Development of Simple Diolution Refrigerators free from Cryogen refilling

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Dilution refrigerator is the very low temperature refrigerator continuously operable down to much below 1K and in the extreme case to 2 milli Kelvin [1]. The complication of the machine and the operation often limits the versatility of such a refrigerator covering the wide temperature range over a factor of a few hundred or so, which is essential for basic research in many fields [2].The first simplification of the refrigerator was brought about by the extinction of the pumped He bath for the condensation of the circulating 3He, instead the enthalpy of the 3He pumped out from the still is utilized in the JT precooling heat exchanger [3]. We have been using a number of JT precooled Dilution Refrigerators and One of them is being used for experiments under world fastest rotation for the fundamental study of physics of quantized vortices for years because of the reliability and the simplicity of the refrigerator [4].The second important simplification is achieved by the elimination of the cryogen usage. The development of the Cryocoolers enables us to operate a dilution refrigerator without cryogen [5], yet the viblation limits the ultimate low temperature as well as type of experiments.We have achieved continuously operating dilution refrigeraor which runs over many weeks without reffiling the cryogen while keeping the temperature below 40mK.This was made possible by combination of the hybrid He refrigerator, consists of Gifford McMahon cycle with solid displacer as the 1st stage and a double inlet pulse tube refrigerator as the second with a JT precooled dilution refrigerator which does not have 1K pot nor liq. ^4He bath. Further study of the combination of vibration isolation and good thermal contact is being carried out in order to obtain the condition to perform experiments below 10mK.