Capillary Purging Procedure for SPARO
Cryostat
The general philosophy of this procedure is to start to warm SPARO and then
halt the procedure around 100 K and then cool it back down again. The
hope is that any frozen atmospheric gasses that may be plugging the
capillary will be evaporated at 100 K and forced out by a flushing pressure of
4He.
Step 1: Close Both Heat Switches
Step 2: Begin flushing the capillary
Before the 4He runs out (level ~2" or so), begin flushing the
capillary with about 10-15 psi of UHP 4He. The procedure is the
same as in the cool down.
- Connect the Pumped Pot line to the fixture which has two valves- one which
goes to a UHP Helium line and one which goes to the Alcatel 2015 pump.
- Disconnect the UHP hose and allow 4He to
flow through it in order to flush out any air.
- Reconnect the line and
pinch off a section of the UHP 4He flushing line(tygon) and open the valve so
as to pump out the pinched off section of the line.
This step has the importance
of clearing the valve volume of any gases other than helium which may be trapped.
- Reclose this valve and unpinch the hose.
- Close the valve connecting the pump to the pumped pot line and begin flushing
UHP He gas through the pumped pot(use a pressure of 10-15 psi).
Step 3: Open the 3He valves
Open the two green-knobbed valves which separate the external 3He
tanks from the internal systems. This gives the expanding gas more volume
in which to expand and prevents a rupture of the delicate internal
plumbing should the relief valves fail.
Step 4: Connect the pump diode thermometer
It is
necessary to use the lab breakout box in order to monitor the temperature
of the dewar. Connect this breakout box to the cycling connector via
the RF choke. Connect a voltmeter to the Pump Diode BNC connector in parallel
with a 10 microamp battery supply. This voltage will give you the temperature
of the reservoir since it now should be heat sunk to the charcoal pumps.
Step 5: Allow the system to warm
The system will slowly warm. As it does so, you will notice the
Ashcroft gauges on the 3He systems slowly rising between 25 and 40 K.
Allow the system to warm to around 100 K. This should take 1-2 days.
Step 6: Pump on the reservoir
This step is designed to remove any remaining atmospheric gasses which
may have been frozen from the first fill(namely Nitrogen). It is
probably a good idea to pump for several hours.
Step 7: Backfill and Cool
Backfill the reservoir with UHP 4He. It is probably a good idea
to do this gently, being careful to purge the backfill line of any non He gasses.(e.g. see pinch-off technique described in step 1.) Once the reservoir is at ambient pressure, begin
cooling SPARO by transferring LHe. It should take between 60 and 75 L
to cool back down to 4 K. Note also that the cryostat will take about a
day to cool entirely, thus it will take time for the 4He hold time
to return to its maximum. Keep an eye on the level. The flow may be higher
when flushing helium through the capillary, but it should still be onscale
on the 11 lpm flow meter.
Here is some hard data on the time to cooldown: From logbook 6, page12.
-
cooling from 135 K
-
using 250 liter at pressure of 2-5 lbs. (use heater element to keep pressure)
-
saw plume after two hours had elapsed since beginning the fill
-
at the point when we saw plume, Si-diode thermometer on charcoal pump
was reading 30 K.
Step 8: Begin Pumping on Pumped Pot
Close the flushing valve. Pump out line from pump to Pumped Pot valve.
Open valve slowly. The pressure should drop as the pumped pot cools and
begins to fill with liquid. The pressure should come back up again after
a few hours.