superconductors (HTS). The system contains an HTS excitation coil and a power source. The HTS excitation coil comprises a plurality of turns containing the HTS material and a metal stabilizer, as well as an electrically conductive layer separating the turns so that current can be distributed between the turns through the conductive layer. The power source is configured to supply the first current to the HTS excitation coil during loading of the HTS excitation coil and to supply the second current to the HTS excitation coil, opposite in direction to the first current, during unloading of the HTS excitation coil.

EFFECT: optimization of control of a magnetic system with a high-temperature superconductor with the possibility of controlled unloading of HTS magnets.

21 cl, 14 dwg

Description

Technical field

The present invention relates to high temperature superconductor (HTSC) magnets, HTS magnets. In particular, the invention relates to a method for unloading HTS magnets, for example, in response to the detection of a violation of superconductivity, and a device that implements this method.

Background of the invention

Superconducting materials are typically divided into "high temperature superconductors" (HTSC) and "low temperature superconductors" (LTSC). LTSC materials such as Nb and NbTi are metals or metal alloys whose superconductivity can be described by the BCS (Bardin-Cooper-Schrieffer) theory. All low-temperature superconductors have a critical temperature (the temperature above which a material cannot be superconductive even in zero magnetic field) below about 30 K. The behavior of an HTS material is not described by BCS theory, and such materials may have critical temperatures above about 30 K (although it should be It should be noted that it is the physical differences in the work and composition of superconductors, and not the critical temperature, that determine the HTSC and LTSC materials). The most commonly used HTSCs are "cuprate superconductors" - ceramics based on cuprates (compounds containing a copper oxide group) such as BSCCO or ReBCO (where Re is a rare earth element, usually Y or Gd). Other HTS materials include iron pnictides (eg FeAs and FeSe) and magnesium diborate (MgB ₂ ).

ReBCOs are typically produced as tapes with the structure shown in FIG. 1. Such a tape 100 is typically about 100 microns thick and includes a substrate 101 (typically an electropolished Hastelloy about 50 microns thick) on which is deposited by ion beam deposition (IBAD), magnetron sputtering, or other suitable technique, a sequence of buffer layers, known as buffer pack 102, with an approximate thickness of 0.2 microns. An HTS-ReBCO epitaxial layer 103 (deposited by metal organic chemical vapor deposition (MOCVD) or other suitable method) covers this buffer package and is typically 1 micron thick. A 1-2 micron layer 104 of silver is deposited on the HTS layer by sputtering or other suitable method, and a stabilizing layer 105 of copper (copper stabilizer) is applied to the tape by electroplating or other suitable method, which often completely encapsulates the tape.

The substrate 101 provides a mechanical support that can be fed down the production line and allows subsequent layers to be grown. The buffer package 102 is required to provide a biaxially textured crystal template on which the HTS layer will grow, and to prevent chemical diffusion of elements from the substrate into the HTS, which destroys its superconducting properties. The silver layer 104 is required to provide a low resistance transition from the ReBCO to the stabilization layer, and the stabilization layer 105 provides an alternate current path in the event that any part of the ReBCO ceases to be superconductive (goes to the "normal" state).

In addition, "delaminated" HTS tape can be made which does not have a backing and buffer package, but instead has silver layers on both sides of the HTS layer. A tape that has a backing will be referred to as "backed" HTS tape.

HTS tapes may be bundled into HTS cables. An HTS cable comprises one or more HTS tapes that are connected along their length through an (electro)conductive material (usually copper). The HTS tapes may be stacked (ie, stacked so that the HTS layers are parallel), or they may have some other tape arrangement that may vary along the length of the cable. Notable special cases of HTS cables are single HTS tapes and HTS pairs. HTS pairs comprise a pair of HTS tapes arranged so that the HTS layers are parallel. When tape with a substrate is used, HTS pairs can be type 0 (with the HTS layers facing each other), type 1 (with the HTS layer of one tape facing the substrate of the other), or type 2 (with the HTS layers facing each other) . Cables containing more than 2 tapes may place some or all of the tapes in HTS pairs. Stacked HTS tapes may comprise various arrangements of HTS pairs, most commonly either a stack of Type 1 pairs or a stack of Type 0 and (or equivalently, Type 2) pairs. HTS cables may contain a mixture of backed tapes and peeled tapes.

Generally speaking, there are two types of construction of magnetic coils - by winding or by assembling several sections. Wound coils, as shown in Fig. 2 is made by winding the HTS cable 201 around the frame 202 in a continuous helix. The frame is shaped to provide the required inner perimeter of the coil and may be a structural part of the final wound coil or may be removed after winding. Sectional coils, as shown schematically in FIG. 3 are composed of several sections 301, each of which may contain several cables or preformed busbars 311 and will form an arc of a common coil. The sections are joined by connections 302 to form a complete coil. While the turns of the coils in FIG. 2 and 3 are shown spaced apart for clarity, there is usually material connecting the turns of the coil, for example, they can be combined by pouring epoxy.

Fig. 4 shows a section through a particular type of wound coil, known as a "disk coil" (or "biscuit" coil), where the HTS cables 401 are wound to form a flat coil in a manner similar to a tape spool. Disc coils can be made with an inner perimeter that has any two-dimensional shape. Often the disc coils are provided as a "double disc coil", as shown in the sectional view of FIG. 5, which contains two disc coils 501, 502 wound in opposite directions, with insulation 503 between the disc coils and with inner terminals 504 connected together. conclusions 521, 522, which are usually more accessible.

Coils can be "insulated" - having an electrically insulating material between the turns of the coil, or "non-insulated", when the turns of the coil are electrically connected radially, as well as along the cables (for example, by connecting the copper stabilizing layers of the cables by soldering or by direct contact). "Partially insulated" coils are also possible, where the turn-to-turn resistance is less than that of a traditional insulator such as a ceramic or organic insulator, but greater than the resistance between the ribbons in the coil cable, for example ^100-1015 times the resistance between the ribbons in the cable. Lack of insulation, or partial insulation between turns, slows down the rate at which the temperature of a local "hot spot" (normal zone) rises, as this allows current to be shared between the turns.

A bare HTS coil can be modeled as having three current paths - two helical paths that follow the HTS tapes (one in the HTS and one in a metal stabilizer) and a radial path through the metal (and any other resistive material) connecting bare or partially insulated turns of HTS cable between coil terminals (although this may be modeled as a single path, it actually represents the sum of all radial resistive paths through the magnet). Only the current flowing through the helical paths creates a significant axial magnetic field at the center of the coil. The spiral path of an HTS can be modeled as an induction coil having a large inductance and zero or negligible resistance when the entire tape is superconducting. The helical path of the copper stabilizer is parallel to the HTS path and has the same inductance but significant resistance. For this reason, a negligible current flows through it, unless superconductivity begins to break in parts of the HTSC spiral path. If this happens, the excessive current above the critical current Ic of the HTS spiral path is distributed between the stabilizer spiral path and the radial path according to their relative resistances and time constants L/R. The radial current path can be modeled as having negligible inductance and much more resistance than the helical path, while the HTS is superconducting throughout.

One potential application of HTS excitation coils exists in tokamak plasma chambers, in particular for fusion reactors. WO 2013/030554 describes an approach involving the use of a compact spherical tokamak for use as a neutron source or energy source. The shape of the plasma with a small aspect ratio in a spherical tokamak improves the particle confinement time and provides usable effective power generation in a much smaller setup. However, a small diameter central column is required, which presents problems for the design of the plasma confinement magnet. High temperature superconductor (HTSC) excitation coils are a promising technology for such magnets.

Another potential application of HTS excitation coils is in proton beam therapy devices. Proton Beam Therapy (PBT), also known as proton therapy) is a form of charged particle radiation therapy used in the treatment of cancer (and other conditions that respond to radiation therapy). In PBT, a beam of protons is sent to the site of treatment (for example, a tumor).

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

Proton beams for PBT and PBCT are created by particle accelerators such as cyclotrons or linear accelerators. Accelerators typically used for PBT and PBCT typically produce protons with energies in the 60-250 MeV range, with the most powerful facility currently in operation having a maximum energy of 400 MeV.

There are, generally speaking, two types of designs for PMT devices that provide the ability to change the beam angle. In the first type of construction, which is illustrated in FIG. 6, the accelerator 3001 is mounted on a gantry 3002 which allows it to rotate around the patient 3003 (typically around a horizontal axis). The patient is placed on a movable bed 3004, which provides additional degrees of freedom (for example, translational movement and rotation around a vertical axis).

The second type of construction is illustrated in Fig. 7. The accelerator 4001 is stationary and the beam is directed towards the patient by means of steering magnets 4002 (typically including both four-pole and two-pole magnets), at least some of which are located on the gantry 4003 so that the beam can rotate around the patient 4004 (for example, around horizontal axis). The patient is placed on the movable bed 4005.

Either design requires the gantry to hold electromagnets capable of driving protons with beam energy that can be up to 400 MeV. This requires very strong magnetic fields and, in fact, the use of HTS field coils can greatly reduce the mass and size of the electromagnets and gantry needed to move them. HTS excitation coils can be used in the accelerator, four-pole magnets for control magnets or two-pole magnets for control magnets.

The essence of the invention

The invention is characterized in the appended claims.

Brief description of the drawings

Fig. 1 is a schematic representation of an HTS tape;

Fig. 2 is a schematic representation of a wound HTS coil;

Fig. 3 is a schematic representation of a sectioned HTS coil;

Fig. 4 is a section of a disk coil;

Fig. 5 is a section of a double disk coil;

Fig. 6 is a schematic representation of a first PMT device;

Fig. 7 is a schematic representation of a second PMT device;

Fig. 8 shows the simulation results of a partially insulated coil;

Fig. 9 shows simulation results of an exemplary magnetic system;

Fig. 10 shows graphs of the magnetic field and temperature for the coil after simply turning off the power supply;

Fig. 11 shows the voltages for the coil during the same process as shown in FIG. 10;

Fig. 12A and B show an example of leaky insulation;

Fig. 13A and B show a further example of leaky insulation;

Fig. 14 shows a circuit equivalent to a two-turn coil.

Detailed description of the invention

Fig. 8 shows the current, voltage and power in an uninsulated coil during loading and steady state operation. During loading of a bare coil, the current will initially flow in a predominantly radial path (period A in Fig. 8) and then stabilize. The amount of current flowing in the radial path is higher for faster loading rates (since the voltage developed in the helical path, L.dI/dt, is higher, this is the period B). At the end of loading, dI/dt drops to zero and the current will transfer to the helical path of the HTS with time constant L/R (period C). The current will, for the most part, be transferred to a helical path with several (approximately five) L/R time constants after the end of loading. As such, the time constant must be chosen to result in a reasonable loading time, for example a time constant of 5-10 hours would be acceptable for a tokamak TF coil (giving a loading time of about 1-2 days).

In a large magnet, in order to avoid damage from superconductivity disruption (either in an insulated or non-insulated coil), an active superconductivity protection circuit can also be implemented. In this arrangement, the accumulated energy of the magnet can be dumped into some component other than the magnet's superconductivity failure region before sufficient temperature rise can occur in the superconductivity failure region to cause damage. Such other component may be an external resistance or a separate section of the magnet in which superconductivity is broken over most of the magnet's cold mass (thereby distributing the stored energy of the magnet over a large volume and reducing the overall temperature rise). However, the proactive approach requires that the time between the occurrence of a normal zone (also called a "hot spot") and the start of unloading ("reset") the current of the magnet be short enough that the final temperature of the hot spot is less than the temperature at which damage to the coil can occur, for example , about 200 K. This approach can also be used in small magnets, providing additional protection against superconductivity violations.

The above, as well as a discussion of suitable designs to achieve the desired time constant, are discussed in more detail in co-pending GB1818817.7.

Although the use of PI coils (partially insulated coils) extends the time available to reset the magnet current, it is still important that this operation be done as quickly as possible after a hot spot is detected. There has been relatively little discussion of current shedding methods for PI coils, as in the literature PI coils are generally only used for smaller coils where the total coil energy is relatively low and superconductivity disturbance will tend to propagate throughout the coil relatively quickly. , meaning that the energy dump is dissipated through the coil. In addition, PI coils are inherently robust compared to insulated coils, and therefore can often be operated without protection from superconductivity disturbance because the risk is low. However, in large coils with significant stored energy, coils with a geometry that allows the hot spot to cover all turns in only a relatively small portion of the coil winding, and/or coils designed for long-term operation in harsh environments (for example, field excitation coils for a fusion reactor ), active protection against violation of superconductivity is important.

The use of PI coils provides additional benefits in releasing energy from the magnet. Fig. 10 shows graphs of the magnetic field (proportional to the spiral path current) and temperature for the coil after simply disconnecting the power supply (i.e. making it an open circuit). During period A, the power supply is on (shown by current 1001). At the beginning of period B, the power supply is turned off, forcing the inductance of the coil to generate a voltage that continues to drive current in each turn in the closed loop, shorting back to the beginning of the turn through the turn-to-turn resistance. This causes ohmic heating, which reduces the critical current in the HTS. This process continues for a period B (typically a few seconds, depending on many factors, 14 seconds in this experiment) until superconductivity is broken in the HTS turn and enough voltage is created to release the current of its circuit into the metal spiral path stabilizer (period C). Because it has a much higher resistance than the turn-to-turn resistance, the coil's magnetic field energy is rapidly converted to heat in the stabilizer, and the coil's temperature rises evenly as the spiral path current rapidly drops to 0 (<1c).

However, for effective protection against superconductivity breakdown, a longer period B is still undesirable - in the case of superconductivity breakdown, this period can be long enough to cause significant local heating to occur at a local hot spot elsewhere in the magnet, resulting in unacceptably high peak temperature in this hotspot. In addition, a magnet, such as a TF magnet in a tokamak, may contain multiple coils (eg, separate legs) that are weakly thermally and magnetically coupled. It is desirable that when triggering a reset by turning off the power supply, all coils experience the same delay before breaking superconductivity. Variations between coils can be due to manufacturing differences in local temperature or magnetic field differences. If the coils do not break superconductivity simultaneously, the resultant very large electromagnetic forces between the coils can cause damage to the mechanical support structure of the magnet and/or the coils themselves. To minimize this, it is desirable to minimize the variation between the delay between PSU (power supply unit) shutdown and the breaking of superconductivity in each coil, so that any variation between delay times is also minimized. The duration of this period can be reduced by increasing the resistance of the partially insulating layer (and thereby increasing the heating due to the passage of current along the radial path in period B), but this will have domino effects on other electrical properties of the coil, such as changing the time constant for loading the coil, or making it more difficult to distribute the current between the turns (which would increase the likelihood of a hot spot causing global disruption of superconductivity).

Instead of turning off the power supply, it is proposed to apply a large reverse current to the magnet coil (i.e. in the opposite direction to the current flowing through the coil before unloading), for example, using a four-quadrant PSU that is able to drain current from the magnet coil . The superconducting path has a large inductance, and therefore such a reverse current will flow mainly in a radial path in all coils of a TF magnet. This large increase in radial current causes significant heating of all coils, quickly "extinguishing" the entire magnet (and thus dissipating the waste energy over a large area).

While applying reverse current instead of simply turning off the PSU may seem counter-intuitive, an important difference is that excess radial current will heat the entire volume of the magnet, which means that the disruption of superconductivity will rapidly propagate through the magnet, and the waste energy will be dissipated throughout the volume (or at least to a significant extent). A concentrated release of energy, which will occur without any intervention, will cause unacceptable temperature rises in this small area, damaging the HTS. If superconductivity is broken in a significant area of the magnet, the same amount of energy (plus a small contribution from the reverse current itself) is dissipated throughout the magnet, which limits the temperature rise in the HTS. In addition, the more uniform heating of the magnet prevents the formation of steep temperature gradients in the magnet. If the temperature gradient is too high, then the differential thermal expansion of adjacent regions of the magnet will cause structural damage.

In existing magnets, uniform heating can be achieved using "heaters to break superconductivity", i.e. heating elements placed next to the HTS cable that can be turned on to supply heat to the coil. However, such heaters take up space and essentially reduce the available space for the HTS conductor or for the metal stabilizer. The "reverse current" method actually uses the radial current path as a "superconductive disruption heater", which means that the heat is evenly distributed throughout the coil and no additional heating elements are required.

The reverse current can be limited to the operating current of the magnet or set at the level of the operating current of the magnet. Thus, maximum heating is achieved with reverse current without exceeding the design parameters of the components external to the magnet compared to what would be required for normal operation of the magnet.

The above has been described with reference to protection against superconductivity breaking, but it should be noted that the above energy reset method can also be applied to other situations where the magnet is unloaded, such as turning off the magnet under normal conditions, in the absence of any detected superconductivity breaking ( or conditions likely to lead to disruption of superconductivity).

Fig. 9 shows simulation results of an exemplary partially insulated coil during loading, steady state operation, and unloading. In this case, the PSU (power supply) is modeled as a current source, i.e. the current supplied by the PSU is given in the simulation and the voltage across the PSU is calculated.

During loading, the PSU current increases steadily to 2.2 kA at a constant rate. The PSU voltage is positive, on the order of 0.1 V. The current in the radial path is approximately proportional to the PSU voltage (because the radial path can be modeled as a simple resistance), and the current in the spiral path increases at a constant rate. At time T1, when the desired current is reached, the magnet switches to steady state, the PSU is set to a constant current, and the radial path current decreases with the time constant L/R _radial as described previously. The PSU voltage decreases to a value on the order of a few millivolts as the radial path current decays because the helical path has negligible resistance (in this simulation it is modeled as having zero resistance, so the PSU voltage tends to zero. In practice, it will typically be stabilize around a few tens to hundreds of millivolts). During loading and steady state operation, the temperature of the HTSC is substantially constant below 20 K.

At time T2, a magnet reset is initiated (either in response to the detection of a superconductivity violation or otherwise). The PSU supplies a reverse current (with a rapid current change, modeled in this case as a downward current change ten times faster than the initial load), which flows primarily along a radial path. The PSU voltage is negative during the application of this current, on the order of -0.5 V. The temperature of the HTS rises rapidly. The simulation ends when the HTS temperature has reached approximately 55 K, as the entire coil will break superconductivity and the temperature rise becomes too fast for the model being used. However, in reality, the stored energy of the magnet will quickly be converted into heat, dissipating relatively evenly throughout the magnet, safely shutting it down.

The reverse current may be applied for a predetermined time or until a particular condition is reached, such as when a violation of superconductivity is detected in a significant area of the magnet, a specific temperature is detected in a significant area of the magnet, or when a spiral path current (or generated by a coil) magnetic field) has decreased below the threshold value.

The unloading rate depends on the rate of rise of the reverse current in the PSU.

It is desirable to be able to control the rate of change of the current both in the loading phase of the magnet and in the reset phase. For this reason, a PSU with a feedback controlled current output is preferred. The PSU current can be adjusted based on the current in the spiral path, the temperature of the magnet, the magnetic field generated by the coil, or any other suitable property of the coil.

The power supply may contain several power supplies, each of which supplies power to the coil for a different period. In particular, the power source may include a first unit for supplying power to the coil during loading and steady state operation and a second unit for supplying reverse voltage to the coil during unloading.

The power supply (or one or more power supply units) may be partially located in a cryostat containing an HTS magnet and may include a transformer configured to carry power through the cryostat without the presence of cables passing through the cryostat, as described in PCT/ GB2018/050337.

When magnet unloading is triggered in response to the detection of a superconductivity violation or conditions likely to lead to a superconductivity violation, this detection may be by any practical method. For example:

- detection of excess voltage on the HTSC material in the magnet;

- the use of auxiliary HTS tapes, which are provided adjacent to the main coil and are capable of destroying superconductivity before the main coil, for example, as described in International Patent Application PCT/GB2016/052712 or British Patent Application GB1812120.2;

- detection of temperature, strain, magnetic fields or other conditions in the magnet coil, for example, by means of Role dissipation in fiber optic cables, as described in International Patent Application PCT/GB2017/053066, or by other temperature, strain or magnetic field detectors that known in the art.

It is important not to apply reverse current for too long, otherwise an active reset can heat the magnet above 200K and cause problems. Ideally, the reset system should limit the total energy supplied to the magnet to the energy required to raise the temperature of the entire magnet above its critical temperature (ie, turn all coils from superconducting to normal). This is a small fraction of the total energy required to raise the temperature of the entire magnet to ~200K. As superconductivity begins to break in the coils, the magnet's own stored energy will be dissipated, causing a global breakdown in superconductivity.

A simple way to supply the correct amount of energy is to discharge the capacitor bank into the magnet. It also eliminates the need for a four-quadrant PSU. A single quadrant PSU can be used to load the magnet. When a magnet needs to be reset, it is simply disconnected using active switches and a pre-charged capacitor bank attached to the magnet to induce a reverse radial current. Note that when the PSU is disconnected, no large voltages are created, since the high inductance of the magnet is shunted by its radial resistance.

Much of the invention described above has focused on unloading the magnet after a superconductivity violation has been detected, when the speed of unloading is of critical importance. There are also methods using the same fundamental principle that are applicable to magnet control in conditions where unloading time is not a primary factor.

As an example, the PSU may be configured to supply a discharge current that is less than the current in the magnet but flows in the same direction. This will cause a current equal to the difference between the currents of the magnet and the PSU to flow along the radial path, heating the magnet as before. This will result in slower discharge compared to reverse current (or simply disconnecting the PSU) and reduced temperature rise in the magnet.

As a further example, the PSU may be configured to supply AC superimposed on the DC delivered to the coil (either during steady state, loading, or unloading). When the period of an alternating current is significantly less than the time constant L/R, this alternating current will flow entirely in a radial path. This results in heating of the magnet without otherwise affecting the current along the helical path (compared to the case where only direct current is supplied).

The superimposed alternating current can also be used in combination with any of the previous examples. For example, a combination of reverse DC and AC (i.e. when the total current is a sinusoidal current with an average value that is opposite in sign to the coil current and the period is less than the magnet time constant) can be used to unload the coil with additional heating. Alternatively, an alternating current can be combined with a DC unloading current that is less than the current in the magnet but flows in the same direction (i.e. when the total current is a sinusoidal current with an average value that is less than the coil current and the period is less than the time constant of the magnet ). As a further alternative, pure alternating current can be applied to unload the magnet - this will force a current equal to the magnet current in addition to the alternating current to flow along the radial path. In each case, the addition of alternating current results in more current in the helical path (and hence faster unloading) compared to using direct current alone.

A surprising feature of unloading a partially insulated coil, as discussed in the examples above (either by turning off the PSU or by applying a modified current) is that the turn-to-turn voltage of the coil remains low (on the order of a few volts, even for large coils) throughout the entire process. Large insulated superconducting coils require reinforced insulation that can withstand very high voltages, but the small voltages experienced by a partially insulated coil can be effectively insulated by a much wider variety of materials (or even a simple vacuum or air gap).

Fig. 11 shows the voltage across each coil in the same magnet as used in FIG. 10 (which contains 6 disk coils with a total inductance of 0.12 H). The peak voltages that occur at the time corresponding to the onset of global superconductivity breakdown (i.e., at the end of period B and the beginning of period C in FIG. 10) are approximately 0.1 V. In contrast, the expected voltage for an equivalent insulated coil under the same conditions will be on the order of 2 kV, and the expected voltage for a large coil will be at least 5 kV (the voltage can be calculated as V=-L dI/dt, where dI/dt is approximately the transfer current (1.4 kA for the coil under test) divided by the time taken to break the superconductivity (0.1 kA for the coil under test)). With a partially insulated coil, the coil can handle high transfer current and inductance without developing a particularly high voltage, which is most practical when operating a large coil with low transfer currents (a few kiloamperes compared to typically 50 kA) and high inductance (i.e. more high number of turns).

The peak voltage developed between two turns of a partially insulated coil during PSU open circuit current reset can be approximated as V=I ₀ R _stab , where I ₀ is the transfer current and R _stab is the resistance of the resistive material in the coil's helical path. Superconductivity breakdown propagates between the coils by mutual inductance, which means that the peak voltage of the coil does not exceed the peak voltage for a single turn. The peak voltage will not exceed 10V for any realistically large coil.

Fig. 14 shows the equivalent circuit for a coil with 2 turns (and can be expanded to more turns by adding more coils in series and mutual inductors between each coil). In this figure, I ₀ is the transfer current, R _tt is the turn-to-turn resistance (i.e., the radial path resistance divided by the number of turns), R _stab is the resistive helical path resistance, L _turn is the inductance of each turn, and R _HTS is the HTS resistance. in each turn (i.e. 0 during normal operation and only non-zero during superconductivity breakdown or near superconductivity breakdown). M is the mutual inductance between the two turns and K is the coupling factor between the turns.

This is especially important for coil insulation, both for isolating the coil as a whole from other components (the "ground wrap"), and for partial insulation designs that use an insulating material that has a number of conductive channels through it (known as "leakable insulation" and discussed in more detail below). The low voltage means that the insulation does not need to be a reinforced material such as Kapton™ (which is used in traditional insulated magnets), but simpler materials such as paint, varnish or even paper can be used, or the insulating material can be replaced with air or vacuum gap (with suitable support structures, also insulators if required).

Insulating structures can be characterized by a "breakdown voltage" above which the structure no longer insulates and the resistance of the structure drops from on the order of a few megaohms to on the order of a few ohms or milliohms. For an insulated coil, this breakdown voltage must be at least 2 kV (at least 5 kV for large coils, e.g. with a radius greater than 50 cm), which severely limits the materials that can be used while still keeping the insulation reasonably compact (voltage breakdown is approximately proportional to the thickness of the material, with a constant of proportionality ("dielectric constant") depending on the material).

A partially insulated coil only requires a breakdown voltage greater than 10V or so, which will allow any material that can reasonably be called an "insulator" to be used.

Alternatively, in environments that are unfavorable to materials (e.g., in a fusion reactor where materials will be bombarded with neutrons), this will allow the insulation to be used longer before it breaks down to the point where it is no longer insulating, since neutron bombardment will have the tendency to cause changes in the insulation that will reduce its dielectric constant or introduce physical gaps.

An example of leaky insulation is shown in FIG. 12A and B. The metal strip 901 is provided with a thin insulating coating 902 on at least the sides facing the HTS cables, where the insulating coating is removed or absent over windows (or "through holes") 903 at intervals on each side of the metal strip. The windows may be of any shape and may extend to the edges of the tape. The windows on both sides of the metal strip are staggered, as shown in FIG. 12B, which increases the resistance (compared to an uninsulated strip or a strip where the windows on each side were directly opposite each other) because the current must make a path 910 along a portion of the length of the metal strip.

By varying the distance between the windows so that they are closer together in the return legs ("bypasses") and farther apart in the core, the desired difference in turn-to-turn resistance between the return legs and the core can be achieved. Further customization can be done by using a different metal for the metal strip in the core compared to the return legs, or by changing other aspects of the strip geometry.

To enable even finer tuning, instead of a solid metal strip, a layer having several metal tracks can be used, effectively forming an insulating layer with conductive radial tracks placed therein, where the distance between the tracks and their length determine the resistance of such a partially insulating layer.

Fig. 13A-E show a further example of a leaky insulation layer. The leaky insulation layer contains 5 layers, which are, in order:

- the first metal connecting layer 1611;

- the first insulating layer 1621;

- electrically conductive layer 1630;

- the second insulating layer 1622;

- the second metal connecting layer 1612.

Fig. 13A-C show the layout of the first metal bonding layer 1611, the electrically conductive layer 1630, and the second metal bonding layer 1622, respectively. Fig. 13D and E are sections taken along lines D and E in FIG. 13A-C.

A bonding layer is present to allow connection to HTS cables by low temperature soldering.

Unlike the previous example, where the electrically conductive layer is a continuous metal strip, in this example, the electrically conductive layer is divided into several conductive regions. These areas are of two types. The square regions 1631 (although in practice they can be of any shape) are connected by interlayer connections 1606 to only one of the metal bonding layers. These areas do not affect the electrical properties of the partially insulating layer, but provide a heat path (thermal channel) through the corresponding insulating layer. By varying the size of these areas and the number of connections between them and the metal bonding layer, regardless of the electrical properties, the thermal properties of the partially insulating layer can be changed.

Other regions 1632 each connect window 1601 of first insulation layer 1621 to window 1602 of second insulation layer 1622. The resistance between windows can be adjusted by changing the geometry of regions 1632, such as when region 1632 contains a track 1633 that is elongated as shown in FIG. 13B, increasing the track width will decrease the inter-window resistance, and increasing the track length (eg, by providing a non-linear track or by moving the windows) will increase the inter-window resistance.

Windows 1601 in the first insulating layer are formed through holes drilled through the first bonding layer and the first insulating layer, which are then plated with metal 1603 (or other electrically conductive material) to form interlayer connections between the first bonding layer and the electrically conductive layer. Windows 1602 in the second insulating layer are formed by drilling a through hole 1602 through all layers, which is then plated with metal 1604 (or other electrically conductive material) to form an interlayer connection. To prevent the formation of a connection with the first bonding layer through the windows 1602 of the second insulating layer, the first bonding layer is etched around the interconnect 1602, electrically insulating it, and an insulating cap 1605 is placed on the end of the interconnect 1602, ensuring that a short does not occur due to soldering or contact with HTS cable.

Alternatively, windows 1602 may instead be drilled into the other side of the partially insulating layer so that they pass through the second bonding layer, the second insulating layer, and the electrically conductive layer and do not pass through (or not completely through) the first insulating layer. As a further alternative, all windows may be formed from interlayers that extend through all layers by etching the second bonding layer and an insulating cap on the second bonding layer used for the windows 1601 of the first insulating layer.

Another unexpected benefit of partially insulated coils is that the added stability of superconductivity disruption allows for a greater choice of materials for the non-superconductive conductive elements in each cable. In traditional coils, both the HTS stabilizer (i.e. a thin layer of metal or metal sheath on each tape) and any material connecting the tapes will be copper as it has very low resistivity and higher resistivity materials will cause excessive heating. However, copper is also a relatively soft metal, so that at high pressures it can extrude from the tape or deform under shear stresses, which is likely to be responsible for damage to the HTS layers after superconductivity is broken.

Therefore, it is preferable to reduce or eliminate copper from coil turns and partial insulation. The reduced amount of copper can be, for example, less than 10 microns of copper thickness per HTS tape in a reel (i.e., reduced compared to traditional HTS tapes) or less than 5 microns of copper thickness (i.e., less than half of this). Metals or other electrical conductors used in place of copper may have one or more of:

- reduced ductility compared to copper;

- increased modulus of elasticity in shear compared to copper;

- increased Young's modulus compared to copper;

- increased bulk modulus compared to copper;

- increased Brinell hardness number compared to copper.

Suitable materials include stainless steel.

The above disclosure can be applied to a variety of HTS magnetic systems. In addition to the tokamak fusion reactor mentioned above as an example, it can be applied to HTS coils in nuclear magnetic resonance imaging (NMR/MRI) devices, manipulation of magnetic devices in a non-magnetic environment by means of magnetic fields (e.g. robotic magnetic navigation systems). for manipulation of medical devices inside the patient) and magnets for electric motors, for example, for an electronic aircraft. As a further example, the disclosure may be applied to proton beam therapy (PBT) devices comprising HTS magnet systems that include the disclosed features, these HTS magnet systems being used in the PBT device's accelerator, four-pole, or two-pole control magnets of the device. PMT or any other magnet of the PMT device.

Claims (54)

1. Magnetic system with a high-temperature superconductor (HTSC), containing an HTSC excitation coil and a power source, while: The HTS excitation coil contains: a plurality of turns containing HTS material and a metal stabilizer; an electrically conductive layer separating the turns so that current can be distributed between the turns through the electrically conductive layer; the power supply is configured to: during loading of the HTSC excitation coil, apply the first current to the HTSC excitation coil; during the unloading of the HTSC excitation coil, apply a second current to the HTSC excitation coil, opposite in direction to the first current. 2. The HTS magnetic system according to claim 1, comprising: a superconductivity violation detection system configured to detect superconductivity violation in the HTS material and/or detect conditions likely to cause superconductivity violation in the HTS material; wherein the power supply is configured to unload the HTS drive coil in response to detection of a superconductivity violation or conditions likely to cause superconductivity violation by the superconductivity violation detection system. 3. The HTS magnetic system according to claim 1 or 2, wherein the second current is direct current. 4. The HTS magnetic system according to claim 1 or 2, wherein the second current is a combination of direct and alternating currents so that the current varies sinusoidally with an average value opposite in sign to the first current and a period less than the time constant of the HTS excitation coil. 5. The HTS magnetic system according to any one of the preceding claims, wherein the power supply comprises a four-quadrant power supply unit (PSU). 6. Magnetic system with HTSC according to paragraphs. 1, 2, or 3, wherein the power supply comprises a single-quadrant power supply unit (PSU) and a capacitor, wherein the single-quadrant PSU is configured to supply a first current and the capacitor is configured to supply a second current. 7. Magnetic system with HTSC according to any one of paragraphs. 1-6, wherein the power supply is configured to supply a second current to one of: detecting a violation of superconductivity in a predetermined section of the HTS excitation coil; detecting a specific temperature in a predetermined section of the HTS excitation coil; detecting that the magnetic field generated by the magnet has decreased below a threshold value; And detecting that the current in the HTS material of the HTS drive coil has decreased below a threshold value. 8. The HTS magnetic system according to any one of the preceding claims, wherein the power supply comprises a feedback system configured to control the current supplied to the magnet depending on one or more of: current in the HTSC material of the magnet; temperature of the HTSC excitation coil; And magnetic field of the HTSC excitation coil. 9. Magnetic system with a high-temperature superconductor (HTSC), containing an HTSC excitation coil and a power source, while: The HTS excitation coil contains: a plurality of turns containing HTS material and a metal stabilizer; an electrically conductive layer separating the turns so that current can be distributed between the turns through the electrically conductive layer; the power supply is configured to: during loading of the HTSC excitation coil, apply the first current to the HTSC excitation coil; while unloading the HTS field coil, apply a second current to the HTS field coil, which has the same direction as the first current and is less than the current in the HTS field coil material. 10. The HTS magnetic system of claim 9, wherein the second current is a combination of DC and AC such that the current varies sinusoidally with an average greater than zero and less than the first current and a period less than the time constant of the HTS drive coil. 11. Magnetic system with a high-temperature superconductor (HTSC), containing an HTSC excitation coil and a power source, while: The HTS excitation coil contains: a plurality of turns containing HTS material and a metal stabilizer; an electrically conductive layer separating the turns so that current can be distributed between the turns through the electrically conductive layer; the power supply is configured to: apply direct current to the HTSC excitation coil; And supplying alternating current to the HTS field coil in addition to direct current in order to heat the field coil, wherein the alternating current has a period less than the time constant of the field coil and a value less than the direct current. 12. A tokamak comprising a magnetic system according to any one of the preceding claims, wherein the HTS excitation coil of the magnetic system is one of a toroidal field excitation coil or a tokamak poloidal field excitation coil. 13. The device for proton beam therapy (PBT), containing the magnetic system according to any one of paragraphs. 1-10, while the HTSC excitation coil of the magnetic system is one of: excitation coils of the accelerator of the PMT device; two-pole or four-pole magnet of the proton beam control system of the PMT device. 14. A method for unloading a high-temperature superconductor (HTSC) excitation coil, wherein the HTS excitation coil comprises a plurality of turns containing an HTS material and a metal stabilizer, and an electrically conductive layer separating the turns, so that current can be distributed between the turns through the electrically conductive layer, when In this method, the method comprises applying a second current to the HTS excitation coil opposite to the first current flowing in the HTS material. 15. The method of claim 14, wherein the second current is applied in response to detection of a superconductivity violation or conditions likely to cause a superconductivity violation in the HTS drive coil. 16. The method according to claim 14 or 15, wherein the second current is applied to one of: detecting a violation of superconductivity in a predetermined section of the HTS excitation coil; detecting a specific temperature in a predetermined section of the HTS excitation coil; detecting that the magnetic field generated by the magnet has decreased below a threshold value; And detecting that the current in the HTS material of the HTS drive coil has decreased below a threshold value. 17. The method according to any one of paragraphs. 14-16, wherein the second current is a combination of direct and alternating currents such that the current varies sinusoidally with an average value opposite in sign to the first current and a period less than the HTS drive coil time constant. 18. A method for unloading a high-temperature superconductor (HTSC) excitation coil, wherein the HTS excitation coil comprises a plurality of turns containing an HTS material and a metal stabilizer, and an electrically conductive layer separating the turns, so that the current can be distributed between the turns through the electrically conductive layer, and the method comprises applying a second current to the HTS excitation coil, which is less than the first current flowing in the HTS material. 19. The method of claim 18, wherein the second current is a combination of DC and AC such that the current varies sinusoidally with an average greater than zero and less than the first current and a period less than the HTS drive coil time constant. 20. A method for heating a high-temperature superconductor (HTSC) excitation coil, wherein the HTS excitation coil comprises a plurality of turns containing an HTS material and a metal stabilizer, and an electrically conductive layer separating the turns so that current can be distributed between the turns through the electrically conductive layer, wherein the method comprises applying a combined direct and alternating current to an HTS excitation coil. 21. The method of claim 20, wherein the DC current is equal to the current in the HTS material of the HTS drive coil.

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