American Magnetics 420 Power Supply Programmer - Manual (Page 16)

xiv Rev. 7
Foreword
Safety Summary
The properties of many materials at extremely low temperatures may be
quite different from the properties that these same materials exhibit at
room temperatures. Exercise extreme care when handling materials cooled
to cryogenic temperatures until the properties of these materials under
these conditions are known.
Metals to be used for use in cryogenic equipment application must posses
sufficient physical properties at these low temperatures. Since ordinary
carbon steels, and to somewhat a lesser extent, alloy steels, lose much of
their ductility at low temperatures, they are considered unsatisfactory and
sometimes unsafe for these applications. The austenitic Ni-Cr alloys
exhibit good ductility at these low temperatures and the most widely used
is 18-8 stainless steel. Copper, Monel
®
, brass and aluminum are also
considered satisfactory materials for cryogenic service.
Magnet Quenches
When an energized superconducting magnet transitions from
superconducting state to normal state, the magnet converts magnetic
energy to thermal energy thereby rapidly converting the liquid helium to a
vapor. When this phase transformation occurs, pressures can build rapidly
in the cryostat due to the fact that one part of liquid helium will generate
782 parts of gaseous helium at STP. The cryostat must be designed to
allow the generated vapor to rapidly and safely vent to an area of lower
pressure. Cryostats are designed with pressure relief valves of sufficient
capacity so as to limit the pressure transients within the container in
order to prevent damage to the vessel. Operating a superconducting
magnet in a cryostat without properly sized relief mechanisms or disabled
relief mechanism is unsafe for the operator as well as for the equipment. If
there is any doubt as to the sufficiency of the pressure relief system,
contact the manufacturer of the magnet and cryostat for assistance.
Safety Summary
Superconducting magnet systems are complex systems with the potential
to seriously injure personnel or equipment if not operated according to
procedures. The use of cryogenic liquids in these systems is only one factor
to consider in safe and proper magnet system operation. Proper use of
safety mechanisms (pressure relief valves, rupture disks, etc.) included in
the cryostat and top plate assembly are necessary. Furthermore, an
understanding of the physics of the magnet system is needed to allow the
operator to properly control the large amounts of energy stored in the
magnetic field of the superconducting coil. The Model 420 Programmer has
been designed with safety interlocks to assist the operator in safe
operation, but these designed-in features cannot replace an operator’s
understanding of the system to ensure the system is operated in a safe and
deliberate manner.