Patent Description:
A tokamak features a combination of strong toroidal magnetic field, high plasma current and, usually, a large plasma volume and significant auxiliary heating, to provide hot, stable plasma. This allows tokamaks to generate conditions so that fusion can occur. The auxiliary heating (for example via tens of megawatts of neutral beam injection of high energy H, D or T) is necessary to increase the temperature to the sufficiently high values required for nuclear fusion to occur, and/or to maintain the plasma current.

A problem is that, because of the large size, large magnetic fields, and high plasma currents generally required, build costs and running costs are high and the engineering has to be robust to cope with the large stored energies present, both in the magnet systems and in the plasma, which has a risk of 'disrupting' - mega-ampere currents reducing to zero in a few thousandths of a second in a violent instability.

The situation can be improved by contracting the donut-shaped torus of a conventional tokamak to its limit, having the appearance of a cored apple - the 'spherical' tokamak (ST). The first realisation of this concept in the START tokamak at Culham, UK demonstrated a huge increase in efficiency - the magnetic field required to contain a hot plasma can be reduced by a factor of <NUM>. In addition, plasma stability is improved, and building costs reduced.

To obtain the fusion reactions required for economic power generation (i.e. much more power out than power in), a conventional tokamak would have to be huge so that the energy confinement time (which is roughly proportional to plasma volume) can be large enough that the plasma can be hot enough for thermal fusion to occur.

<CIT> describes an alternative approach, involving the use of a compact spherical tokamak for use as a neutron source or energy source. The low aspect ratio plasma shape in a spherical tokamak improves the particle confinement time and allows net power generation in a much smaller machine. However, a small diameter central column is a necessity, which presents challenges for design of the plasma confinement vessel and associated magnets.

The magnet coils on a tokamak can be divided into two groups. The poloidal field coils are horizontal circular coils wound with their centre lying on the central column of the tokamak, and produce a poloidal field (i.e. one which is substantially parallel to the central column). The toroidal field coils are wound vertically through the central column, and around the outside of the plasma chamber (the "return limbs") to produce a toroidal field (i.e. one which is circular around the central column). The combination of the poloidal and toroidal fields produces a helical field within the plasma chamber which keeps the plasma confined.

The electrical currents required in the toroidal field are very large. For a compact spherical tokamak, the central column should be as small in diameter as possible. This presents conflicting requirements, as the current density that can be achieved, even with superconducting materials, is limited.

The large electrical currents, in combination with the large magnetic fields in a tokamak generate large Lorentz forces that may deform or otherwise damage parts of the tokamak.

Superconducting materials are typically divided into "high temperature superconductors" (HTS) and "low temperature superconductors" (LTS). LTS materials, such as Nb and NbTi, are metals or metal alloys whose superconductivity can be described by BCS theory. All low temperature superconductors have a critical temperature (the temperature above which the material cannot be superconducting even in zero magnetic field) below about <NUM>. The behaviour of HTS material is not described by BCS theory, and such materials may have critical temperatures above about <NUM> (though it should be noted that it is the physical differences in superconducting operation and composition, rather than the critical temperature, which define HTS and LTS material). The most commonly used HTS 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, commonly Y or Gd). Other HTS materials include iron pnictides (e.g. FeAs and FeSe) and magnesium diboride (MgB<NUM>).

ReBCO is typically manufactured as tapes, which are generally approximately <NUM> microns thick, and include a substrate (typically electropolished hastelloy approximately <NUM> microns thick), on which is deposited by IBAD, magnetron sputtering, or another suitable technique a series of buffer layers known as the buffer stack of approximate thickness <NUM> microns. An epitaxial ReBCO-HTS layer (deposited by MOCVD or another suitable technique) overlays the buffer stack, and is typically <NUM> micron thick. A <NUM>-<NUM> micron silver layer is deposited on the HTS layer by sputtering or another suitable technique, and a copper stabilizer layer is deposited on the tape by electroplating or another suitable technique, which often completely encapsulates the tape.

The substrate provides a mechanical backbone that can be fed through the manufacturing line and permit growth of subsequent layers. The buffer stack is required to provide a biaxially textured crystalline template upon which to grow the HTS layer, and prevents chemical diffusion of elements from the substrate to the HTS which damage its superconducting properties. The silver layer is required to provide a low resistance interface from the ReBCO to the stabiliser layer, and the stabiliser layer <NUM> provides an alternative current path in the event that any part of the ReBCO ceases superconducting (enters the "normal" state).

Commercially available HTS tapes start to degrade at internal stresses larger than about <NUM> MPa.

It is an object of the present invention to provide a central column for a toroidal field coil that addresses, or at least alleviates, the problems described above.

According to a first aspect of the present invention there is provided an electromagnet comprising one or more channels. Each channel has provided therein a conductor element comprising one or more layers of superconductor material for conducting electrical current along an axis of the channel. The conductor element is arranged to contact sidewalls of the channel through respective first and second wedge surfaces that are inclined with respect to one another such that a force biasing the conductor element in a direction perpendicular to the axis (e.g. into the channel) generates opposing contact forces on the first and second wedge surfaces that act to compress the conductor element along a direction with a component perpendicular to the force.

The electromagnet may be configured so that stress generated during operation of the electromagnet does not exceed a threshold stress associated with the superconductor material. The threshold stress may be, for example, determined based on a degradation of the HTS material and/or a degradation of the HTS material's ability to function as a superconductor.

The or each channel may be an "open" channel such that the conductor element is not encircled by the walls of the channel. Alternatively, the channel may be "closed", such the walls of the channel surround the conductor element, e.g. the channel may be a through hole in the support member.

In some cases, the electromagnet may have a central axis, with the axes of the one or more channels being arranged parallel to the central axis. In this case, the one or more layers of superconductor material are arranged to conduct electrical current parallel to the central axis. The force biasing the conductor element in a direction perpendicular to the axis may be directed towards the central axis.

Each conductor element may comprise one or more stacks of High Temperature Superconductor, HTS, tapes, each HTS tape including a layer of HTS material extending along the channel. The HTS material may be ReBCO.

Each conductor element may comprise a first wedge member provided between the stack of HTS tapes and the first sidewall of the channel, the first wedge surface being provided on the first wedge member. The first wedge member may be fixed to the stack of HTS tapes.

The first wedge member may comprise a surface in contact with the stack that is substantially perpendicular to the layers of HTS material.

For one or more of the conductor elements, an acute angle between the first wedge surface and the surface in contact with the stack may be greater than <NUM> degree, greater than <NUM> degrees, or greater than <NUM> degrees.

Each conductor element may comprise a second wedge member provided between the one or more stacks of HTS tapes and the second sidewall of the channel, the second wedge surface being provided on the second wedge member.

A static friction coefficient between each of the wedge surfaces of the conductor element and the respective sidewalls of the channel may be from <NUM> to <NUM>, or greater than <NUM>, or greater than <NUM>.

Each of the conductor elements may be potted.

The electromagnet may further comprise a support member, the channels being provided in the support member. The support member may comprise one or more through holes extending in a direction having a component parallel to one or more of the channels. The one or more through holes may be located adjacent to one or more of the sidewalls of the channel. The one or more through holes may be used for cooling the electromagnet.

According to a second aspect of the present invention there is provided a central column for a toroidal field coil of a tokamak plasma chamber. The central column comprises a plurality of electromagnets according to the first aspect of the present invention. The channels are spaced around a central axis and the one or more layers of superconductor material are arranged for conducting electrical current parallel to the central axis. The force is a radial force biasing the conductor element towards the central axis.

The channels may be provided in a single support member.

The central column may comprise one or more holes through the or each support member, each hole extending parallel to the central axis. The one or more holes may be located radially outwards of a radially inner edge of the conductor element. The holes may be used for cooling the central column, e.g. by flowing a cooling fluid through the holes. Despite the one or more holes, the toroidal field coil(s) may be operated without causing the central column to deform as a result of lower internal stresses in the central column compared to known designs of central columns.

The or each support member may comprise one or more removable angular segments, each angular segment comprising one or more of the plurality of channels.

Also described herein as an example a toroidal field coil for a tokamak, the toroidal field coil comprising a central column according to the second aspect Also described herein is a tokamak comprising the toroidal field coil.

<FIG> shows a vertical cross section through a spherical tokamak <NUM> comprising toroidal field coils <NUM>, poloidal field coils <NUM> and a toroidal plasma chamber <NUM> located within the toroidal field coils <NUM>. The tokamak <NUM> also comprises a central column <NUM>, which extends through the centres of the plasma chamber <NUM> and the toroidal <NUM> and poloidal <NUM> field coils. Each of the toroidal field coils <NUM> comprises a straight section <NUM> that extends along the axis A-A' of the central column <NUM> and a curved section <NUM> that is electrically connected to either end of the straight section <NUM> to form a closed loop.

<FIG> shows an axial cross section of the central column <NUM> viewed looking along the axis A-A'. The tokamak <NUM> in this example comprises <NUM> toroidal field coils <NUM> and the respective straight portions <NUM> of each of the toroidal field coils <NUM> are angularly spaced about the axis A-A' of the central column <NUM> in an equiangular arrangement. The central column comprises a support member <NUM> made from copper (although other metals and/or alloys can be used) that extends along the axis A-A' and which has a plurality of channels <NUM> in which the straight sections <NUM> of the toroidal field coils <NUM> are housed. The straight portions <NUM> each comprise a stacked arrangement of lengths of HTS tape extending along the central column <NUM> and are arranged to contact one another through their respective faces. The HTS layers of the tapes run parallel to the axis A-A'.

<FIG> is an axial cross section of an angular segment of the central column <NUM> showing one of the straight portions <NUM> of one of the toroidal field coils <NUM>. In this example, the straight portion <NUM> comprises six stacks of HTS tapes <NUM> A-F with four inner stacks 201B-E having the same thickness as one another (i.e. the same length in a direction along a radial line B-B' of the central column <NUM> which passes through the centre of the straight portion <NUM> and is transverse to the tapes) provided between two outer stacks 201A, F, which each have a smaller thickness than the inner stacks 201B-F. The stacks 201A-F are bonded to one another with their radially outermost edges aligned. Each channel <NUM> has a cross sectional profile that corresponds to the outline (i.e. the shape of the perimeter in the cross-section of the channel <NUM> perpendicular to line A-A') of the corresponding straight section <NUM> of the toroidal field coil <NUM>.

In use, the stacks of HTS tape 201A-F carry axial electrical current in an azimuthal magnetic field, and hence experience a Lorentz force <NUM> acting towards the centre of the column <NUM> (i.e. along the radial direction indicated by the broken line B-B'). These Lorentz forces act radially on the HTS tapes 201A-F to compress them against the support member <NUM> of the central column <NUM>, thereby creating contact forces <NUM> between the radially innermost edges of the stacks of HTS tape 201A-F and the walls of the channel <NUM>. The combination of the Lorentz force <NUM> and the contact forces <NUM> therefore generates compressive stress within the HTS tape 201A-F that may damage the HTS tapes 201A-F or otherwise decrease their ability to carry large electrical currents.

<FIG> is similar to <FIG> except that is shows a central column <NUM> in which wedge members 301A-D are fixed to the outermost faces of the stacks 201A, B, 201E, 201F, to form circumferentially-outer portions of the straight portions <NUM> of the toroidal field coils <NUM>. The channels <NUM> through the support member <NUM> are enlarged to accommodate the increased size of the straight portions <NUM>. The wedge members 301A-D in this example are triangular prisms that extend with translational symmetry parallel to the axis A-A'. The triangles which form the cross-section of the wedge members may be substantially right angled triangles, and the wedge members may be arranged such that the largest, outer face of the each prism (i.e. the hypotenuse of the triangular cross-section) is directed away from the HTS tapes 201A-F, with the narrower (pointed) end of the prism being located radially inwards of the wider end of the prism (i.e. the side of the triangle furthest from the most acute angle of the triangle). That is, the wedge members 301A-D taper radially inwardly along direction B-B'. Each of the channels <NUM> also tapers radially inwardly so that the sidewalls of the channel <NUM> are parallel to the corresponding outer faces of the wedge members 301A-D. Indeed the sidewalls of the channel <NUM> lie against the corresponding outer faces of the wedge members 301A-D. In this example, the wedge angle <NUM> (the interior angle between the two largest faces of the wedge member 301A-D, at the narrower end of the prism) is <NUM> degrees.

In use, the Lorentz force <NUM> acting radially on the straight section <NUM> of the HTS tapes 201A-F generates contact forces <NUM> between the outer face of the wedge members 301A-D and the sidewalls of the channel <NUM>. The contact forces <NUM> are oriented normal to the outer face of the wedge members 301A-D, i.e. the contact forces <NUM> have a radial component (directed along the radial axis B-B') and a circumferential component (directed towards the radial axis B-B'). The wedge members 301A-D therefore provide a mechanism whereby radial stress within the central column <NUM> is exchanged for hoop stress, i.e. the radial stress within central column is decreased while the hoop stress increases. This trade-off allows the stacks of HTS tapes 201A-F to conduct higher currents without causing mechanical damage to the HTS tape.

<FIG> shows a graphical plot of the results of a finite element method (FEM) calculation of the radial stress generated within the stacks of HTS tapes 201A-F when the tokamak is operated with a current of <NUM> MA passing through the central column <NUM>. This current leads to a peak magnetic field of <NUM> T at the radially outermost edges of the HTS tapes 201A-F. The magnetic field decays monotonically away from the central column <NUM> and has a magnitude of <NUM> T at a distance of <NUM> from the central column <NUM>. The plotted results show the radial stress in MPa (vertical, Y axis) in the coils as a function of the friction coefficient (horizontal, X axis) between the outer face of the wedge members 301A-B and the corresponding sidewalls of the channel <NUM>.

The two uppermost sets of data points <NUM>, <NUM> (shown as filled circles) indicate the calculated radial stress in the outer stacks 201A, F for wedge angles of <NUM> degrees and <NUM> degrees respectively (the same wedge angle being used for each wedge member 301A-D). These results show that the radial stress is lower for the larger wedge angle and that for both angles, increasing the friction coefficient reduces the radial stress monotonically by progressively smaller amounts. Similar results (not shown) are obtained for other angles in the range from <NUM> degrees to <NUM> degrees, with the calculated radial stresses having values that are intermediate between the radial stresses for the sets of data points <NUM>, <NUM>. Angles greater than <NUM> degrees can also be used, e.g. angles up to any one of <NUM> degrees, <NUM> degrees, <NUM> degrees or <NUM> degrees. Depending on the desired radial stress, the friction coefficient may be varied from <NUM> to <NUM>, or from <NUM> to <NUM>, or greater than <NUM>, e.g. up to <NUM>.

The two lowermost sets of data points <NUM>, <NUM> (shown as filled triangles) indicate the calculated radial stress in the inner stacks 201B-E for wedge angles of <NUM> degrees and <NUM> degrees respectively (the same wedge angle being used for each wedge member 301A-D). The calculated radial stress decreases monotonically with increasing friction coefficient for the wedge angle of <NUM> degrees (data points <NUM>), but reaches a minimum value at a friction coefficient of <NUM> for the wedge angle of <NUM> degrees. Calculations of the radial stress for wedge angles of <NUM>, <NUM> and <NUM> degrees provide values that are generally between the values obtained for wedge angles of <NUM> and <NUM> degrees.

A target maximum radial stress of <NUM> MPa in the outer stacks of HTS tape 201A,F can be achieved using a friction coefficient of <NUM> and wedge angles greater than <NUM> degree.

<FIG> show contour plots of the calculated radial stress and the calculated Von Mises stress within an axial cross section of the central column <NUM>, calculated for a wedge angle of <NUM> degrees and a friction coefficient of <NUM>. The calculation is performed for one half of the angular segment shown in <FIG> for computational efficiency, with boundary conditions applied to enforce the symmetry of the FEM solution with respect to the symmetry of the central column <NUM>.

The radial stress within the stacks of HTS tapes <NUM> is calculated to be less than <NUM> MPa. The maximum values of the radial stress for each of the stacks of HTS tape 201A-C are located at the radially innermost faces of the stacks with calculated values of <NUM> MPa for stack 201A, <NUM> MPa for stack 201B and <NUM> MPa for stack 201C.

The calculated Von Mises stress in the support member <NUM> is mostly below about <NUM> MPa adjacent to the wedge members 301A, B. This is sufficiently low that optionally cooling holes can be provided in this part of the central column <NUM>, i.e. between the channels <NUM> and extending parallel to the axis A-A', without weakening the central column excessively.

Although the above description has focussed on a central column comprising HTS tape, other forms of HTS material such as wires can also be used or LTS materials can be used instead of, or as well as, HTS material. The HTS tapes may be "potted", e.g. by encasing the stack of HTS tapes in solder.

Claim 1:
An electromagnet comprising one or more channels (<NUM>), each channel having provided therein a conductor element (<NUM>) comprising one or more layers of superconductor material for conducting electrical current along an axis (A-A') of the channel, the conductor element being arranged to contact sidewalls of the channel through respective first and second wedge surfaces that are inclined with respect to one another such that a force (<NUM>) biasing the conductor element in a direction (B-B') perpendicular to the axis (A-A') generates opposing contact forces (<NUM>) on the first and second wedge surfaces that act to compress the conductor element along a direction with a component perpendicular to the force.