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magnetizing a superconducting ring

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DrZoidberg

Thu Mar 20 2008, 04:08AM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

Hi,
if you have a ring made from superconducting material is there a way to make a current flow in that ring without damaging it?
You could bring a magnet close to the ring and a current would start flowing but as soon as you remove the magnet the current will stop.
I'm searching for a way to get a current flowing and keep it flowing even after the removal of the magnet, Maybe you could bring a copper coil close to the ring and let current flow through it in such a way that when you turn off the current there will still be a current in the ring and so the ring will have turned into a permanent magnet.
That could be used to store energy in superconductors.
I know you could simply make a hole in the ring and bridge it with a superconducting switch. Then you connect a battery and once the current has reached it's maximum you close the switch and remove the battery.
But I wonder if it can also work with a solid ring and magnetic fields to transfer the energy into the ring.

Tesladownunder

Thu Mar 20 2008, 04:54AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I suspect (from a position of no knowledge of superconductors) that it is related to di/dt. ie put the magnet in fast and withdraw it slowly. Or sawtooth wavefom on an electromagnet.

Alternatively have it in a magnetic field before cooling to superconductivity, then remove the magnet.

Could someone tell me the real way it is done?

TDU

Spedy

Thu Mar 20 2008, 05:25AM

Registered Member #964 Joined: Wed Aug 22 2007, 12:39AM
Location: Stockton, CA
Posts: 134

It seems to be there is a much simpler answer:

Imagine the superconducting ring. Imagine part of it is "hinged" and can swing and break the complete ring.
When the ring is "open" it is connected with a DC power supply. When the ring is "closed" the power supply is disconnected, and the started current continues withing the completed ring. This makes sense to me, though I imagine a much more complicated switch is used...

Bjørn

Thu Mar 20 2008, 10:57AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I have seen a diagram of a superconducting solenoid that had the PSU in series with the solenoid at all times. Also there were resistors and diodes that would discharge the solenoid safely if the PSU failed.

A superconducting solenoid can store a lot more energy than a capacitor and there has to be a really fail safe way to discharge it because a superconductor may for different reason suddenly drop out of the superconducting state. Then you might end up with an explosion or a large pool of boiling metal. So having the supeerconductor in a unbroken ring with no way of discharging it quickly is not usable except on a small scale.

A superconducting switch sounds very complicated and unreliable. Even making a reliable superconducting wire is complicated.

In the case of a simple one turn ring then TDU's methods sounds reasonable to me.

Dr. Slack

Thu Mar 20 2008, 11:33AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you start off with zero current in the ring, then there is no way to induce a permanent current into it while it stays superconducting. It may be possible, depending on the type of superconductor, to force field into it by exceeding the maximum field strength with localised vortex creep, IIRC this works with HTS but not with metal.

However if you have a field present first, then cool it, magnetic field will only be expelled from the material, not from the interior of the loop, and you will "trap" the field there. Although there was no current flowing when it froze, current will begin to flow as you remove the magnetising field.

There is a video kicking around somewhere, with links on this very forum, of a model train with high temp superconductor block instead of wheels. You set it X amount from a magnetic track, then add LN2, and it freezes in the flux. Then it will stay hovering X amount away.

The way it's done in commercial magnets (I've working in MRI and played with the Oxford magnets) is this. The magnet really is an unbroken loop of SnNb, with no way to discharge it quickly. In the event of a quench, a large amount of helium boils off explosively, through several 300mm diameter bursting disks on the top of the magnet. The theory is there's enough helium to boil to absorb all of the quench energy in latent heat. If the quench happens through temperature rise due to lack of coolant, then I suppose the magnet's stuffed.

To control the field, a small probe-accessible part of the winding is fitted with a heater. The job of changing the field is measured in hours, not minutes. The probe takes 10s of minutes to cool first with LN2, then He, while part inserted. Then it's pushed into contact with the link and heater. You can't just whack a room-temperature probe onto the link without risking a quench. The current is increased in the probe to the same value of the magnet current, adjusting until the link current drops to zero. The heater then makes the link resistive, and the probe power supply can now apply a voltage. IIRC, it was 20/30 volts, and it stayed there for 10s of minutes as the current leisurely ramped up or down - that was one h*ck of an inductance. Then the process is reversed, it's a bit quicker on the way out, applied voltage to zero, allow the link to cool and go superconducting, probe current ramp down to zero, and withdraw.

Because of this long drawn-out procedure, it was normal to work on a live magnet, it was just too expensive in time to run the field down to zero. So, tools were made of BeCu, not steel, and you learnt not to whip your head around too fast when it was at high field. We once got a fork-lift truck stuck to the magnet while inserting a heavy GRP patient tube, and even then it was more efficient to round up a rugby team to push it off than to take the flux down.

DrZoidberg

Thu Mar 20 2008, 07:08PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

That's impressive.
How high is the inductance on that thing? It must have a really large number of windings.

Bjørn

Thu Mar 20 2008, 07:36PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If I remember correctly the largest superconducting magnet at CERN stores more than 1 GJ in the field, that is over 270 kWh.

Bored Chemist

Thu Mar 20 2008, 07:57PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Or about quarter of a ton of TNT. I hope they are careful with it.

Steve Conner

Fri Mar 21 2008, 11:10AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

NeilThomas, what does happen if you whip your head round too quickly in the field?

Shaun

Fri Mar 21 2008, 03:26PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I'd imagine that has something to do with diamagnetism in water, and the huge amount of water in the brain, right?

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