Confirmation of the topology of the Wendelstein 7-X stellarator’s magnetic field
nature.com
nature.com
Here's an industrial view of the construction project.[1] Considerable new manufacturing technology had to be developed to build this. One minor component is a diamond window 120mm across and 1.8mm thick, soldered into a stainless steel frame with copper-silver-titanium solder.
Cost €370M. Look at the construction pictures and you can see why.
I wonder how the Skunk Works fusion project is coming along. They have an even stranger geometry. The only word out of Lockheed is that there's been enough progress to justify spending more of Lockheed's money on the project.
And a factor of 66 within the valuation of Uber :)
There's just so much stuff there, impossible to summarize.
Not an expert, but ... imagine trying to successfully build a stellarator as irregular and sinuous as this in the 1960s! You can see why they didn't try.
This is a stellarator which is the same idea as a tokamak: Confine hydrogen with magnets and get fusion.
The magnetic field in a stellarator is crazy complicated, and was impossible to model and design before computers. With computers they were able to design it, but were unsure if they could build it.
This is a test showing that yes, they can actually build it.
In theory stellarators are simpler to get right than tokamaks, but only if you can actually design, and then make one.
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".
Ah, my favorite theorem, The Hairy Ball Theorem!
To phrase it another way, could you explain why the magnetic field is tangent to the sphere and not normal to it?
The force magnetic fields apply is always perpendicular to the field: F ~ v × B (right hand rule, and all that). So if you want F to point inward, the one component of B that doesn't matter is its radial component---ie. the tangent component is all that counts. But it's not clear to me that you can't compensate for the restrictions of the hairy ball theorem by having a nonuniform velocity distribution. Of course, there's no "hairy torus" theorem (because it's not true!) and this immediately suggests the tokamak design.
It's also not possible to take a sphere and have the radial magnetic field point inward everywhere (or outward everywhere), because Maxwell's equations prohibit magnetic monopoles.
And of course, the last point about no magnetic monopoles should be sufficient to make a spherical field impossible.
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.
Wiki: However, stellarators, unlike tokamaks, do not require a toroidal current, so that the expense and complexity of current drive and/or the loss of availability and periodic stresses of pulsed operation can be avoided, and there is no risk of toroidal current disruptions. It might be possible to use these additional degrees of design freedom to optimize a stellarator in ways that are not possible with tokamaks.
But for complicated reasons, it just isn't efficient enough to actually produce net power output. Of course, neither is any other design right now, but magnetic confinement appears to be the more promising path forward.
Now I'm really puzzled why no one has tried a more naive approach. I'm picturing a hollow metal sphere with a high positive charge with positive ions inside of it. Wouldn't it push them all together and with a high enough charge get them hot enough to fuse?
Fusion reaction has pros over fission in that a fusion reaction produces a lot more energy than a fission - and that it can't 'melt down' - unlike fission it doesn't have to be kept in check. If Homer Simpson messes up at the fusion plant, the reaction will just stop. Why? Because for fusion to happen, the atoms (hydrogens) need to overcome their electrostatic repulsion of eachother - the Coulomb barrier generated by the protons in the nuclei (at fusion temps the atoms have all become ionized - electrons have shot off from the atoms, so you have electrons and positively charged nuclei flying around - aka a charged gas aka a plasma).
Anyway that coulomb force has the same dependence on distance as gravity (1/r^2) - but unlike gravity - is a repulsive force. If you can force the atoms close enough together (by putting them under high temperature and pressure) then they will be close enough that the nuclei of the atoms will be pulled together by a stronger attractive force - the nuclear force - which only works at a very short range. As mentioned, you have to get the atoms going really fast and really close together for this to happen (put the gas under high temp and pressure, which incidentally will ionize the gas atoms into electrons and positively charged ions).
The fact that it is now 'electric' is the key to both approaches (tokamak and stellarator) at containing the plasma. These are just two approaches at Magnetically Confined Fusion, which operates on the simple principle that charged particles (both electrons and the ions that you are trying to fuse w other ions) travel along magnetic field lines. They spin in tight little circles around magnetic field lines, the stronger the magnetic field, the tighter the circle. Tokamak, which is simpler than a stellarator, is basically a solenoid https://en.m.wikipedia.org/wiki/Solenoid that wraps around into a donut shape. Just as the magnetic field lines in a solenoid go along the length of the solenoid, the magnetic field lines in a tokamak will go around the donut (toroidal field). Just imagine that the tokamak magnets are really powerful so the magnetic field lines (not really lines) are really strong - then your ions will gyrate around and bump into other ions that are gyrating around B-lines. There are problems though.
It's been a while since I studied but basically, in addition to ions and electrons very fast giration around the magnetic field 'line' and it's slower movement along the field line (around the donut) - it also 'drifts' https://en.m.wikipedia.org/wiki/Guiding_center - this is the slowest movement. There are a few mechanisms that cause this drift, but to the great misfortune of mankind, these drifts are 'out' - the ions drift away from the plasma core, and towards the wall of the tokamak.
To stop this drift from happening, you need the toroidal (around the donut) B field lines to also twist, so they look like a twizzler that has been wrapped around on itself. In a tokamak this is generally done by running a current through the plasma around the donut. Plasmas are highly conductive, so you can do this. It is like a big coil of wire. Just like the coils on a solenoid generate a straight B field through the center of a solenoid, and the coils of a tokamak (donut shaped solenoid) generate a donut shaped B field, running a current through the donut shaped plasma will generate a B field from the ceiling to the floor thru the donut hole (the poloidal field). The poloidal and toroidal fields vector add to make your twirly twizzler shaped field that prevents drifts. This running a current thru the plasma is how you 'twist' the field in tokamaks.
Stellerators, on the other hand, generate the field with complex (very) magnetic configuration (look up images of WX-7). There is no need to run current thru the plasma, and this in theory, and I think now in practice, with WX-7, leads to a more stable plasma. This stability issue is very important. Scientists have known about drifts since year 0 of fusion research, but there are many instabilities that cause the plasma to break down.
As mentioned in the article - there is a triple product that basically describes the success of the plasma - plasma density, plasma temperature and confinement time. This is more or less equivalent to 'are we getting more energy out than we are putting in' and I believe the advantage of stellerator would be confinement time. Hopefully these brainiacs can get it happening.
However, it is too small to break even. A smaller device will loose plasma faster (bigger surface area/volume), so too make it produce enough energy it needs to be big. See this picture of the planned ITER, note the human figure for scale:
https://www.iter.org//doc/www/content/com/Lists/Stories/Atta...
Achieving these objectives does not require producing
an energy-yielding fusion plasma. This is because the
properties of an ignited plasma can largely be
transferred by the ITER tokamak to stellarators.
Wendelstein 7-X can therefore dispense with the use
of the radioactive fusion fuel, tritium, thereby
greatly reducing costs.
Example of flame vortex: https://www.youtube.com/watch?v=JJPwxBf3rjk . Plasma vortex will be similar to flame vortex, except that it must be cycled to avoid losses.
I mean... I know that testing complex software projects is a pain in the ass. But... this? I mean... how do you even begin to plan where all the wires and other elements go and then test that it's a sound design?
I'm guessing they do a lot of simulations and such, but what if something fails? how much do you have to backtrack to fix a potential bug?
It just boggles the mind. I had no idea these things were so complex. Thanks for the links.
Edit: This video explains the elements quite in an understandable way [0]
That is very complex, and a large part of the design process is to make sure it actually is testable.
The article actually describes the result of some of the system testing of the device. They have now measured the magnetic field and found that it reproduced what they predicted it should be.
The engineering simply makes my jaw drop...sick achievement, congratulations to everyone involved. Surprised that it isn't pushed more in the media here. I think it's really hard to communicate the achievement to the masses but this needs to be on the news way more than it is (and presented better...somehow)
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.
Also, of course we could do it cheaper now. The project began around 20 years ago and had much luck, because there were many occassions were the techhnology needed to build the Wendelstein was just invented.
Source: A german 2h30m podcast with the project lead: https://alternativlos.org/36/ Extremely interesting!
> This project is going to
You seem to imply it's still in construction. It's finished already, in fact they finished testing it a year ago.
That's 15+ years of research, breakthroughs and inventions in plasma confinement physics and engineering that have had to happen first.
As a noncommercial research reactor, W7-X will continue to be useful for many years to come. For material testing alone I would imagine access to reactor capable of producing a stable, continuous fusion reaction is invaluable.
The ARC is a design proposal from 2015. As such they have access and can utilize all the achievements, results and processes from W7-X, ITER and other material science advances of the last 15 years. If their proposal was not better than already built specimens, it would be a bad proposal. To declare it a race against the research foundation they built upon seems ignorant at best.
In 10-20 years, when the ARC is built, a new design proposal will emerge, based on even newer advances in material science and the lessons learned from building the ARC. And it again will be better than the then current, assembled reactors. That is how it is supposed to be.
stellarator |ˈstɛləreɪtə|
noun - Physics
a toroidal apparatus for producing
controlled fusion reactions in hot plasma,
where all the controlling magnetic fields
inside it are produced by external windings.
ORIGIN
1950s: from stellar (with reference to the
fusion processes in stars), on the pattern
of generator.This news today is saying that the hairy, combed, billion-dollar metal donut is the right shape, which is very important because nobody ever made a donut like this before, and if it was the wrong shape you couldn't comb it and that means the hydrogen would escape and you couldn't make any energy.
[0] https://en.wikipedia.org/wiki/Hairy_ball_theorem
[1] http://www.sciencemag.org/news/2015/10/bizarre-reactor-might...
For anyone reading who don't get just how simple it is, there are literally hundreds and possibly thousands of people who have built their own DIY fusion reactors at home. Many high schoolers, some of them as young as age 14.
Fusion is really easy by scientific standards.
plasma physics jokes ftw
http://www.lockheedmartin.com/us/products/compact-fusion.htm...
Lockheed claims that fast iteration will make for fast progress, but they're not the only fusion startup taking that approach. Others include General Fusion, Helion, and Tri Alpha. And Tri Alpha, at least, gets a lot of respect for openly sharing experimental data in papers and conferences.
Whereas the Stellarator has no DC current flowing in the plasma and can operate continuously with no pulsing/ripples.
If you have information that does rule this out, I would like to hear it, since I hear this claim repeated often.
Edit: there's also lower-hybrid current drive. This combined with boostrap means that ITER may be able to operate at steady-state.
What does that mean? What is a 'magnetic surface'?
> Each magnetic field line meanders around on its magnetic surface; it never leaves it. In general, if one follows a field line from one point on a magnetic surface, one never comes back to the same exact location. Instead, one covers the surface, coming infinitely close to any point of the surface.
Does 'magnetic surface' just mean this, the manifold generated by following a field line?
In that case, is it really meaningful to call these 'nested'? If what they're saying is that at any point in the field, following the field will trace out a surface, then there aren't actually discrete surfaces here, and it's not entirely meaningful to call them nested. It's like talking about the contour lines on a hill being nested, when what you really mean is that the hill has a smooth gradient.
I've never heard that "nested" need only refer to a finite number. It's a way to illustrate, with words, how such surfaces relate to a 3d torus. I don't see anything wrong with this.
http://iopscience.iop.org/article/10.1088/1367-2630/15/1/013...
https://en.wikipedia.org/wiki/File:Stellarator_magnetic_fiel...
In the hill metaphor, saying that the surfaces are nested corresponds to there being a single of peak of the hill.
So there's probably an ideal size
I wouldn't want to stand too close to that sucker though.
Woosh.
A star is a fusion reactor using only gravity for containment.
Can't wait for them to run plasma through it.
AFAIK, in Tokamaks you need to permanently increase the plasma current to keep it stable, which puts an upper bound on the time you can sustain the plasma (you can't increase the current forever). In a Stellarator, by contrast, you can keep the current constant, and thus have plasma durations measured in hours instead of minutes.
If you make a doughnut twizzler it doesn't happen.
A Top Fuel dragster's supercharger takes around 600 horsepower to drive at full bore, but that enables an engine that's loosely derived from (read: shares core dimensions with) a ~350HP production car to produce more power than a freight train.
Sometimes you've got to spend power to make power.