Lake Shore 625, Руководство пользователя

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Lake Shore Model 625 Superconducting MPS User’s Manual
2-4 Magnet System Design
2.4 MAGNET CURRENT LEADS
The power supply should be placed close to the magnet to reduce the length of the lead wires. The resistance of the wires
becomes very important when such large currents are being supplied to the magnet. The rate at which a magnet can be
charged depends on the voltage that can be supplied across the terminals of the magnet given by the equation V = L
(
di/dt ). The voltage is limited by the maximum voltage the power supply can output minus the voltage that is lost
through the magnet leads. Use lead wires heavy enough to limit the voltage drop to less than 0.5 volts per lead and keep
conductor temperature under 85 °C for a 35 °C ambient temperature. Table 2-1 lists the current capacity and total lead
lengths for load connections.
Table 2-1. Current Capacity and Total Lead Lengths
AWG
Area
(mm
2
)
Capacity
(A)
Resistivity

/1000 feet
Total Lead Length (feet)
60 A 120 A
0 53.5 245 0.09827 170 85
2 33.6 180 0.1563 107 53
4 21.2 135 0.2485 67 34
6 13.3 100 0.3951 42
—
8 8.4 75 0.6282 27
—
The Remote Voltage Sense connection can be used to monitor the voltage directly across the terminals of the magnet.
This will give a more accurate voltage reading across the terminals of the magnet by eliminating the voltage drop in the
leads. Some magnets manufacturers provide voltage sense connections directly at the terminals of the magnet. If these
are not available, they can be added and the signals can be brought out of the dewar to be connected to the power supply.
If it is not desirable to add wiring inside the dewar, the sense leads can be connected to the magnet current leads at the
dewar. The remote voltage sense input can only be used to read the voltage at the magnet terminals and cannot be used to
control the voltage limit.
2.5 HELIUM DEWARS
1
Since superconducting magnets need to be run at cryogenic temperatures, they are installed in dewars filled with liquid
helium. A dewar usually consists of one or more reservoirs surrounded by a vacuum jacket. This vacuum jacket insulates
the reservoir from room temperatures. In dewars with multiple reservoirs, the outside reservoir is normally filled with
liquid nitrogen as a way to further reduce the heat transfer from the liquid helium filled inner reservoir. Most dewars are
made from stainless steel although they can also be made from glass or epoxy-fiberglass and aluminum. Stainless steel is
used because it is very rugged, has low thermal conductivity, and can easily be welded to different types of metals.
The most basic dewars are of an all welded construction with an opening in the top for direct access to the cryogen
reservoir. The dewar will have an evacuation valve to evacuate the vacuum jacket surrounding the cryogen reservoir.
There will also be a pressure relief valve to protect the vacuum jacket in case a leak should develop. This leak would
allow cold cryogen into the vacuum jacket where it will expand upon contact with the room temperature wall. This
pressure relief valve is set to open between 2 and 5 psi to safely vent the leaking gas.
The superconducting magnet can either be supported by the insert or supported by a base in the bottom of the dewar. If
the magnet is in the base of the dewar, it is usually installed when the dewar is built and can only be removed or serviced
by cutting the dewar apart. Any insert that is placed in the helium reservoir should contain a number of radiation baffles
in the neck region of the dewar. These baffles are normally made from copper and are cooled by the escaping helium
gas. This will help cut down on radiation losses from the room temperature top flange on the insert. It will also help to
cut down on conductive heat loss being transferred down the neck of the dewar. The high current leads used for charging
the magnet should be vapor cooled to reduce the amount of heat that is transferred into the helium reservoir.
1
Information gathered from Introduction to Laboratory Cryogenics, M.N. Jirmanus, Janis Research Company, Inc.