Alaric B Snell wrote:
jim rozen wrote:

In article , Alaric B Snell says...


The interesting thing (from a sci fi fan's point of view) is that
such a system stores energy in the fabric of space - the energy isn't
in the coil, it's in the space in which the coil sits.



And very real energy it is. Anyone who's seen a superconducting
magnet quench (go normal) while up at field, will attest to that.


anecdote

A friend of mine did a stint as a research physicist, and saw a few
interesting things in his time.

The department had got a new NMR machine. An NMR machine is a big
superconducting coil, in liquid nitrogen, which is charged with current
over a period of days until many millions (billions?) of amps are
flowing around it, to create an intense magnetic field. You put a sample
in the field and do stuff to it with RF energy that I won't go into in
order to analyse its chemical composition.

Anyway, they'd just charged the field up on this thing, after many days
of piping in electricity, so they were holding a little demo and
analysing random objects and so on, when the guy who delivers cylinders
of oxygen went past the door with an oxy cylinder on a trolley.

Now, the people who sited the NMR machine didn't know the oxy guy's
route went past the door. And the oxy guy didn't know that, unlike last
week, there was now a superconducting magnet in the room. So everyone
was surprised when the door slammed open to admit an oxygen cylinder
which flew across the room and slammed into the end of the coil.

The magnetic field, up until that point fairly evenly spread around the
room, concentrated itself into the nicely permeable iron cylinder, which
raised the field strength at two points in the coil beyond the field
strength the superconductor could take, so two bits of it stopped
superconducting and started shunting electrical and magnetic energy into
heat - boiling the liquid nitrogen.

This presented a problem. They couldn't bring the ring current down fast
enough to prevent the LN2 boiling off. Once the coolant level fell below
critical, the ring would start shedding energy as heat, thus boiling
away the remainder even faster - leaving a non-superconducting ring
carrying a vast current, which would turn into heat, which would stop
the current, which would cause the energy stored in the magnetic field
to try to turn back into current, which would turn into more heat... it
would probably violently explode.

LUCKILY, they were prepared for this kind of event, and had a
non-ferrous scissor jack to hand. The strongest members of the group
managed to roll the cylinder onto the jack and jack it away from the
ring housing enough to get shoulders behind it and walk it out of the door.
/anecdote

Nice story but I don't believe it. If the magnetic field were
following the inverse square law and the field was strong enough
to pull in the cylinder from the hall, they'd never get it off with
a non-metallic scissor jack. Just my opinion.