宇宙用スターリング冷却機の開発

Abstract

For earth observations and for astronomical observations in space, there is a need to extend observation periods and improve detector performance. Development of mechanical cryocoolers with longer lifetimes and improved cooling capacity is anticipated. The free piston-type Stirling cryocooler, which applies 1) the Stirling cycle for refrigeration, 2) linear motors for driving, 3) magnetic bearings for support systems, and 4) fine gaps for clearance seals, is the most suitable design for a mechanical cryocooler utilized in a space environment. Development of this type of cryocooler began in 1987 with Bread Board Model 1. Its major components, the linear motors and magnetic bearings, were studied in order to identify the technical problems. In the present model, Bread Board Model 2, design criteria which enhance operating life, compactness, and light weight are being studied. The thrust force of moving magnet-type oscillatory linear motors satisfies the required specifications. Radial vibration amplitudes were reduced to less than 25μm with the use of PID control for magnetic bearing control. Further, the total amplitude of axial vibration was decreased to only 17μm when a dynamic damper was utilized to reduce cryocooler vibration. A cooldown time of about 17min, to a surface temperature of 70K, and an ultimate surface temperature of about 50K were achieved. The cooling power at a temperature of 70K is 2W, which is not sufficient compared with the design specification of 5W. It is supposed that stroke shortage in a piston is the primary reason for this. It was also confirmed that the stroke of a piston can be significantly increased by decreasing the clearance between the piston and the cylinder, as well as by decreasing the passage resistance of a connecting section. Further investigation of cooling power improvement to satisfy the design specifications and durability tests are planned on Bread Board Model 2, which is redesigned and now being manufactured based on the forementioned results.