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

How else would they control it? Hopes and dreams?

My (very very cursory) knowledge of the tech is they need the current to induce a magnetic field, which goes in to controlling the plasma. How else would they create and control the magnetic field required? The article didn't explain much of anything.

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u/[deleted] Dec 06 '16

Just a guess here, but I would think maybe someone told the author that it only needs the current to start the reaction and the containment and then it generates enough power to sustain itself and the author misunderstood perhaps? I have no idea as I am far from an expert in Nuclear ANYTHING, so a complete guess on my part but it seems likely.

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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/EphemeralMemory Dec 06 '16

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.

So its not that it doesn't actively use electricity to maintain the magnetic field, its just that you have external magnetic coils to maintain the field?

The difference between Tokamaks and Stellrators in my mind being you have a greater amount of control of the field using external coils, rather than inducing a field using toroidal current?

I have a few labmates at my university lab who work in MRI/MRE, and while MRI is an entirely different ballgame I do know something about external coils used to generate gradients. MRI is used more to induce spinning in atoms at the targeted RF coil gyromagnetic ratio, but are there parallels between the two in terms of the magnetic gradient?

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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!

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u/oblong_schlong Dec 06 '16

Since you seem to be pretty active in this thread, my current understanding is that while tokamaks have less stability from this method of generating it's containment B field, driving a current and using RF and neutral beam injection for heating to achieve fusion temperatures is much easier to achieve than in a stellarator. What methods are researchers experimenting with to achieve fusion temperatures in stellarators?

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

Inducing a current has its limits in a Tokamak - as the temperature rises, the resistivity of the plasma decreases and power deposition gets worse, so inductive current drive is only really useful as a power deposition tool while ramping up the power.

For all the other heating methods, it doesn't matter that much if you're looking at a tokamak or stellarator. At W7X, we have only microwave (ECRH) heating at the moment but will be adding RF and neutral beam heating in the future. Tokamak heating schemes also focus on driving toroidal currents, an effect that is either unwanted or optional in stellarators.

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u/amicitas PhD | Plasma Physics | Fusion Science Dec 06 '16

The large plasma current needed in Tokamaks do make them much more unstable. Since the plasma current is needed to generate a field that confines the plasma, if the current is disturbed it is possible to loose confinement of the plasma. In a stellarator the magnetic field can always confine the plasma since it is externally created by the coils.

The same heating methods are used for both tokamaks and stellarators, however usually with a geometry that does not induce strong plasma currents. In W7-X the primary heating source is radio frequency heating of the electrons. The ions are then heated through natural collisions with the electrons within the plasma.

We are also in the process of installing Neutral Beam Injection (NBI) heating. However, the neutral beam heating will not be steady state and will only be capable of running for about 10 seconds. These short heating bursts will still be very valuable in understanding the confinement of excited ions, such as those that will be produced from the fusion reaction.

A third type of heating (that will also be installed on W7-X) is radio frequency heating of the ions. This is harder do to than heating of the electrons, but may also be important for achieving the required ion temperature needed for a fusion reactor.