winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
This is in response to a comment in http://forums.mikeholt.com/showthread.php?t=187018 but so totally off topic that I figured it should be in another thread.
Even with no resistance, you still have inductance. The direction of current flow is determined by whatever initiated the current flow in the first place.
The 'common' use here on earth is for superconducting magnets used in things like MRI instruments.
You have a superconducting coil connected to an external power supply, with a 'superconducting switch' shunting the connection to the external supply. The coil gets chilled, and external power is applied to start current flowing and build up the magnetic field. These are large inductors, subject to significant mechanical stresses, and so you need to ramp up and adjust things slowly. Once the proper current flow and magnetic field is established, the switch is closed and the power supply disconnected. The current simply continues to flow in the superconductor.
Interestingly high magnetic fields can cause superconductor to cease to be super, and the mechanical stresses can cause wires to move and thus cause frictional heating, which will also cause the superconductor to cease to be super.
At the same time designers are always trying to push the limits with stronger and stronger fields, pushing the magnet right up to the edge. So with large magnets there is a very real possibility of 'quenching'. This means that you've pumped lots of energy into the magnetic field, when something happens to stop the superconductivity in a bit of conductor. Well as soon as a bit of conductor is causing ohmic heating, the surrounding conductors will heat up and stop superconducting. Pretty suddenly _all_ of the energy stored in the magnetic field is dissipated as heat and em radiation.
One of the spectroscope operators that I worked with as a student described seeing stars from being in the same room as a 5T magnet that quenched, just from being in the large changing magnetic field.
When large accelerator magnets are built, part of the process is called 'training' the magnet, where the field is intentionally built up until the magnet quenches. This has the effect of hammering the bits of coil that can move into place, so that the next time you can get to a higher field before the magnet quenches. You keep doing this until the field reaches the design requirements.
-Jon
Even with no resistance, you still have inductance. The direction of current flow is determined by whatever initiated the current flow in the first place.
The 'common' use here on earth is for superconducting magnets used in things like MRI instruments.
You have a superconducting coil connected to an external power supply, with a 'superconducting switch' shunting the connection to the external supply. The coil gets chilled, and external power is applied to start current flowing and build up the magnetic field. These are large inductors, subject to significant mechanical stresses, and so you need to ramp up and adjust things slowly. Once the proper current flow and magnetic field is established, the switch is closed and the power supply disconnected. The current simply continues to flow in the superconductor.
Interestingly high magnetic fields can cause superconductor to cease to be super, and the mechanical stresses can cause wires to move and thus cause frictional heating, which will also cause the superconductor to cease to be super.
At the same time designers are always trying to push the limits with stronger and stronger fields, pushing the magnet right up to the edge. So with large magnets there is a very real possibility of 'quenching'. This means that you've pumped lots of energy into the magnetic field, when something happens to stop the superconductivity in a bit of conductor. Well as soon as a bit of conductor is causing ohmic heating, the surrounding conductors will heat up and stop superconducting. Pretty suddenly _all_ of the energy stored in the magnetic field is dissipated as heat and em radiation.
One of the spectroscope operators that I worked with as a student described seeing stars from being in the same room as a 5T magnet that quenched, just from being in the large changing magnetic field.
When large accelerator magnets are built, part of the process is called 'training' the magnet, where the field is intentionally built up until the magnet quenches. This has the effect of hammering the bits of coil that can move into place, so that the next time you can get to a higher field before the magnet quenches. You keep doing this until the field reaches the design requirements.
-Jon
