Since around 1970, cryogenic systems have required the use of cold compressors. The requirement appeared for two reasons, The first was reduce the pressure of liquid helium with the intention of realizing the stable operation of large superconducting magnets with the best superconducting ability by decreasing operating temperature. The other was to improve the reliability of helium refrigerator compressors by introducing a turbo-compressor with oil-free bearings. This paper describes the circumstances and particular development of cold compressors, requirements related to helium refrigeration systems and cold compressors and future prospects.
The wire motion in a superconducting magnet has been analyzed by means of the Monte Carlo method. A sufficient analysis of the instability originating from wire motion has not been made because wire motion has not been clarified at a singular area in the coil, such as a “cross over” position where one wire crosses over other wires in the lower layer. A winding compression test was also performed and the location of disturbances in the coil was determined by the acoustic emission method. It was clarified that, in the singular area of a coil, wire motion is easily generated. The experimental results were compared with those calculated. It was confirmed that the instability in a coil originates from wire motion mainly in the singular area.
To standardize the test method for the critical current (Ic) measurement of three-component (Nb-Ti/Cu-Ni/Cu) superconducting wires, we have extracted many problems such as quenching, which often occurs in the wires, and have established a new measuring method. It was found that adding tension corresponding to a 0.1-0.2% strain applied to the wires is effective for preventing quenching than using silicone grease for fixing the wires to a mandrel, and that we can measure Ic's for all magnetic fields without quenching by soldering the wire to an alloy mandrel. It was also found that no trouble occurs when common Cu/Nb-Ti wires are measured based on the Ic test method of three-component wires. Therefore it is favorable to fix the wires by tension rather than using silicone grease in the case of wires which are easily quenched. Round robbin tests were carried out to check the validity of the draft for the test method. As a result of checking the coefficients of variation (COV) concerning individual experimental results, it was confirmed that the COV's reduce less than 2% in most cases. Based on these results, the draft of a test method for three-component wires was prepared.
The YBaCuO superconductors prepared by the Melt-Powder-Melt-Growth (MPMG) process show high Jc characteristics at high magnetic fields and high temperatures. Since the high Jc property is very important for high-field applications, these materials are expected to be applied to high-field applications such as for bulk superconducting magnets, current leads, magnetic levitation systems and so on. In this study, we investigated the temperature and magnetic-field dependences of the critical current density, Jc, in YBaCuO superconductors prepared by the MPMG process at a magnetic field of up to 5.5T in the temperature range of 5 to 90K. We estimated the scaling parameters of Jc (B, T) based on the present result that the precipitates consisting of the 211 phases work as effective pinning centers. We also estimated the Jc anisotropy of the superconductor between the c axis direction and directions in the (a, b) plain.
Since superconductors do not require voltages, a high-current power supply could run with low power if the voltage is sufficiently reduced. Even a battery-powered power supply could give as much as 2, 000A for a superconductor. To demonstrate this hypothesis, a battery-powered 2, 000A power supply was constructed. It uses an IGBT chopper and Schottky diode together with a specially arranged transformer to produce a high current with low voltage. Testing of 2, 000A operation was performed for about 1.5hr using 10 car batteries. Charging time for this operation was 8hr. Ramping control was smooth and caused no trouble. Although the IGBT frequency ripple of 16.6kHz was easily removed using a passive filter, spike noise remained in the output voltage. This ripple did not cause any trouble in operating a pancake-type inductive superconducting load.