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 ti...

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