Wendelstein 7-X

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Re: Wendelstein 7-X

Postby Izawwlgood » Tue Feb 09, 2016 1:56 pm UTC

Frenetic Pony wrote:
Copper Bezel wrote:Same as everything else: heat a kettle.

Or a longer explanation "giant ring of superhot plasma gives off a lot of heat, lets flush water around it (to cool the superconducting magnets and etc.) then dump the hot water below a turbine."

The engineering specifics are a lot more complex, but the as Copper pointed out, the idea is basically the same :D

Is that basically what's done?

I've wondered about this as well - presumably if you can contain plasma that is hot enough for fusion, the released heat contributes towards continued fusion and reduces the amount of energy required to heat the plasma (right?). That's well and good, but how do you then extract that extra energy?
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Re: Wendelstein 7-X

Postby Neil_Boekend » Tue Feb 09, 2016 2:22 pm UTC

The plasma in a fusion reactor would be very hot. Plus there is a lot of neutron flux from the fusing plasma. Ergo even while it is insulated from the wall by a near-perfect vacuum the walls need cooling anyway because otherwise they'd melt. Just from radiation and neutron flux. That heat can be used in a normal modern steam based powerplant, of which we have thousands and thousands.
It also allows for a heat buffer, because if the Steralator design used in the Wendelstein doesn't pan out we're "stuck" with the Tokamak or the laser based ICF design tested in the NIF.

Since plasma is basically a bunch of electrical charged particles and hot plasma means these particles are moving FAST there may theoretically be a way to harvest the energy more directly, by using the particle stream itself as a coil in a transformer, but I haven't seen any serious research in that direction.
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Re: Wendelstein 7-X

Postby Copper Bezel » Tue Feb 09, 2016 2:32 pm UTC

Ninja'd, but I'm posting it anyway, because I enjoyed typing this.

Thermodynamics? If something is hot, and it's next to something that is less hot, heat is going to move from the one to the other, whether conductively or radiatively. Unless you can create an opaque vacuum, there's nothing on Earth that can completely stop that heat flow. My understanding was that the purpose of the reactor was to slow down the heat dispersal enough to allow the fusing plasma to become fusing plasma while the reactor remains not liquid - remove the magnetic confinement, and the plasma expands and cools into a hot gas, so that as a result the reactor expands and warms into a tepid pool, and pretty soon everything settles down to room temperature and you don't have a reactor anymore.

Relatedly, if there wasn't heat escaping the plasma, then you wouldn't need any additional heat from fusion to keep it hot. = ]
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Re: Wendelstein 7-X

Postby KarenRei » Tue Feb 09, 2016 4:21 pm UTC

So, you have a variety of things carrying energy out of the reactor. Examples include:

* The divertor (think "exhaust pipe") juts into the outer edges of plasma stream where it gets heavily bombarded by alpha particles (read: the helium it's trying to remove)
* The first wall is subject to a huge radiative load by the superhot plasma, and to a lesser extent, direct bombardment
* Neutron radiation (which carries most of the energy of the reaction) impacts primarily the lithium blanket (nextmost the first wall), transferring their energy in the process.

In each case you have to work very hard to get the heat away as quick as possible so that everything doesn't... well, melt and/or vaporize (except for that which you want to melt!). For example, there are some liquid metal or FLiBe (molten lithium-beryllium salt) divertor concepts divertor concepts, and some blanket designs also call for FLiBe. Thus it can double as your primary coolant. But the key point is that either way, you have to have coolant in some form move in and draw the heat out, otherwise you're going to have a Bad Day(TM) ;) You then transfer your heat to a secondary coolant (either that or only use a single coolant loop) which you then run through a gas turbine at as hot of a temperature as you can in order to get the maximum thermodynamic efficiency. Your reactor then consumes its chunk of the generated power and the rest goes to market.

Note that it's not impossible to have non-thermalized fusion power generation - I've played around with this a bit in simulations. It doesn't help any with neutrons, but any power left in ions can be directly decelerated against an electric field for nonthermal power generation if you separate what you don't want to be involved in collisions from what you want to be involved in collisions. It leads to some interesting "energy recycling" possibilities, getting back power from collisions that didn't go the way you'd prefer so that you can try again. Easier said than done, though. And it does nothing to let you get the neutron energy in a non-thermal manner. The best you can do with the neutrons is multiply them in beryllium and breed your fuel, or if you don't need them for breeding purposes, put them to some other value-added function - research, production of isotopes for sale, transmutation of fission waste, fusion-driven fission, capture and transmutation to an isotope that gives off lots of decay heat (optionally trying to non-thermally capture that... very difficult), etc. The fact that they're energetic isn't really important for the majority of those things, apart from thermal power generation.

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