Sunday, December 11, 2016

Fusion Power from Stellarator vs. Tokamak

In discussion on HN about the stellarator vs. tokamak this morning about Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000, published November 30, 2016 (see the whitepaper describing the work or the Wikipedia article), ars summarizes the difference in layman's terms as the following:

If you squish hydrogen atoms together hard enough, and hot enough, they release energy.

But it needs to be really really hot - so hot that anything you made it out of would melt.

So what do you do?

You use a magnet. The magnet squishes the really hot hydrogen without actually touching it.

But if you squish one side, the hydrogen will want to go out of the other side. So you have to squish all sides exactly the same amount.

It turns out, it's impossible to make a magnet in the shape of a ball that squishes on all sides equally.

But, it is possible to make one in the shape of a doughnut! That's a tokamak. They are complicated because you also have to use the hydrogen inside the tokamak to help make the magnet work and keep the hydrogen inside.

Another shape that works is a kind of twisted doughnut, this is called a stellarator, if you do that, you don't need to also use the hydrogen inside as a magnet, and this makes it easier. But the twisted shape means it's harder to build because you have to put the magnets in exactly the right place.

The name "tokamak" comes from a sentence in Russian describing the machine, and the name "stellarator" comes from a word that means "sun".

stevep98 says the impossibility of the magnet in the shape of a ball is described by the Hairy Ball Theorem.

snarfy then describes a difference of the Tokamak (donut/toroidal shaped) vs. Stellarator:

The main difference is the plasma circulates.

With the donut, the problem is similar to trying to inflate a balloon that has a weak point in it. Blowing causes the weak point to inflate instead of the balloon.

Plasmas are electrically charged and have their own magnetic field. Trying to squeeze down on the plasma inside the donut shape to cause fusion causes the plasma's own magnetic strength to increase and counter the field being applied to it.

The stellarator works with the plasma's magnetic field instead of against it by spiraling it around in circles.

visarga writes:

Is it true that this reactor doesn't use the latest REBCO superconductors? I watched a talk where they claim they can make reactors 10x smaller now, because they increased the strength of the magnetic field (can go up to 10-20T, instead of 3T). And the superconducting tape is cheap, much more flexible and only needs to be cooled to 100K instead of 4K.

If that's true and I'm not mistaken, the MIT ARC reactor would be much better posed to win the race than Wendelstein 7-X, especially that W7X doesn't aim to generate surplus energy. A commenter above was wondering at the expensive diamond window they had to use. This project is going to be too expensive and with dated technology. We could do it cheaper now.

mtgx mentions Lockheed is using the Tokamak design in the ITER and XorNot notes that the Wendelstein 7-X was not designed to have a net generation of energy.

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