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====== MPMS helium transfer ======







===== About Helium Transfer =====

 
The following procedure is recommended for initial and subsequent transfers.

We have made appointments for the ordering of Helium at the cryogenic department of Hans van Kuyk. When you get the vessel, please book it under //nr 64, password gerrit// for the PPMS and MPMS-5S, for MPMS-XL book it under //nr 72 password JOS//. 
The vessel can stay upstairs. Never empty it completely!

Collect the following near the MPMS: \\ 
liquid helium dewar \\ 
helium transfer tube (on the back wall) \\ 
a pair of gloves  \\ 
heat gun 




===== Initial Transfer =====

The initial transfer will require between 75 and 90 liters of liquid helium to cool down the system and fill the dewar. For the first transfer, the output end of the transfer tube should be fitted with one of the extensions provided to reach to within a few centimeters of the bottom of the MPMS dewar, about 112 cm below the top surface of the dewar. Just before the transfer tube is inserted into the dewar, the plug should be removed from the MPMS top plate, and the transfer fitting installed. It should seat in the tube sticking up from the top plate, and the horizontal tube should be directed away from the head. If a helium recovery system is being used, you may wish to connect it to this exhaust fitting. Open the cooling valve from the Diagnostic Gas Controls menu. Go to the Diagnostic Menus, move cursor to Gas Controls and press Enter. Next move the cursor down to the Cooling Valve selection and press Enter. Select Open and press Enter again. This will provide full helium gas flow and help prevent blocking the impedance tube.
Insert one end of the transfer tube into the storage dewar and the other end into the transfer port. Insert the transfer tube into the MPMS dewar slowly, making sure that its extension does not bump into the magnet or other parts attached to the lower part of the probe. The transfer should be started slowly to take maximum advantage of the cold helium gas. Since the initial  transfer will take 2 to 3 hours, it is helpful to have a small fan or a hair dryer directed across the head during the transfer to help keep excessive moisture from condensing on the head of the probe. The transfer rate can be set by adjusting the pressure in the storage dewar. While the dewar and probe are cooling down, the pressure should be held at about 200 mm of mercury. This should produce a steady gas flow out of the transfer fitting. When liquid begins to collect, normally after about 1 hour, the flow will momentarily drop to zero, then resume. Also, if the helium level is being plotted on the computer, it will begin to increase at this time. As the liquid helium level increases, the gas flow out of the pump will increase to a maximum of 2.5 liters/min. Once this flow rate has been achieved and the helium level is greater than
25%, the cooling valve can be closed. Close the cooling valve from the Diagnostic Gas
Controls menu. Go to the Diagnostic Menus, move cursor to Gas Controls and press Enter. Next move the cursor down to the Cooling Valve selection and press Enter. Select Close and press Enter again. The SQUID should also be tuned at this time as described in the MPMS Software User's Manual Chapter 9. Continue the transfer until the helium level reading above 90% or until the level stops increasing indicating and empty storage dewar. It will take 24 to 48 hours for the dewar to completely cool down and equilibrate, and a high but decreasing boiloff should be expected during this period. The quiescent boiloff of the system with the sample region cold (less than about 200 Kelvin), is about five liters of liquid per day. This loss is higher when the sample tube is warm or when the system is being cooled down in the high-power cooling mode.




===== Subsequent Transfers =====


The helium level in the system should be kept at about 40% or higher to provide cooling for
the superconducting magnet. If the level drops below 40%, the magnet should not be used.
Subsequent helium transfers into a cold MPMS dewar are essentially the same as the initial
transfer with the following exceptions:

1. The magnetic field should be set to zero before beginning the transfer. If the magnet is
left at a high field when the transfer is started, the initial process of inserting the warm transfer tube into the dewar may cause the magnet to quench. 

2. The output end of the transfer tube only needs to get into the belly of the MPMS dewar
about 40 cm below the top plate, so no extension should be used in
subsequent transfers.

3. Before the transfer tube is inserted into the storage dewar, it should be cooled by
blowing cold gas through it until a plume appears at the outlet. If the warm tube is
inserted first, the initial warm gas from the tube will boil off liquid already in the
MPMS dewar.
At the beginning of the transfer, before inserting the transfer tube into the storage dewar, remove the screw cap, compression ring, and o-ring from the top of the transfer fitting, and slip them up over the output end of the transfer tube. With the transfer fitting in place, after the tube has been precooled, the tube can be slipped into the transfer fitting, and the screw cap tightened to seal the o-ring around the transfer tube.
Note: If you try to insert the transfer tube through the o-ring while cold gas is coming
out, the o-ring will freeze to the transfer tube, causing it to jam as it is going through the fitting. Be sure the o-ring, the compression ring, and the screw cap are installed over a warm transfer tube. After the transfer tube has been inserted into the MPMS dewar, the pressure in the storage dewar can be set to about 0.1 - 0.2 bar.The exhaust of the transfer tube is connected to the helium recovery system, a pressure greater than 0.1 bar will be required to transfer helium. This higher pressure may be needed to overcome backpressure from the helium recovery system; 
The progress of the transfer can be monitored on the computer with the helium level plot. It will take 15 minutes to fill the dewar when starting at a level of 40 percent.