Superconducting circuits

The middle leg could certainly be constructed so that there is zero (or even negative, if that turns you on) mutual inductance between the two loops.

John

Yes, since in the first circuit the voltage across both inductors is always the same and in the second they are independent.

Huh?

John

Dooh!!

That's what happens when I react instead of thinking.

There will be current if there is resistance in the wires or if the wires are concidered to be transmission lines.

If everything is ideal, there can not be any voltage across either inductor and there can not be any current anywhere.

There can, and should be, current in L1 and L2.

I = E/R. If E and R are both zero, we have the mathematical freedom to circulate any arbitrary I.

Superconductive magnets, like in MRI machines, are cooled down then charged with a big power supply, to hundreds or thousands of amps. Then a superconductive shorting link is closed, the power supply loaded back onto the truck, and the current, and the mag field, persist for years. They just need to top off the liquid nitrogen and helium every few months.

These magnets typically lose a couple of PPM of field strength per day, in small jumps, as things shift around.

John

Sure there can be current.

Steady state for a pure inductor is any constant current and zero volts developed or impressed across it. (perhaps "static state" would be a better term, to distinguish from a the AC circuit notion of "steady state" for a sinusoidal driving function). The energy is stored in the mag field.

In a similar fashion, for a pure capacitance steady state has a fixed voltage and zero current, energy being stored in the electric field of the capaictor.

Even with a steady DC current (and thus zero voltage drop from inductance) and superconducting wires (and thus zero voltage drop from resistance)? IANASCEE, but it seems to me that in both cases the voltage across both inductors is always the same (zero). An ineresting question is whether the current going around each loop is independent in the upper circuit. Why would an electron coming out of L1 prefer the path through the zero ohm center leg rather than the zero ohm L2 leg?

Cool. I wonder how much stored energy is in there? I^2 and all...

Cheers, James Arthur

Yep, absolutely... although I suspect it might be a little difficult to nail down the exact value of "L" with such large coils -- the room you place the MRI machine in possibly has a non-negligible effect.

This is sort of how supercurrent switches are constructed. You put a heater on the centre leg to drive it normal, ramp up the drive current, and turn off the heater. As you ramp down the drive current again, the supercurrent switch takes over, so when it's down to 0, you can disconnect the power supply and the supercurrent will flow until you run out of helium.

Cheers,

Phil Hobbs

This is sort of how supercurrent switches are constructed. You put a heater on the centre leg to drive it normal, ramp up the drive current, and turn off the heater. As you ramp down the drive current again, the supercurrent switch takes over, so when it's down to 0, you can disconnect the power supply and the supercurrent will flow until you run out of helium.

Cheers,

Phil Hobbs

Here's a quench of a small NMR magnet...

This boils a lot of expensive helium, but probably not enough to asphyxiate people in the room. Bigger ones can get dangerous.

NMR and MRI magnets run in the 10's or even 100's of megajoules.

Superconductive energy storage has been played with, with experimantal systems running into the 70 Gj region.

John

Impressive.

YouTube is handy...

...err... what does "IANASCEE" mean?

...and useless if one is on dial-up!

I Am Not A SuperCondutor Electrical Engineer. In some newsgroups it is common for someone who is not a lawyer to write IANAL (I Am Not A Lawyer) when discussing legal issues. They do so for the same reason I use IANASCEE and IANARFEE -- I know some things about a subset of the field, but I am not passing myself off as an expert.

age

I've used a few 6-10 Tesla SC magnets for research. These had fairly small bores (only small volume of high field) I think the inductance was in the 1's to 10's Henry range with maximum currents of 100 Amps. When you recieved a new high field magnet it would never get to it's highest field on the first run. You had to 'train' the magnet by taking it up in field and letting it quench. I little bit nerve racking because you never knew when it would quench... a little more current.. and then Wooshh!

George Herold

How big were they? The smallest NMR magnets I've seen were horizontal-bore things that are shaped like rugby balls, about the size of a medium watermelon. The biggest ones, the GHz guys, have built-in spiral staircases to help you load samples; ballpark a megabuck and up.

The trend lately is to have pumps that recycle/reliquify helium boiloff, rather than paying for refills. That shifts money from a lab's "supplies" budget to the often-unaccounted-for facility electric budget.

John