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<kbd>Figure 2: Section view of the ALPACA cryostat along a row of antennas. The radio frequency transparent window consists of HDPE (blue) over 4 layers of Rohacell 71 HF foam (pink). The 20 K antennas pass through the 1st stage (red highlight) and into the foam with no thermal contact. The 2nd stage is completely shielded from room temperature heat loads. The vacuum vessel solid top half is welded aluminum.
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The functional requirements for the window are that it must be vacuum tight, RF transparent, structurally sound, and transmit a manageable heat load to the internal cryogenic stages. Our solution is to functionally separate the vacuum from the structural and thermal requirements. As demonstrated with our PAF prototype[^2], a durable vacuum tight structure is achieved with a welded HDPE top hat that is only ~3 mm thick. An RF transparent foam, Rohacell 71 HF, fills the vacuum side of the HDPE, supporting it from within and keeping it from collapsing. The foam transfers the atmospheric load to the 1st cold stage, which in turn transmits it to the top plate of the vacuum vessel where it is balanced by the atmospheric load on the other side. This design also solves the black body heat load problem because the foam is opaque in the infrared. This changes the window heat transfer method from radiation to conduction, and since the thermal conduction of the foam is rather poor (0.025 W/m/K), the heat load on the 1st stage is limited to a manageable 50 W.
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The functional requirements for the window are that it must be vacuum tight, RF transparent, structurally sound, and transmit a manageable heat load to the internal cryogenic stages. Our solution is to functionally separate the vacuum from the structural and thermal requirements. As demonstrated with our PAF prototype, a durable vacuum tight structure is achieved with a welded HDPE top hat that is only ~3 mm thick. An RF transparent foam, Rohacell 71 HF, fills the vacuum side of the HDPE, supporting it from within and keeping it from collapsing. The foam transfers the atmospheric load to the 1st cold stage, which in turn transmits it to the top plate of the vacuum vessel where it is balanced by the atmospheric load on the other side. This design also solves the black body heat load problem because the foam is opaque in the infrared. This changes the window heat transfer method from radiation to conduction, and since the thermal conduction of the foam is rather poor (0.025 W/m/K), the heat load on the 1st stage is limited to a manageable 50 W.
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Since it is more efficient to intercept environmental heat loads at higher temperatures, an intermediate first cold stage “surrounds” the 20 K second cold stage, sinking room temperature thermal radiation before it reaches the 2nd
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stage, and providing a temperature guard for wiring and structural components. This two stage configuration is readily aligned to commercial two-stage Gifford-McMahon cryocoolers. In our case, three CTI-1020 cold heads from Trillium are sufficient to handle the load. However, the distance between the antennas and LNAs must be small to minimize signal loss and added noise. This results in a unique design where the antennas at 20 K pass through the ground plane at the first cryogenic stage temperature with no thermal contact (see Figure 2 below).
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[Section 3.3 >>](/3-front-end-design/3.3)
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[^a]: Please refer to [Parshley et al. 2020](../uploads/7c6759a47aba883325e3d4aabe9049e8/ASME-PVP2020-21818-Parshley-vFinal.pdf) for more details of the cryo-mechanical design of the ALPACA Cryostat |
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[^a]: Please refer to [Parshley et al. 2020](../uploads/7c6759a47aba883325e3d4aabe9049e8/ASME-PVP2020-21818-Parshley-vFinal.pdf) for more details of the cryo-mechanical design of the ALPACA Cryostat |
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