Titanium and its alloys have a high yield strength, a high specific strength, good physical properties favorable for cryogenic structural materials. Comparisons of their properties with those of ferrous alloys are made and titanium materials standards for low temperature use are collected in order to characterize the current cryogenic appplication of the titanium materials. Tensile, Charpy impact, fracture toughness and fatigue properties are reviewed in reference to the effects of alloy type, test temperature, impurity level, and heat treatment.
We have designed and constructed a superconducting four-winding power transformer in which main windings were wound by NbTi wires developed for AC use. The maximum capacity of 72kVA was attained with high performance of efficiency and voltage regulation. The lifting weight of the transformer was about one seventh compared with conventional ones with the same scale of capacity. We also investigated the processes of quench and recovery in main windings in an accidental short test. In this test, instantaneous partial quenches occured in the main windings, and transport current was shifted from the main coils to the auxiliary ones. It was also confirmed that the short-circuit current was limited by loose coupling between the auxiliary windings. After release of short, the system was stably recovered by temporal break of the main windings, because the temperature of the main windings after quench was suppressed by an abrupt decrease of the transport current.
Time constant of decay of coupling current was measured on cable-in-conduit conductors which consist of twelve Nb3Sn strands with chrome plating. The time constant measurement was performed by observing exponential decrease of induced coupling current in conductors. The measured time constant of conductor composed of bare strands was 4.5 times larger than that of the single strand, in case that a void fraction of conductor was 30%. However, the time constant of conductor composed of strands with chrome plating of more than 2μm thickness was equal to that of the single strand. These results show that chrome plating on strands is effective for decrease of coupling losses in Nb3Sn cable-in-conduit conductors.
Ericsson magnetic refrigeration has been studied in the temperature range from 20K to 77K. We have designed and built an Ericsson magnetic refrigerator which has a lot of difference from Carnot magnetic refrigerators. In this temperature range, it is necessary to use ferromagnetic material as the magnetic refrigerant, because it has the internal magnetic field which arranges the spin system. Moreover, above 20K since the lattice entropy grows almost comparable to or larger than the magnetic entropy, we must apply Ericsson cycle for the thermal cycle. In our apparatus, lead was used as regenerator material, and gaseous helium was used to transfer heat between the two solids, magnetic substance and regenerator material. Our device operated on Ericsson cycle with fixed magnetic substance and moving regenerator, and magnetization and demagnetization of the magnetic substance were obtained by charging and discharging the magnet. In this paper, the primary experimental results are described. The magnetic working substance was DyAl2.2 sintered compound which had the Curie temperature of 49.8K, and two kinds of regenerator were tested. At the cycle of 300s and a field of 5T, the magnetic substance changed in temperature between 48.3 and 59.1K and regenerator produced a temperature of 50.3 and 58.7K at the cold and hot end, respectively. The temperature change of the magnetic substance was about 1.7 times as large as the maximum adiabatic temperature change with the magnetic field of 0 and 5T. Some losses were discussed in our paper. The Brayton like cycle and very slow cycle period obtained in our experiment were caused by insufficient heat transfer between the regenerator and the magnetic substance. In addition to this problem, thermal conduction in the regenerator material which caused heat leak to low temperature end of the regenerator was thought to limit the temperature span produced in the regenerator.
One of the major problems in helium cooling of field windings of superconducting generators is liquid helium level control in the rotating helium pool. A unique helium flow circuit with positioning the mouth of a liquid helium transfer tube under helium level in the rotating helium pool is proposed for the solution of this problem. This flow circuit is expected that it provides the ability to externally control the liquid helium level in the rotor by adjusting the helium supply pressure. However there has been few papers which have proved clearly that the helium level control device works well and so the feasibility of this device is not clear. Accordingly we constructed a large scale rotating cryostadt which allows to simulate a cryogenic rotor of a superconducting generator and intended to prove stable helium level control by use of this test rig. This paper at first outlines the rotating cryostadt and then describes experimental data. This paper further presents calculation method of liquid helium level and also comparison between calculations and experimental data. We have achieved stable helium level control successfully and proved that calculations agree well with experimental data. These results confirmed that this helium level control device is feasible and applicable to the rotors of large scale superconducting generators in the future.
In order to reveal the future conductor structure for a high Tc superconductor, a stability evaluation was conducted. Flux jumping, minimum propagating zone (MPZ) and full stabilization with copper were discussed, based on parameters obtained for YBaCuO (YBCO) at 77K. Obtained results were as follows. Flux jumping can not take place in a YBCO core whose diameter is less than 0.8mm. Therefore, the conductor structure does not have to be like a multifilamentary composite. The MPZ for YBCO at 77K is nearly equals to that for NbTi at 4.2K. However, specific heat of YBCO is 200 times larger than that for NbTi. Therefore, the YBCO stability against thermal disturbance is almost 200 times larger than NbTi. Current density in a fully stabilized conductor with a copper stabilizer at 77K is as low as that for a normal copper conductor.