New high-Tc oxide compounds, whose superconducting transition Tc's exceed 100K, were discovered in Bi-Sr-Ca-Cu-O (BSCCO) and Tl-Ba-Ca-Cu-O (TBCCO) systems. The stoichiometric composition of the high-Tc phase was found to be Bi2Sr2Ca2Cu3Ox and Tl2Ba2Ca2Cu3Ox. Upper critical field Hc2's of these compounds were evaluated for bulk sintered specimens. The Hc2 (T=0K) defined at the midpoint of the resistive transitions are 160T and 220T for BSCCO and TBCCO high-Tc phases, respectively. High values of critical current density Jc at 77K have been already attained for sputtered BSCCO and TBCCO, while the Jc of the bulk specimens still remain less than 104A/cm2 at 77K and 0T. The parameters restricted the Jc in the bulk polycrystalline materials are considered to be (1) anisotropy in Hc2 and Jc, (2) weak link between grains, (3) weak pinning force, and (4) flux creep. The effect of each parameters is discussed with the experimental results of Jc and magnetic measurements.
Experimental and theoretical investigations of time dependent heat transfer in pressurized He II channel terminated by a copper rod have been carried out. Heat flux is applied in the form of a sinusoidal wave. As the frequency of heat flux increases, the change of copper and He II temperatures converge for the case of constant application of the effective value of heat flux. And no dispersion of Kapitza conductance is observed within the frequency range from 0 to 40Hz. On the basis of the mutual friction counterflow combining with the heat conduction in copper and Kapitza conductance, a one-dimensional heat transfer model has been developed, which can well predict the experimentally obtained time dependent heat transfer characteristics.
He II under pressures from 1atm to saturated vapor pressure and constant bath temperature was investigated as a possible coolant for superconducting magnets. The cooling strength is described in terms of a critical frequency fc at which the peak transport currents corresponding to successive constant voltage pulses decrease abruptly due to a breakdown of superfluidity. Maximum fc was found under the pressure at the lower λ-point (He IIλ) i.e. with a hydrostatic pressure head of about 2m. This allows the use of ordinary cryostats while retaining the efficiency of saturated He II systems. He IIλ does not give full stabilization in the classical sense, but is a promising coolant for pulse magnets or meta-stabilized DC magnets.
A fundamental consideration of a closed-thermosiphon type cold neutron source (CNS) has been presented on the basis of the non-equilibrium thermodynamics. In order to simplify the treatment, a discontinuous thermodynamic model consisting of two large sub-systems connected by a small transport sub-system is employed. The analyses are especially concentrated in the self-regulation property, that is, the amount of liquid hydrogen in the moderator cell can be kept almost constant against thermal disturbances from reactor output fluctuations. It is shown that this property results from a thermodynamic process producing an entropy of a total system by a temperature increment in which an evaporation in one sub-system is compensated by a liquefaction in the other sub-system linked together by a common quantity of latent heat.
We have studied experimentally quench processes in a 72kVA superconducting four-winding power transformer. Quenches in main windings were initiated by excess current due to sudden short-circuit or a local heating. An instantaneous decrease in the transport current originated from appearing extensive normal regions just after the short-circuit was observed in both of the primary and the secondary main windings. Similar phenomena were generated by a local heating in the secondary main winding. The equivalent propagation velocity of normal zones along the winding during the first half cycle after the initiation of the quench was about two orders larger than the existing theoretical predictions. Two dimensional propagations of the normal region were observed after the instantaneous transitions to the normal state in the main windings. We discussed the mechanism of these abnormal transition to the normal state in relation with possibility of quench protection in AC superconducting windings.
This paper introduces a new temperature measurement method, which can convert temperature change to phase change in applied AC current, and gives us frequency change as a final output of the measurement. The new method does not require any type of the special thermometers like a capacity thermometer. So the method is applicable to conventional temperature measurement system as an alternative one, and also suitable to the temperature measurement in a rotating cryostat just like a superconducting generator, since the method realizes non-mechanical contact measurement. In this paper, the principle of the new measurement method is studied, and the experimental data of temperature measurement in an actual rotating system is demonstrated, and the results show that the method is applicable for the measurement of rotating system, since the output is quit stable for any rotating speeds (less than 80rps).
A simple cryostat has been made to observe the transition properties of high temperature superconductors. Since the cryostat utilizes the latent heat of liquid nitrogen under reduced pressure, temperatures ranging from room temperature to 61K can be obtained with larger cooling capacity than other cryostats using liquid helium or miniature cryocooler. In this note, the construction and the ability of the simple cryostat are described.