Cooling capability of JT coolers during the cool-down phase for space science missions

•The cooling down experiments were performed for the 4 K-JT and the 1K-JT.•It has been confirmed that the 4K-JT can cool down to 4.5 K with 21 mW.•It has been confirmed that the 1K-JT can cool down to 1.7 K with 5 mW.•The 4K-JT cooling capability between 4.5 K and 20 K was calculated.•The cooling times of large payloads cooled by the 4K-JT were estimated. In space science missions that uses cryogen-free mechanical cooler systems to cool the low temperature telescope and detectors, a cooling time from room temperature to low temperature should be considered in the required mission life time, because of a limited cooling power of mechanical cooler. For instance, in the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), the 4K-class Joule Thomson cooler (4K-JT) and the 1K-class Joule Thomson cooler (1K-JT) are being considered to cool a 2.5 m telescope and provide a 1–4 K environment for scientific instruments. The cooling capability of these JT coolers, below 20 K and even lower than 5 K, is critical for initial cooling. The 4K-JT cooling from precooling to operating temperatures under the heat input was measured and confirmed that the 4K-JT has enough cooling power to cool with these heat load. The 1K-JT cool down measurement with heater was also performed. These results enabled us to estimate the cool down time for the heat capacity provided by the telescope and scientific instruments assumed in SPICA, LiteBIRD and the ESA X-ray mission ATHENA.