American Magnetics AMI Superconducting Magnets - Manual (Page 13)

12
Installation
Cooldown
Cooldown
Be sure the magnet and support assembly are free of all moisture and contaminants
prior to magnet cooldown.
1. Precooling the magnet to 77K in liquid nitrogen (LN
2
) will reduce the amount
of liquid helium (LHe) required for cooldown to 4.2K. This can be
accomplished several ways.
a. Immerse the magnet in LN
2
prior to inserting the magnet in the system
dewar. Styrofoam cooler containers work well for this process. This
process may not be practical for complex magnet systems.
b. Immerse the magnet in LN
2
while installed in the system dewar. This may
be accomplished by adding LN
2
to the LHe chamber to cover the magnet.
Once the magnet has cooled to 77K, all LN
2
must be removed from the
LHe chamber before helium is transferred.
c. In a nitrogen-jacketed dewar system where cooldown time is not critical,
filling the dewar jacket with LN
2
will slowly cool the magnet and support
stand to 77K.
Note
If the magnet is precooled in LN
2
, it is important that all LN
2
is removed from
the magnet assembly and dewar before cooling the magnet and dewar to 4.2K.
Residual LN
2
will require an excessive amount of liquid helium (LHe) to
freeze the LN
2
and also may adversely affect magnet operation. Removing all
residual LN
2
can easily be accomplished by allowing the system to warm-up
several degrees above 77K to evaporate any LN
2
.
2. Connect a LHe transfer line that will reach to the bottom of the dewar and
commence slowly transferring LHe. To make sure there is an efficient transfer
of LHe, it is important the transfer tube (or an extension of the transfer tube)
introduce the LHe at the very bottom of the dewar below the magnet. This
allows the magnet to be efficiently cooled by the cold helium gas as it rises.
Transferring LHe should be done slowly until the system is cooled to 4.2K
and liquid helium starts to collect in the dewar.
Caution
To increase the transfer rate of the LHe, an external pressure source may be
used on the LHe storage vessel. Make sure that only dry helium gas is used for
this purpose. Air or nitrogen will freeze in liquid helium and contaminate the
storage vessel and/or the transfer system and dewar.
3. Vent some of the exhausting helium gas through the vapor-cooled current
leads to prevent blocking the gas vent holes with condensing moisture and
freezing air.
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