Comment by bostik | Hacker News Reader

Comment by bostik | Hacker News Reader

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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...

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?

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

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

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.

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).

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-...

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.

Thank you for your explanation. Another thing learned today.

The Breakthrough: The Race for the Superconductor http://www.amazon.com/The-Breakthrough-Race-Superconductor/d... is a nice account of the initial discovery and characterisation of HTSs in the 1980.

Oh, thank you. That went to my read queue.

I wonder if safety is also a factor seeing that liquid nitrogen to gas expansion ratio is 1:696 and liquid hydrogen is a whopping 1:851.

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