Spinning Sphere of Molten Sodium

Thermometry is kind of a key safety diagnostic to make sure that we're well controlled. Thermometry, thermometry! What if it gets too high? Here in trouble! Or sodium expands when it heats, the vessel has a certain volume. There's a temperature above which it's over full, and it's better not to be over full. What temperature is that?

Uh, I think it's around a hundred and thirty degrees. Have you ever been slows? No, no, because of our mama trees. We have mana tree and backup from our tree, and backup to the backup. The wrong tree.

Let's back up and explain what is this thing. This is a three-meter sphere of molten sodium that is 12.5 tons of hot liquid metal designed to spin at up to four revolutions per second. At that rate, the outer surface of the sphere is moving at over a hundred and thirty kilometers an hour. That's over 80 miles per hour. Basically, it's representing the earth.

What they're trying to do is find out how the Earth's magnetic field is generated. Some of the first scientists to study this, starting with William Gilbert in 1600, proposed that the earth was a huge permanent magnet. Now, today we know that the inner core of the earth is solid, and it's mostly made up of iron and nickel, which are ferromagnetic elements.

However, the temperature of the inner core is nearly 6,000 Kelvin, which is way above their Curie temperatures. So there is no way that they could maintain a permanent magnetic field. The same is true of the solid mantle of the earth; much of it is way too hot for permanent magnets, which leaves only the liquid metal outer core as the place where Earth's magnetic fields could be generated.

And this is what is modeled by the 3-meter spinning sphere of molten sodium. It needs to be as large as practicable, it needs to be as high-speed as practicable, and we use liquid sodium because it's the best electric conductor of any liquid. So, it has the downsides of being hazardous and flammable, but the failure would be modestly catastrophic.

It's gonna weird twist a phrase, but it's modestly catastrophic. Most buildings have sprinkler systems in case of fire, but not this lab. To see why, take a look at a single drop of water falling on a single drop of molten sodium. Now, multiply that by 12.5 tonnes and you have modestly catastrophic.

But the researchers have thought about this, and they have a plan. There is a liquid nitrogen Dewar that sits up top. So, when one of these is mostly full, you can just hit a switch and dump the entire contents of liquid nitrogen on top of the experiment. So this is like a physicist's super fire extinguisher.

Then, you know, it's the backup plan. The plan is no leak, no fire. The backup plan is to put the fire out with cryogenics. It actually works great; this is three hundred degrees colder than the sodium, and so sodium cannot burn in a nitrogen gas atmosphere.

The cold freezes it, and any leak gets frozen, and everything just chills. But when everything's working normally, how is a spinning sphere of liquid metal meant to generate a magnetic field? Well, for the earth, the thinking goes like this: first, as the earth radiates heat into space, its core cools.

This means the solid inner core is growing as iron and nickel from the outer core solidify at the boundary. Now, this leaves some lighter elements, like sulfur, for example, behind in the outer core. And so patches with those lighter elements experience a buoyant force and rise away from the core in turbulent convection currents.

Now, add to this the fact that the earth is rotating. The rotation causes those convection currents to spiral around and access roughly parallel with the Earth's axis of rotation. These spiraling turbulent flows of liquid metal can trap magnetic field lines.

As they move and stretch the field, they create electric currents that produce more magnetic fields. If the magnetic fields are generated faster than they dissipate due to the resistance of the metal, well then you have a self-sustaining electric generator or a dynamo.

We talk about the dynamo generation of magnetic fields to mean when you have a turbulent conductor, let's say convecting like the core, that's spontaneously generates a magnetic field. If you have a little bit of current that happens, it causes a little bit of magnetic field that couples into the motion, causing more currents, causing more magnetic fields, which couples into the motion, and then you're just converting motion to currents.

But it goes unstable. It's kind of counterintuitive, though, because you started out with no magnetic field. So how do you get the current in the first place? You don't! I mean, like, no, no! Oh, you know, all you need is a little infinitesimal amount, a thermal variation. A random current will just grow to full size.

So one sun is unstable. Actually, you don't really care where the tickle came from. The states we have now don't generate their own magnetic field, so we've not reached the dynamo threshold, although the hope is to do so in a modified version of the experiment.

So we apply magnetic fields from the outside and then look at the amplification in those magnetic fields by the rotating turbulence. We get copious amounts of amplification depending on where we are in the operating conditions. For instance, we apply these external magnetic fields that sort of go through the experiment like that. They get twisted up by the flow, and the magnetic fields in the angular direction can be up to ten times larger than the fields we apply, so we actually have lots of gain even though we've not yet had a closed-loop dynamo.

So why is it important to understand the Earth's magnetic field? We know the Earth's magnetic field is part of what makes Earth a habitable planet. One of the ways is that it forms a shield, the magnetosphere, which deflects the worst parts of the solar storms around.

But lately, that shield appears to be weakening. The Earth's magnetic field has dropped 10% in the last 170 years. We don't know why. There's a very weak spot in the Earth's magnetic field in the South Atlantic, the South Atlantic anomaly, that's growing weaker. We do know the Earth's magnetic field has reversed many hundreds of times in its history.

We don't know where it's going now. We actually don't currently have a predictive science of those magnetic fields. Why is that important? So, the Earth's magnetic field has weathered changes. You might want to have a forecast for planning purposes. Will this spinning sphere make that forecasting possible?

It's an experiment, so no one has run a device like this ever. It's world unique. So you don't entirely know what's gonna happen. We run it, but that's why we do experiments.

So, in fact, we don't know whether it's possible or not to predict the Earth's future magnetically. It's an open question.

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