The cooled receiver components are located in a
hybrid dewar which utilizes a CTI 22 refrigerator head and a CTI
compressor. During normal operation the compressor should indicate
a supply pressure of 
lbs.
If the dewar is at atmospheric pressure, it should be pumped down to
below 
Torr before the refrigerator is turned on.
The refrigerator cools the two radiation shields down to 
and
K. Lakeshore temperature diodes are located on
the two radiation shields and also directly on the cold plate and on top
of the mixer. These temperatures can be read out with a Lakeshore temperature
monitor. The 70 K and 14 K shield temperature sensors are at monitor positions
1 and 2. The cold plate and mixer temperature sensors are at positions
3 and 4, respectively. The temperature of the 4th stage should be
checked several times a day to prevent an unforeseen warm-up.
The vacuum window on the bottom of the dewar is made of a 32.0 mil thick piece of polyethylene. Antireflection grooves have been milled into both sides of the window. Never work with sharp tools under the dewar; one false move and Whamo! If moisture is present on the window, it can be gently wiped away with a Kim-Wipe. We suggest placing a small muffin fan under the dewar so that it blows past the window. The air circulation will prevent the accumulation of moisture.
If the dewar's vacuum is ever compromised,
the helium will boil off quickly. If this happens
don't panic! Simply hook the vacuum pump up to the dewar and let it
pump down to a few times 
Torr. If the helium is all gone,
seal off the vent and fill tube with a couple of pieces of rubber
and rubber bands. Sealing off the vent and fill tube will prevent
the helium can from warming-up too much while you are pumping down.
If the dewar is pumped back down and the can temperature is between
80 and 100 K, you can refill it directly with helium. If the
temperature is much below 80 K, there is a chance ice could be on
the bottom of the helium can. I would suggest blowing helium gas
through the can until a temperature of 80 K is achieved before adding
the liquid helium. If the temperature is much above 100K, you might
wait until the can cools down before adding more helium.
If an ice plug forms in the vent or fill tube, stick a funnel in the blocked tube and blow warm helium gas through the funnel. I usually do this by ramming the plastic hose from the helium bottle down the throat of the funnel. If the ice block is stubborn, you can use a heat gun to warm the gas passing through the funnel just before it enters the blocked tube. Be patient, the ice block may take a few minutes to clear.