High temperature superconducting combined function magnet for carbon-ion radiotherapy
Baird, Yvonne Turid Eiking
With an increase in cancer cases seen year on year, developing and improving treatment forms becomes increasingly important. Treatment of cancerous tumours using carbon-ion radiotherapy has shown favourable results compared to conventional treatment methods. However, enabling this treatment form to become more widely available on a world-wise basis remains a key challenge, due to the facility size and cost. In recent years, material and manufacturing developments of superconductors has improved the performance of high temperature superconductors (HTS) while their price has come down. This has allowed more compact accelerator and gantry magnets to be designed. The inherent characteristics of second generation (2G) HTS enable a drastic increase in the current density and magnetic field generated, compared to normal conductors and low temperature superconductors (LTS). For future superconducting applications, HTS has therefore been recognised as the solution. This thesis aims to address the issue of magnet size and associated costs, by employing 2G HTS in the design of a compact magnet through a layer-by-layer design algorithm. This new design resulted in a reduction of 26.3% of superconducting material used compared to other known designs, thus a cost saving of $115835. The thesis begins by presenting a brief overview of particle therapy, and the application of superconducting technology in these facilities, followed by a concise review of superconductors. The important steps and design considerations to design a magnet are then presented. This includes cross-section and coil end design, yoke design, field quality, and field computation modelling. The thesis goes on to present the designed HTS combined function magnet, designed for use in carbon-ion radiotherapy. The six-layer combined function magnet realises both bending and focusing/defocusing components in each layer, thus utilising space and materials effectively. This resulted in a precise and compact magnet, which uses considerably less HTS material compared to other designs. A further size reduction was achieved by the yoke design and optimisation which followed, which utilised COMSOL Multiphysics and Magnet Infolytica to simulate the magnet and iron yoke, using the finite element method (FEM). FEM is implemented for both stationery and time-dependent simulations. An anisotropic homogeneous-medium bulk approximation is adopted with a power law E – J relationship to model the combined function magnet during magnet ramp. This allowed a comprehensive profile of the HTS tape blocks to be obtained, in addition to several important issues such as magnetisation and critical current of the superconducting coils to be addressed.