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The ground cryostat (C2CC - Cryostat for 2 K core Calibration) being developed at INTA for cryogenic tests and future missions includes a 300 K vessel, intermediate shields at 40 and 4 K (temperatures achieved with Pulse-Tube crycoolers) and final 2 K shield. The heat switch is made up with a conductive core consisting of graphite (pyrolytic) embedded at its two extremity in a copper fitting to provide a high coupling between the interface and the graphite core. The graphite material is chosen because the thermal conductivity reduction with temperature, being at 1 K 10000 times lower than at 300 K. As a consequence, when the cool down starts, higher heat loads can be removed from the 2 K shield by the 4 K shield coolers through the heat switch. While the stages are being cooled, this heat removal decreases little by little, and when the operating temperatures are reached, the heat switch barely conduces heat, so the heat loads from the 4 K shield on the Joule-Thomson cooler are so low that they don’t affect the nominal behaviour of the cryostat. At INTA, a heat switch was produced and tested, in order to obtain a thermal conductivity curve of the material through experimental means, and the total conductance of the thermal switch. This characterization should enable to validate this design, and conclude on the introduction of this element in the thermal design of the cryostat. |
