Refrigerator

The 1 K refrigerator for the polarized target consists of a LHe bath pumped to a pressure of approximately 0.2 mbar by a combination of rotary vane pumps and Roots blowers. Liquid helium is supplied to the evaporation chamber (or ``tailpiece'') from the separator through a 5 mm copper tube. The tube is heat sunk to a series of ten perforated copper plates that act as liquid-gas heat exchangers between the incoming liquid and the outgoing vapor. The flow of liquid through the heat exchangers is metered by needle valve EV91120 (aka the ``Run'' valve). A PID loop controls the Run valve to maintain a constant level of LHe in the evaporation chamber. A second needle valve, EV91121 can be used to bypass the heat exchangers but is usually kept closed.

There are no computer controls for these pumps, although a remote control station is located in the counting house. Strain gauge PI91140 (0-1000 mbar) measures the pressure in the main pumping line, while PI91145 monitors the pressure at the inlet of the rotary vane pumps. Liquid level indicator LL91112 measures the amount of LHe in the evaporation chamber, or ``tailpiece''. Stepper motor-controlled valves EV91120 and EV91121 meter the flow of LHe through and around the heat exchangers, respectively. Both valves are equipped with optical encoders for position readback.