Development of a closed-cycle dilution refrigerator for astrophysical experiments in space

Orateur : Angela VOLPE (Institut Néel, équipe HELFA)

Several satellite missions are being proposed to resolve many of the unanswered questions regarding the Universe. Instrume nts on some of these missions will require temperatures below 0.1 K to operate effectively. This requirement means that the cooling chain is a crucial element of the satellite’s design. The cryogenic specifications of these future missions are more demanding than the current state of the art : they will require a cooling power of 1 µW at a temperature of 50 mK for a lifetime of 5-10 years. This has motivated the development of a closed-cycle 3He-4He dilution refrigerator. This design is based on the open-cycle dilution refrigerator used on the Planck satellite, whose lifetime and cooling power were limited by the on-board supply of 3He and 4He, since the mixture was ejected into space after the dilution process. To overcome these limitations, the cycle has been closed by separating the helium isotopes at low temperatures and re-injecting them into the refrigerator. This thesis describes the progress in the development and comprehension of this new closed-cycle system and shows that the cooling requirements (1 µW at 50 mK) can be met under the required conditions. This work also attempts to solve a problem related to a micro-gravity environment : the vapor-liquid phase separation in the still. Our experimental results show that liquid confinement in the still under required conditions and negative-gravity is possible. These results have driven the design of a novel gravity-insensitive still, the last step in developing a closed-cycle dilution refrigerator adapted to zero-gravity.

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