RU2772438C2 - Double poloidal field coils - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: double poloidal field coils comprise an internal and an external poloidal field coils and a controller. The internal poloidal field coil is configured to be installed inside a toroidal field coil of a tokamak. The external poloidal field coil is configured to be installed outside a toroidal field coil. The controller is configured to cause current supply to the internal and external poloidal field coils so that the total magnetic field generated by the internal and external poloidal field coils has the value of zero on the toroidal field coil. The tokamak is made spherical and comprises poloidal and toroidal field coils.

EFFECT: expansion of the range of technical means.

5 cl, 5 dwg

Description

Field of invention

The present invention relates to tokamak plasma chambers. In particular, the invention relates to the positioning of poloidal field coils relative to a toroidal field coil.

Background of the invention

The task of generating thermonuclear energy is very difficult. In addition to tokamaks, many alternative devices have been proposed, but none has yet yielded any results comparable to the best tokamaks currently in operation, such as the JET fusion reactor.

Worldwide fusion research has entered a new phase with the start of construction of the ITER reactor, the largest and most expensive (about 15 billion euros) tokamak ever built. A successful approach to building a commercial fusion reactor requires long pulse duration and stable operation, coupled with the high efficiency required to generate electricity economically. These three conditions are especially difficult to meet simultaneously, and the planned program will require many years of experimental research at the ITER reactor and other fusion facilities, as well as theoretical and technological research. The general consensus is that a commercial fusion reactor based on this approach will not be built until 2050.

In order to achieve the fusion reactions required for economical power generation (i.e. producing much more energy than the energy expended), a conventional tokamak would need to be so large (like the ITER reactor, for example) that the energy confinement time (which is approximately proportional to the plasma volume) could be large enough that the plasma could be hot enough for fusion to occur.

WO 2013/030554 describes an alternative approach using a compact spherical tokamak for use as a neutron or energy source. The low aspect ratio of the plasma shape in a spherical tokamak increases the confinement time of thermal energy and allows for useful power generation in a much smaller plant. However, a small diameter central column is needed, which makes it difficult to design the toroidal magnet required for plasma stability. To provide sufficient current density to produce the required high magnetic fields, superconducting magnets are used for at least the toroidal field (TF) coils of the spherical tokamak.

Superconducting materials are usually 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 using the BCS (Bardeen-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 HTS material is not described by BCS theory, and such materials may have critical temperatures above about 30 K (although it should be (Note that physical differences exist more in the operation and composition of superconducting materials than in the critical temperature that describes the HTSC material). The most widely used HTSCs are "cuprate superconductors", namely ceramics based on copper salts (compounds containing a group of copper oxides), such as BSCCO or ReBCO (where Re stands for a rare earth element, usually Y or Gd). Other HTS materials include iron pnictides (eg FeAs and FeSe) and magnesium diboride (MgB2).

The ReBCO compound is typically made into tapes with the structure shown in Figure 1. In general, such a tape 500 typically has a thickness of about 100 micrometers and includes a substrate 501 (typically an electropolished Hastelloy alloy with a thickness of approximately 50 micrometers), on which the IBAD method (ion beam deposition), magnetron sputtering, or other suitable method, deposit a series of buffer layers, known as buffer stack 502, with a thickness of about 0.2 micrometers. An epitaxial layer of HTS-ReBCO 503 (deposited by MOCVD or other suitable method) covers the buffer stack and is typically 1 micrometer thick. A layer 504 of silver 1-2 micrometers thick is deposited on the HTS layer by sputtering or other suitable method, and a layer 505 of copper stabilizer is deposited onto the tape 505, which often completely encapsulates the tape.

The substrate 501 provides a mechanical support that can be fed down the production line and allows the buildup of successive layers. The buffer stack 502 is required to provide a biaxially textured crystal template onto which the HTS layer is built up and to prevent chemical diffusion of elements from the substrate into the HTS that reduces its superconducting properties. The silver layer 504 is required to provide a low resistance interface between the ReBCO and the stabilization layer, and the stabilization layer 505 provides an alternative current path in the event that any part of the ReBCO loses superconductivity (goes to the "normal" state).

To form conductors with high current carrying capacity, HTS tapes can be positioned to form cables. There are several tapes in each cable, and the copper stabilizing layers of all the tapes are connected (usually by means of an additional copper coating). There are two general approaches to forming cables, namely, the HTS tapes can be crossed and/or twisted, or the cables can be stacked. Crossed or stranded cables are often used in AC or fast cycling electromagnets because this design greatly reduces the I/O loss for the electromagnet. Stacked cables are often used in magnets with a slow change in magnetic field, such as tokamak TC coils, as this allows the ribbons to be positioned relative to the local magnetic field in such a way as to maximize the critical current I _C .

The voltage across the length of the HTS tape depends on the transport current I in a highly non-linear way, which is usually parameterized as follows:

where E ₀ =100 nV/m is a predetermined critical current criterion and n is an experimental parameter that models the abruptness of the transition from superconducting to normal; n is typically in the 20-50 range for ReBCO. Depending on the value of n, the voltage can be neglected for values of I/I _C < ~0.8.

When the current in the tape reaches the critical current, the HTS tape loses its superconductivity. This can be done either by increasing the transport current I or by decreasing the critical current I _C . A decrease in the critical current can be caused by several factors, most significantly temperature, external magnetic fields and voltage. Reducing any of these factors will increase the stability of the HTS tape.

The essence of the invention

According to a first aspect of the invention, a poloidal field coil assembly is provided for use in a tokamak. The poloidal field coil assembly contains an inner and outer poloidal field coil and a controller. The inner poloidal field coil is configured to be installed inside the tokamak toroidal field coil. The outer coil of the poloidal field is configured to be installed outside the coil of the toroidal field. The controller is configured to cause current to be applied to the inner and outer poloidal field coils such that the total magnetic field generated by the inner and outer poloidal field coils is zero at the toroidal field coil.

According to a second aspect, a magnetic assembly is provided for use in a tokamak. The magnetic assembly comprises a poloidal field coil assembly according to the first aspect and a toroidal field coil containing a high-temperature superconductor. The inner and outer poloidal field coils are located inside and outside the toroidal field coil, respectively.

In accordance with the third aspect, a tokamak is provided, containing a toroidal plasma chamber and a magnetic assembly according to the second aspect.

Brief description of the drawings

Figure 1 is a schematic representation of an HTS tape;

Figure 2 - cross section of the tokamak in the poloidal plane;

Figure 3 is a diagram of the magnetic field generated by a coil of wire;

Figure 4 is a cross section of an exemplary tokamak; and

Figure 5 is an enlarged view of one of the poloidal field coil assemblies shown in Figure 4.

Detailed description

A cross section of one side of the tokamak is shown in Figure 2. Tokamak 200 comprises a toroidal plasma chamber 201, a toroidal field (TF) coil 220 having return branches 221 and a central column 222, and poloidal field coils 230, 231, 232, 233, 234, 235 (PP). The TF coil 220 provides a toroidal magnetic field within the plasma chamber 201. The FC coils perform various functions, for example, the compression fusion (CF) coils 230 provide the pulse to initiate the plasma, and the divertor coils 231, 232 extend the plasma zone 202 during tokamak operation. The PP coils can be roughly divided into two groups, of which the first group (which includes the KC 230 coils) is activated only for short periods, usually during plasma initialization, and the second group (which includes the 231 divertor coils) is activated during long periods during tokamak operation.

Each PP coil is usually structurally made in the form of a single conductor ring. The conductor can be either a superconductor or a normal conductor, depending on the properties required of the coil, for example, coils that carry alternating current will typically have high losses if they are made from a superconducting material, so normally conducting materials are preferred. Figure 3 is a cross-sectional view (along plane 300) of the magnetic field 301 generated by the loop of electrical wire 302. The same field pattern will be produced by each PCB coil. As can be seen, the field near the PP coil is relatively strong.

Figure 2 also shows that some of the PP coils of the tokamak are located near the TP coil. This means that the PP coils will apply an external magnetic field to the TC coil, which will reduce the critical current of the superconducting material in the TC coil.

To eliminate this effect, an alternative design is proposed. This design is shown in figure 4 for one set of diverter coils (equivalent to divertor coils 231 in figure 2), but can be extended to any of the PP coils with suitable corrections. Figure 4 depicts an exemplary tokamak 400 containing a toroidal plasma chamber 401 and a TP coil 420 containing a central column 421 and return legs 422. Tokamak 400 also includes upper and lower poloidal field coil assemblies 430, 440, each of which contains internal 431, 441 and external 432, 442 coils PP. Each element shown in Figure 4 is cylindrically symmetrical with respect to the central column 421. Within each node, the inner coil 431, 441 is located inside the coil TP 420 (i.e. between the coil TP 420 and the toroidal plasma chamber 401), and the outer coil PP 432, 442 is located outside the coil TP 420.

Figure 5 is an enlarged view of the upper poloidal field coil assembly 430, showing the magnetic field generated by the inner 431 and outer 432 PP coils during operation, in the poloidal plane (fields from other components in the tokamak are not shown to simplify the figure; but in the course of calculation and during operation, these fields will be included and taken into account). The PCB coil assembly 430 is designed such that the current in the inner and outer coils flows in the same direction ("inside the page" in the example). The magnetic field between the two coils is zeroed at point 502, which is located between the two coils at a distance that depends on the ratio of the currents of the two coils. Outside the two coils, in the far field 501, the magnetic fields generated by each coil will be mutually reinforcing. The current control of each coil is such that the far-field field 501 produced by the PV coil assembly is substantially the same as for the single equivalent field coil in Figure 2, and such that the ratio of the currents of the two coils results in a null at point 502 on the coil. TP 420. This means that the influence of the magnetic field of the PP coil assembly on the critical current of the TP coil is significantly weakened compared to the effect of an equivalent single PP coil.

The exact location of the zero point on the TC coil can be selected based on the design of the TC coil. For example, if there are any "hot spots" on the TC coil where less stability or a decrease in l _C , (for example, connections) would normally be expected, then the inner 431 and outer 432 coils of the PP can be positioned so that the zero point is located in the place of local overheating (for example, connections).

This design is most preferable for replacing PV coils that are constantly active during the operation of the tokamak, and which are located near the TC coils, for example, divertor coils. However, this design can also be used to replace other PP magnet coils.

The design can be applied either to a spherical tokamak or to a conventional high aspect ratio tokamak.

Claims (11)

1. A magnetic assembly for use in a tokamak, comprising: poloidal field coil assembly; a toroidal field coil containing a superconducting material; and controller, and: an internal poloidal field coil is installed inside the toroidal field coil; an external poloidal field coil is installed outside the toroidal field coil; and the controller is configured to cause current to be applied to the inner and outer poloidal field coils such that the total magnetic field generated by the inner and outer poloidal field coils is zero at the toroidal field coil. 2. A magnetic assembly according to claim 1, wherein each poloidal field coil contains a high temperature superconductor. 3. The magnetic assembly of claim 1, wherein the toroidal field coil comprises a junction, and wherein the inner and outer poloidal field coils are positioned such that the total magnetic field generated by the inner and outer poloidal field coils is zero at the junction. 4. Tokamak containing a toroidal plasma chamber and a magnetic assembly according to claim 1. 5. Tokamak according to claim 4, wherein the tokamak is a spherical tokamak.

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