SPIRE Design Description - Research Services

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