RESUMEN
We report the design, construction, and operation results of an 18 T 70 mm cold-bore high temperature superconductor (HTS) no-insulation (NI) magnet, which is developed for an axion haloscope experiment. The magnet consists of 44 double-pancake coils wound with multi-width and multi-thickness REBa2Cu3O7-x (RE = rare earth) tapes. Owing to the NI feature, the magnet is highly compact; is 162 mm in outer diameter and 476 mm tall; and provides an environment of 0.22 T2 m3 within the cold-bore target space of 66 mm in diameter and 200 mm in length. After an initial performance test at SuNAM Co. Ltd., the magnet was installed at the Center for Axion and Precision Physics Research (CAPP) of the Institute for Basic Science in Daejeon, South Korea, in August 2017. The magnet has been successfully operating at the CAPP since then, except for maintenance in October 2018. The magnet may represent the first high field HTS user magnet that experienced long-term operation of over one year.