One of the major technological challenges inherent to the tokamak, a toroidal chamber inside which a high temperature plasma is magnetically confined to produce fusion reactions, concerns the very large Lorentz forces (of the order of tens of MN) acting on the set of coils devoted to the generation of the toroidal component of the magnetic field (the Toroidal Field Coils, TFCs). If not properly counteracted with reinforcing structural material or other solutions, these forces can lead to the rupture of the coil.
Drastically reducing the force acting on the TFCs would lead to a simpler, more reliable, and more economical tokamak design. The Lorentz force is proportional to the intensity of the current running in the coil, the magnetic field produced by the coil, and the angle between the current and the magnetic field vectors: F = I x B x cos(I x B). It follows that the intensity of this force can be modified if the various regions of the coil (and thus the direction of the coil current running through it) are tilted in the toroidal direction by appropriate angles.
Supported by a Finanziamento d'Ateneo 2016, the P.I. of the present proposal has written a code which finds the force-minimizing angle along the entire perimeter of the TFC, and has conducted a series of initial studies on the tilted TFC concept [Reference 1: R. Gatto, F. Bombarda, "The differentially-tilted toroidal field coil concept for tokamaks", Fusion Engineering and Design, 147, 2019 (https://www.sciencedirect.com/science/article/pii/S0920379619307082?dgci...)].
Additional funds would make it possible the continuation of this line of research. In particular, next steps would include a more definitive design of the tilted TFC, a study of the equilibrium and stability properties of the plasma confined inside the tilted TFC system, the proposal of operational scenarios which take advantage of the vertical field due to the tilting, and a thermo-mechanical stress analysis of the tilted TFC.
To our knowledge, nobody is at present working on the idea of differentially-tilted TFCs in the context of tokamak research. Doing that is however, to our judgment, very much appropriate in a period in which several research groups around the world are discovering the important benefits of increasing the toroidal magnetic field component in tokamaks. This interest is testified by several recent proposals of compact, high field devices, like for example the ARC tokamak proposed by the Massachusetts Institute of Technology, and the IGNITOR device, a tokamak under advanced design status, planned to be built and operated by an Italian-Russian governance. The tilted TFC system, if proved to be feasible, would attract the attention because of its ability to produce a much higher field for the same structural load on the coils, or the same field for a reduced stress load. Due to simultaneous ability to increase the amount of poloidal flux available to drive the plasma current, the adoption of tilted TFCs would lead to much more performing tokamaks with respect to conventional designs (extended duration of plasma discharges).
The computational studies proposed in the present research project, if successful, would need to be followed by an application phase in which prototypes of the tilted coil systems must be built, in scale, to verify the theoretical predictions. Such phase requires the involvement of a company specialized in the building of magnets, like for example the Ansaldo Superconductors (ASG) in La Spezia, a company that has been working in the field of tokamak devices for a long time (presently, the ASC is building the toroidal field coils of ITER).
In the most optimistic case, the tilted TFC concept could be adopted in the design of future experimental compact high-field tokamak devices.
Possible spin-offs of the proposed research could involve many fields outside fusion energy research. A tilted toroidal coil system produces both a strong toroidal field in the toroidal volume inside the chamber, and a strong vertical field inside the central bore of the torus. Two possible applications of the tilted toroidal coil system in isolation are therefore envisaged, each one exploiting one or the other of these two magnetic field components. The toroidal field can be used to store large amounts of energy under the form of magnetic energy. Since the field in this case has to be maintained for a long time, to avoid large resistive losses this application requires the adoption of superconducting coils. To have a uniform field, the torus in this case should be rather large. The strong vertical field present in the central bore of the torus can be used in the context of pulsed magnets, the objective of which is to generate the highest possible field in a small volume for a very short time (for example, for testing materials under extreme conditions). This application can be pursued with conventional resistive magnets. In the traditional approach, the high magnetic pressure is contained by the structural material located in the same region of the field, with obvious problems of space. In the tilted coil approach, on the contrary, the magnetic pressure of the vertical field is contained by the corresponding magnetic pressure of the toroidal field. The mechanical problems are thus transferred to the generation and containment of the toroidal field, and the tilted geometry leads to a much more favourable situation.