A close-packed coil with a 72 mm aperture is developed using the formvar-insulated NbTi wire made from the WST company. The micrometer-level inter-turn spaces are filled with CTD-101 K utilizing a vacuum pressure impregnation method with a controllable flow speed of the epoxy resin. The cryogenic excitation results indicate that the coil inter-turn spaces are well filled, so that it can generate a 7.35 T central magnetic field at an operation current of 176 A with no coil quench. The maximum loadline of the coil is proved to be 98.96%. The maximum hoop-stress is approximately 64.1 MPa at the center of the inner most coil layer. The VPI method is applicable to the close-packed coils (t_coil ≤ 28 mm) wound from the formvar-insulated NbTi wire.

Differ from the fiberglass-insulated Nb₃Sn wire, the formvar insulated NbTi provide a relative smooth wire surface, which does not absorb the epoxy resin. This significantly increases the difficulties of vacuum pressure impregnation (VPI) for a thick close-packed NbTi coil. With the purpose of understanding the applicable of current VPI procedure on formvar insulated NbTi coils, a close-packed solenoidal magnet wound from the formvar insulated NbTi is wound, vacuum pressure impregnated and cryogenic measured at the Institute of Plasma Physics (ASIPP), Chinese Academy of Sciences (CAS). The design aperture of the magnet is 72 mm, with coil thickness of 28.75 mm and coil height of 180.5 mm, the magnet is expected to generate a 7.04 T central magnetic field with an operation current of 170 A at 4.2 K. The excitation results show that the central magnetic field of the NbTi coil impregnated with current VPI procedure is able to reach 7.35 T with no quench under an operation current of 176 A. The loadline is close to 98.96% of the NbTi wire.