Mr. Miguel Fernández Sánchez Profile

Miguel Fernández Sánchez

at INTA

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Proceedings Article | 26 August 2024 Presentation + Paper

Low stiffness thermal straps for cryogenic applications

J. Torres, M. Martinez, M. Fernandez, A. Diaz, S. Martin, A. Sanchez, M. Reina

Proceedings Volume 13100, 131002W (2024) https://doi.org/10.1117/12.3017979

KEYWORDS: Vibration, Design, Accelerometers, Cryostats, Cryogenics, Prototyping, Cryocoolers, Aluminum, Industrial applications

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In cryogenics, working together with cryocoolers, the thermal straps are a critical thermal element, since they thermally link the instrument with the cold source. In optical elements that work at cryogenic temperatures, the thermal path, and therefore the straps, must isolate them from the vibrations of the cryocoolers. Consequently, its stiffness is a characteristic to consider in the design. But, in the case of optical focal plane arrays, working at very low temperatures such as 50 mK, it can be resonant with the low frequency range of the cryocoolers, and its dissipation can break the superconducting state of the sensors, the basis of its functioning. It is therefore an undesired thermal effect derived from structural behavior. Design limitations impose the need for straps where stiffness is just as important as its conductance. The stiffness value needed is much lower than the one found in commercial straps. Therefore, this work highlights the design, and characterization tests of different strap prototypes in order to achieve the necessary resistivity and thermal conductivity for low cryogenic temperatures. The thermal straps were initially designed for the Cryostat facility for 2K Core Calibration (C2CC) a ground equipment for the ATHENA project (Advanced Telescope for High - ENergy Astrophysics, ESA).

Proceedings Article | 26 August 2024 Poster + Paper

Heat switch to reduce cooldown times for a ground cryostat

M. Fernández-Sánchez, L. Bastide, A. Sánchez, M. Reina

Proceedings Volume 13100, 131006W (2024) https://doi.org/10.1117/12.3016954

KEYWORDS: Switches, Cryostats, Graphite, Electrical conductivity, Copper, Sensors, Design, Cryogenics, Tolerancing, Calibration

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A passive heat switch is an element used to conduce heat flow at high temperatures (from 300 to 100 K) and to insulate at low temperatures (< 10 K). Its goal is to accelerate the cool down transient phase and reduce the waiting time from the cryostat closure until the temperature stabilization required to perform the tests.
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.

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