He dewars

Superconducting magnets must be operated at liquid helium temperature. Consequently, they require the use of some cryogenic apparatus in addition to the magnet itself. A liquid helium dewar, a means of mounting the magnet in the dewar, a power supply, a liquid helium level meter/sensor, and a helium transfer tube are basic and necessary.

Dewars supplied by AMI are constructed from non-magnetic materials with copper and aluminum radiation shields. All non-demountable joints are inert gas welded providing a rigid, mechanically strong dewar for long and reliable use. All joints are checked with a helium leak detector at every stage of construction, and vacuum integrity is guaranteed for one year. The completed dewar is temperature cycled and tested to confirm that it meets AMI's high standards for quality.

Many of our liquid helium dewars utilize liquid nitrogen shielding for reduced liquid helium consumption. In these cases, the vacuum jackets of the nitrogen and helium reservoirs are connected via a common vacuum pumping port through the radiation shield. Where liquid nitrogen shielding is undesirable, a vapor shielded dewar can be provided with multilayer superinsulation, reducing the radiation heat load of the helium reservoir to a very low value.

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He level meter

He cooled leads

The AMI Model 420 Digital Programmer is a sophisticated power supply controller which allows an operator to manage a superconducting magnet system with unprecedented accuracy and ease of use. The Model 420 is the heart of a modern superconducting magnet system; when it is used in conjunction with a four-quadrant power supply, it provides for a degree of flexibility and accuracy previously unavailable in an economical commercial product.

AMI vapor cooled current leads are the industry standard for introducing high currents into the liquid helium dewar environment. State-of-the-art techniques provide the most efficient way to transfer high currents from 300K to 4.2K. Standard and custom designs from 25 amperes to 75,000 amperes have been built and used.

The heat capacity of helium vapor is used in these counterflow leads to minimize the liquid helium consumption for a specified operating current. Leads of this type have been shown to evaporate 2.8 x 10^-3 liters per hour per ampere for each lead pair when operated at the design current (40% less at zero current). AMI leads are expected to provide this performance under normal conditions, however a more conservative value of 3.2 x 10^-3 liters per hour per ampere is recommended for system design calculations.