

Are We a Step Closer to Nuclear Fusion? - Sami_Lehtinen
http://www.xprize.org/news/are-we-step-closer-nuclear-fusion

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bostik
Hidden in the middle of the article there is an interesting tidbit: _" When
cooled to liquid nitrogen temperature, the superconducting tape can carry as
much current as the larger copper conductor."_

Material science and superconductivity have come a long way, and they benefit
from the feedback loop with fusion research.

All of which reminds me of what a friend told me quite a few years back. At
the time, she was doing her PhD in superconductivity at Cern, and while there,
her instructor had a very nice project result. The instructor and team
discovered an alloy composition that happened to keep superconductivity at
temperatures nearly 20 Kelvin higher than other state-of-the-art compounds. At
the time most promising superconductors required temperatures in the range of
-210°C.

The discovery sounded neat, and a temperature bump like that certainly was
cool but it didn't sink in until a bit further just what they had achieved.
Superconductivity at temperatures in "low -190's" Celcius meant that suddenly
it was possible to use liquid nitrogen (boiling point ~-196°C).

The availability and cost of coolant improved quite a bit.

There have been huge improvements since - the state-of-the art high
temperature superconductors now keep up at temperatures around -135°C.[0]
Cheaper and more abundant coolant, along with lower delta-T, means that
research into _applying_ superconductivity is much more accessible.

Small-scale fusion is likely going to be just a start; I expect to see major
improvements in energy transfer, particularly on power loss, over the next
20-30 years.

0: [https://en.wikipedia.org/wiki/High-
temperature_superconducti...](https://en.wikipedia.org/wiki/High-
temperature_superconductivity)

~~~
rsp1984
Maybe that's an entirely stupid question and please overlook my complete
ignorance in this matter, but it certainly sounds quite challenging to keep a
superconductor at around -190 deg C while just some feet away there is
confined hot plasma at millions deg C. Would this be a problem?

~~~
bostik
Not a stupid question at all. One could say the foundations of the entire ITER
programme circle around need to answer that one. :) [0]

But on a more practical note: the containment for the plasma is a vacuum. All
the matter inside the magnetic container is held within the plasma, and in
simplified terms, between the "surface" of the plasma and the wall of the
container there is _nothing_. Vacuum is a very good insulator, and the
magnetic fields that control the plasma serve a dual purpose. Always, to
prevent plasma from getting into contact with the walls of the container - but
both to keep the energies within the ring, and to make sure any contact with
the vessel can't cool the plasma down.

The reason why vacuum is such a great insulator is easy to explain. Heat is
transferred by conduction (contact), convection (heat exchangers), and
radiation (ejected particles). When surrounded by vacuum, matter isn't in
contact with anything and heat transfer by conduction is 0. Because there is
nothing for heat to flow to, also convection is effectively 0. Heat loss by
radiation is far, far less than by conduction - and since plasma is a cloud of
charged particles, even that is subject to the strong magnetic fields pushing
the matter back.

0:
[https://en.wikipedia.org/wiki/ITER#Vacuum_vessel](https://en.wikipedia.org/wiki/ITER#Vacuum_vessel)

~~~
danmaz74
Very interesting. What about the high-energy neutrons produced by D+T fusion
though? Wouldn't they escape the magnetic containment?

~~~
bostik
Yes.

They are also a major source of excess energy once the reaction becomes self
sufficient. Capture the high-energy neutrons in a suitable material and it
heats up. Use heat exchangers to run turbines, produce electricity.

If you read up on fusion energy, you'll soon encounter the note that fusion
reactor waste is short-lived and therefore easier to manage + store than
fission reactor waste. The material used to capture neutrons is transmuted to
radioactive isotopes, but due to the choice of materials* used the half-life
of those isotopes is pretty low.

Yes, it also means that the fusion reactor waste is more energetic and more
active radiation source than fission waste. But because it's going to become
safe in just a couple of hundred years, it causes much less of a headache in
terms of storage.

*: We don't necessarily know what the radioactive isotopes will be. I think the jury and researchers are still out on that one but I dare say it's guaranteed that the neutron absorbent will NOT be uranium or radium.

~~~
danmaz74
To clarify, when talking about the neutrons I wasn't thinking about the
radioactive waste problem, but about keeping the superconductor's temperature
low. I just took a look at the Iter schema and noticed that there is a
"blanket" that covers the interior surface of the vacuum vessel exactly to
absorb the neutrons. The vessel then has a water cooling system.

At this point, I was also wondering if you have any idea about how much of the
heat transfer from the plasma to the vessel would be be through those neutrons
(ie if it's the main heat transfer medium).

~~~
bostik
By my understanding practically all of the generated energy is due to neutrons
hitting the containment vessel. Highly energetic neutron, impacting solid
matter, will generate quite a bit of heat in the violent collision.

I found one fairly decent source [0] which goes into somewhat greater detail.
It's worth noting that if you follow the link to first page of the series,
that page explicitly states that what fusion reactors call "neutron flux"
would be called "heat flux" in traditional reactors.

And it makes sense even when thinking logically. Energy and matter are
interchangeable, and the reactor turns matter into energy; majority of matter
escaping from the fusion reactor will be helium and neutrons. These particles
interacting with external solid matter turns kinetic energy into heat. That's
the power plant's heat source.

Now, as to the fraction of heat transferred by neutrons compared to that
transferred by the newly formed helium - I have no idea.

0: [http://www.visionofearth.org/industry/fusion/how-do-we-
turn-...](http://www.visionofearth.org/industry/fusion/how-do-we-turn-nuclear-
fusion-energy-into-electricity/)

~~~
danmaz74
Thanks for the update :)

Regarding helium, considering that it's plasma we're talking about, I think it
can't escape at all - the separated nuclei and electrons should be contained
by the magnetic field.

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smoyer
An ARC reactor? (Affordable, Robust, Compact) - I can't believe they went
there. What percentage of the royalties for this design will go to Marvel
Comics?

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ehuna
Interesting! For an active Nuclear Fusion project in the US (San Francisco Bay
Area), check out the 'National Ignition Facility' -

[https://en.wikipedia.org/wiki/National_Ignition_Facility](https://en.wikipedia.org/wiki/National_Ignition_Facility)

For more links and my modes overview of the project see
[http://blog.ehuna.org/2009/05/national_ignition_facility_192...](http://blog.ehuna.org/2009/05/national_ignition_facility_192.html)

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DrBazza
Someone posted this the other day (apologies, I can't remember who).
[http://i.imgur.com/sjH5r.jpg](http://i.imgur.com/sjH5r.jpg)

The image appears to be from 2012. The current funding is 'fusion never'. I
hope that's not true.

~~~
giarc
That was first published in 1976 and perhaps updated when re-published in 1986
so perhaps the curves have changed a bit due to technology changes.

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MichaelMoser123
wasn't some form of graphene superconductive at room temperatures? Is it
possible to turn that into a coil - superconductive at room temperatures?

~~~
dghughes
I think I read a few days ago when lithium is added to graphene it does
something like that, only discovered recently.

I don't think graphene was superconductive until this lithium method was
discovered or maybe this allows it to be super conductive a a higher
temperature.

