This paper deals with bubble behavior in pool-cooled HTC superconducting coil, which is developed for superconducting transformers to understand the state of the insulation medium in the coil at the quenching condition and to find an effective suppression method of vicious bubble effects on electrical insulation and cryogenic cooling. The tested coil system immersed in liquid nitrogen consisted of two coaxial coil layers wound on FRP frames and a cylindrical ITO film electrode deposited on the outerside of a glass cylinder by ion sputtering, and the axis of electrode system was set vertically. The experiments show that bubbles generated thermally by a heater mounted in the superconducting wire leave from the wire discretely at lower applied voltages, but they turn bubble columns locked in cooling channels at higher applied voltages under the layer-to-layer insulation condition, i.e., boiling on the wire covered with FRP tape changes from nuclear boiling to film boiling by the application of a high electric field to the coolant. At the turn-to-turn insulation condition, bubble coagulation phenomena appear near the inner surface of the glass cylinder, and the coagulated bubbles spout through the cooling channel between coil layer and glass wall. The formation of vapor locking by an electric field can be minimized or controlled by making an inclined path crossing over the barrier between coil turns or pressurizing liquid nitrogen to a subcooled state.
