Superconducting magnets require liquid nitrogen (N2) and liquid Helium (He). Because it is difficult to make a perfect dewar to hold these cryogens, they need to be periodically replenished.

• Liquid nitrogen is typically filled every 7 to 10 days and depending on your type of magnet, liquid helium fills can be required as often as once a month to as infrequently as only once or twice per year.
• Cryogen fills must be performed correctly to avoid injury to you and the magnet.
• Injury to a magnet could include breaking a seal on a dewar or quenching a magnet.
• Both forms of magnet injuries are repairable, but at the least entail recharging the magnet; at the most, they can entail replacing the magnet.

• When filling the magnet with liquid nitrogen, you must be sure not to exceed the recommended fill pressure and rate for your magnet.
• If your magnet has two liquid nitrogen ports, one should be used for filling and the other for venting the boil-off gaseous nitrogen and overfill liquid nitrogen.
• A piece of tubing is typically placed on the vent port to direct the overfill liquid nitrogen away from the magnet seals, probe, and electronics.
• It is highly recommended that your liquid nitrogen tanks be made of non-magnetic stainless steel.
• Liquid helium fills are typically a awkward operation. Because they are done so infrequently, it is good to review the process before each fill.
• The fill requires a supply dewar of liquid helium, a special liquid helium transfer line, and a tank of pure compressed helium gas.
• Liquid helium is transferred from the liquid helium supply dewar up through the transfer line, into the helium dewar of the magnet.
• The transfer line goes into the top of the liquid helium supply dewar, but should never rest on the bottom of the dewar.
• The bottom of the dewar may contain frozen water, oxygen, and nitrogen which will be forced into your magnet if the transfer line touches the bottom during the transfer process.
• The compressed helium gas, mentioned earlier, is for pressurizing the liquid helium supply dewar with about 2 to 4 psi of pressure.
• Gauges on helium supply dewars can be very inaccurate, so do not count on them to give you an accurate reading.
• A helium pressure above the liquid forces the Helium into the magnet dewar.
• The transfer line is usually inserted into the magnet until it contacts a transfer flange in the bottom of the magnet.
• The nitrogen ports on the magnet should be plugged with a check valve during filling of the helium dewar of the magnet.
• This step prevents cryopumping, a process whereby nitrogen, water, and oxygen are condensed out of the atmosphere into the nitrogen dewar due to the magnet stacks being cooled by the helium.
• Many labs loosely plug the helium vents with tissue during the fill. This cuts down on cryopumping should the flow of the venting He drop.
• The best way to determine if the magnet is full is to look for a change in the gas cloud coming out of the magnet vents.
• When the magnet is full the cloud becomes very thick with a deep white center plume with a slight blue tint.
• The helium vents on the magnet should be closed promptly after the magnet is full.

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