

Another barrier to fusion falls? - Sapient
http://www.sciencedaily.com/releases/2012/04/120423143128.htm

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ars
The sun is an extremely diffuse energy source. If we wanted to power the US
using a power source with the same volumetric efficiency as the sun you'd need
a device that's about 3000 cubic miles in size.

Assuming a 100 foot tall building, that would be a building 400 miles on each
side. For comparison the amount of solar energy delivered to that same area is
20 times higher! (Assuming you could capture all of it.)

[http://www.wolframalpha.com/input/?i=%283.3991%C3%9710^12+wa...](http://www.wolframalpha.com/input/?i=%283.3991%C3%9710^12+watts%29+%2F+%28%28%283.846%C3%9710^26+watts%29+%2F+3.388%C3%9710^17+cubic+miles%29%29)

So to make fusion practical for human use it's not enough to duplicate the sun
- they actually need to make it work far better than what happens in the sun.
That's one of the reasons they use D-T reactions instead of H-H - it's much
faster.

~~~
Sapient
But you are talking about volumetric efficiency - the portions of the sun
where fusion is actually taking place are far more efficient than that surely?

I always believed they were trying to duplicate the suns core, not the whole
ball of gas.

~~~
ars
That's easy enough to calculate. You end up with 46 cubic miles. That's
equivalent to 100 foot building 50 miles on each edge.

Much smaller - still pretty large, but doable. Your energy output in
comparison to capturing solar energy is now 3 times as much rather than 1/20
as much.

~~~
Sapient
In which case, I can only assume we are aiming for greater efficiency than the
sun. Thanks for your answer!

~~~
Retric
Jet is 1980's technology and it's already produced 16 megawatts from 200 cubic
meters in 1997. <http://www.ccfe.ac.uk/JET.aspx> Compared to "the center of
the sun, fusion power is estimated by model to be about 276.5 watts/m3"
<http://en.wikipedia.org/wiki/Solar_core>

Which works out to 290x the energy density of the core. Direct comparisons of
the Continent vessel vs the volume of the sun are even higher.

~~~
arethuza
Not to mention getting 5.4 yottawatts from roughly the same volume, if not for
the same amount of time:

<http://en.wikipedia.org/wiki/Tsar_Bomba>

------
gjm11
Original article: <http://www.pppl.gov/polPressReleases.cfm?doc_id=1242>
(includes link to podcast with authors)

Article in _Physical Review Letters_ :
<http://prl.aps.org/abstract/PRL/v108/i16/e165004> (paywalled)

Freely accessible preprint:
<http://www.pppl.gov/pub_report/2012/PPPL-4728.pdf>

~~~
drostie
It's really unfortunate that the "podcast" amounts to the authors reading a
press release. Good physicists -- and I think these guys probably are good
physicists -- can often give an intuitive picture of what such a density limit
is, why it exists, what we think is going wrong in the field today. Because
they were confined to just two minutes, it seems, they were forced to
alternate awkwardly between reading a prepared statement which explains little
and name-drops a couple of ridiculously broad fields ("radiation physics" and
"magnetohydrodynamics") briefly.

I mean, there is always a problem with oversimplifying these things, but
talking to a layperson doesn't have to be too much different from talking to a
fellow physicist; the key difference for me seems to be presenting two or
three perspectives on the same thing before going forward.

Here is the backstory to this paper, as I would explain it to laypeople: "So
we've got this plasma -- plasmas are like a metallic gas, lightning is a good
example; the electrons get shared between the gas molecules so the gas becomes
an amazing conductor. Same thing happens whenever we superheat any gas,
really. So we've got this superheated gas in a tokamak, which is a big ring
which uses magnets to keep the plasma spinning around in a circle without
hitting the edges. It's a neat trick, a bit hard to do. Now if we could get
the plasma hot enough and dense enough, we could recreate the conditions in
the Sun with it, and solve the energy crisis -- this is what's called fusion
energy, much cleaner than nuclear fission. But how do we heat it up? Well like
I said, this plasma is metallic, it's just like the wire in a light bulb, so
we just put a lot of electricity through it, and it heats up and glows. We
call that "ohmic heating". But there's a problem. Before it gets hot enough
for controlled fusion, we basically run into a wall, we try to pour more
current into it and it doesn't get hotter, doesn't get more dense.* And until
now we didn't really know why this happened. It's as if the laws of nature
just said, 'Eff you, I'm not letting you solve the world's energy problems
that easily.' We have an explanation, now, why..."

I haven't read the paper yet but I hope it can rise a bit to a better
technical level than the press releases. The press releases are a little
elliptical about what their solution entails but it sounds like you have to
understand what's happening with certain observed impurities in the plasma.

* From what one of the press releases says it sounds like it might actually kill the containment field and fly apart, but that sounds really strange -- as if it would destroy the tokamak.

~~~
ars
> kill the containment field and fly apart

It's _really_ hard to keep the magnetic field right. It's not a stable system,
it needs to be dynamically controlled. Presumably these defects mess up the
magnetic field to the point that it's impossible to contain it.

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JumpCrisscross
The article's title alludes to scientists seeing a "solution" that is nowhere
to be found in the article itself. It seems more like we understand the
problem better rather than have found a solution. Am I mis-reading?

~~~
starwed
I have no idea if the article is _accurate_ , but yes, it does mention a
possible solution:

>"Among other things, they intend to see if injecting power directly into the
islands will lead to higher density. If so, that could help future tokamaks
reach the extreme density and 100-million-degree temperatures that fusion
requires."

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pilom
I am pretty sure that an energy bill in the US from 2008 killed all US funding
for the future fusion reactor in France. I can't find a source though. Anybody
have any insight on that?

~~~
juiceandjuice
It was a refocusing of funds, but yeah. The US decided to put more effort into
the National ignition facility instead of ITER. Still, we do research for/with
ITER, just not as a partnering country.

