Future Upgrade of the Superconducting High Field Facility HOMER II to 25 T

In 2006, the superconducting high field facility HOMER II of the Karlsruhe Institute of Technology was commissioned. It provides a magnetic field of 20 T in a large bore of 185-mm diameter, using a coil system with sections made of NbTi and (NbX)3Sn wires. With regard to the increasing availability of high-quality second-generation high temperature superconductors (2G-HTS) in longer lengths, in 2014, the project for an intended upgrade of HOMER II to a total field of 25 T has been started. The objective is to build up a 5-T insert made of REBCO HTS to be operated routinely in the background of 20 T produced by the basic magnet configuration of HOMER II. In this paper, we report on the schedule of the project, the preliminary design of the HTS-insert, and the requirements for the HTS wire. Furthermore, the results of the preparatory operation of small solenoid HTS coils in the 20-T background field are presented.