Optimization of Poloidal Coil System for the Volumetric Neutron Source (VNS) Fusion Reactor

The Volumetric Neutron Source (VNS) is a facility designed to replicate DEMO-relevant conditions to advance Tritium Breeding Blanket (TBM) technology in fusion reactors. This study focuses on optimizing the superconducting Poloidal Field (PF) magnet system of VNS to achieve efficient and reliable plasma performance. The machine features a small plasma with a magnetic field strength of 5.4 T at a major radius of 2.53 m, characterized by an aspect ratio of 4.6. Due to the considerable separation between the plasma axis and the PF coils, which are primarily designed for shaping and stability, the magnet system must support exceptionally high current and stored magnetic energy. The study uses the MADE algorithm to analyze the relationship between size, shape, and performance of the six PF coils and the optional extra-vessel coils. To control plasma operation, two machine solutions were proposed in the pre-conceptual design phase: a PF-out-TF solution (conventional tokamak), where the long distance from the plasma center to the PF coils is mitigated by implementing ex-vessel Window Frame coils, and a PF-in-TF solution, a first-of-its-kind in a superconducting tokamak in which by bringing the PF coils closer to the vessel, plasma controllability is improved. The study concludes by providing insights into the performance and implications associated with each option. It highlights the trade-offs between the PF system in a compact tokamak and the figures of merit for plasma equilibrium.