Development of superconducting motor producing circular flow of superfluid helium

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We report the results obtained in the present study as follows, for the purpose of developing a motor with a rotor working in superfluid helium.(1) We employ a brushless motor with a rotor separated from driving coils. A brushless motor requires a sensor of rotor position. We measured the features of some position sensors, finding that a sensor using induced voltages between ends of driving coils is appropriate for low temperature usage. (2) We have succeeded in developing a brushless motor working at low temperatures above -203 C (70 K), though the motor could not work below the temperature. (3) We find that a rotation loss increases with decreasing temperature. This is because magnetic hysteresis of the core material of the coils increases. (4) By measuring the drag force of an obstacle moving in superfluid helium, we find that the obstacle produces quantized vortices at a velocity above 100 mm/s.