Slush hydrogen is a two-phase solid-liquid cryogenic fluid consisting of solid hydrogen particles in liquid hydrogen. Compared to liquid hydrogen, the density is about 16% greater at a solid mass ratio (solid fraction) of 50%, and the cryogenic heat capacity (enthalpy) is about 18% higher. Various applications are anticipated, including fuel for reusable space shuttles, coolant for cold neutron generation, as well as the transport and storage of hydrogen as a clean energy source. At a solid fraction of within 50%, piped transport can be conducted in the same way as for normal fluids. This paper reports on the slush hydrogen technology in terms of the measurement of the density and the mass flow rate. (Translation of the article originally published in Cryogenics 44 (2004) 59-68)
HTS wires have been improved to the extent that they have practical properties, and various HTS coils have been designed and fabricated. One of the features of an HTS coil is high thermal stability against thermal disturbances. The minimum quench energy (MQE) of an HTS coil is several orders higher than that of an LTS coil. However, thermal runaway, which shows almost the same phenomenon as quenching observed in an LTS coil, is observed in a conduction-cooled HTS coil when the HTS coil is operated under a high load factor. Thermal runaway causes degradation of an HTS coil, so it is important to quantitatively evaluate the thermal runaway currents of a conduction-cooled HTS coil. This paper shows the thermal runaway evaluation test results for a conduction-cooled HTS coil wound with an Ag-sheathed Bi2223 wire.
In order to quantitatively evaluate the thermal stability of conduction-cooled HTS coils, thermal runaway currents of a conduction-cooled HTS single-pancake coil were measured at various temperatures and several cooling conditions, and numerically calculated using a calculus of finite differences. Calculated results were in good agreement with the experimental ones. A sensitivity analysis of thermal runaway currents for various physical values was also carried out. The thermal conductivity, specific heat and n-value of an Ag-sheathed Bi2223 wire do not significantly affect the thermal runaway currents of the coil. However, the critical current of an Ag-sheathed Bi2223 wire strongly affected the thermal runaway currents. Therefore, more precise critical current data should be prepared in the thermal stability analysis of conduction-cooled HTS coils.
Reducing AC loss in superconducting apparatus is one of the most important issues, and the precise measurement and estimation of AC losses are essential to reduce them. The four-terminal method is universally used as an electric measurement method of AC losses for superconducting tapes and wires. In this method, noise and inductive voltage superposed on the terminal voltage of the superconductor are eliminated by a lock-in amplifier and cancel coil, respectively, and then measurement of very small resistive voltage is achieved. However, using this conventional method, a plurality of measuring instruments and apparatus are needed, and therefore the measuring system becomes complicated and much time is consumed in the calibration process. In this paper, we present a simple and precise measurement system based on an active power detection method, which is proposed as a quench detection method. The proposed system consists of a small number of instruments and apparatus and is less susceptible to noise. Its usefulness is verified by comparing the proposed method and the conventional four-terminal method in measuring the AC transport current loss of a Bi2223/Ag tape.
High-temperature superconducting (HTS) cables have been studied because of their low loss and compact properties as compared to conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, a tri-axial cable composed of three concentric phases has been intensively developed, because it has advantages such as reduced HTS tape, small leakage field and small leakage heat loss as compared to three single-phase cables. However, there is an inherent imbalance in the three-phase currents in tri-axial cables due to the differences in the radii of the three-phase current layers. The imbalance of the currents causes additional loss and a large leakage field in the cable, and deteriorates the electric power quality. Therefore, a model composed with two longitudinal sections is proposed. This model allows us to determine cable construction parameters such as winding pitches and the radii of the three concentric phase layers, thereby satisfying both conditions: three balanced concentric phase currents and a homogeneous current distribution in each phase of the tri-axial cable. We formulate and derive general equations for the three-phase current distribution in the tri-axial cable as functions of the winding pitches of the three concentric phase layers. The equations are applied to tri-axial cables composed of one or two layers per phase.