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.

No, it is really just hundreds of amps. They have to "ramp up" such a
magnet with a non-superconducting power
supply, with non-superconducting cables, so the currents outside can't
be enormous. Therefore, there's no
way to get enormous currents INSIDE the superconducting part. But, it
doesn't matter, as they just add more
turns of the superconducting wire. The flux can add up to millions of
amp-turns, however.


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

Yup, it's an anecdote, all right. Superconducting magnets quench in a
few seconds under these conditions.
And, it doesn't take a gas bottle to do it, a wrench in the wrong place
can affect the field enough, especially
when it is moving at a couple of hundred miles an hour. The cryogens in
the magnet are enough to absorb the
heat generated (remember, the active part of the magnet winding is at
about 4 Kelvins, (4 degrees C above
absolute zero temperature) so it takes a lot of heat to warm it up just
to room temperature.

The danger of an explosion is not the magnet itself, but the double
dewar jackets filled with liquid helium
and liquid nitrogen. When this stuff boils, it generates a LOT of gas.
They have 6" and bigger vent stacks
to carry away the gas, which would otherwise suffocate everyone within a
couple of rooms.

Since the magnet was quenching anyway, there would be nothing to save,
so it would be wise for
everyone to just get the heck out of there. Once the quench starts
(some spot in the winding becomes
a normal conductor) the quench will spread to the rest of the magnet
very quickly, as only a slight rise in temperature
will change the superconductor to normal.

Jon