High temperature superconductor accelerator magnets

Abstract

For future particle accelerators bending dipoles are considered with magnetic fields exceeding 20T. This can only be achieved using high temperature superconductors (HTS). These exhibit different properties from classical low temperature superconductors and still require significant research and development before they can be applied in a practical accelerator magnet. In order to study HTS in detail, a five tesla demonstrator magnet named Feather-M2 is designed and constructed. The magnet is based on ReBCO coated conductor, which is assembled into a 10 kA class Roebel cable. A new and optimized Aligned Block layout is used, which takes advantage of the anisotropy of the conductor. This is achieved by providing local alignment of the Roebel cable in the coil windings with the magnetic field lines. A new Network Model capable of analyzing transient electro-magnetic and thermal phenomena in coated conductor cables and coils is developed. This mode is necessary to solve critical issues in coated conductor accelerator magnets, such as thermal stability, quench propagation and field quality. The electrical part of the Network Model is validated using a series of benchmark simulation tests and measurements. The model is then used to calculate the dynamic field quality in coil structures. It is concluded that dynamic compensation of the field quality is needed. This can be achieved by a series of persistent current shim coils that are inserted inside the aperture, which is a novel concept introduced in this thesis. The thermal part of the Network Model is validated by comparing it to an earlier experimentally validated one-dimensional single tape model. Using this a quench is modeled in a section of Roebel cable and later in the full Feather-M2 coil. Is it shown that the initialization of the quench consists of three phases: Thermal Drift, Pre-Quench and Quench. The Drift phase can be detected using temperature sensors and the Pre-Quench phase using an array of pick-up coils. These quench detection techniques, provided sufficient margin in maintained, could allow for safe operation of HTS coils up to very high current densities. An iterative layout optimization algorithm is implemented and used to study layouts at the magnetic field limit of Nb3Sn. It is shown that grading of the Nb3Sn is essential for reaching magnetic fields beyond 14T with 20% margin and that operation at 1.9K and improving the critical current density of the Nb3Sn conductor are highly recommended. Additionally, preliminary results are shown for layouts using HTS in the high magnetic field region of the coil in 20T magnets.