Hello,

Interestingly enough, I was talking to someone on MRI coils. Anyways he explained that to turn the superconducting coil off you must match the coils current with external supply and slowly reduce the current until it reaches zero. Now MRI coils at 1.5T use up to 700 AMPS! If I match the coils current with an external supply and slowly turn it down, how does the coil turn off while still being connected to the main power supply?

Thanks,
limited knowledge on this

 

Hello,

MRI magnets are superconducting magnets.
The current will stay in the magnet as long the liquid helium is there.
To de-energize the magnet, you will need an external powersupply that can handle the current inside the magnet.
As you say, you must run up the external powersupply to the same current flowing in the magnet.
To connect the magnet to the external powersupply, you have to open the cyoswitch.
This cryoswitch is a piece of superconducting wire with an heater next to it.
By putting on the heater, the piece of superconducting wire will loose its superconducting property and "open" the coil to connect it to the external power supply.
Now you can slowly ramp down the current in the coil.
This must be done slowly to avoid a quench.
When a quench happens ALL the helium is blown out of the magnet in a VERY short time.
If no quenchpipe is mounted, all helium will fill the room with helium in the matter of seconds.

http://www.youtube.com/watch?v=T7UOgjvZAVs

Bertus