The feasibility study of a superconducting level sensor for liquid hydrogen with a magnesium-diboride (MgB2) wire is carried out from an experimental point of view. The sample wire consists of a mono-cored MgB2 superconductor and a cupronickel sheath, and several potential taps are attached to it at even intervals in order to understand the position of a threshold between the superconducting and resistive states roughly. The fabricated sensor is vertically located in a glass dewar vessel with an infill of liquid hydrogen, and the position of a preselected potential tap is adjusted by eye and hand to liquid level before starting a new measurement. Simulated operations with constant currents finally yield the future possibilities as the level sensor for liquid hydrogen with MgB2 wire although the fabricated sensor has a few problems at present. In order to improve the performance of the sensor, the specifications required for MgB2 wires will be reported elsewhere by applying the stability theory in superconductor composites and by simulating the operation with a numerical code.

We fabricated a superconducting level sensor with a magnesium-diboride wire and carried out experiments with liquid hydrogen. First, we observed a normal zone propagation phenomenon in the gaseous hydrogen. We also measured the terminal voltage of the sensor with a constant current depending on the relative change in the liquid level, and recognized that input power during normal operation must be suppressed to realize a level sensor for liquid hydrogen. Operation of the level sensor was numerically simulated based on the experimental results. The time evolution of temperature distribution along the wire was calculated using a heat balance equation including the cooling effects of liquid hydrogen and vaporized gas. The influences of wire size and material properties on minimum propagating current and power consumption in the gaseous hydrogen were evaluated to achieve the optimum design for the level sensor.

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