Lake Shore 625, User Manual

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Lake Shore Model 625 Superconducting MPS User’s Manual
4-14 Operation
4.16 QUENCH DETECTION
A magnet quenches when a part of the superconducting wire in the magnet becomes resistive. When a section of the
magnet becomes resistive, it will begin to heat and 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. There are typically three
ways in which a magnet can quench. One way is if the cryogen is allowed to boil off to the point in which the magnet is
no longer covered. Another way is to ramp the current in the magnet at a rate that is greater than specified. The final way
is to exceed the maximum current specified for the magnet.
Although a magnet quench is undesirable, it typically is non-destructive. Most magnets are designed to be able to handle
dissipating the heat generated with the magnet at rated current. Exceeding the maximum current could cause damage to
the magnet if it were to quench in this state since there is more energy in the system than it was designed for. If a magnet
quench occurs, the level of the cryogen should be checked before operating the magnet system again since much of it
may have boiled off.
Quench detection is important to alert the user and to protect the magnet system. When a quench is detected, the output
current setting of the supply is immediately set to 0 A. If the power supply was still supplying current to the magnet
system after a quench, it is possible that the protection diodes in the magnet would turn on and then the diodes would
take the full current load, destroying themselves in the process. During a quench, the current in the magnet will quickly
drop to zero. The 625 constantly monitors the output current and calculates an actual current ramp rate. If the current
changes at a rate greater than the current step limit then a quench is detected. The current step limit should be set to a
value greater than any expected ramp rate.
4.16.1 Quench Detection Enable
Quench detection can be enabled or disabled. If quench detection is disabled, another form of quench detection should be
used to avoid damage to the magnet system.
To configure quench detection, press Quench Detect . The first quench detection setup screen appears as a prompt for the
quench detection mode.
Quench Detection
Select With °®
Quench Detection: Enable
Use the
s
or
t
key to select the quench detection mode, either Enable or Disable. Press Enter to accept the new
selection and continue to the next screen. Press Escape to cancel the new selection and return to the normal display.
4.16.2 Current Step Limit
In order for a quench to be detected, the output current must change at a rate greater than or equal to the current step
limit setting. The current step limit should be set to a rate greater than what the magnet is capable of ramping at if it were
operating correctly. The formula V = L (
di/dt ) can be used to calculate the maximum ramp rate. For example, the Model
625 has a maximum compliance voltage of 5 V, so for a 10 H magnet, the maximum ramp rate that is possible is 5/10 or
0.5 A/s. The current step limit should be set slightly above the maximum theoretical ramp rate, typically above 0.2 A/s
more. Therefore, a good setting for the current step limit for a 10 H magnet would be about 0.7 A/s.
To enter a value for the current step limit, continue from the quench detection mode screen or press Quench Detect then
Enter until the following quench detection setup screen appears as a prompt for the current step limit value. Quench
detection must be enabled to set the current step limit.
Quench Detection
Enter a Value For
Current Step Limit: 0.7000 A/s