Lake Shore 625, Руководство пользователя

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Lake Shore Model 625 Superconducting MPS User’s Manual
Magnet System Design 2-5
With the magnet submerged in liquid helium, it will be at a temperature of 4.2 K at atmospheric pressure. Some magnets
are rated to work at 2.2 K allowing a larger field to be generated. This temperature can be achieved by lowering the
pressure over the helium reservoir thereby lowering the boiling point of the helium. Some dewars will have a pumping
port that can be attached to a vacuum pump to reduce the pressure and lower the temperature but will increase the rate of
liquid helium consumption.
The amount of energy that can be stored in a magnet is given by the equation E = ½LI
2
. Typical laboratory magnets have
inductances of 10 to 100 Henries and can have maximum currents of 40 to 120 Amps. The energy stored in a typical
magnet can be anywhere from a few thousand Joules to over one hundred thousand Joules. In the case of a magnet
quench, all of this energy is going to be dumped into the liquid helium within a matter of a few seconds creating a large
amount of helium gas. Any dewar that is used with a superconducting magnet should have a pressure relief port on the
helium reservoir to allow the helium gas to be dissipated in the case of a quench.
The level of the liquid helium can be monitored by using a liquid helium level sensor installed in the dewar. The level of
the helium should never be allowed to drop below the top of the magnet while the magnet is in operation. Allowing the
magnet to become uncovered can cause a quench. Also, the level of the helium should never be higher than the inlets of
the vapor cooled current leads. If the inlets are submerged in liquid helium, the helium gas can no longer cool the leads
and extra heat will be transferred into the helium reservoir increasing the rate of helium boil off.
2.6 MAGNET QUENCH
A magnet quench occurs when part of the superconducting wire in the magnet becomes normal and has resistance. When
a section of the magnet becomes resistive it will begin to heat and will cause more of the magnet to become resistive.
This causes an unstoppable chain reaction that will result in the magnet dissipating all of its energy into heat. This can
happen if the critical temperature, critical current, or critical field are exceeded. Refer to Paragraph 2.1 for a description
of superconductor properties. Even though a quench is not necessarily destructive to the magnet, it should be avoided at
all costs. Always check the level of liquid helium and make sure that the magnet is completely covered before operating
the magnet. Never ramp a magnet at a ramp rate greater than what is specified by the magnet manufacturer. Never
exceed the current rating of the magnet since a quench in this case can easily damage the magnet.
Typically the current in the magnet will be completely dissipated in about a half of a second causing the magnet to heat.
It may then take several minutes before the liquid helium cools the magnet back to its superconducting state. Since a
quench can boil off a significant amount of helium, always check the helium level before operating the magnet after a
quench.