Warmup Procedure for SPARO Cryostat

Step 1: Close both heat switches

This step essentially makes sure that everything is in thermal contact in the inside of SPARO. This way, everything warms up at the same rate. The heat-switch with green tape is the charcoal pump heat switch. It takes 40 turns to close. The heat-switch with the red tape is the Helium-3 pots heat switch. It takes more like 90-100 turns to close, I think. For both heat switches, clockwise closes them.


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Fig 1 - SPARO heat switches: Charcoal pump heat switch (green) is at 11 hours. Helium-3 pots heat switch (red) is at 4 hours.

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 pumped pot line should already be connected to the "flushing fixture". This fixture has two valves: one is supposed to go to a UHP Helium bottle, and the other goes to the Alcatel 2015 pump. (see Figures 11, 12, and 13 of "Procedure for Removing SPARO from Viper"). At this point in time, the valve to the UHP bottle should be closed, and the valve to the Alacatel 2015 pump should be open.
• Attach a small diameter hose running from a UHP ⁴He bottle (with regulator attached) to the "flushing fixture". Make sure to use a regulator that is graduated in steps of 2 lbs. to avoid overpressurizing.
• With the hose to the UHP bottle disconnected from the "flushing fixture", open the regulator to allow ⁴He to flow through the hose - in order to flush out any air.
• Reconnect the UHP line from the UHP bottle to the flushing fixture. Set the pressure on the UHP regulator to 10-15 psi. [Note for 2003: use 5 psi to avoid making the leak worse.]
• Use your fingers to close off (pinch off) a section of the UHP ⁴He flushing line at a point near to the flushing fixture, and then briefly open the valve on the "flushing fixture" 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 release the hose.
• Close the valve on the "flushing fixture" that connects to the mechanical pump, and open the valve on the "flushing fixture" that goes to the UHP line. Now you are flushing UHP He gas through the pumped pot and into the reservoir.

Step 3: Open the ³He valves

Open the two green-knobbed valves (see Figure 2) 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 in case the relief valves fail to open.

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Fig 2 - Location of one of the two green-knobbed valves is shown. One is on the guard 3He stage, and one is on the detector 3He stage.

Step 4: Connect the pump diode thermometer

We need to monitor the temperature inside SPARO. We will do this by monitoring the Si Diode thermometer on the charcoal pumps in SPARO. It is necessary to use the "lab breakout box" (see Fig. 3) in order to monitor this thermometer. Connect this breakout box to the cycling connector via the strange-looking RF choke (see Fig. 4). This is how you read out the Si diode thermometer: First find the Si diode current source box. You can borrow one from the SPARO rack. This box is powered by 9 V batteries. The way to hook it up is to put a BNC T on the current source box, and then connect the two ends of the T to (a) The Charcoal pump Si Diode thermometer BNC output on the breakout box, and (b) a DVM. The chart for converting voltage to temperature for a Si diode thermometer is in the blue binder of SPARO documentation. (also on the SPARO web page under "Cryostat" menu item.)

larger	Fig 3 - Lab Breakout Box and Si Diode Current Source Box
larger	Fig 4 - how to connect the lab breakout box to the cycling connector on SPARO.

Step 5: Allow the system to warm

Allow SPARO to run out of Helium in the main reservoir. Then it should slowly warm - in a day or perhaps two. When the temperature is above 280 K, you can (gently) admit air into the vacuum jacket, but then reclose the valve once you have 1 atm in SPARO. Then wait a few hours for things to equilibrate, and then you can open SPARO.

Note that as the system warms between 25 K and 40 K, you will notice the Ashcroft gauges on the ³He systems slowly rising

If you get impatient waiting for SPARO to warm, you can use a copper rod that should be in the SPARO cabinet. This rod is long enough and skinny enough to stick down the neck tube of SPARO. Use of the copper rod is amazingly efficient at warming. You can even use it to get rid of the remaining liquid Helium if you don't want to wait for dewar to run out of liquid Helium naturally. Here is how rod is used: Just insert rod in necktube- wait 5 min. - remove rod - apply heat gun for 10 min. or until rod is warm - dry rod well - insert rod in necktube - wait 5 mins - etc. You will find it takes very little time to warm the first few tens of degrees in this way. I think we warmed SPARO to ambient in 6-12 hours once using this method.

A note on safety. There is a danger that a small leak over a period of months could cause a lot of frozen gas to adhere to the reservoir. Upon warmup, this gas evaporates and could cause an overpressure of the dewar. Fortunately, the snout, which is held on by an o-ring and only two plastic screws, allows a safety blowout valve.