Lake Shore 625, User Manual, Page: 23 / 132

Lake Shore Model 625 Superconducting MPS User’s Manual
Magnet System Design 2-3
The resistance of the leads must be taken into account when calculating charge rate since a voltage drop across the leads
will limit the voltage that can be delivered to the terminals of the magnet. This becomes especially important as the
charging current rises since the voltage drop across the leads will increase.
2.2.3 Maximum Ramp Rate
Not only is the rate at which a magnet can be charged limited to the magnet’s inductance but it is also limited by the
magnet’s construction. When a magnet is charged or discharged, heat is generated in the coils. The faster the ramp rate,
the more heat that will be generated. If the heat cannot be conducted out of the magnet fast enough, a section of the
superconducting windings can go normal and cause a quench. The magnet manufacturer should state the maximum ramp
rate of the magnet. In some magnets, the current cannot be changed at the same rate over the entire current range of the
magnet. These magnets need to be charged at a slower rate as they reach their maximum current rating.
2.2.4 Maximum Magnet Current
Although superconducting wire can carry more current than non-superconducting wire of the same size, the amount of
current that it can carry is not unlimited. If the critical current of the wire is exceeded, the wire will no longer be
superconducting and will revert to its normal state causing the magnet to quench. Commercially purchased magnets have
been designed to work up to a maximum stated current. The magnet should be able to handle a quench up to the rated
current of the magnet. Do not exceed the maximum current rating of the magnet or the magnet can quench and possibly
be damaged.
2.2.5 Magnet Quench Protection Diodes
Many superconducting magnets have protection diodes installed across the terminals of the magnet. These diodes will
turn on in the event of a quench and will help dissipate some of the magnets energy. Typically the diodes are attached to
the magnet itself and are submerged in the cryogen. At 4.2 K the forward voltage of the diodes may be on the order of 10
volts. If the magnet quenches, the energy dissipated in the diodes will warm them, resulting in a decrease in their forward
voltage. If this voltage drops below the compliance voltage limit of the power supply, the power supply will continue to
source current to the diodes eventually damaging them and causing them to short. This would require that the protection
diodes be replaced which could be a significant expense especially if the magnet is in a sealed dewar.
To ensure that the diodes are not damaged by the power supply, the compliance voltage limit of the supply should be set
to a voltage below the protection diode range. It is also recommended that some form of quench detection be used to
force the output of the power supply to 0 amps when a quench is detected ensuring that no additional current is being
supplied to the diodes. The Model 625 offers both internal quench detection and a remote inhibit line that can be
connected to an external quench detection circuit.
2.3 PERSISTENT SWITCHES
Some superconducting magnets are constructed with a persistent switch. A persistent switch is a length of
superconducting wire that shorts across the terminals of the magnet. This length of wire can be heated and drives it into a
resistive state so that a voltage can be applied across the magnet terminals and the magnet can be charged or discharged.
When the heater is shut off, this section of wire will cool and become superconducting again and the magnet will be in
persistent mode. At this time, the power supply can be ramped to zero current and even removed from the system while
the magnet holds its charge.
One of the reasons to use a persistent switch is when a very stable field is required. When the magnet is in persistent
mode, all of the current is being circulated within the magnet with no interference from outside sources. Another reason
to use a persistent switch is when it is desired to hold a particular magnet field for an extended period of time, such as in
a MRI system. Once the magnet is in persistent mode, the power supply can be removed from the system and used
elsewhere. It is also possible on some systems to remove the vapor cooled leads from the dewar to further reduce the
amount of helium boil off.
The magnet manufacturer will specify the current necessary to turn on the persistent switch heater. Do not use any more
current than is necessary since that will result in excess helium boil off. It is important when turning on the persistent
switch heater that the current setting of the power supply is equal to the current in the magnet. If the current does not
match, the current in the magnet will ramp to the current setting of the power supply at the compliance voltage limit.
This may cause the power supply to incorrectly detect a quench.