
Confirmation of the topology of the Wendelstein 7-X stellarator’s magnetic field - bcaulfield
http://www.nature.com/articles/ncomms13493
======
Animats
Wow. I'd seen renders of the proposed magnet geometry, and it looked like a
nightmare to build. They actually built it successfully. It was built by
welding curved parts together, not 3D printing the thing in metal. It uses a
superconducting magnet, which means having to wind coils from somewhat brittle
niobium-titanium alloy. These are strangely-shaped non-planar coils which
operate at liquid helium temperatures a few centimeters from 100 million
degree plasma.

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.

[1]
[https://www.ipp.mpg.de/987655/w7x_and_industry_en.pdf](https://www.ipp.mpg.de/987655/w7x_and_industry_en.pdf)

~~~
2sk21
The attached PDF is wonderful. Is there any other cluster of companies in the
world capable of such high-precision mechanical and electrical engineering
outside of northern Europe?

~~~
tsomctl
If you like the pdf, you might like this tour of the shop that makes a lot of
the stuff for Lawrence Livermore labs:
[https://www.youtube.com/watch?v=FmmNRaKpBTI](https://www.youtube.com/watch?v=FmmNRaKpBTI)

------
ars
Bit of a summary:

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.

~~~
mynewtb
Any chance for an ELI5 of what a stellarator and tokamak are?

~~~
ars
Sure.

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".

~~~
mrfusion
Say why could t you use an electric field instead of a magnetic field?

~~~
gliese1337
You can. That's what a Farnsworth fusor does (there are other related designs,
but that's the original, and the kind you can build in your garage).

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.

~~~
mrfusion
Thanks for the explanation. I looked up some of those designs.

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?

~~~
tedsanders
Counterintuitively, this setup would have zero force inside the sphere. It
turns out that being closer to charge on one side of the sphere is always
exactly balanced out by the more distant side's greater total charge.

------
gnarbarian
Here are two videos that do a great job of conveying the incredible
accomplishment of building it.

[https://www.youtube.com/watch?v=u-fbBRAxJNk](https://www.youtube.com/watch?v=u-fbBRAxJNk)

[https://www.youtube.com/watch?v=lyqt6u5_sHA](https://www.youtube.com/watch?v=lyqt6u5_sHA)

~~~
saganus
How the _hell_ do you test such a monstrosity?

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]

[0]
[https://www.youtube.com/watch?v=lyqt6u5_sHA](https://www.youtube.com/watch?v=lyqt6u5_sHA)

~~~
alephnil
> How the hell do you test such a monstrosity?

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.

------
kriro
First project in a while where I feel my tax money is used well (my guess is
the overall cost will end up being around 1.5 billion).

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)

------
visarga
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.

~~~
mioelnir
The design for ITER was finalized in 2001, amended in 2007. Construction for
W7-X started 2003. W7-X is now built, fully functional and operational. At
several points during its construction, the construction process for its
required parts had to be invented first.

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.

~~~
visarga
But, the catch is that ARC reactor is smaller, cheaper, faster to build and
could possibly be developed by industry, which would speed up the research
cycle.

------
sdfjkl
Stellarator is a beautiful word. I think it's my word of the year.

    
    
      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.

------
owenversteeg
ELI5 explanation: Because of something called the "hairy ball theorem" [0],
which says you can't comb hairy balls (but you can comb hairy donuts),
scientists are making a hairy, combed, billion-dollar metal donut [1] to
squish hydrogen atoms evenly on all sides. Once you turn on the machine, the
squished hydrogen atoms release energy!

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](https://en.wikipedia.org/wiki/Hairy_ball_theorem)

[1] [http://www.sciencemag.org/news/2015/10/bizarre-reactor-
might...](http://www.sciencemag.org/news/2015/10/bizarre-reactor-might-save-
nuclear-fusion)

~~~
dom0
An important thing about the 7-X is that it's not about the fusion
(squishing), because fusion is fundamentally a very simple process. It's all
about the combing (plasma physics), which is very difficult and the reason we
don't have squishing-plants yet.

~~~
wolrah
> fusion is fundamentally a very simple process.

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.

~~~
FreeFull
Right, it only becomes significantly harder when you care about getting more
energy out than you're putting in.

------
noobermin
I was at the 2016 APS Department of Plasma Physics Meeting a month or so ago.
I work in Laser Plasma, so I have only a surface understanding of the magnetic
confinement stuff, but I did attend a keynote by this group, and my impression
after the talk was that they were approaching a respectable result but hit a
snag. Looks like they couldn't get this out before DPP but fine enough, they
can confirm it today, so cheers to them.

~~~
clebio
> surface understanding

plasma physics jokes ftw

------
kensai
Amazing. And remember, this stellarator is our "Plan B". The main effort is in
the alternative method, the Tokamak (in ITER).

~~~
threeseed
And Plan C is ARC/SPARC out of MIT which I like because it is taking more of a
startup approach and addresses issues such as financial return on investment,
maintenance etc.

[https://www.youtube.com/channel/UCMFsHceLUJNkXMiCMFNnvkA](https://www.youtube.com/channel/UCMFsHceLUJNkXMiCMFNnvkA)

~~~
andrewflnr
Between stellarators working out and these guys, I'd actually put ITER down as
plan C, not A.

~~~
mtgx
Isn't plan A what Lockheed Martin is building? At least they made it sound
like they were the closest to real fusion.

[http://www.lockheedmartin.com/us/products/compact-
fusion.htm...](http://www.lockheedmartin.com/us/products/compact-fusion.html)

~~~
chillydawg
Yeah, but Lockheed could just be running a scam to fleece the US govt out of
millions in research grants and tax credits. Obviously it's a huge bet that
could pay out $lol if it succeeds, but I suspect it's more of an accounting
trick than serious R&D.

~~~
XorNot
They're not running a scam, but they don't have the results to be in more then
the research curiousity stage. Keep in mind too, the W-7 isn't designed to be
net energy positive either whereas ITER is.

~~~
drjesusphd
Are you aware of ANY results at all?

------
mkj
Is the strange geometry just one possible arrangement (current best
optimisation) or is there something more specific about it? To an outside
observer the apparent lack of symmetry etc is surprising.

~~~
chillydawg
It has 5 fold symmetry. It's broadly a pentagonal shape (viewed from above)
made up of the same piece repeated five times. It's actually rather simple and
beautiful (to my eyes). I'd not have been surprised if the model they ran spat
out some ridiculous random-looking shape that happens to fit some local
minima. As it is, it's a fairly reasonable-looking twisted ribbon like shape.

------
twic
> In a stellarator, nested toroidal magnetic surfaces are created from
> external magnetic coils

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.

~~~
drjesusphd
A "magnetic" (or "flux") surface is one in which particle orbits remain
confined. The only way they move from surface to surface is via colliding with
other particles, or by being carried away by turbulent fields.

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.

------
clebio
What is the process to generate the physical design for this (or for the
Tokamak)? Is it a sort of reverse-statics equation solver, where you define
the objective (confined field lines, etc.)? There has to be several layers of
solving involved, though, since you first determine the forces needed, and
then determine the physical hardware needed to create those forces (as well as
other systems like material-inlet). I imagine the article and its citations
actually contain this information, but might take a while to read through.

------
mozumder
How small can these stellarators get? Is there a necessary minimum strength
magnetic field that basically creates a minimum size for a stellarator?

~~~
raverbashing
I'd say the smaller the Stellerator is, the stronger the magnetic fields have
to be, as the curve radiuses are smaller

So there's probably an ideal size

~~~
krashnburn200
my math suggests something approximating the size of a star could achieve
fusion by using only gravity for containment.

I wouldn't want to stand too close to that sucker though.

~~~
pavlov
If someone had built such an object, say 10,000 light years away, what would
it look like to us?

~~~
krashnburn200
nothing for 10,000 years - however long ago they built it.

------
ChuckMcM
Nicely done! To me at the W7-X is at least as impressive as the LHC. I
particularly like the ways they measure the magnetic topology by shooting
electron beams through a neutral gas. That had me wondering if you could build
a 3-D vector display that way. Sort your plasma lamp but with figures inside
rather than randomness.

Can't wait for them to run plasma through it.

------
crispyambulance
It all looks very impressive, but why all the funky curves? I thought the idea
is to create a magnetic field to contain a plasma. Could that not be done with
a torus of circular cross-section? Instead we see a very complex undulating
torus, what's the reason for that?

~~~
luca_ing
A simple toroidal magnet creates a complicated field, while this complicated
magnet creates a much simpler and more benign field.

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.

------
dghughes
I love the technology but I find it amusing to get free energy from fusion we
just have to build the most complex energy hogging device ever made.

~~~
wolrah
Think about it like a supercharger on a car. They consume power to do their
job, but they gain you more than they consume so the net result is more power
delivered to the crankshaft.

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.

------
i2amsam
What is an optimistic timeline from here? When could this be energy positive /
used commercially?

~~~
asmithmd1
The same as it has been for the past 50 years - about 10 years before
commercial use.

~~~
zachrose
Presumably each estimate is made with fewer unknown unknowns and thus higher
real chances of success?

------
intransigent
So, magnetic fields can be used to confine fusion plasma with "five nines"
reliability?

~~~
taneq
No, they just generated the fields they wanted to within that accuracy.

------
foota
Anyone know why you can't just build a really long solenoid?

~~~
wtallis
That would allow you to keep a quantity of plasma confined for a longer period
of time than a short solenoid, but doesn't help you keep the plasma
density/pressure up high enough for sustained fusion.

------
hughes
This is a very confusing title. 1:100,000 is not a magnitude of a magnetic
field. It refers to the approximate deviation from the desired magnetic
topology that the stellerator experiment was targeting.

~~~
noobermin
Hmm, I read it specifically as deviation from what they tuned for.

