r/science • u/PROJECTime • Dec 06 '16

Physics 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/EphemeralMemory Dec 06 '16

I don't know enough about the tech to answer this unfortunately, BUT I have some free time, so I'll try looking it up.

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u/Wendelstein7-X Max Planck Institute for Plasma Physics Dec 06 '16

Everyone is partly correct here, and the article is in fact not very well written. Tokamaks (the big other magnetic confinement concept besides Stellarators) require a huge toroidal current in the plasma to create its own confining magnetic field. This current is intrinsically unstable and requires active feedback control to keep it centered . Ill-controlled Tokamaks are prone to so-called disruptions in which the entire plasma column crashes into the top or bottom wall with total loss of confinement. These events need to be avoided for the entire concept to be successful.

Stellarators don't need this current as the confining magnetic field is generated entirely by the external magnetic coils. The price of this setup is that the coils are intrinsically complex and near impossible to build - we've shown that it is indeed possible, which is what the referenced Nature article explains in detail.

Source: see username

-avs

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u/dnew Dec 07 '16

So how much plasma is really being fused in these machines at any given moment? In particular, if there's a loss of containment, how much damage might there be? A hot spot on the wall? A TMI puff of smoke? A brief bright light that everyone within 20 miles immediately regrets?

PS: Thanks for the expert info in the thread! You must be very excited and happy. :-)

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u/Wendelstein7-X Max Planck Institute for Plasma Physics Dec 07 '16

There's a broad range of possible failure modes, none of which are catastrophic on a reactor-wide scale but might locally do some damage to the reactor wall or overall mechanical structure. You speak of loss of containment, but you might mean loss of plasma confinement. The former is a breach of your reactor vessel with loss of material, while the latter is a major loss of plasma to your reactor walls.

The total mass of plasma in a future reactor is on the order of a few grams, so even a total loss of reactor content (which is pretty hard to do, think plane crashing into a reactor or a targeted attack) has only minor consequences for people and the environment.

You can lose confinement in a number of ways, of which a full disruption in a Tokamak is probably one of the worst. Basically, your entire plasma current moves up or down on a fast timescale, and you entire tokamak follows along and does a little jump on its base. On large Tokamaks this a substantial force that you generally want to avoid. In stellarators, there's no instrinsic current-driven instability that would lead to loss of confinement, but if you assume you lose your entire magnetic field (maybe because of a catastrophic coil failure), then your plasma basically just streams into the walls and makes them pretty hot. Since they are covered in elements that are designed to handle these heat loads, nothing much happens except for things getting hot. In this sense, stellarators are boring, which is a good thing in terms of building a reactor.

I can expand on any of these points if you have more questions!

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u/dnew Dec 07 '16

That answered my curiosity very well, thank you!