The Big Quench

I like Chad Orzel's True Lab StoriesTM series so much that I've decided to be inspired by (read: steal from) him and tell the only vaguely worthy story from my short researching experience.

Not too long ago, I was but a wee undergrad doing her senior research in physics. The project had started with vast ambition ("Build a variable capacitor that will work under these extreme conditions!") and, over the year, turned into something more modest ("Figure out whether this one piece of equipment could possibly be a component in a variable capacitor that will work under these extreme conditions!").

We were using a metal probe to insert our test equipment into a very cold (100 mK), very hardcore (12 T) magnet. At the beginning of our experiment I had been casually warned by my advisor not to pull the probe out of the magnet too quickly. I laughed: I am a weak little girl, and this was a fairly heavy probe being pulled into the magnet by a strong field. It was all I could do to get the thing out of there at a snail's pace.

Well, one day, magically, the probe started coming out easily. I gleefully let it come to me at its own pace, thrilled that my arms wouldn't be aching, and when it popped out it swung a little to the side and down and tapped the top of the magnet. Great. Probe removal: successful. I climbed down the ladder leading to the top of the magnet, ready to adjust the equipment in the probe, when suddenly, I heard it: A quiet but very distinct hissing noise.

"Dale," I said, interrupting the lab's grad student, "Is that an OK noise or a bad noise?"

Dale paused and listened for about two seconds, and then his face dropped.

"Oh no. It's quenching."

At this point the lab broke into chaos as the four grad students, professor next door, and soon, my adviser, started running around, looking at gauges and talking wildly amongst themselves, as I stood dumbly staring at them.

It's quenching? What the heck is quenching?

It turns out that quenching is Slow Magnet Death, the process by which a superconducting magnet ceases to be both superconducting and a magnet.

Inside our NMR magnet was a superconducting wire spool that yields a magnetic field when a current runs through it. On top of the coil, a little piece sticks out from the cold liquid helium into the warmer helium gas. This acts as a superconducting switch: When you want to adjust something inside the magnet, you can warm up this switch and normalize part of the wire and drive current with a power supply.

When I a) pulled out the probe to quickly or b) hit it against the magnet, I warmed up the superconducting switch. Unfortunately, whatever I did kept this switch open, normalizing the entire wire of the magnet and giving it a high resistance. The current in the coil then took a path through another wire (the one of least but non-zero resistance), discharging and starting a decay of the magnetic field.

In our case, the magnet, over the course of quite a few hours, dropped from 12 Tesla—obscenely strong—to .07 Tesla—could still beat up your dad, but won't do our experiments for us. After the first few hours, the boiling helium built up so much pressure that the cap of the magnet blew straight off, hitting the ceiling. It took several weeks before we were able to get the magnet back up to its normal strength and work on the project again. By that time, we had pretty much just decided to analyze the data we already had.

And for the kicker, the rapidly decaying magnetic field not only ruined our lab but also completely threw off the sensitive experiment of the group across the hall.

I was totally mortified!


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