SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Gap ( 100 µ m) Copper parts Filling tube 104 Thin walled tube Heater Cold heat sink side Miniature sorption pump Figure 4-45 - Schematic representation of the gas gap heat switch. 48 hours recycle hard mounted to 4K FPU Optical Bench Separate straps from Pump and Evaporator to L0 Helium Tank, both via heat switches. Separate straps necessary to prevent warming of evaporator during cooler recycling. Evaporator strap has high conductance requirement (>100mW/K @ 2K) in order to limit temperature at end of recycling. Leak before burst titanium alloy (Ta6V Eli 3He cooler : 264 mK ultimate T, 20 µW @ 300 mK 2-K I/F Kevlar Suspension Sorption Pump (SP) Thermal Shunt (TS) Figure 4-46 - Illustration of the 2-K cooler. Pump tube Evaporator (EV) Support Structure (SST) Figure 4-47 illustrates the operation of the cooler during recycle. At the end of the cooling cycle, there is no liquid 3 He remaining in the evaporator and therefore no more cooling power can be delivered to the cold tip. At this point in time, all the 3 He will be contained in the sorption pump. Current is fed to ES1 (see Figure 4-48) and the Helium absorbed in the miniature sorption pump is driven off and fills the gap between the copper cylinders in the gas gap heat switch. The thermal conductivity of the switch rises greatly and the evaporator becomes thermally shorted to the 2-K stage of the cryostat. At the same time, no current is passed
Draft SPIRE Design Description Document to the evaporator heat switch and all the Helium remains in the gas gap miniature sorption pump. A vacuum exists between the copper cylinders in the gas gap switch and the evaporator remains thermally insulated from the 2-K stage. The evaporator heater (PH1, Figure 4-48) is switched on and the temperature rises to approximately 40 K. This state can be seen in Figure 4-47A. A shunt halfway along the pump tube prevents heat leaking from the pump to the evaporator. At 40 K, the 3 He that has been held in the pump is driven off and recondenses in the evaporator. Once all the 3 He has condensed in the evaporator, the pump heat switch is closed to cool the pump back down to approximately 2-K while the evaporator heat switch is opened to thermally isolate it and allow it to cool below the 2-K stage. This state is represented by Figure 4-47B. Once the pump nears 2 K, it starts to pump on the evaporator and liquid 3 He starts to boil off. The latent heat of evaporation of the 3 He causes the temperature of the liquid to drop below 300 mK and simultaneously provide cooling power to the BDAs. (see Figure 4-47C). OFF Pressure A 40 2+ε 2 C 0.3 K T (K) ON B A 2 K Temperature B ON OFF 2
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