Differences

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--- stm:manual_cryostat [2009/05/18 12:28] – galli
+++ stm:manual_cryostat [2009/05/18 12:36] (current) – galli
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   IMPORTANT!: Do not pump the Helium tank if ALSO the OVC is NOT pumped!
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   - The long hose that was used for the initial refill should be left to air or better to an empty LN2 Dewar, so that you don't waste the LN2 coming out of the cryostat. It is very important now that the siphon is really touching the bottom of the helium tank, otherwise some LN2 will remain.
   - Open the gas outlet of the LN2 Dewar, slowly first than increase the pressure. LN2 should immediately start coming out of the tank. When no liquid comes out anymore, flow some dry N2 gas for 5-10 minutes to be sure that no liquid remains. The magnet temperature might/should rise a bit too (380 Ω or so).
-  - Remove transfer line and close the tank. Remove "Xmas tree" piece and blind the tank with a flange. Open the valve to recovery, after having switched to the helium pump (1K-pot pump). Pump the tank and keep an eye on the pressure (use 1K-pot P gauge): if the recovery tube freezes and the pressure does not drop below 10mbar, STOP PUMPING. It means that there is still LN2 in the tank, and it should NOT be removed by pumping but by flowing more warm gas longer.
+  - Remove transfer line and close the tank. Remove "Xmas tree" piece and blind the tank with a flange. Open the valve to recovery, after having switched to the helium pump (1K-pot pump). Pump the tank and keep an eye on the pressure (use 1K-pot P gauge): if the recovery tube freezes and the pressure does not drop below 10mbar, STOP PUMPING. It means that there is still LN2 in the tank, and it should NOT be removed by pumping but by flowing more warm gas longer. In any case, if the siphon was lowered fully till the bottom in the previous steps and warm nitrogen gas was flown for 10 minutes after all nitrogen came out, it's very unlikely that liquid nitrogen is still inside.
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   It is very important to flush the helium tank, the capillary and rinse the needle valves with dry-clean helium gas before cooling down, or the capillaries and needle valves will freeze. The whole tank should be pumped and flushed several times in order to pump away all Nitrogen gas which will otherwise freeze during liquid helium transfer.
-  - Be sure that there's enough helium overpressure in the recovery and in the tank.
+  - Be sure that there's enough helium overpressure in the recovery and in the tank. Then insert liquid helium transfer line (the half that goes in the cryostat) with the usual procedure that avoids transfer line blockage.
-  - Open 1K-needle valve (something like 80%). You might also open the UHV-sock needle valve a bit. The 1K-pumping line and the UHV-sock pumping line should be open to helium recovery.
+  - Transfer helium initially slowly - the magnet takes time to cool and it should not be subject to thermal shocks (NbSn magnets are brittle!).
-  - Transfer helium initially slowly - the magnet takes time to cool and it should not be subject to thermal shocks (NbSn magnets are brittle!)
+  - At the beginning of the transfer, open 1K-needle valve (something like 80%). You might also open the UHV-sock needle valve a bit. The 1K-pumping line and the UHV-sock pumping line should be open to helium recovery.
   - The magnet temperature can be monitored by reading the thermometer resistance: ~400 Ω at 77K, when liquid helium starts to collect at the bottom the resistance immediately rises to 1 KΩ and finally when the magnet is fully submerged in liquid helium the resistance should read ~ 4 KΩ.
   - Initial transfer normally requires ~ 100 liters (12 Kg).