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wendelstein 7-x

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I'll ELI15 for now. If you need something more basic see what I wrote here. The idea behind a stellarator is that you are holding a plasma with magnetic fields. However this requires a large amount of precision in your construction. We're talking millimeter accuracy on a machine that is several meters large. We already knew that the magnets were in the correct locations since they tested the field lines, which you can see here.

So this is more a celebration of a beginning of operation. There wasn't too much of a doubt that things would work. But now they can start the campaign in earnest. They can see how well the plasma is confined. Whether it has the desired properties. How much heat gets deposited on the walls. And other important measurements.

edit: Ok, here's an ELI20 and I've had a college course in physics, but want to know more. Ok, so now you've got the basics of what magnetic confinement is (if you don't go ahead and read the ELI5 linked above) and know basically what a stellarator is, but what is the deal with W7X. What makes it special?
What is an Optimized Stellarator? - They Confine Particle Drifts

The main thing that makes W7X special is that it's an optimized stellarator. Optimized means a lot of things, but in the context of stellarators we tend to use it to describe very specific design parameters. The first optimization of importance is that it particles do not drift off of flux surfaces. That might be hard to understand, first we need to figure out what a flux surface is. If you follow a magnetic field line around your machine, it can do several things. It can hit the wall, in which case we say the field line is unconfined or "open". It can bite it's own tail, in which case it's both confined and "rational." It can never hit the wall but still go all over the place, in which case we'd say it's confined and stochastic. Or it can never hit the wall, never return on itself, but remain on a 2-D surface. If all the field lines bite their own tails or remain on nice 2-D surfaces we say it has good flux surfaces. Here are what they look like in a calculation of W7-X. What you're seeing is that we take a point in space and follow it around the machine once until it reaches the same location and then we put that point on the figure. We keep on doing this until we have lots of points. If everything works well we should have a closed surface. Most of the surfaces look like that, and that's good. W7-X is calculated to have good flux surfaces. They've tested this out and mapped the flux surfaces in a vacuum, and you can see them here. Don't worry that the shape is different, they are just measuring different parts of the machine, and the shape of the plasma changes as you move around the machine. For comparison, here's a picture of some flux surfaces in a tokamak.

Ok, now that we know what a flux surface is what do we mean by drifts. Well, the zeroth order calculation says that particles that are on a magnetic field line will always stay very close to that field line, gyrating around it. However, if the field line bends or is stronger on one side of the field line than the other you start getting drift effects. The particle will move off the field line over time. This drift is a first order correction. In a tokamak, because of the symmetry, it turns out that that the particles will just drift around the machine toroidally (this means the long way around the donut). They will stay on the same flux surface, but precess around. But in a stellarator, because there is no symmetry, it's not clear that they will stay on the flux surfaces. In fact in most earlier stellarators, particles just drifted right off into the walls. Confinement was terrible, and even though stellarators and tokamaks started at the same time, this problem made stellarators an inferior alternative.

This changed in the 1980s because then we had enough computational power to design stellarators where the particle drifts kept them on the flux surfaces. We call these "optimized" stellarators. There are very few optimized stellarators around. The precursor to W7-X, W7-AS was partially optimized. There is a small stellarator at the university of Wisconsin, called HSX which is optimized. And now there's W7-X. That's it.
W7-X Allows for Maximum Control of the Plasma Shape - Necessary to Exhaust the Plasma Energy

But W7-X isn't just optimized for this confinement. It turns out you can also try to improve other things. What W7-X has tried to do limit the amount of self-generated or "bootstrap" current in the plasma. The reason is that it wants to strongly control the shape, and if the plasma generates a lot of its own current, it will alter the shape. This type of optimization is called isodynamicity, and it's the main goal of the Wendelstein design idea.

One more question and we're done. Why is controlling the shape so important? There are a lot of reasons, but the one I want to focus on is the edge problem. In a hot plasma, some of it will invariably leak out, and you need a way to handle that plasma. The solution from tokamaks, which you can see in the image I linked above (this one) is the "divertor". If you look at that right hand figure you'll see that everything inside the orange section is "confined." When a plasma particle gets bumped out across the boundary, (called a "separatrix") it moves along the open field lines and it hits the wall. The divertor allows you to place the wall, farther away from the confined plasma. This allows you an opportunity to cool the plasma a bit, but also keeps junk that gets knocked off the wall from entering the plasma. Compare this to the left hand figure which has a "limiter" (in black on the right side) where the wall is right next to the confined plasma. The divertor was a major improvement to tokamak performance.

Stellarator divertors are much more difficult, and to solve the problem, the Wendelstein team pioneered the concept of the "island divertor." Here's a schematic of what they look like. The left is a standard tokamak. The right is a stellarator. The black lines are the separatrices. Anything inside is confined, anything outside is unconfined. Here's what the island divertor looks like in W7X. See those five separate blobs in the figure that look like closed mini confined plasmas? Those are magnetic islands. Generally these are bad, and you don't want them inside your plasma. But if you have them at the edge you can use them as a divertor. A plasma particle that crosses into the island from the inside, will get swept around the outside of the island. If you put your wall on the outside, ta-da, you have a divertor.

So, now we can understand why controlling the current is so important. It turns out if you have a lot of current, you will move those islands around. And in doing so, you can really negatively impact the performance, either by melting the wall, because now energy is going where you didn't want it to, or hurting the plasma, because now the confined region is too close to the wall and junk is getting in.

Whew that was a lot. If you read this far, I'm impressed.

edit2: added some formatting for ease of reading

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[–]Phil_EV 60 puan 1 gün önce

Thanks for the great response. In an ideal scenario, with everything working as it should on this machine, what sort of developments could it lead to? What is the desired aim for the machine? Is it just a proof of concept?

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[–]TymedOut 79 puan 1 gün önce*

Nuclear fusion is the opposite of nuclear fission.

In fission, large atoms (like Uranium, for example) are broken apart into smaller atoms, which produces energy. This is what nuclear bombs and reactors operate off of.

In fusion, small atoms are slammed together to produce larger atoms, which also produces energy. This is how stars "burn". The difficulty with this so far has been to be able to replicate the pressures and temperatures necessary for fusion to occur (essentially temp/pressure at the core of the sun). It's virtually impossible to contain these sorts of conditions under physical containment, so most experimental fusion reactors (like this one I believe) use very strong electromagnetic fields to contain the superheated, pressurized plasma. The other problem with that is that these fields often times use more energy than they produce.

So the current goal is to amp up the heat and pressure within the reactor to the point at which the fusion produces more energy than the field uses (since more heat/pressure will increase the reaction rate and thus energy production).

Fusion would be massively important because it would allow us to take very abundant elements like Hydrogen and produce energy from them, giving us a VERY clean energy source (only byproduct is Helium from H+H fusion) with a virtually limitless supply of fuel.

It's basically the energy source of the future. No nasty radioactive waste or materials (like fission). No carbon emissions. Cheap, abundant fuel.

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[–]munk_e_man 46 puan 1 gün önce

What's the downside? If someone knocks a magnet loose do we send out the equivalent of a solar flare through central Europe or something?

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[–]defrgthzjukiloaqsw 59 puan 1 gün önce

The plasma needs constant heating, if that goes out it will simply stop fusion.

It's far safer than fission reactors.

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[–]pulifrici 7 puan 1 gün önce

does the reactor produce more energy than it's required to heat the plasma?

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[–]Baloneykilla-420 33 puan 1 gün önce

Not currently, this is the kicker. The moment we can create more energy than we use to create the energy- we have an energy surplus (as opposed to our current energy deficit using this technology). The day we are able to create surplus our world is going to change dramatically. nuclear fusion (with energy surplus) would completely change our world.

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[–]pulifrici 11 puan 1 gün önce

I understand. Does the theoretical model show that we can get more energy out of it then it's required for it to work?

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[–]The_Last_Y 15 puan 1 gün önce

Yes. The tricky part is getting enough plasma that it reaches self-sustaining fusion. At this point the fusion reaction is hot enough that it continues to trigger more reactions. As long as it has fuel, which you can continually inject into the plasma, it will keep burning. There are several reactors in construction which should be big enough to achieve this and once they do that design can be used to develop commercial grade power systems.

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[–]iheartanalingus 3 puan 1 gün önce

Is the US joining in or are we, once again, lagging behind in development?

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izleğin devamını göster

[–]NFB42 16 puan 1 gün önce*

Yes. If you want a ELI5 on nuclear power:

Atoms, as you may know, are made up out of electrons, protons, and neutrons. The protons and neutrons are fused together in the atom's nucleus, while electrons move around the nucleus.

The number of protons (and to a lesser extent neutrons) in the nucleus is what decides the main property of the atom. For example if it has only one proton that means it's a Hydrogen atom. If it has 94 Protons that means it's a Plutonium atom.

But, an atom's nucleus also has something else in addition to protons and neutrons. This something else is binding energy that is keeping the protons and neutrons together. This is also called Nuclear binding energy and is the source of Nuclear Energy.

In Nuclear Fission, heavy atoms like plutonium are split apart and as a result their binding energy is released. This is the energy that drives most nuclear bombs and all currently functional nuclear power plants.

And I'm guessing this makes sense intuitively, it must take a lot of binding energy to hold a lot of protons and neutrons together, so of course breaking them up releases a lot of energy.

But the funny thing is, the amount of binding energy required doesn't just linearly go up the larger an atom gets. In fact, it is shaped like a valley. Around iron (56 protons) is the lowest point. Any atom bigger than iron requires increasingly more binding energy the bigger they get. But any atom smaller than iron requires increasingly more binding energy the smaller they get.

So when you split atoms larger than iron it releases energy. But any atoms smaller than iron have the reverse. They cost energy to split apart, and they release energy when you do the opposite of splitting: fusing them together. Here's a simple graph, if that helps. Fe = Iron

The problem is, fusing atoms is a lot harder than splitting them. Nuclear Fusion happens naturally in stars, because the stars' are so enormous their gravity exerts humongous pressures on the atoms inside, enough to cause them to fuse. This fusion then produces light which is how stars 'burn'.

In principle, harvesting fusion energy is no different than oil or gas. At some point energy was stored in these atoms, and by fusing them we can release that energy. The main difference though is that oil or gas are very finite and you have to burn a lot of it to get a lot of power, with Nuclear Fusion you only need to 'burn' relatively little to get a lot of power and the basis for your fuel is water (as in, the water that covers 2/3rd's of the planet). So it has the potential to truly revolutionise our access to power.

The difficulty is finding a way of harvesting fusion energy that's cost-effective. Scientists believe that there is probably a way to do it, but it will require extremely advanced technology. The Wendelstein 7x is one of dozens top level science initiative developing technology that we hope will eventually lead to profitable nuclear fusion. Another initiative, ITER, is done jointly by Europe, Russia, China, India and the US and is building a reactor in France which hopes to successfully produce small amounts of fusion energy by 2027 (which if successful would be followed by successor reactors scaling up till they reach commercially viable levels of output).


https://www.reddit.com/r/Futurology/comments/3w7ujk/wendelstein_7x_germanys_experimental_nuclear/cxu2i0f
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