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We evaluated critical current density and mechanical properties of the Y123 coated conductor (CC) tapes on buffered Hastelloy substrates prepared by the chemical vapor deposition method in high magnetic fields and low temperatures. The Jc values of the tape are about 1.1 MA/cm at 77.3 K, 0 T and 1.8 MA/cm2 at 4.2 K and 17 T for B//c. In addition, the hoop stress test of the single layer coil shows that the stress limit of the CVD-Y123 CC tapes on Hastelloy is over 1 GPa. On the basis of those experimental data, we designed the innermost high temperature superconducting insert coil for upgrading of the 18 T cryogen-free superconducting magnet (18 T-CSM) under the condition of the stress limit of 600 MPa. The coil, which consists of the 22 double pancake coils can generate 9.4 T with the operation current of 295 A in the backup field of 15.6 T. In this case, the central field of the 18 T-CSM can be improved up to 25 T from 18.1 T.