The paper describes the design, construction and long-term perfomances of the 6m-superconducting solenoid and associated supercritical He cooling system for the muon channel of the Meson Science Laboratory, University of Tokyo. The basic new and novel concept of this system compared to the SIN system stays in the following points: 1) the supercritical He is generated by the “Single” heat exchanger located inside the refrigerator cold box, therby simplifying the total cooling system; 2) the iron return yoke is placed at room temperature, reducing substantially the cold weight; 3) after the careful design of the system, a large mass flow of 5.0gr/s is adopted, minimizing cool-down time, etc. In this report, the emphasis is given for the potential use of this “Simplified” supercritical He cooling for the large-scale superconducting solenoid. Associated design principle as well as required thermodynamical calculations are given. Through the successful and maintenance free operation in the period of 1980 to 1984 amounting to 9000h, we have confirmed the excellence of the whole system.
In JAERI, as one of superconducting toroidal coil development works for a tokamak fusion machine, the Cluster Test Program is under way. The first Test Module Coil (TMC-I) in this program has been constructed in 1982. The TMC-I, whose winding inner diameter is 60cm, has been fabricated with a double pancake winding method using a reacted multifilamentary Nb3Sn conductor. Up to now, experiments of the TMC-I were carried out on cooling-down, current charge, stability, manual dumping and out-of-plane force. The TMC-I was stably operated in 11.1T with the current of 6kA and the current density in the winding of 30A/mm2. A total stored energy was 46MJ at 11.1T. 192cm length normal zone, generated by heat-input in the innermost turn, recovered spontaneously to superconducting state in 6.3s. From these results, it was demonstrated that a multifilamentary Nb3Sn conductor is applicable to large-current and large-size coils.
The test module coil (TMC-I) is the first test coil wound with multifilamentary Nb3Sn conductor in order to demonstrate high field superconducting toroidal field coils for fusion. The TMC-I was constructed in 1982 and the electrical, mechanical, and thermal characteristics were tested at the magnetic field of 10.2T by using the Cluster Test Facility (CTF). Thereafter, the TMC-I was successfully charged up to 11.1T in the Cluster Test Facility reinforced by adding two more back-ground coils (CBC) to the existing Cluster Test Coils (CTC). This paper describes thermal characteristics obtained in the cool-down, the heat load measurements, and the manual dump test of the TMC-I.
Superconducting cable is one of the promising ways for transmitting the huge electric power in the future. In the terminal bushing of superconducting cable, huge heat influx flows from the ambient temperature region into the cold region through conductor. Huge Joule heat arises because of large electric current in the normal conducting part. To minimize the refrigerator load of terminal bushing in superconducting cable system under steady operation, the sum of Joule heat and heat influx should be minimized. In this paper we discuss the optimum thermal design of the bushing conductor for superconducting cable system. The conclusions are (1) It was shown that, to minimize the sum of Joule heat and heat influx in the terminal bushing with constant cross section along the length, it is required that heat influx is equal to the Joule heat in each part of bushing conductor. (2) We obtained the optimum cross section and optimum temperature distribution from the optimum conductor design based on the above condition. The sum of heat was compared with ones from other design to show the validity of this optimum design. (3) By the design technique combining the optimum design and simulation of temperature distribution, it is possible to take account of temperature dependences of electric resistivity and thermal conductivity, and heat transfer coefficient between gas helium and bushing conductor. (4) With regard to conductor configuration, configuration with high heat transfer coefficient, as braid conductor, makes conductor dimension and helium consumption small.
Centrifugal helium compressors were developed to improve efficiency and reliability of helium refrigerators. These comprssors are operated almost below the temperature of liquid nitrogen. The rotor of the compressor is supported by helium lubricated gas bearings. A controlled attractive magnetic bearing is adoptted as auxiliary. Two compressors were manufactured. The 1st. trial one was made simply for experiment to study the possibility of low-temperature helium compressors. The 2nd. trial one was made taking practical use into consideration in respect to flow rate and compression ratio. This paper describes mainly the 2nd. trial compressor. Its typical specifications are as follows: type: two-stages centrifugal compressor with intercooler, flow rate: 75g/s, total compression ratio excluding the pressure loss in intercooler: 1.69, inlet helium temperature: 80K, rotational speed: 700rps (42, 000rpm), motor output: 10.2kW, type of bearings: helium lubricated internal tilting pad radial bearings and a spiral grooved thrust bearing with a controlled attractive magnetic bearing. The compressor was installed in a vacuum chamber for cold test, and the performance was examined in a closed loop. Items to be measured are rotational speed, electric input power, flow rate, pressure and temperature of helium gas at each inlet and outlet ports, vibration of the rotor and the current of the magnetic bearing. The results showed that the compressor excelled the expected values in performance. The following is the principal performance at the rated rotational speed, total compression ratio at the flow rate of 74.8g/s: 1.76, adiavatic efficiency: 1st. stage; 71%, 2nd. stage; 73%, total efficiency: 62%. The rotor supported by the bearing above mentioned operated stably up to 44, 000rpm. The controlled attractive magnetic bearing gave an adequate clearance to the thrust gas bearing in start and stop condition, so that the rotor was able to start many times without any damage to the gas bearings. The compressor has operated for a long time.
A helium refrigerator system equipped with low-temperature centrifugal compressors is proposed. This refrigerator, named L-system refrigerator, is operated almost below the temperature of liquid nitrogen. It has higher reliability and capability of long time continuous operation free from the contamination of helium with oil and from wear of moving part. It also has an advantage that the control of refrigeration power is easily achieved by adjusting the speed of compressors. The direct input power to the L-system refrigerator is lower, but it thermal efficiency is almost same as the conventional one, when the energy necessary to make liquid nitrogen is taken into account. A L-system refrigerator with refrigeration power of 1, 500W at 20K level is designed based on the experimental results of the low-temperature centrifugal compressor developed at Electrotechnical Laboratory. Its refrigeration power at 20K varies from 930W to 2, 000W almost lineary by changing the rotational speed of the compressors from 500rps to 720rps. The direct input power is 17kW to 35kW, but it consumes 310kg to 630kg of liquid nitrogen par hour. Large capacity helium refrigerators with high reliability and efficiency will be materialized when L-system refrigerators are built near the region where a large bulk of coolant is easily obtained, such as a LNG repository.
The systematic error in the pressure measurement is estimated quantitatively for establishing the precise helium vapor pressure temperature scale in the temperature range from 0.5K to 5K. From this estimate, a design of the cryostat, in which the temperature scale can be realized with the uncertainty of 0.1mK, is described. The thermomolecular effect and the hydrostatic pressure head are considered as the sources of the systematic error. The uncertainties in the correction are also estimated for both sources of error. The Weber-Schmidt equation is used to estimate the thermomolecular effect for both 4He and 3He, which gives negative correction to the pressure value measured by the pressure gauge at room temperature. The thermomolecular effect for 4He is shown to be negligible, if the temperature range is restricted above lambda point. In the case of 3He, in turn, the uncertainty in the correction for the thermomolecular effect at about 0.5K is shown to be critical unless properly designed cryostat is used. From the analysis of the temperature dependence of thermomolecular effect, an example of a set of tubings for the measurement of 3He vapor pressure is shown to give uncertainty of only 0.05mK in the correction for the thermomolecular effect. In this example, the diameter of the tubing is changed by three steps according to the temperature range to minimize the heat input into the 3He pot. The hydrostatic pressure head gives, on the contrary, positive correction of about 0.3mK at most on the temperature scale. The correction is made by measuring the gas pressure in the capillary thermally attached to the sensing line used in the vapor pressure measurement. This technique is simpler than the conventional method which is based on the measurement of the temperature at several points along the sensing line and on the deduction of temperature between those points by interpolation. The uncertainty of the correction following the present technique is estimated to be less than 0.04mK for both 4He and 3He, assuming the probable errors in the several parameters. As the systematic error resulting from the non-ideality of gaseous helium can't be measured by this technique, this error should be added to the uncertainty of the correction. The total uncertainty is shown to be less than 0.09mK. In conclusion, we give a design of a pressure sensing line which is expected to limit the uncertainty in realizing the helium vapor pressure scale to 0.05mK between 0.6K and 4.2K and to 0.1mK between 0.5K and 5.2K.