In a fission reactor, I know the energy goes into the water, which creates steam and that turns a turbine. But it looks like fusion reactors use a near vacuum and high powered magnets. Once two atoms actually fuse, where does that energy "go"? Does the new helium atom just bounce around the containment unit? How do we actually extract meaningful energy from such a closed system?
In the same way. Most proposed fusion systems use deuterium-tritium fusion where a significant amount of the energy is carried away as neutrons, so direct energy conversion wouldn't be possible anyway.
From the article you referenced:
> ITER will not produce enough heat to produce net electricity and therefore is not equipped with turbines to generate electricity. Instead, the heat produced by the fusion reactions will be vented.
So in a fusion plant, the particle energy would turn into heat (by the particles interacting with matter), this would heat up water (or some other carrying fluid), turning a turbine that produces electricity. See also https://en.wikipedia.org/wiki/DEMOnstration_Power_Plant which contains some diagrams showing just how that would be done.
More exotic reactions (e.g. p-B11) have been proposed, where almost no energy is in the form of neutrons. Theoretically, you could then use electrostatic devices to capture the energy directly without any of the mess with Carnot efficiency. However, getting p-B11 fusion going is much harder than d-t.
Where/how does the heat extraction take place? Are they planning to put water pipes in the walls of the confinement unit? I'm struggling to understand because in the diagrams I see a powerful magnetic field containing the particles, and then a near vacuum. Those two things alone must make heat transfer very difficult, no? I don't see where the produced energy gets extracted.
Sure, but how? What physical processes are being used? Most of us can understand how hot fision encased in water turns to steam and runs a steam engine (even if the reality is still quite complex). But how does the thin-plasma get converted to steam?
Fast neutrons leaving the confinement field. Most of the energy goes into the blanket between the first wall of the vessel and the confinement coils. This blanket is made of Be and Li. Be to act as a neutron multiplier and Li to breed tritium. The Li fission is actually energetically favorable and is a significant percentage of the power output. It is also the source of tritium fusion fuel. The byproducts of these reactions are helium. Liquid lithium can also double as a coolant in an efficient (high temperature differential) thermal cycle.
The rest of the power gets dumped into the divertor. This is a pretty violent section of the hull that has an enormous power density slamming into it (in ITER it will be similar to an atmospheric re-entry vehicle). There are games to play here though and the strike points can be blown up in size.
In both cases energy comes from neutrons. In the case of a fusion reactor, some neutrons escape plasma containment, presumably then heat up something outside, then heat transferred to water as usual.
Edit: https://en.wikipedia.org/wiki/ITER for visuals.