Central pillar for tokamak plasma chambers.

Abstract

A toroidal field coil for a tokamak plasma chamber having a central pillar, the toroidal field coil comprising first and second high temperature superconductor (HTS) assemblies each comprising one or more HTS tapes for conducting electrical current parallel to an axis of the central pillar, each HTS tape comprising an HTS material having an associated critical current that is dependent on a magnetic field in the HTS tape during use of the central pillar, the central pillar further comprising a cooling mechanism configured to preferentially cool the first HTS assembly relative to the second HTS assembly to reduce or eliminate a difference in the critical current of the HTS tape or each HTS tape of the first HTS assembly relative to the critical current of the HTS tape or each HTS tape of the second HTS assembly.

Description

[Technical Field]

[0001] The present invention relates to a central pillar for a toroidal field coil of a tokamak plasma chamber, such as one used in a fusion reactor. In particular, the present invention relates to a central pillar comprising a high temperature superconductor (HTS) material. [Background technology]

[0002] Superconducting materials are typically classified as "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 explained by BCS theory. All low-temperature superconductors have a critical temperature (the temperature above which the material no longer becomes superconducting, even in zero magnetic field) below approximately 30 K. The behavior of HTS materials is not explained by BCS theory, and such materials may have critical temperatures above approximately 30 K. (Note, however, that it is the physical differences in superconducting behavior and composition, not the critical temperature, that define HTS materials.) The most commonly used HTS are "cuprate superconductors," ceramics based on cuprates (compounds containing a copper oxide group), such as BSCCO (bismuth strontium calcium copper oxide) or REBCO (where Re is a rare earth element, usually Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diboride (MgB2).

[0003] REBCO is typically manufactured as a tape having a structure as shown in Figure 1. Such a tape 100 is typically about 100 microns (micrometers) thick and includes a substrate 101 (typically electropolished Hastelloy® about 50 microns (micrometers) thick) on top of which is deposited by IBAD, magnetron sputtering, or other suitable technique a series of buffer layers about 0.2 microns (micrometers) thick, known as a buffer stack 102. An epitaxial REBCO-HTS layer 103 (deposited by MOCVD or other suitable technique) covers the buffer stack and is typically 1 micron (micrometer) thick. A 1-2 micron (micrometer) silver layer 104 is deposited on the HTS layer by sputtering or other suitable technique, and a copper stabilization layer 105 is deposited on the tape by electroplating or other suitable technique, often completely encapsulating the tape.

[0004] Substrate 101 provides a mechanical backbone that can be fed through a manufacturing line and enables the growth of subsequent layers. Buffer stack 102 is required to provide a biaxially oriented crystalline template on which to grow the HTS layers, preventing chemical diffusion of elements from the substrate into the HTS that would impair its superconducting properties. Silver layer 104 is generally required to provide a low resistance interface from the REBCO to the stabilization layer, and stabilization layer 105 provides an alternative current path in case any portion of the REBCO becomes non-superconducting (becomes "normal").

[0005] HTS tapes can be arranged into HTS cables, also referred to herein as HTS assemblies. HTS cables, as referred to herein, typically comprise one or more HTS tapes connected along their length via a conductive material (usually copper). HTS tapes can be stacked (i.e., arranged so that the HTS layers are parallel) or have other tape arrangements that can 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 substrate-attached tapes are used, HTS pairs can be Type 0 (HTS layers face each other), Type 1 (the HTS layer of one tape faces the substrate of the other tape), or Type 2 (the substrates face each other). Cables with three or more tapes can have some or all of the tapes arranged in HTS pairs. Laminated HTS tapes can comprise various arrangements of HTS pairs, most commonly either stacks of Type 1 pairs or stacks of Type 0 pairs (or equivalently, Type 2 pairs).

[0006] An important property of HTS tapes (and superconductors in general) is the "critical current" (I c ), which is the current at which, at a given temperature and external magnetic field, the HTS generates enough voltage to drive a portion of the current into the stabilizing layer. The critical point of the superconducting transition, at which the superconductor is considered to have "gone normal," is somewhat arbitrary, but is usually considered to be when the tape generates E0 = 10 or 100 microvolts / meter. The critical current can depend on many factors, including the temperature of the superconductor and the magnetic field at which the superconductor is placed. In the latter case, both the magnitude of the magnetic field and the orientation of the superconductor's crystallographic axes in the field are important.

[0007] FIG. 2 shows a cross-sectional view of an exemplary REBCO tape 200 in the xz plane. The REBCO layer itself is crystalline, and the major axes of the REBCO crystal are shown for a point in the tape. The REBCO tape is shown in simplified form with an HTS layer 201, a copper cladding 202, and a substrate 203. The REBCO crystal structure has three mutually perpendicular major axes, referred to in the art as a, b, and c. For purposes of this disclosure, any dependence of the critical current on the orientation of the magnetic field component in the ab plane is ignored, and therefore the a and b axes can be considered interchangeable, and therefore they will be considered only as the "ab plane" (i.e., the plane defined by the a and b axes). In FIG. 2, the ab plane of the REBCO layer 201 is shown as a single line 210 perpendicular to the c-axis 220. In many tapes, the ab plane 210 exactly coincides with the plane of the HTS layer 201, but this is not a common condition.

[0008] The critical current of a tape depends on the thickness and quality of the REBCO crystal. It also has an approximately inverse dependence on the ambient temperature and the magnitude of the applied magnetic field. Finally, it also depends on the orientation of the applied magnetic field relative to the c-axis. When the applied magnetic field vector is in the ab-plane 210, the critical current is significantly higher than when the applied magnetic field vector is aligned along the c-axis 220. The critical current varies smoothly between these two extremes for "out-of-ab-plane" magnetic field orientations. (In reality, there may be multiple angles at which the critical current peaks. Furthermore, the amplitude and width of the peaks vary with both the applied magnetic field and temperature, but for the purposes of this discussion, we can consider a tape with a single dominant peak that defines the optimal orientation of the applied B-field that gives the maximum critical current.)

[0009] REBCO tapes are typically manufactured with the c-axis as nearly perpendicular as possible to the plane of the tape, however, some commercially available tapes have their c-axis at angles up to 35° from normal to the x / y plane.

[0010] For HTS cables, assuming the cable is at a uniform temperature and in a uniform magnetic field along its entire length, the critical current of all tapes in the stack will be relatively uniform. In this case, when the cable is connected to a power source, the current will be distributed among the tapes according to Ohm's law in proportion to the termination resistance at the ends of the cable. However, current distribution can often be affected by many factors, including variations in the local magnetic field magnitude along the length or across the width of the tapes in the cable, or variations in the magnetic field angle relative to the c-axis of the REBCO layer.

[0011] Magnets containing high-temperature superconductors can be used in fusion reactors, such as spherical tokamaks (STs), to confine plasma at extremely high temperatures. Spherical tokamaks offer important advantages for commercial fusion power plants, including higher thermal power output per unit plasma volume and large bootstrap currents. These advantages enable the development of smaller, more efficient machines, shortening development time and reducing recycled power. Understanding of ST physics continues to advance worldwide in experimental devices such as MAST, NSTX, and ST40, which use pulsed resistive magnets.

[0012] Commercial power plants require superconducting magnets for either long-pulse or continuous operation and to maximize net power production. This has previously been an obstacle for STs, as the slender central column of a toroidal field (TF) magnet results in a magnetic field on the superconductor that exceeds the capabilities of conventional low-temperature superconductors (LTS). The recent commercial availability of high-performance REBCO-coated conductors ("tapes") from multiple suppliers has made it possible to realize high-field STs with the mission of demonstrating net power gain (Q>1) using DT fuel on a scale smaller than conventional aspect ratio tokamaks using LTS. A 1.4 m major radius HTS ST with an on-axis 4 T magnetic field could accomplish this mission, provided that an adequately thick neutron shield (>25 cm) can be realized.

[0013] 3A shows a vertical cross-section of a spherical tokamak 300 comprising toroidal field coils 301, poloidal field coils 303, and a toroidal plasma chamber 305 located within toroidal field coils 301. Tokamak 300 also comprises a central pillar 307 extending through the centers of plasma chamber 305 and toroidal field coils 301 and poloidal field coils 303. Each of D-shaped toroidal field coils 301 comprises a generally straight portion 309 (the "inner rim" of TF coil 301) extending along axis A-A' of central pillar 307, and a curved portion 311 (the "outer rim" of TF coil 301) electrically connected to both ends of straight portion 309 to form the D-shape. In this example, spherical tokamak 300 has a semi-major axis of 1.4 m, and central pillar 307 has a radius of approximately 0.6 m.

[0014] 3B shows an axial cross-section of central pillar 307 as viewed along axis A-A'. Tokamak 300 includes twelve toroidal field coils 301, with the respective straight sections 309 of each toroidal field coil 301 angularly spaced equiangularly about axis A-A' of central pillar 307. The central pillar includes a support member 313 extending along axis A-A' and having a plurality of channels 315 in which the straight sections 309 of toroidal field coils 311 are accommodated. Support member 313 may be formed from a plurality of interlocking angular segments like the segments of an orange, with each segment accommodating the inner rim 309 of one of the TF coils 301.

[0015] FIG. 4 is an axial cross-sectional view of an angle segment 400 of central post 307, which constitutes one half of a segment of support member 313 that houses the inner limb 401 of one of toroidal field coils 301. Only the “top” half of the angle segment is shown in FIG. 4; the omitted “bottom” half is a mirror image of the top half. Multiple angle segments 400 are assembled to form a substantially cylindrical central post 307. The inner limb 401 of toroidal field coil 301 is formed by winding multiple turns of HTS cable 402 (multiple turns (“windings”) can be collectively referred to as a “winding” or “coil” section), with each turn comprising HTS tape extending parallel to the axis of central post 307 (i.e., into the page with respect to FIG. 4 ). A portion of winding section 401 showing the four individual turns of HTS cable 402 that make up the winding section is shown in more detail in FIG. 5.

[0016] Existing designs for HTS assemblies (cables) 402 generally follow those used for low-temperature superconductors. These designs assume a "cable-in-conduit conductor" (CICC) construction, in which the HTS cable 402 comprises a stack of HTS tapes 501 surrounded by a stabilizing material 502 (such as copper or aluminum) with cooling channels 505. Because the stabilizing material 502 and cooling channels 505 are weak, a high-strength "jacket" with structural supports 503 made of a high-strength material such as Inconel is used to prevent mechanical deformation of the HTS assembly 402 under the electromagnetic pressure generated when the coil is energized. Insulators 504 are provided between the HTS cables 402 to electrically insulate them from each other. The stack of HTS tapes 501 is cooled by flowing a cryogen through a central cooling channel 505 through the stabilizing material 502. The introduction of cooling channels 505 and a large amount of flexible, highly conductive stabilizer 502 into the HTS assembly 402 weakens the HTS assembly 402, requiring a relatively strong (i.e., thick) structural support 503. The stack of HTS tapes 501 are evenly spaced around the central cooling channel 505 to ensure uniform cooling of the stack of HTS tapes 501. Conventionally, the HTS tapes are provided in a "twisted" or "transposed" configuration, where the orientation of the HTS tapes changes along the axis of the central post.

[0017] Referring again to FIG. 4, the angular segment 400 of the central pillar 307 has a vacuum gap 403 separating the cryogenic components (HTS cable 402 and support member 313) from the neutron shielding material 404, which is located farther from the axis of the central pillar 307 than the windings 401 and support member 313.

[0018] The use of cable-in-conduit conductors in the HTS assembly 402 typically requires 100A / mm 2 The current density (J wp), which means that for a given central pillar 307 diameter, the area of ​​central pillar 307 available for neutron shielding 404 is limited, especially in small tokamaks. As a result, the CICC configuration may expose the HTS coil section 401 to higher nuclear heating than is desirable when operating the tokamak. Summary of the Invention [Means for solving the problem]

[0019] According to a first aspect of the present invention, there is provided a central pillar for a toroidal field coil of a tokamak plasma chamber. The central pillar comprises first and second high temperature superconductor (HTS) assemblies, each comprising one or more HTS tapes for conducting electrical current parallel to the axis of the central pillar. Each HTS tape comprises an HTS material having an associated critical current that is dependent on the magnetic field at the HTS tape during use of the central pillar. The central pillar further comprises a cooling mechanism configured to preferentially cool the first HTS assembly relative to the second HTS assembly to reduce or eliminate a difference in the critical current of the HTS tape or tapes of the first HTS assembly relative to the critical current of the HTS tape or tapes of the second HTS assembly.

[0020] For example, due to the magnetic field generated during operation of the toroidal field coil, the critical current of the or each HTS tape of the second HTS assembly may be greater than the critical current of the or each HTS tape of the first HTS assembly. As described below, the critical current may depend on the strength and / or field angle of the magnetic field at the HTS tape. Specifically, the magnetic field strength and / or field angle at the or each HTS tape of the first HTS assembly may be greater than the magnetic field strength and / or field angle at the or each HTS tape of the second HTS assembly. As a result, the critical current of the or each HTS tape of the first HTS assembly may be less than the critical current of the or each HTS tape of the second HTS assembly. The cooling mechanism may be configured to cool the first HTS assembly to a lower temperature than the second HTS assembly to compensate for the difference in critical current.

[0021] By reducing or preferably eliminating the difference in critical current between the first and second HTS assemblies, transport current can be more evenly distributed between them. For example, the cooling mechanism can be configured to ensure that the critical current of the HTS tape of the first HTS assembly is within 20%, preferably within 10%, more preferably within 5%, or even within 1% of the critical current of the HTS tape of the second HTS assembly.

[0022] The HTS material can be, for example, REBCO.

[0023] The critical current of each HTS tape can be inversely dependent on the strength of the magnetic field in the HTS tape. The strength of the magnetic field in the first HTS assembly can be greater than the strength of the magnetic field in the second assembly. Generally, the critical current decreases with increasing magnetic field strength (i.e., the critical current is inversely dependent on the magnetic field strength) and with increasing temperature (i.e., the critical current is inversely dependent on the temperature); for example, the critical current can be inversely proportional to the magnetic field strength (B) and the temperature (T), and the cooling mechanism can be configured to generate a temperature distribution across the first and second HTS assemblies that compensates for the difference in magnetic field strength in the first and second HTS assemblies. For example, if the magnetic field strength in the first HTS assembly is greater than the magnetic field strength in the second assembly, the cooling mechanism can be configured to cool the first assembly to a lower temperature than the second assembly.

[0024] For example, the cooling mechanism can be configured to compensate for a positive radial magnetic field gradient (dB / dr, r being the radial distance from the axis of the central pillar) by creating a negative radial temperature gradient (dT / dr) between the first and second HTS assemblies. The temperature gradient can be used to compensate for a critical current I generated by the magnetic field gradient. c It can be chosen so that the variations in (B,T) are approximately cancelled out.

[0025] Each HTS tape can have an associated plane defined by the crystalline structure of the HTS material of the HTS tape. The plane can be, for example, the ab plane, as described above in connection with the REBCO tape 200 of FIG. 2 . The critical current of each HTS tape depends on the magnetic field angle between the magnetic field in the HTS tape and the plane of the HTS tape, with the critical current decreasing as the angle increases. The HTS assemblies can be arranged such that the magnetic field angle between the magnetic field and the plane of the or each HTS tape of a first assembly is greater than the magnetic field angle between the magnetic field and the ab plane of the or each HTS tape of a second assembly. For each HTS assembly, the respective planes of the HTS tapes of the HTS assemblies can be parallel to each other. Optionally, the plane of the HTS tape of the first HTS assembly can be parallel to the plane of the HTS tape of the second HTS assembly. For example, the first and second HTS assemblies can each be part of a respective planar pancake coil comprising nested windings of HTS tape centered about an axis, the pancake coils stacked adjacent to one another in a face-to-face arrangement. In one example, the maximum critical current of each HTS tape can occur when the magnetic field (B) is parallel to the ab-plane of the HTS tape. For example, the cooling mechanism can be configured to cool the first HTS assembly to a lower temperature than the second HTS assembly when the magnetic field angle between the magnetic field of the or each HTS tape of the first assembly and the ab-plane is greater than the magnetic field angle between the magnetic field of the or each HTS tape of the second assembly and the ab-plane.

[0026] The distance between the first HTS assembly and the axis of the central pillar can be greater than the distance between the second HTS assembly and the axis of the central pillar, each distance being measured in a plane perpendicular to the axis.

[0027] The cooling mechanism may comprise one or more channels for flowing a cryogenic fluid, preferably helium, more preferably supercritical helium.

[0028] The or each cooling channel can be substantially straight (i.e., the centerline of the channel is straight) (or can include a portion that is straight) and can extend in a direction that has a component that is parallel to the axis of the central post. For example, the or each cooling channel and the HTS tape can all be (substantially) parallel to the axis of the central post.

[0029] The thermal impedance between the or each cooling channel and the first HTS assembly may be less than the thermal impedance between the or each cooling channel and the second HTS assembly.

[0030] The shortest distance between the or each cooling channel and the first HTS assembly can be shorter than the shortest distance between the or each cooling channel and the second HTS assembly, each distance measured in a plane perpendicular to the axis. This configuration allows the or each cooling channel to preferentially cool the first HTS assembly relative to the second HTS assembly (at least in the plane along which the distance is measured). In some examples, the or each cooling channel can be closer to the first HTS assembly than to the second HTS assembly along the entire central column.

[0031] In some embodiments, the or each cooling channel can be located further from the axis of the central post than both the first HTS assembly and the second HTS assembly. Preferably, the or each cooling channel is located further from the second HTS assembly than from the first HTS assembly to provide preferential cooling to the first HTS assembly compared to the second HTS assembly.

[0032] The density of the cooling channels adjacent to the first HTS assembly can be greater than the density of the cooling channels adjacent to the second HTS assembly. Alternatively, or in addition, the cross-sectional area of ​​each of the cooling channels adjacent to the first HTS assembly can be greater than the cross-sectional area of ​​each of the cooling channels adjacent to the second HTS assembly. These configurations allow the cooling channels to provide greater cooling power to the first HTS assembly compared to the second HTS assembly.

[0033] The first and second HTS assemblies can each comprise a plurality of HTS tapes, each having an associated ab plane defined relative to the crystalline structure of the HTS material of the HTS tape, the ab planes of each of the HTS tapes being parallel to each other within each HTS assembly.

[0034] The HTS magnet may further comprise a support member having one or more channels, the or each channel preferably extending in a direction parallel to the axis of the central post, and the first and second HTS assemblies may be provided within the one or more channels of the support member.

[0035] At least a portion of the central post can be made of a thermally conductive material, such as copper, preferably solid copper, i.e., a material that has high thermal conductivity at temperatures below the critical temperature of the HTS material in the HTS tape. In some examples, the material can have a thermal conductivity of greater than 100 W / mK, greater than 300 W / mK, or greater than 7000 W / mK for temperatures ranging from 20 K to 40 K. The cooling mechanism can be configured to cool the portion of the support member through a surface of the support member that is continuous with the main body portion of the portion of the support member (i.e., there is no interface between the main body portion and the surface). The main body portion is in contact with the first and / or second HTS assemblies through one or more walls of the or each channel of the support member in which the first and / or second HTS assemblies are provided, thereby cooling the first and / or second HTS assemblies by the portion of the support member.

[0036] At least a portion of the second HTS assembly can be located radially inward of the first HTS assembly, i.e., extend closer to the axis of the central column than the first HTS assembly. This portion can be in thermal contact with a main body portion of a portion of the support member cooled by the cooling mechanism, thereby transferring heat from the portion of the second HTS assembly to the cooling mechanism through the portion of the support member cooled by the cooling mechanism. The cooling mechanism can be configured to cool the portion of the support member cooled by the cooling mechanism to a temperature lower than the temperature of each HTS assembly during use of the central column. For example, the first and second HTS assemblies can be cooled to a temperature of 25 to 35 K, while the portion of the support member that can be cooled by the cooling mechanism can be cooled to a temperature of 20 to 25 K.

[0037] The support member can include a portion radially inward of the portion cooled by the cooling mechanism and having a higher mechanical strength than the portion cooled by the cooling mechanism. The other portion can be made, for example, from ICONEL®. The increased mechanical strength resists compression of the central pillar by the HTS assembly as a result of Lorentz forces generated during use of the central pillar.

[0038] The cooling mechanism can be configured to cool each HTS tape to below the critical temperature of the HTS material in the HTS tape, preferably to a temperature below 30K, more preferably to a temperature below 25K, for example to about 20K.

[0039] According to a second aspect of the present invention, there is provided a tokamak plasma chamber comprising a central pillar according to the first aspect above and a toroidal field coil comprising a plurality of windings of HTS tape, each winding comprising one HTS tape. The tokamak plasma chamber may further comprise a plurality of toroidal field coils configured to provide a toroidal magnetic field within the plasma chamber when current flows around the windings of the toroidal field coils, and the central pillar comprises respective first and second HTS assemblies for each toroidal field coil (i.e., each winding of the toroidal field coil comprises one HTS tape of each of the first and second HTS assemblies).

[0040] The toroidal field coil can be, for example, a D-shaped coil, with the windings arranged so that an inner rim (corresponding to the straight portion of the D) is formed by the HTS tape on the central column, and an outer rim (corresponding to the curved portion of the D) is formed by the other HTS tapes that make up each winding. Current supplied to the first winding of the toroidal field coil circulates around each of the coil's other windings in turn (like a solenoid), with the current flowing along the inner rim, around the outer rim, and back to the inner rim for each winding.

[0041] According to a third aspect of the present invention, there is provided a method of operating a tokamak plasma chamber according to the second aspect, the method comprising, for each of a plurality of toroidal field coils: passing a current around a winding of a toroidal field coil; using a cooling mechanism to preferentially cool the first HTS assembly relative to the second HTS assembly to reduce or eliminate a difference in critical current of the or each HTS tape of the first HTS assembly relative to the critical current of the or each HTS tape of the second HTS assembly; Includes.

[0042] Where the cooling mechanism comprises one or more cooling channels, using the cooling mechanism may include flowing a cryogenic fluid, such as supercritical helium, through the or each cooling channel.

[0043] The magnetic field generated by the toroidal field coil can be such that, for example, the magnetic field strength in each first HTS assembly is greater than the magnetic field strength in each second HTS assembly. Alternatively, or in addition, the magnetic field angle between the magnetic field in the HTS tape of each first HTS assembly and the ab-plane or each ab-plane can be greater than the magnetic field angle between the magnetic field in the HTS tape of each second HTS assembly and the ab-plane.

[0044] According to a fourth aspect of the present invention, there is provided a central pillar for a toroidal field coil of a tokamak plasma chamber. The central pillar comprises a support member having a plurality of channels spaced about a central axis. Each channel has a conductor element disposed therein, the conductor element comprising one or more layers of superconductor material for conducting electrical current parallel to the central axis. The central pillar further comprises a cooling mechanism configured to cool the superconductor material to create (or maintain) a downward temperature gradient across each conductor element along a radial direction perpendicular to the central axis, before or during operation of the tokamak plasma chamber as a nuclear fusion reactor, such that the temperature of each conductor element decreases along the radial direction away from the central axis.

[0045] The temperature gradient across each conductor element reduces the current-to-critical current (I / I) in the superconductor material of the conductor element by at least partially compensating for the increase in magnetic field strength with increasing distance from the central axis and / or the non-optimal magnetic field angle. c ) ratio more uniformly in the radial direction.

[0046] The cooling mechanism may include one or more cooling channels extending through the support member for flowing a cryogenic fluid, and the density of the cooling channels and / or the cross-sectional area of ​​each of the cooling channels may increase radially across the support member to provide differential cooling to radially inner and radially outer portions of the support member as the cryogenic fluid flows through the cooling channels.

[0047] The cooling mechanism can include a regulator for controlling the flow rate of the cryogenic fluid through the cooling channels, the cooling channels and the regulator configured to provide a greater flow rate through a first set of cooling channels than through a second set of cooling channels, the first set of cooling channels being located farther from the central axis than the second set of cooling channels.

[0048] Each conductor element may be spaced from one or more walls of the channel to define a respective one of the cooling channels.

[0049] Each conductor element may comprise multiple layers of superconductor material, the layers being oriented substantially perpendicular to the radial direction.

[0050] In use, for each conductor element, the average temperature of the first layer of superconductor material can be higher than the average temperature of the second layer of superconductor material, the first layer being located closer to the central axis than the second layer. The first layer can be the radially innermost layer of the conductor element, and the second layer can be the radially outermost layer of the conductor element. The cooling channel can be positioned such that, in use, a cryogenic fluid contacts the second layer of each conductor element.

[0051] Each conductor element can contact a portion (e.g., a wall) of the channel of the support member in which the conductor element is provided, the portion extending perpendicular to the central axis and made of a thermally conductive material, which can be or can include copper, preferably hard copper.

[0052] The superconductor material can be a high temperature superconductor (HTS) material, such as REBCO.

[0053] Each conductor element may comprise multiple stacks of HTS tape arranged side by side in a channel, preferably with insulating material between adjacent stacks. The or each cooling channel may span the face of the respective conductor element.

[0054] The cryogenic fluid can be helium, preferably supercritical helium.

[0055] According to a fifth aspect of the present invention, there is provided a tokamak plasma chamber comprising a central pillar according to the fourth aspect above and a plurality of toroidal field coils, each toroidal field coil comprising one or more conductor elements.

[0056] According to a sixth aspect of the present invention, there is provided a method of operating a tokamak plasma chamber comprising the central column according to the fourth aspect and a plurality of toroidal field coils, each toroidal field coil comprising one or more conductor elements, the method comprising flowing a cryogenic fluid through a cooling channel before and / or while current is being supplied to each toroidal field coil. The cryogenic fluid may be helium, preferably supercritical helium. The flow rate of the cryogenic fluid may be increased before and / or during pulsed operation of the tokamak plasma chamber as a nuclear fusion reactor. [Brief explanation of the drawings]

[0057] [Figure 1] FIG. 1 is a schematic perspective view of a prior art HTS tape. [Figure 2] FIG. 1 is a schematic cross-sectional view of an HTS tape showing the ab-plane and c-axis of the tape. [Figure 3A] 1 is a schematic cross-sectional view of a tokamak. [Figure 3B] FIG. 3B is a schematic axial cross-section of the central pillar of the tokamak of FIG. 3A. [Figure 4]3C is a schematic axial cross-sectional view of a sector (segment) of the central pillar of FIGS. 3A and 3B. FIG. [Figure 5] 5 is a schematic axial cross-sectional view of the windings of the segment of the central post of FIG. 4. [Figure 6] 1 is a schematic axial cross-section of a segment of a central column of a tokamak according to the invention; [Figure 7] 1 is a schematic axial cross-sectional view of a winding portion of a central post according to the present invention; [Figure 8] 1 is a schematic axial cross-section of a segment of a central pillar according to the invention; [Figure 9] 9 is a schematic axial cross-sectional view of a segment of the central pillar of FIG. 8, with the simulation results of the temperature distribution of the central pillar superimposed. DETAILED DESCRIPTION OF THE INVENTION

[0058] An object of the present invention is to overcome or at least mitigate some of the above-mentioned problems associated with existing central pillars of tokamak plasma chambers. In some embodiments, the present invention enables the fabrication of a central pillar that, when a tokamak plasma chamber is operated, exhibits a more uniform distribution of transport current among the HTS cables (i.e., HTS "assemblies") extending along the axis of the central pillar (forming the "inner" leg of the toroidal field coil) compared to existing central pillars. Specifically, the more uniform distribution of transport current can be achieved by providing a cooling mechanism that preferentially cools the HTS tapes in one HTS cable of the toroidal field coil relative to the HTS tapes in the other HTS cable of the toroidal field coil. Such cooling compensates for differences (i.e., imbalances) in the critical currents in the HTS materials of the two HTS cables. By reducing or eliminating the critical current difference, the transport current is more evenly distributed among the HTS cables in the central pillar. For example, the ratio of transport current to critical current can be more consistent for the HTS cables. Differential cooling of the HTS material contrasts with the approach used in existing central pillars, which aims to provide a uniformly high cooling rate to the HTS material regardless of where it is located in the central pillar.

[0059] The use of HTS materials, as opposed to LTS materials, generally means that a larger temperature difference between two (or more) HTS cables can exist without the risk of thermal runaway due to loss (or partial loss) of superconductivity. For example, in existing magnets that use LTS materials, the temperature margin of the LTS material, i.e., the difference between the operating temperature and the critical temperature at which thermal runaway begins, can be less than 1 K. In contrast, with HTS materials, the temperature margin can be an order of magnitude higher, allowing the HTS magnet to withstand a larger temperature gradient across its windings without loss of superconductivity.

[0060] FIG. 6 is an axial cross-sectional view of an angular segment of a central pillar 600 of a tokamak plasma chamber (e.g., tokamak 300 of FIG. 3A ). As with FIG. 4 (and FIG. 8 , described below), only half of the angular segment is shown in FIG. 6 , and the omitted half of the angular segment is a mirror image of what is shown in the figure. The central pillar 600 includes a support member 613 similar to the support member 313 of FIGS. 3B and 4 . The support member 613 includes channels that extend parallel to the axis of the central pillar 600 (i.e., into the plane of the paper in FIG. 6 ) and house multiple HTS assemblies 601 arranged as “windings” 602. Each HTS assembly 601 is elongated in a direction parallel to the axis of the central pillar 600 (i.e., into the plane of the paper in FIG. 6 ). In the embodiment shown in FIG. 6 , each HTS assembly 601 includes multiple HTS tapes, each aligned such that its longest axis is (substantially) parallel to the axis of the central pillar 600. Each HTS assembly 601 also extends in a direction having at least a component toward the axis of the central pillar 600, i.e., along the radius of the central pillar. The HTS assemblies 601 are arranged as a stack, and the lengths of the HTS assemblies 601 are different to efficiently utilize the shape of the angular segments, i.e., the length of the HTS assemblies 601 at the ends of the stack (e.g., the HTS assembly 601 at the top of the stack with respect to Figure 6) is shorter than the length of the HTS assemblies 601 in the middle of the stack.

[0061] Central pillar 600 also includes a vacuum gap 603 between support member 613 and nuclear shielding 604 that surrounds support member 613 to limit nuclear heating of HTS assembly 601 when the tokamak is in use (i.e., operating as a fusion reactor). Support member 613 may be made from copper (although other metals and / or alloys may be used) and may be formed as a single piece or may be formed from two or more pieces, as described below in connection with FIG. 8.

[0062] FIG. 7 is an axial cross-sectional view of the central post 600 showing a portion of the windings 602 disposed within the channel of the support member 613. The windings 602 shown in FIG. 7 comprise a stack of four HTS assemblies 701 (rather than the stack of three HTS assemblies 601 shown in FIG. 6). In general, the stack can comprise any number of HTS assemblies 601, limited only by the size of the central post 600 and the dimensions of the HTS tape. A pair of stabilization layers 702A, 702B, made from, for example, copper or aluminum, are provided on either side of the stack of HTS assemblies 701, between the stack and opposing walls of the channel in the support member 613 that houses the windings 602. The channel walls serve as structural support 703 for the HTS assemblies 701 to prevent deformation and possible damage to the HTS tape. In this example, an electrical insulating layer 704 is provided between each adjacent pair of HTS assemblies 701 to separate them from each other.

[0063] Each HTS assembly 701 comprises an array of HTS tapes arranged opposite each other, the HTS tapes extending parallel to each other and contacting each other through their respective faces, where each array of HTS tapes forms part of a respective pancake coil, which is part of a toroidal field (TF) coil, such as the TF coil 301 shown in FIG. 3A. This arrangement allows for efficient heat transfer between the HTS tapes, such that cooling the end of the HTS assembly 701 farthest from the axis of the central pillar 600 can cool the other end of the HTS assembly 701 through the intervening HTS tapes.

[0064] The use of HTS assemblies ("cables") without twists or transpositions in fusion-scale HTS magnets is controversial. However, these features are nominally inherited from LTS cables for fusion magnets to minimize AC losses and ensure equal current sharing between tapes. However, the relatively large size of coated REBCO conductors means that twist pitches are long and loss reduction is practically minimal. Conversely, the increased thermal stability afforded by operation at high temperatures means that stable operation of large coils without twists or transpositions is feasible. The choice of a stacked tape design (as in the HTS assembly 701 described above) also allows for a 3-5 times higher critical current to be achieved by better aligning the REBCO ab plane with the local magnetic field vector, as is possible in the TF central pillar 600 described above.

[0065] The cooling channel 705 is provided at the radially outermost end of the windings 602, i.e., the central post 600 is positioned such that the windings 602 are provided between the axis of the central post 600 and the cooling channel 603. In this example, the faces of the HTS assembly 701 together form one of the walls of the cooling channel 705, so that when a cryogenic fluid (such as supercritical helium) flows through the cooling channel 705, the fluid contacts the radially outermost surface of the HTS tape and can preferentially cool the radially outermost surface of the HTS tape.

[0066] 6 has the same radius as central pillar 400 of FIG. 4, but has windings 602 that occupy a significantly smaller area, at least in part because cooling channels 705 are provided outside of windings 602. Thus, windings 602 shown in FIGS. 6 and 7 have a significantly higher winding current density (J) than windings 402 of FIG. 4 with CICC-type HTS assembly 402. wp Approximately 350A / mm 2) can be provided. Furthermore, a greater proportion of the central pillar 600 can be used for nuclear shielding 604, which reduces the nuclear heating rate when the tokamak is operating, reducing damage to the central pillar 600 and reducing the risk of neutron-induced degradation of the critical currents in the HTS tapes of the HTS assembly 601. The reduced nuclear heating due to the thick neutron shielding material 604 also means that the radially inner portion of the HTS assembly can be cooled by conduction cooling through the support member 613, for example, by surrounding the support member 613 with a ring of flowing supercritical helium, as described below with reference to FIG. 8. Furthermore, by locating the cooling channels outside the windings 602, the mechanical integrity of the windings 602 remains high, thereby eliminating the need for a thick, high-strength jacket (i.e., support structure) around each HTS assembly 701, thereby allowing the HTS tapes to occupy more space and increasing the thermal conductivity of the HTS assembly 601.

[0067] 8 is an axial cross-section through (half of) a segment of an exemplary central pillar 800, similar to central pillar 600 of FIG. 6, except that the support member includes a radially inner portion 801A, which may be made from an ICONEL® alloy (for example), to withstand the high mechanical loads on the central pillar when the toroidal field coils are in operation. The support member also includes a radially outer portion or “side bar” 801B, which may be made from copper, such as hard copper, that extends across a winding section 802 comprising a stack of six HTS assemblies 802A, 802B, 802C (only three of which are shown in FIG. 8), similar to HTS assembly 701 described in connection with FIG. 7. In this example, HTS assemblies 802A, 802B, 802C are three pancake coils (substantially straight portions of) arranged in a stack, each pancake coil comprising an HTS tape (e.g., HTS tape 100 described above in connection with FIG. 1) that includes multiple layers of HTS material.

[0068] Central pillar 800 also differs from central pillar 600 of Figure 6 in that "internal" cooling channels 805 are contained within side bars 801B. Cooling channels 805 extend in a direction parallel to the axis of central pillar 800, i.e., into the plane of the paper in Figure 8. This configuration allows side bars 801B to be cooled from the inside by cryogenic fluid flowing within cooling channels 805.

[0069] Of course, more than one internal cooling channel 805 can be provided within side bar 801B, and the number and / or density of cooling channels 805 and / or the cross-sectional area of ​​channels 805 can be varied to change the temperature distribution within central column 800 so as to make the critical current of the HTS tapes within HTS assemblies 802A-802C more uniform.

[0070] During tokamak operation, a toroidal magnetic field is generated by the circulation of current around the windings of the pancake coils comprising HTS assemblies 802A-802C (and the pancake coils of the corresponding other segments of central pillar 800, not shown in FIG. 8). The magnetic field varies radially across central pillar 800, beginning at zero on axis A-A' of central pillar 800 and increasing approximately linearly across each of HTS assemblies 802A-802C (i.e., from left to right in FIG. 8).

[0071] Because the HTS assemblies 802A-802C extend generally radially inward (i.e., in a direction having at least a component toward the axis of the central post 800) by different amounts, the HTS tapes of the HTS assemblies 802A-802C experience magnetic fields of different strengths. Because the HTS tapes are all aligned parallel to one another in this example, the angle of the magnetic field at each HTS tape also differs depending on which HTS assembly 802A-802C the HTS tape belongs to. For example, the magnetic field alignment for the HTS tape of HTS assembly 802A, located toward the center of the segment (i.e., toward the bottom of FIG. 8), is more favorable for superconductivity than the magnetic field alignment for the HTS tape of HTS assembly 802C, located closest to sidebar 801B. The combined effect of the different magnetic field strengths and alignments means that the critical temperatures of the HTS assemblies 802A-802C are different. For example, HTS assembly 802A, which has more favorable magnetic field alignment and an overall lower magnetic field strength throughout HTS assembly 802A, may have a critical temperature of approximately 40 K, while the other two HTS assemblies 802B-802C may have lower critical temperatures of approximately 37 K and 32 K, respectively.

[0072] Figure 9 shows the results of Monte Carlo N-particle transport (MCNP) simulations and thermal finite element analysis (FEA) of the temperature distribution in the central column 800 after pulsed operation of the tokamak as a fusion reactor, overlaid on a segment of the central column 800 of Figure 8, taking into account active cooling by supercritical helium flow. The cooling channels 805 are omitted from Figure 9 for clarity. Prior to the fusion pulse, each HTS assembly 802A-802C is cooled to approximately 20 K. During a 35 MW fusion pulse, approximately 50 kW of heat is transferred to the central column 800, raising the temperature of each HTS assembly 802A-802C to approximately 35 K (HTS assembly 802A), 33.5 K (HTS assembly 802B), and 31 K (HTS assembly 802C), respectively. Simulations show that the nuclear heat load varies radially across central pillar 800, with the highest nuclear heat load found to occur at the radially outermost ends of HTS assemblies 802A-802C and decreasing by approximately a factor of two near the axis of central pillar 800. However, the temperature varies in the opposite direction with the location of cooling channel 805, as more heat flows into this channel and is removed from the components closest to the channel to the helium coolant.

[0073] Alternatively or additionally, an "external" cooling channel can be provided outside the side bar 801B, spanning both the winding section 802 (i.e., the face of the HTS assemblies 802A-802C) and the face of the side bar 801B, with one wall of the cooling channel formed at the radially outermost face of the side bar 801B and the HTS assemblies 802A-802C. This configuration allows these faces of the side bar 801B and the HTS assemblies to be cooled by a cryogenic fluid flowing through the cooling channel. In one example, the cooling channel can extend continuously around the central column 800, forming an annulus surrounding each segment of the HTS assemblies 802A-802C and the side bar 801B. In use, supercritical helium flows through the cooling channel to directly cool the side bar 801B and the HTS assemblies 802A-802C. That is, supercritical helium (or other cryogenic fluid) can contact and cool each surface of side bar 801B and HTS assemblies 802A-C. Specifically, the surface of side bar 801B that contacts supercritical helium can be continuous with the rest of side bar 801B, with no interfaces within side bar 801B between different regions of side bar 801B, to ensure high thermal conductivity.

[0074] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to those skilled in the art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.

Claims

1. 1. A central pillar for a tokamak plasma chamber, comprising: a toroidal field coil comprising first and second high temperature superconductor (HTS) assemblies each comprising one or more HTS tapes for conducting electrical current parallel to the axis of the central post, each HTS tape comprising an HTS material having an associated critical current that is dependent on a magnetic field in the HTS tape when the toroidal field coil is in use; a cooling mechanism comprising one or more cooling channels extending in a direction parallel to the axis of the central post, the cooling mechanism configured to preferentially cool the first HTS assembly relative to the second HTS assembly to reduce or eliminate a difference in critical current of the or each HTS tape of the first HTS assembly relative to the critical current of the or each HTS tape of the second HTS assembly; a support member having one or more channels, the first and second HTS assemblies being disposed within the one or more channels of the support member, the support member having a body portion made of a thermally conductive material extending radially across the first HTS assembly, the one or more cooling channels of the cooling mechanism being within the body portion; A central pillar with a

2. 2. The central post of claim 1, wherein the critical current of each HTS tape is inversely dependent on the strength of the magnetic field in the HTS tape such that as the strength of the magnetic field increases, the critical current decreases, and the strength of the magnetic field in the first HTS assembly is greater than the strength of the magnetic field in the second HTS assembly.

3. 3. A central post according to claim 1 or 2, wherein each HTS tape has an associated plane defined with respect to the crystalline structure of the HTS material of said HTS tape, wherein the critical current of each HTS tape depends on the field angle between the magnetic field at said HTS tape and the plane of said HTS tape, said critical current decreasing as said field angle increases, and wherein said HTS assemblies are arranged such that the field angle between the magnetic field and the plane of the or each HTS tape of said first HTS assembly is greater than the field angle between the magnetic field and the plane of the or each HTS tape of said second HTS assembly.

4. 4. The central post of claim 3, wherein for each HTS assembly, the respective planes of the HTS tapes of the HTS assembly are parallel to each other.

5. 5. The central post of claim 4, wherein a plane of the HTS tape of the first HTS assembly is parallel to a plane of the HTS tape of the second HTS assembly.

6. 6. A central post according to any one of claims 1 to 5, wherein a thermal impedance between the or each cooling channel and the first HTS assembly is less than a thermal impedance between the or each cooling channel and the second HTS assembly.

7. 7. A central post according to any one of claims 1 to 6, wherein the shortest distance between the or each cooling channel and the first HTS assembly is less than the shortest distance between the or each cooling channel and the second HTS assembly, each distance being measured in a plane perpendicular to the axis.

8. The central post of claim 1 , wherein the thermally conductive material comprises copper.

9. 9. The central pillar according to claim 1, wherein the cooling mechanism is configured to cool the body portion of the support member through a face of the body portion, the body portion being in contact with the first HTS assembly through one or more walls of the or each channel of the support member in which the first and second HTS assemblies are provided, whereby the first HTS assembly is cooled by the body portion.

10. A central pillar described in any one of claims 1 to 9, wherein the main body portion of the support member is in contact with the first HTS assembly through one or more walls of the channel or channels of the support member in which the first and second HTS assemblies are provided, thereby causing the first HTS assembly to be cooled by the main body portion.

11. 11. The central pillar of claim 1, wherein at least a portion of the second HTS assembly is located radially inward of the first HTS assembly, the portion being in thermal contact with the body portion, whereby heat is transferred from the portion of the second HTS assembly to the cooling mechanism via the body portion.

12. The central pillar according to claim 1 , wherein the support member comprises another part located radially inside the main body part and having a higher mechanical strength than the main body part.

13. 13. The central post of claim 1, further comprising a winding portion comprising the first and second HTS assemblies, the support member extending radially across a side of the winding portion.

14. 14. A tokamak plasma chamber comprising the central pillar of any one of claims 1 to 13, further comprising a plurality of toroidal field coils configured to provide a toroidal magnetic field within the tokamak plasma chamber when current flows around windings of the toroidal field coils, each toroidal field coil comprising a respective first and second HTS assembly.

15. 15. The method of operating a tokamak plasma chamber of claim 14, further comprising the steps of: passing a current around a winding of the toroidal field coil; using the cooling mechanism to preferentially cool the first HTS assembly relative to the second HTS assembly to reduce or eliminate a difference in critical current of the or each HTS tape of the first HTS assembly relative to the critical current of the or each HTS tape of the second HTS assembly; A method comprising:

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