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.
Germanium thin films as resistance thermometers have been tested at liquid helium temperature. Germanium is deposited in vacuum on insulated substrates and then silver onto the germanium films as ohmic contacts. The thermometers with desired resistances and suitable sensitivities can be easily fabricated by choosing proper deposition conditions. The purpose of this work is to develop a thermometer with a fast response time for heat transfer experiments in liquid helium. The characteristics of a typical thermometer are as follows. The film resistance can be expressed as a function of temperature by a simple correlation, logR=C+mlogT, between 4.2K and 20K. The sensitivity is approximately 20Ω/K. After 50 thermal cyclings, the increase rate of the resistance at liquid helium temperature is within 0, 8%. An estimated heat capacity of the film is 2×10-8J/K, and a thermal relaxation time is of the order of 10-12s at 4.2K. These results prove that the germanium thin film thermometers are promising to measure transient surface temperature near 4.2K. A stainless steel foil with the thermometers is heated by a direct current in a liquid helium bath, and the surface temperature of the foil are measured. Large temperature fluctuations due to occasional liquid solid contacts are observed.