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### Overall Risk Assessment: LOW
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We are concerned about the loss in performance of the cold heads between the manufacturer's published data and the actual performance at GBT due to the 600 ft helium lines connecting prime focus to the compressor platform. We plan to experiment with this in the lab by introducing an orifice plate to simulate piping losses and quantify the effect on the system prior to deployment. However, ALPACA is operating in the "easy" cryogenic regime (well above 4 K), where material behavior is well characterized, and bolted joints offer little thermal resistance.
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We successfully modeled the thermal performance of AO-19, the prototype of ALPACA, and carry over much of the design. The cold head cooling capacity increases for increases in operating temperature, while the LNAs (the critical requirement) show only a weak dependence on physical temperature: T<sub>N</sub> changes by ~2 K by changing T<sub>phys</sub> from 20 K to 30 K. Furthermore, the heat budget currently has considerable remaining margin for this level of design maturity, roughly 40% per cold stage. In summary, we feel the overall risk due to the cryogenic system and thermal model design is very low.
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[Section 3.5 >>](/3-front-end-design/3.5) |
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