Ramping the magnet
If the magnet coils are in a closed superconducting loop, how do you get current into them to ramp up the field initially?
|
|
|
The process of ramping a superconducting magnet up to full field is typically a 2-3 day process. After the magnet is sited and ready, it is cooled to superconductive temperatures by being filled with liquid helium (at least up to the ~75% level). The electrical terminals to charge the coil windings are located deep within the cryostat. In many designs these are accessed by inserting plungers into the magnet as shown in the video to the right. |
|
During ramping, a heater circuit is turned on causing a finite resistance to develop in resistor R. Once current begins to flow, the heater is turned off, and the resistance of R falls to zero, allowing the current to continue indefinitely through the superconducting loop in "persistence mode".
|
A simplified diagram of the internal magnet circuitry is shown to the left. In parallel with the main coil windings is a by-pass limb containing a component called a persistence switch. The persistence switch is comprised of a resistor (R) and a small electric heater. |
The persistence resistor (R) is heated above superconducting temperatures so it develops a small finite resistance (~100 Ω typical). The service engineer next attaches a power supply to the plungers and begins pumping current into the system. Because the magnet coils are superconducting (and have essentially zero resistance) virtually all of the applied current flows directly through them and little to none flows through the persistence limb. The external current is slowly increased over a 24-48 hour period (to avoid a quench) until the desired final current level (typically 500-1000 A) and field strength are attained.
At this time the heater element is turned off and as the resistor (R) cools it becomes superconductive, shorting the circuit across the coils. The external power supply is now disconnected and the magnet current continues indefinitely through the superconductive loop in persistence mode. Finally, the plungers are removed and the fully ramped magnet is ready for the next stage of installation (typically shimming).
At this time the heater element is turned off and as the resistor (R) cools it becomes superconductive, shorting the circuit across the coils. The external power supply is now disconnected and the magnet current continues indefinitely through the superconductive loop in persistence mode. Finally, the plungers are removed and the fully ramped magnet is ready for the next stage of installation (typically shimming).
References
Superconducting Magnets. Wikipedia, The Free Encyclopedia.
Overweg J. MRI main field magnets. Presented at ISMRM 2006. Click here for link.
Superconducting Magnets. Wikipedia, The Free Encyclopedia.
Overweg J. MRI main field magnets. Presented at ISMRM 2006. Click here for link.
Related Questions
What causes superconductivity?
Isn't superconduction like perpetual motion? Doesn't the current in the superconducting coils ever slow down?
How are superconducting magnets constructed?
What causes superconductivity?
Isn't superconduction like perpetual motion? Doesn't the current in the superconducting coils ever slow down?
How are superconducting magnets constructed?