American Magnetics AMI Superconducting Magnets - Manual (Page 14)

13
Installation
Instrumentation and control check
Note
An 18-24" piece of tubing, connected to the top vent of each lead, and allowed
to hang over the side of the dewar is an effective way to create a helium gas
trap and minimize the air which enters the leads.
4. It is advisable to monitor the magnet temperature during LHe transfer to make
sure proper cooldown is taking place. Cooldown can be simply monitored by
measuring the resistance of the magnet. LHe will not collect in the dewar until
the dewar and magnet temperature reaches 4.2K. Adjust the storage dewar
pressure for the proper transfer rate best indicated by observing the helium gas
exiting from the dewar vent. The most efficient transfer rate is one where there
is a light flow of vapor from the vessel vent. If the vapor has significant
pressure or spits and sputters, the transfer rate is excessive and should be
reduced.
Note
A transfer tube with a poor vacuum or thermal short can evaporate all the
liquid helium before it reaches the dewar.
5. As soon as the dewar temperature reaches 4.2K and liquid helium begins to
collect in the bottom of the dewar, the exhaust gas should decrease noticeably.
The transfer rate can be increased at this point without excessive helium loss.
6. Once the desired LHe level is reached, stop LHe transfer, remove the transfer
line and cap the transfer port.
Instrumentation and control check
Prior to connecting the power supply cables to the vapor-cooled current leads, the
power supply system should be checked for proper control and indication.
1. Setup the power supply system. Refer to the documentation provided with the
power supply system.
Caution
Make sure the power supply and magnet controller is installed in accordance
with the operating instruction provided with your magnet current
programmer.
Caution
Some non-AMI programmer/power supply systems are capable of producing
voltages up to 30 volts and more than a 100 amperes. These power supplies
can easily exceed the turn-on voltages of protective diodes on
superconducting magnets. Furthermore, these power supplies may not always
!