| Abstract |
Tokamak Energy Ltd is a private company targeting the delivery of fusion as a clean and safe energy source by 2030. The company aims to do this by combining spherical tokamaks, which are a type of magnetic confinement fusion device, with high temperature superconducting (HTS) magnets, which can deliver very strong magnetic fields in compact devices. The company believes that HTS spherical tokamaks are the key route to delivering commercial fusion energy on a rapid timescale. This project aims to address two key challenges in the field of HTS magnet technology, in order to accelerate the development of HTS magnets for fusion energy and other applications. The first challenge is to develop a technical and strategic approach towards the characterisation and quality assurance (QA) of HTS conductors, then implement this on several hundred kilometres of conductor procured over a period of several years. The key difficulty here is that the current capacity of rare-earth barium copper oxide (REBCO) HTS conductors is extremely large and has a very complex dependence on temperature, magnetic field strength, field direction and the crystal's nanostructure. Measurement of conductor performance under the end-use conditions in fusion magnets is extremely challenging due to the high magnetic fields and currents involved. Therefore, complete characterisation cannot be carried out routinely despite magnet designs relying crucially on their knowledge. This project will establish the necessary performance indicators (balancing cost, risk and depth of information), develop the methods required to measure them, and implement this on the real conductor as it arrives. The second challenge is the development of dismantlable coil structures for HTS fusion magnets. HTS magnets can be operated at relatively high temperatures (>~20 K) at which substantial heat loads from joints between conductors can be accommodated by cooling systems. Unlike conventional low temperature superconductors (LTS), HTS conductors operated at high temperatures are extremely thermally stable and can therefore tolerate substantial temperature variations of several degrees Kelvin around their structures. This enables dismantlable coil structures to be considered, in which the turns of the magnet can be connected and disconnected from one another during assembly and disassembly. This is an extremely attractive design option for tokamak magnets, where it is advantageous for some coils (e.g. poloidal field coils (PFs) ) to be threaded inside other coil sets (e.g. the toroidal field coils (TFs)). The wider assembly process for tokamaks is also greatly simplified if the coils are dismantlable, for example the assembly of vacuum chambers and neutron shields. Development of dismantlable coils is a multifaceted problem involving development of novel low resistance jointing methods, practical implementation methods in a tokamak assembly hall environment, and design of the wider magnet system to accommodate the joints (including insulation methods and magnet operating principles). |
