MgB₂ Wire Optimization Guidelines for a Liquid Hydrogen Level Sensor

Abstract

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.

Numerical optimization of monofilamentary MgB2 wires with different metal sheath materials for liquid hydrogen level sensor

“…The optimum current for SC level sensor is given by a minimum propagating current in the gas, which is inversely proportional to the square root of the resistivity of SC wire in the normal state [5,6]. Therefore, the optimum currents for the MgB 2 wires with SS, Cu-30Ni, and Cu-10Ni sheaths are fixed at 200 mA, 250 mA, and 300 mA, respectively.…”

“…In order to simulate the operations of SC level sensors, a one-dimensional heat balance equation along thin wire is discretized by means of a finite difference method. The details of the numerical analysis have already been explained [5,6]. The specifications of MgB 2 wires used for numerical calculations are listed in Table 1.…”

“…Such an each situation may be characterized by a heat transfer coefficient h of gas, vertical temperature gradient g in gas, and boiling temperature T b of liquid hydrogen under pressure. The reference case for calibration of the level sensor in Table 2 corresponds to the spontaneous evaporation or slow refill of liquid hydrogen at atmospheric pressure, and the parameters of h = 100 W/m 2 K, g = 0.20 K/mm, and T b = 20.4 K are fixed on the basis of the previous works [5,6]. Case 1 in Table 2 corresponds to the rapid refill of liquid hydrogen, and the heat transfer coefficient and temperature gradient may become larger and smaller, respectively, as compared with the reference.…”

“…Liquid hydrogen would be utilized in the hydrogen economy especially for its transport and storage because the density of liquid hydrogen at atmospheric pressure is much larger than that of gaseous hydrogen compressed up to 70 MPa, which is a maximum pressure for a storage tank mounted in a fuel cell vehicle to become available soon [1]. Superconducting (SC) level sensors with MgB 2 wires have been developed to observe the amount of liquid hydrogen in a container continuously [2][3][4][5][6][7]. Figure 1(a) shows the illustration diagram of a conventional type of SC level sensor [2,4].…”

Dependence of Transport-Current Losses in ${\rm MgB}_{2}$ Superconducting Wire on Temperature and Frequency

“…This means that the present MgB 2 wires would be suitable for applications in low magnetic fields. Level sensors for the liquid hydrogen with some kinds of MgB 2 wires have been investigated as one of the low-field applications [4]- [11]. Furthermore, a fully superconducting motor to drive a cryogenic electric pump for circulation or transfer of the liquid hydrogen has also been proposed [12].…”