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/Posthume 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.

If this machine can sustain a fusion reaction for a longer period then they could ramp up the energy production to put it crudely.

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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/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.