A dilution refrigerator of 80-730μmol/s3He circulation rate has been built and the lowest temperature of 2.85mK is continuously attained. In this refrigerator, the discrete heat exchangers with pressed metal (Cu and Ag) powder are used. These discrete heat exchangers are newly developed based on the analysis of Radebaugh and Siegwarth. The machine is designed for many uses such as a base of nuclear cooling and studies of general solid state physics. The design and construction of the heat exchangers and the mixing chamber are described in part I. And the design and construction of the other parts and the performance of the refrigerator will be described in part II.
The details of the design and construction of the heat exchangers and the mixing chamber for the dilution refrigerator have been described in the part I of our papers. In this paper (part II), the technical points in the design and construction of the still, 1K pot, flow impedances, dewar vessel and safety system for long period of operation and also the performance of this machine are described.
Materials for the F. R. P. cryostat should be selected out of various Fiber Reinforced Plastics. A F. R. P. cryostat for pulsed superconducting magnets has been made of F. R. P. by filament winding method. Its inner diameter is 200mm and the depth is 800mm. The evaporating rate of liquid helium was 0.42l/H initially and was 0.45l/H after one month.
A fiber-glass reinforced plastic (FRP) cryostat has been developed for a 1MJ pulsed magnet. The FRP cryostat consists of two tanks with 100 superinsulation layers. The inner tank has an inner diameter of 62cm, a depth of 150cm, and a wall thickness of 1.2cm. Both tanks are made of fiber-glass reinforced vinyl polyester resin by a hand lay up method without any joints. The steady state heat leak measured was about 1.4W. A 10-7Torr vacuum was achieved when liquid helium had been pooled in it.
The plastic dewar is suited to the use of the large pulse magaet and SQUID by means of its properties as the electrical insulator and non-magbetism. These are three problems which should be solved, i.e. 1) permeability of helium gas through the plastic wall, 2) cracking by means of thermal stress between room and liquid helium temperature, and 3) mechanical properties of gluing at liquid helium temperature. These problems are discussed in this paper. Consequently, it is found that the plastic materials made by the methods of “filament winding” or “vacuum injection” for the liquid helium dewar are effective. The toughness of gluing part is sufficient at cryogenic temperature. The evaporation rate of liquid helium in the plastic dewar made on an experimental basis is measured, and it is found that its evaporation rate is equivalent to that of the metal dewar.