NIRC Cooldown Characteristics

There are two main categories of cooldowns: from ambient dome temperatures to LN2, from LN2 to LHe, and from ambient temperature all the way to LHe. This last is not necessarily a concatenation of the first two, because we are talking about a nonequilibrium process.

Ambient to LN2 (90 hours)

The following shows a warmup from LHe temperatures to ambient, followed by a cooldown from ambient to LHe. Concentrating only on the cooldown from ambient to LN2 (hours 130 through 220) we see a rpiad drop in the LN2 temperature. This is from the initial LN2 dewar fill. Whenever liquid is in a dewar, the dewar temperature plummets rapidly. Of course, a warm dewar will also burn liquid off quickly.

Once the detector and LHe dewar are below ~190 K, it is safe to fill the LHe dewar with LHe. This will accelerate the cooldown at the expense of a modest amount of LHe. LHe fills can be seen at ~160 hours and again just past 200 hours below. The first fill does not last long; the LHe dewar only gets to 155 K before the cryogen has boiled off.

The total time for the LN2 dewar to reach LN2 temperatures is small. The time for the detector to reach LN2 temperatures is 90 hours.

 

LN2 to LHe (20 hours)

Below is a cooldown from an equilibrium, LN2-only state (the normal "warm" condition, where only the LN2 dewar is kept filled) to LHe temperture. The initial LHe fill occurs at 43 hours. Note that the LHe dewar pulls the LN2 dewar down five or six degrees. The detector reaches operating tempertures (25-35 K) after ~20 hours, and gets close to equilibrium after less than 30 hours.

 

Ambient to LHe (145 hours = 6 days)

A full cooldown from ambient to LHe is shown below. Note the acceleration fills at 64, 87, and 90 hours. More agressive LHe fills in this time period would accelerate the curve even more. This curve does not follow the detector all the way to LHe temperatures, but after 155 hours (145 hours after the initial LN2 fill) it is reading near its operating temperature.

Note that no LHe fills occurred in the period between 90 and 130 hours. The LHe dewar remained at nearly constant temperature while the detector slowly followed it. Probably an entire day or more could have been saved during this cooldown if LHe fills had continued during this time period. LHe fills during the time period between 64 and 87 hours would shave even more time off the curve.

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