EVACUATING AND COOLING THE
Rafael Millan Gabet, March 2000
We have had instructions on how to do this for a while, however I think
that they don't emphasize enough the importance of BAKING the dewar while
evacuating, in order for the detector to properly cool to about 77K.
If the dewar is not warm, empty the LN2 and let it warm up to ambient temperature
(about 2 hours min.), perhaps speeding up the process by allowing some
air molecules into the cavity: cover the input with your finger and open
the valve for one second, or inject some gaseous nitrogen.
Do step (4) until the test port pressure reads about 10 mTorr.
Once the dewar is warm, follow the instructions elsewhere to:
1 - start the pump
2 - connect the dewar to the pump
3 - evacuate the lines
4 - open the dewar valve to pump on the dewar cavity
Wrap the dewar in heating tape (be careful with the window when you do
this). Set the heating tape variac to 40-50 Volts.
Pump and heat for several hours. If you can hold the dewar with your hands
for several seconds, then it's temperature is OK (not too hot).
There is a temperature sensor close to the detector. However, connecting
the readout circuit to it is still not documented. In the meantime, contact
you want to use the T-sensor. Under normal circumstances though, the heating
& cooling times given here can be trusted.
If time permits, it is ideal to heat&pump for several hours and then
let the dewar outgas overnight (see instructions elsewhere for overnight
shutdown of the pump). I also recommend leaving the heating tape on during
the night, at a lower 30 volt. The next day, resume heat&pump
(40 volt variac) for 2 - 3 more hours.
Turn-off the heating tape, close the dewar valve and shutdown the pump
Let the dewar cool down a little, about 1/2 hour.
Pour LN2 into the dewar cans (inner and outer). The pressure will be too
high for the ion pump meter to be in range, but its voltage can be read
with a voltmeter between the blue and black leads, and should go down to
under 7 volt relatively quickly (a few minutes). In about 1 hour the ion
pump meter should be in range, reading about 5x10-6
Torr, or about 5 volt on the voltmeter. In about 2 more hours the ion pump
meter should be reading a final value of about 5x10-7
Torr, which corresponds to about 3 volt.
The 1/e cooling time of the detector is about 1 hour, so a couple of hours
after pouring the first batch of LN2, the array should be ready to operate.
The inner LN2 can will hold more than 24 hours, so it is enough to re-fill
it once per day (for example every afternoon).
The outer LN2 can has a hold time of less than 24 hours, therefore it must
be re-filled twice a day (for example once in the afternoon at the same
time as the inner can, plus once more at the end of each night).
Use a funnel with a long neck to fill the dewar inner can, so that it
basically touches the bottom of the can. Otherwise, the LN2 will just
boil on its way down and fool you into thinking that the can is full.
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