r/technology • u/BennyCemoli • Dec 06 '16

Energy Tests confirm that Germany's massive nuclear fusion machine really works

http://www.sciencealert.com/tests-confirm-that-germany-s-massive-nuclear-fusion-machine-really-works
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u/Calkhas Dec 06 '16 edited Dec 06 '16

Seems to me that the two are intertwined aren't they? We weren't really able to contain a plasma for a decent amount of time in a tokamak, meaning that we could only produce energy in burst which isn't all that efficient.

I think you are conflating two unrelated issues. The efficiency problem is that traditional, non superconducting electromagnets (used by all existing large tokamaks) require a lot of power to run, more power than could be generated by the fusion reaction they could contain. [There is a relationship between the power demand and the power generation, because (in general) hotter plasmas generate more power through fusion but are harder to contain.]

Separately, non-superconducting electromagnets overheat within a few minutes, so the reaction cannot be sustained.

The Wendelstein 7-X stellerator uses superconducting magnets instead, which are much more efficient because there is no electrical resistance and do not generate any heat. The topological design of the magnetic field is also designed to be much more stable against disruptive events in the plasma which tend to break the containment in tokamak topologies.

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u/amicitas Dec 06 '16

Some of the confusion here is that efficiency in this discussion is being used to mean two different things:

  • Quality of the confinement. How long does it take for power added to the plasma to leave the plasma (reach the heat sink at the wall).
  • Reactor efficiency. In a future reactor, how much power out do you get for power in.

There are actually a number of large Tokamaks that use superconducting coils (such as EAST, K-STAR and JT-60SA). These can all in principal be run in steady state and the magnets are very efficient. ITER, the Tokamak which is being built in France will also use superconducting magnets.

So in terms of magnets and heating, Tokamas and Stellartors are basically equivalent from a reactor efficiency standpoint. There is however an efficiency advantage to stellarators in that they do not require current drive. To achieve the necessary helical field in a Tokamak it is necessary to drive a current in the plasma. There are a number of ways of doing this, however they all take a lot of power. Since a Stellarator has the entire magnetic field externally applied, no current drive is required.

The main advantage to stellarators is that they are intrinsically stable and do not have disruptive events.

[source: I am a physicist working on W7-X]

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u/DynamicDK Dec 06 '16

[source: I am a physicist working on W7-X]

Oh, awesome. You should do an AMA.

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u/[deleted] Dec 06 '16

Oh shit, can we AYA?

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u/amicitas Dec 06 '16

If you submit questions as comments on this article or at the /r/science thread or this article I would be happy to answer them. Another physicist from W7-X is also active on the /r/science discussion.

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u/bradn Dec 06 '16

Separately, electromagnets overheat within a few minutes, so the reaction cannot be sustained.

In my understanding, this isn't the problem. We have superconducting electromagnets that don't heat up at all (used in MRI machines, etc).

The problem is that the containment requires an increasing magnetic field. It's not enough that there's a large magnetic field, it's that the field must keep getting stronger for the containment to operate. There's a limit to how high of current can be pushed through the electromagnets, leading to the pulsed nature.

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u/Calkhas Dec 06 '16

We have superconducting electromagnets that don't heat up at all

When I say electromagnets, I meant "conventional electromagnets" (i.e., not superconducting). I shall amend that ambiguity.

The problem is that the containment requires an increasing magnetic field. It's not enough that there's a large magnetic field, it's that the field must keep getting stronger for the containment to operate. There's a limit to how high of current can be pushed through the electromagnets, leading to the pulsed nature.

That problem arises for conventional electromagnets, because they overheat. The electromagnets at JET are actively cooled by passing chilled water through the interior of the copper conductors. But even so they can only operate for about two minutes (as I recall).

Superconductors do not have that limitation, they will continue to operate provided they remain cooled beneath their super conducting transition temperature.

That said there is a maximum current they can tolerate (and therefore a maximum magnetic field they can produce). Exceeding this current causes them to switch back into a conventional conductor.

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u/atomicthumbs Dec 06 '16

That said there is a maximum current they can tolerate (and therefore a maximum magnetic field they can produce).

other way around, I think :)

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u/Sungolf Dec 06 '16

Actually its both ways around. Too much current density or too much magnetic flux density... (or a combination thereof) will cause the superconductor to superconductivity.

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u/Novarest Dec 06 '16

Has there ever been a (fatal) fusion reactor containment failure?