Developments of 1-4 K Class Space Mechanical Coolers for New Generation Satellite Missions in JAXA

Mechanical coolers are a key enabling technology to utilize low temperature scientific instruments for the new generation of space science missions, such as the new X-ray astronomical satellite Astro-H, and the new infrared astronomical satellite SPICA. In particular, 1-4 K class mechanical coolers are required as a precooler for the low temperature detector’s cooling system and to cool a space telescope with a very low radiation background. In JAXA, two-stage Stirling coolers (2ST) for 20 K and a 4He Joule-Thomson (J-T) cooler for 4.5 K have been successfully developed and operated in space, e.g. Akari (Astro-F) and the Japanese Experiment Module/SMILES. Based on this heritage, the 2ST coolers and the 4He J-T coolers have been modified and upgraded to achieve high reliability, more cooling power, lower mechanical vibration, and longer life time. In performance tests of upgraded prototypes of the mechanical coolers, the required cooling power of each cooler has been obtained. The engineering model of the 4 K class cooler, consisting of two 2ST coolers and a 4He J-T cooler for the Astro-H payload will be fabricated, and undergo performance tests, including the level of vibration and cooling behavior at the expected heat load in Astro-H. In this paper, the R&D status and performance test results of the 2ST coolers and the 4He J-T coolers are reported. We also report on the development of 3He J-T cooler for the SPICA detectors. INTRODUCTION Mechanical cryocoolers are required to maintain a low temperature with longer orbit time in comparison to a solid or liquid cryogen in space. In past space missions, a large cryogen tank was installed to cool a telescope or detectors, and the cryogen’s lifetime limits the mission life of the astronomical satellite. Instead, the use of the mechanical coolers can provide a mission life of 3-5 years or longer with lower mass, and with no lifetime failure caused by a thermal short in the stored cryogen.1 However, reliability and vibration must be considered, since moving parts exist in mechanical coolers. The Soft X-ray Spectrometer (SXS) on-board the 6th Japanese X-ray astronomy satellite, AstroH, is a high resolution spectrometer utilizing a microcalorimeter array.2-3 A resolving power of 1000 or larger at 6 keV of X-ray energy can be achieved by cooling down and operating at very low 2 CRYOCOOLERS FOR 1-30 K AEROSPACE APPLICATIONS temperature of 50 mK. The SXS cooling chain from room temperature to 50 mK, provided by an adiabatic demagnetization refrigerator (ADR), includes a 1.3 K pumped liquid He and a mechanical cryocooler as a precooler for the liquid He.4-5 In the baseline design of the SXS cooling system, two 2ST coolers are used to cool the radiation shields (IVCS and OVCS), while one 4He J-T cooler is used to reduce the parasitic heat load to the liquid He tank. The Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mission has been proposed as the new-generation infrared (IR) space telescope led by JAXA with the participation of ESA.6-7 The thermal environment at a halo orbit around the Sun-Earth second Lagrangian point (L2) enables the large IR telescope to be cooled down to 4.5 K with advanced mechanical cryocoolers and effective radiation cooling instead of a massive and short-lived cryogen. In the thermal design study of SPICA, two 4He J-T coolers are needed to cool down the IR detectors as well as the large telescope, while the 1.7 K cold temperature region provided by two 3He J-T coolers are also required to cool the far infrared detector and act as a precooler for the lower temperature cooling devices. This paper describes the current development status of 1-4 K class mechanical cryocoolers serving as an engineering model for the new generation of Japanese satellites. We initially describe the improvements to the 2ST cooler and the performance test results. The development status of the 4He J-T cooler is also shown. Then, the requirements, the preliminary test results of the prototype model and the current status of 3He J-T cooler will be described. DEVELOPMENT OF DOUBLE-STAGE STIRLING COOLER Original Design The 20 K class 2ST cooler was originally developed for the first Japanese infrared astronomy satellite Akari8-10 launched on February 2006 based on a two-stage small Stirling-cycle cooler with temperature below 20 K for space applications developed in 1991.11 Figure 1 shows a cross-sectional view of the existing 2ST cryocooler for Akari. This cooler is a split-Stirling cycle cooler composed of a double-staged cold head with two expansion stages, a linear compressor and a connecting tube. The compressor has two opposing pistons to cancel dynamic vibrations. The piston drive shaft is supported by two sets of linear ball bearings to maintain the clearance seal, and is driven by a voice coil motor and coil springs. The cold head consists of a two-stage displacer and an opposing active balancer that works as a counterweight to cancel the dynamic vibration of the displacer. The displacer uses stainless steel meshes for the regenerator and is supported by Teflon contact seals. It’s driven by a voice coil motor and coil springs are the same as the compressor. The total weight, the driving frequency, and the gas pressure are 9.5 kg, 15 Hz, and 1.0 MPa at 240 K, respectively. The input power is 90 W and it provides 200 mW of cooling power at 20 K. The Akari cooler has been in continuous operation for more than 3 years, which exceeds the original specification of 1.5 years.12 Figure 1. Schematic drawing of the Akari double-stage Stirling (2ST) cooler 3 1-4 K COOLERS FOR SATELLITE MISSIONS IN JAXA Requirements for Future Space Mission From the orbital performance result of the 2ST cooler, more reliable and larger cooling power with longer life time are needed for the next generation satellite. Basic requirements are as follows; (1) longer life time (3 years is required, 5 years is a goal), (2) 200 mW of cooling power at 20 K, and (3) lower mechanical vibration. On the other hand, the improvement in the cooling performance of the 2ST cooler can directly increase the J-T cooling power, and can improve the thermal margin which will accommodate design uncertainties and incidents. Fabrication and Performance Test of 2ST Cooler Improvements in the 2ST cooler design are divided into three sections. One is the displacer's support mechanism to reduce the risk of mechanical abrasions based on the experience with the Akari 2ST cooler. Since the 1st and the 2nd stage displacer design of Akari 2ST cooler are supported by contact seals, mechanical abrasions can occur in orbit due to the slide contact on the inner surface of the displacer cylinder, which significantly affects the degradation of the cooling performance during long term operation. As a new design, flexure springs are used for the displacer drive shaft support to keep the driving axis aligned. As a consequence, the compressor's pistons have to be supported by linear ball bearings for long piston strokes with 15 Hz of low-drive frequency. The second improvement is the selection of components to reduce outgassing in the 2ST cooler. Increases in the working gas impurities can strongly affect cooling performance, making it critical to minimize outgassing from internal components to maintain the nominal cooling power for more than 3 years with margin. We selected low-gassing materials and the working gas was substantially purified before assembly. The amount of glue used to fix the permanent magnet was reduced, and the baking process for degassing in the cooler was also optimized. The third improvement is the diameter of the 2nd stage displacer. The diameter was increased to 8 mm to enlarge the gas expansion volume. A cooling power of 325 mW at 20 K was achieved with this design change, while the heat load through the 2nd stage cold head must be increased in the case of the failure of the 2ST cooler. The upgraded design of the 2ST cold head is shown in Figure 2. In a performance test of a breadboard model (BBM) of the upgraded 2ST cooler, a cooling power of 200 mW at 16 K was achieved at the 2nd stage at the same time that 1 W of cooling power at 83.6K was successfully obtained at the 1st stage, by optimizing the drive conditions of 90 W of input power. The working gas pressure was 0.9 MPa (1.0 MPa in nominal) and the voltage phase angle difference between the compressor and the displacer in the cold head was 170 degree (180 degree in nominal) for the best performance. The engineering model of the 2ST cooler with upgraded design was then fabricated as an evaluation test model for Astro-H/SXS. The cooling performance test was conducted under the thermal conditions of its operating temperature of -70 to +30oC. The 2ST cooler test model was mounted on the test plate where the temperature of these interfaces to the 2ST cold head and the compressor were controlled using a laboratory Gifford-McMahon (GM) refrigerator in the vacuum vessel. A working gas pressure of 1.4 MPa was charged at room temperature to ensure the cooling performance at a pressure of 1.0 MPa and a temperature -70oC. The test results at each condition are shown in Figure 3. Figure 2. Photograph of cold head of upgraded 2ST cooler 4 CRYOCOOLERS FOR 1-30 K AEROSPACE APPLICATIONS The relationship between the 2ST 2nd stage temperature (lower temperature stage) and 1st stage in the case of 50 W and 90 W of input power was successfully obtained. A nominal cooling power of 200 mW at 20 K of the 2ST 2nd stage temperature under the condition of 90 W of input power was achieved within the operating temperature range. In this measurement, the lowest temperature of the 2nd stage with 200 mW of cooling power with 1 W of cooling power of 1st stage (83.4 K) was 17.0 K. A life test was also begun, and the detailed status is described in the next section. Mechanical vibration induced by the 2ST coolers as well as the 4He J-T coolers is one of the critical issue for