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The transfer tube optionally provided with the system is of a stainless steel construction. It takes the form of a tube surrounded by a second tube with a vacuum of better than 10^-4 mbar maintained between them. The assembly of the two tubes usually takes the form of a large 'n' shape.

Occasionally re-pumping of the tube will be necessary in service, particularly during the first few months while the materials in the tube are still outgassing.

The ST9 Siphon Evacuation Fitting

The transfer siphons supplied by Oxford Instruments are supplied pre-evacuated, however re-evacuation may become necessary after a period of operation. To evacuate an Oxford Instruments standard siphon, an ST9 fitting is needed to operate the vacuum valve.

1. Remove the yellow nylon dust cap from the transfer tube valve. Connect the ST9 fitting to the high vacuum pumping system.
2. Place the ST9 fitting over the transfer tube valve. Evacuate the pumping lines and check the system for leaks.
3. Using the red anodized aluminium knob, which is connected to the hexagonal key internally, open the transfer tube valve. Pump out the siphon to 10^-4 mbar or better.
4. Close the transfer tube valve using the red knob, isolate the pump and remove the ST9.
5. Replace the dust cap.
6. Try to avoid getting dirt in the ST9 fitting.

Note: The cryostat overpressure relief valve must be in position and not restricted. If the cryostat is connected to a recovery system any flow meter should be capable of high flow rates and should not introduce a restriction (it may be sensible to fit a bypass flap valve to accommodate the high flows during a possible quench, ensuring that all the helium is recovered).

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Initial Filling with Liquid Helium

1. Check that the transfer tube has the correct leg lengths and diameters to be compatible with the cryostat and storage dewar. Connect the cryostat and storage dewar to the helium recovery system or put a one-way valve on the cryostat exhaust port (if the system is large and a one-way valve is found too restrictive, it may be replaced by a 2 m length of convoluted tubing). Position the liquid helium storage vessel so that the transfer tube can be inserted easily and is close to the cryostat to be filled.
2. Remove the plug from the cryostat transfer tube entry port and also from the top of the storage vessel. Insert the transfer tube legs into the cryostat and, slowly, into the storage dewar, allowing it to cool gradually. Ensure that the end of the transfer tube in the cryostat is fitted into the cone on top of the magnet. In this way, cold gas and then liquid is introduced at the bottom of the magnet which is then cooled by the enthalpy of the gas as well as by the latent heat of evaporation.
3. Start transferring the liquid helium by pressurizing the storage vessel. (This is generally done by gently squeezing a rubber bladder). The transfer rate should be such that the vent pipe is frozen for not more than 2 m of its length. The initial transfer rate should be equivalent to about 10 liters of liquid per hour. This rate can be increased as the magnet cools and the boil-off reduces. Typically the cool-down from 77 K to 4.2 K will take between 10 and 60 liters depending on the system size and the care taken in the transfer.

By monitoring the Allen-Bradley sensors, when the magnet temperature falls below 10 K, the transfer rate can be further increased in order to fill the liquid helium container. This should occur when a further 10 to 50 liters of liquid have been transferred, depending on the size of the magnet and dewar.

4. When the liquid helium reservoir has been filled, stop the transfer by releasing the pressure in the storage vessel. Remove the transfer tube and replace the plug. Inspect the liquid helium level at appropriate intervals.

Refilling with Liquid Helium