Evaluation of Screening Current-Induced Field Generated by No-Insulation REBCO Coil System for Multi-Frequency ECR Ion Source

We are developing a high-temperature superconducting skeleton cyclotron (HTS-SC), that is, an ultracompact, high-intensity accelerator. The coil system of the skeleton cyclotron consists of air-core no-insulation (NI) REBCO coils without an iron core, which is conventionally adopted, and can allow variable output energy. In parallel, we have been developing an NI-REBCO coil system for the electron cyclotron resonance ion source for the HTS-SC. It is necessary to change the magnetic-field configuration according to the operation frequencies: 2.45 and 10 GHz. Although both high thermal stability and high current density can be achieved by adopting NI-REBCO winding technology, the screening current-induced in the REBCO-coated conductor generates an irregular magnetic field. In addition, there is an excitation delay caused by a current flowing in the radial direction with coil excitation in the NI coil winding, which reduces the magnetic-field accuracy. In this paper, we focus on the accuracy of the spatial distribution of the generated magnetic field and present the results of evaluations based on experiments and numerical analysis. A newly developed screening current-induced field analysis method for NI-REBCO coils based only on circuit analysis was used for numerical simulations.