Many long distance cooling channel systems have recently been constructed for superconducting power cables and hollow conductor magnets. For safe operation of these systems, an automatic control system is required. However, the use of conventional feed back control systems for the long distance cooling channel system has been restricted to a local-pressure, local-temperature or a sequential control method, since the supercritical helium shows nonlinear thermo physical properties. Authors have so far examined temperature control methods for cooling systems, applicable to superconduting power cables and hollow conductor magnets. In this paper, a microcomputer control method for temperature of the long distance cooling channel is shown. The remarkable feature of this control method is an application of system identification, which makes the microcomputer work like a skillful operator even when the cooling system is much complicated and shows nonlinear properties. The control method is suited for temperature control of superconducting power cables and hollow conductor magnets, since it does not demand accurate thermophysical properties of the supercritical helium, and microcomputers are not expensive today.
The criteria of selecting magnetic materials for a refrigerant of the magnetic refrigerator in the temperature range from 4.2K to 300K have been considered. First, by assuming the molecular field approximation, we have calculated the magnetic entropy SM(T, H) of the typical 3-dimensional ferromagnetic substance near the Curie temperature as a function of temperature and magnetic field strength. In order to examine the usefulness of the above theoretical results, the magnetocaloric effects of Cr3Te4 and Gd near the room temperature have been investigated experimentally. As a result of comparison of the calculated results with the experimental ones, it is confirmed that both of those results are in good agreement with each other. Second, by using the Debye approximation, the temperature variation of the lattice entropy SL(T) as a function of the Debye temperature has been made clear. Using the above SM(T, H) and SL(T), the temperature and magnetic field dependences of the calculated total entropy of the several typical ferromagnets are obtained. In view of the above results we have discussed the several criteria for the magnetic refrigerant used near the room temperature to be satisfied.