CENTRAL COLUMN DESIGNS FOR TOKAMAKS

Abstract

There is described a central column for a toroidal field coil of a tokamak plasma chamber. The central column comprises a support member extending vertically along a central axis of the central column made from material with high compressive strength, and current carrying elements located radially outward of the support member. The support member is in structural contact with the current carrying elements so as to support them radially against compressive Lorentz stress.

Claims

1. A central column for a toroidal field coil of a tokamak plasma chamber, the central column comprising: a support member extending vertically along a central axis of the central column made from material with high compressive strength; and current carrying elements located radially outward of the support member; wherein the support member is in structural contact with the current carrying elements so as to support them radially against compressive Lorentz stress.

2. A central column according to claim 1, wherein the support member is tubular.

3. A central column according to claim 2, and comprising a coolant channel within the support member.

4. A central column according to claim 3, and comprising electrical insulation around each current carrying element, and cooling ribs in thermal contact with the electrical insulation, wherein the cooling ribs extend through the support member into the coolant channel.

5. A central column according to claim 1, wherein the support member comprises steel, Inconel and/or titanium.

6. A central column according to claim 1, wherein the support member comprises a carbon composite.

7. A central column according to claim 1, wherein the support member comprises ceramic.

8. A central column according to claim 7, wherein the ceramic has neutron shielding properties.

9. A central column according to claim 8, wherein the ceramic comprises tungsten carbide.

10. A toroidal field coil for a tokamak, the toroidal field coil comprising a central column according to claim 1.

11. A tokamak comprising a toroidal field coil according to claim 10.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a cross section of a conventional central column;

[0019] FIG. 2A is a graph of the stress in the central column of FIG. 1;

[0020] FIG. 2B is a graph of the stress in a central column with a support; and

[0021] FIG. 3 is a cross section of a central column according to an exemplary construction.

DETAILED DESCRIPTION

[0022] One issue which must be considered in a tokamak is the stress on the superconducting core. This stress arises from the Lorentz force acting on the conductors. In the centre column the conductors carry axial current in an azimuthal magnetic field, and hence experience a force acting towards the centre of the column. These Lorentz forces act radially to compress the centre column, leading to compressive hoop and radial stresses. The return limbs are also subject to forces acting in the direction of the axis of the column. Where these are transferred to the centre column, they can produce axial tension in it. However, these axial forces arising from the return limbs can be supported by external structures.

[0023] In a spherical tokamak, there is no space for coolant channels in the current carrying elements (core conductors). These are therefore made from copper with embedded HTS tapes, as described in PCT/GB2017/053065. A high fraction of copper is needed for quench protection (i.e. to temporarily carry the current if one or more HTS tapes quench, as described in PCT/GB2017/053065). The conductors may be electrically insulated from one another with material sufficient to withstand voltages induced during a quench. Heat deposited in these conductors (e.g. from neutron heating, current sharing around cracks in the HTS, or during the onset of a quench) is conducted radially through the turns to an external annulus 104 carrying cryogenic coolant, for example, helium gas at high pressure (typically 15 bar) and low temperature (eg: 15 K), as previously discussed and shown in FIG. 1. The central column is thus primarily made from copper (in particular, the cooling ribs and current carrying elements are primarily copperin superconducting central columns the superconducting material is normally provided within copper cladding for thermal and electrical stabilisation).

[0024] The yield stress of copper depends on its purity and the degree of work hardening. Since the magnetic field in the central column is high, the resistivity of copper is dominated by magnetoresistance, and little advantage is gained by using a high purity copper, as often specified for use in magnets. In the following description it is therefore assumed that fully hard copper is used. This has a yield stress of approximately 350 MPa. Allowing a reasonable safety margin, the maximum acceptable limit for stress in the centre column is therefore roughly 250 MPa.

[0025] FIG. 2A shows a plot of radial (.sub.r), hoop (.sub.), and Von Mises stress in the centre column as a function of radius for a notional spherical tokamak generating 3.5 T at 2 m major radius. The centre column must carry 35 MA to generate this field. The inner radius of the superconducting core is assumed to be 120 mm. This value is chosen because it becomes hard to package the tapes into the narrow sectors if a lower inner radius is chosen, so we are forced to accept this rather large hole in the core for practical assembly reasons. The superconducting core is modelled as 100% copper taking Young's modulus as 130 MPa and 0.3 for Poisson's ratio. Assuming a uniform current density of 90 A/mm.sup.2 the outer radius of the core is 370 mm. This current density is chosen by quench protection considerations.

[0026] FIG. 2A is a plot based on the arrangement of FIG. 1, where the compressive radial stress is supported by the copper of the column itself. It peaks at 450 MPa on the inside radius. Since the yield stress of full-hard copper is 350 MPa this is unacceptable.

[0027] FIG. 2B shows a similar plot for an arrangement in which a 100 mm thick high strength steel (eg: heat treated Inconel 718) support tube or rod is inserted into the central hole. This has yield stress 1200 MPa, and the Young's modulus and Poisson's ratio for the support tube were taken to be 200 MPa and 0.3 respectively. With the support tube or rod, the peak stress in the copper falls to 200 MPa, which is acceptable for full hard copper.

[0028] FIG. 3 shows a central column 300, similar to that shown in FIG. 1, but including additional support. The central column 300 includes current carrying elements 301, neutron shielding 302, electrical insulation 303, coolant channels 304, and cooling ribs 305 which are equivalent to those in FIG. 1 except as noted below. A support member 306 is provided along the central axis of the central column. The support member 306 could be formed as a tube (as shown in FIG. 3) or a rod, and is made from a strong material such as steel, Inconel, or titanium so as to bear the stresses in the central column.

[0029] The support member 306 is in direct contact with the inner surfaces of the current carrying elements 301. The purpose of the support member is primarily to reduce the hoop and radial stresses in the centre column. The current carrying elements all generate forces acting towards the central axis of the column. These accumulate radially, such that the radial and hoop stresses increase as the distance to the central axis reduces. The addition of a central tube (or rod) of strong, relatively stiff material allows some of this load to be taken by a material chosen for its high structural strength, rather than for its thermal and electrical conductivity.

[0030] The fact that the stress in the support element 306 is entirely compressive also means that, in addition to (or instead of) a metal such as steel, Inconel or titanium, a carbon composite or ceramic could be used to form the central support member. Although ceramics are not reliable in tension, they can be used in compression. This potentially makes it possible to use a material such as tungsten carbide which also has neutron shielding properties. Although the primary neutron shield 302 is around the outside of the central column, it can still in some circumstances be helpful to prevent neutrons which have passed through the primary neutron shield and the conductors in one side of the central column from interacting with conductors on the other side. For a ceramic the concept of yield strength does not normally apply: in general (for any material) the most important feature is a high compressive strength.

[0031] As mentioned above, the support member 306 may be formed as a solid rod, or alternatively as a tube as shown in FIG. 3. Where the support member 306 is formed as a tube, the bore of the tube may be used as a supplementary cooling channel 304b. This is only practical where the support member is formed of a material having a high thermal conductivity. As a possible alternative, the cooling ribs 305 could be at least partially be extended through the support member 406 to achieve direct thermal contact with the coolant in the coolant channel 304b. The provision of such an arrangement would require balancing the need for thermal transfer with the provision of a sufficiently strong support element.

[0032] Similar constructions may be used in central columns which lack insulating layers 303 and/or cooling ribs 305.