This article is an extract from a commemorative lecture by the author at the time of his retirement from Tohoku University, March 1997, focusing on his research in the field of cryogenic engineering. The motives, difficulties, and outlines of his studies on the following subjects are described: thermoelectric semiconductors, cooling technology for electronic equipment, natural convection, heat exchangers, air-water mist cooling, gas fluidized beds, heat-transfer control, transcritical thermal phenomena, thermal stability of superconducting coils and visualization of quench, rapid transient boiling of liquid helium, cryoprobes, etc.
A persistent current switch (PCS) in SMES requires not only small ON resistance but also large OFF resistance in order to perform efficiently during SMES operations. Because a mechanical switch has the advantage of infinite OFF resistance, it can operate without generating Joule losses during coil excitation and energy transfer. Furthermore, it will cause an energy storage loss as long as the ON resistance, which mainly depends on contact resistance, is not decreased to zero. In this study, case studies on energy losses due to the ON resistance of a mechanical switch and OFF resistance of a superconducting switch are carried out in order to show the necessity of a mechanical switch for a PCS in a large-scale SMES. Then, the contact resistance characteristics of a mechanical switch using bulk NbTi contact pieces are investigated in liquid helium as a function of switch driving force, switch pressing force and ON-OFF operation number. Additionally, the effects of surface roughness and shape on contact resistance are examined. It was found that a superconducting connection at contact is realized even if the switch is repeatedly opened and closed many times. The tested mechanical PCS supports a superconducting current flow of more than 200A. Finally, the formation mechanism of the superconducting connection at contact is modeled and analyzed based on theoretical considerations.
We present formulas for field-sweep rate (β) dependence of irreversible magnetization (M) in superconductors of various shapes. The expressions of M-β relation were derived for arbitrary current-voltage (electric field (E) versus current density (J)) characteristics taking into account the distributions of E and J. We measured the β dependence of M in a YBa2Cu3O7 (YBCO) disk, and E-J characteristics are estimated from the experimental data of M-β dependence using a theoretical formula. Furthermore, we estimated current distribution in the YBCO disk.
Powder-metallurgy processed Nb3Sn composite has been studied aiming at the practical application of magnetic shielding. The composite was fabricated using a mixture of gas-atomized bronze (Cu-10wt% Sn) powder and hydride-dehydride processed Nb powder which passed through a sieve with an opening size of 50μm. The mixture was compacted by a hot isostatic press and heat-treated to form the Nb3Sn composite. A three-dimensional network consisting of Nb3Sn shells and a three-dimensional network of bronze were observed in the composites. Magnetic shielding properties of the composites were invesitgated using cylindrical samples with an outer diameter of 24mm, an inner diameter of 6.7mm and a height of 43mm. It was found that the cylindrical samples reduced inhomogeneous field without a flux jump in an alternating field with a rate of 0.1T/min up to 1-2T. In three kinds of cylindrical samples, composites with a volume fraction of bronze of 0.2, 0.25 and 0.3 in the starting mixture, the stability of the cylindrical sample for the hysteresis loss increased and the maximum external field strength to be shielded increased as the fraction of bronze rose.
A cryocooler-cooled and thermally controlled type persistent current switch (PCS) using CuNi/NbTi superconductor for a cryogen-free superconducting magnet has been investigated. This PCS was combined with the cryogen-free NbTi superconducting magnet, and we succeeded in generating a magnetic field of 5.0T under persistent current operation. The PCS was covered with a thermal insulator and embedded in a copper cup with a cryogenic grease, and the copper cup was fixed to the second stage of the cryocooler. Heat transfer from the PCS to the cryocooler was controlled by a thermal insulator between the PCS and the copper cup, enabling the PCS to turn off with minimum power input. A practical PCS exhibited stable operation with 40mW of power input, and 18s and 100s of switching time from on-to-off and from off-to-on, respectively. When the magnet is energized, current sharing is induced to the PCS due to self inductance of the magnet. In order to reduce the self-heating of the PCS due to current sharing, enlargement of the off-resistivity of the PCS was important. The cryogen-free NbTi superconducting magnet with 52.6A/5T of constant magnetism and 59.6H of self-inductance was able to energize within 18min from 0T to 5T. The decreasing rate of generating 5T on persistent current operation was 39.5ppm/h (on average) in the initial 100h. This value corresponds well with the results of calculations based on an LR-circuit discharge due to the resistivity of soldered joints in the closed circuit.
In this study, hardnesses measurements were carried out for austenitic stainless steels (JN1, JN2, SUS304 and SUS316L) from 4K in liquid helium to 293K in room temperature. Hardness increases with a decrease in temperature to around 70K. After peaking, the hardness increases again below 70K. Applying the Arrhenius equation to the hardness properties, two different deformation processes are suggested: slip dislocation at higher temperatures and deformation twinning at lower temperatures. The temperature of the peak hardness changes with loading rate. This temperature change corresponds to the change in the intersection between twin stress and slip stress with the loading rate. As a result, it is clear that a change in the deformation mechanism from slip dislocation to deformation twinning occurs.