I'm not that guy, but I can speak to what you're asking. I've followed Wendelstein 7-X for almost a decade.
Nuclear fusion occurs at extremely-high temperatures. As you heat your fusion fuel to sufficiently-high temperatures to allow fusion, the matter transitions into a plasma, which is great: plasmas react to electromagnetic fields. As such, a major challenge with achieving viable nuclear fusion is making a vessel capable of holding the fusion reaction. Because we can't create on-demand gravity wells, the next best option for confinement is using electromagnetic fields to hold the plasma in the air.
So, you now have an "electromagnetic bottle" capable of suspending a fusion reaction above the reactor's walls. Now, you have another issue: how do you ensure the fuel will sufficiently mix to sustain a fusion reaction? One approach is to move the plasma in a loop. The topologically-simplest method to accomplish this loop is the torus. Such a plasma-confinement device is called a tokamak. A tokamak uses two magnetic fields, torodial and polodial, to accomplish its task. The torodial field is driven through the plasma to push it forward, while the polodial field pulls the plasma in toward the center. Proper balance of these fields will allow the plasma to circuit the vessel following a helical path, achieving confinement.
However, driving two separate magnetic fields is energy-intensive, and a successful fusion reactor will want to minimize its own power consumption to maximize the amount available for external usage. Enter the stellarator. The stellarator also drives the plasma around in a circle, it but uses a single magnetic field. How? It "tricks" the plasma into "thinking" there's only one magnetic field by using computer-optimized magnets with highly-complex geometries. This provides stellarators with a major engineering advantage over tokamaks and is a primary reason Wendelstein 7-X would have chosen it.
With the confinement vessel topology largely identified, the next main step is to figure out how to build a vessel able to contain a sustained fusion reaction. For context, fusion experiments traditionally only operate on timescales of milliseconds to maybe a second. The reason? Fusion occurs at millions of degrees, and keeping the reaction vessel cool, ensuring a continuous supply of fuel, and dealing with reaction "exhaust" (e.g., alpha particles) and stray high-energy neutrons from the common deuterium-tritium reaction (which irradiate your reactor walls because neutrons don't react with electomagnetic fields) is a major, major engineering challenge. Any operational, net-positive fusion reactor must be able to operate for days, weeks, and months on end.
What Wendelstein 7-X has been attempting to do for years is demonstrate that building such a vessel is even possible. Their overall goal is to sustain a fusion reaction for about 30 minutes. Such a timescale will show a proof-of-concept system which enables sustained fusion reactions to occur.
Currently, the preferred fuel is deuterium-tritium because the fuel is generally available and has an attainable fusion temperature. The stray neutron issue can be mitigated by lining reactor walls with lithium to breed tritium fuel. Even better is to use the helium3-helium3 reaction, which completely annihilate to produce pure energy as the output (welcome to e=mc^2, enjoy your stay). The main holdups are: (1) the reaction occurs at much higher temperatures than deuterium-tritium, and (2) he(lium)3 is quite scarce on Earth. Once Wendelstein 7-X shows how to engineer a proper confinement vessel at a "lower" temperature, you can then work on the higher temperature levels required for he3-he3. Also, he3 is plentiful on the surface of the moon, so mining the surface of the moon will be performed to obtain the required fuel, which is the fundamental premise of the movie "Moon".
Someone asked for information on electromagnetic plasma containment folding. I recommend reading up on magnetohydrodynamics (MHD). It's the mathematical and physical foundation of your interest.
> Even better is to use the helium3-helium3 reaction, which completely annihilate to produce pure energy as the output (welcome to e=mc^2, enjoy your stay).
It doesn't completely annihilate to produce pure energy. It produces helium-4 and two protons. Or you can react helium-3 + deuterium to produce helium-4 and one proton. The point is that helium-4 and protons are easier to shield against than neutrons, don't turn your reactor radioactive, and at least in theory their energy can be extracted directly (eg through induction) instead of through heat.
Edited to add: except helium-3 + deuterium still produces neutrons, because sometimes the deuterium will react with itself to produce helium-3 and a neutron.
You're absolutely right, thank you for the correction. he3-he3 doesn't produce neutrons, which is the major advantage, in addition to the massive energy output.
One guy asked why the mobius aspect is needed and I couldn't answer. I know a lot of stellarators aren't odd-period like Wendelstein, and the old designs didn't do folding at all. Do you know what improvements the mobius design has over something like TJ-II?
The helical path creates a twist in the plasma which cancels out the drift forces. This is what I meant by "tricking" the plasma. User mjfl gives an even more technical explanation:
> By twisting the plasma into a shape where the curl of B (proportional to J) is parallel to B, i.e. a helix, the cross product is 0, and thus there are no net magnetohydrodynamic forces on the plasma.
Hope all that's a good answer for you.
> Mobius aspect
You might avoid using the word "Mobius" and instead use "helical." A Mobius strip is important because it has two faces which form a single surface. The surface aspect isn't relevant in this context, so a term which refers to the shape would likely dispel confusion in a reader.
As far as I'm aware, each section of a stellarator is periodic in its own right, which means the end and start points of each section are the same. Though I'm not certain, the choice of four versus five is more likely an engineering factor rather than one of physics, whereas the distinction between a tokamak and stellarator is of physics and not just engineering.
If a 'particle' (I don't know a better word) finds itself near one of the top divertors, at the same point in the next orbit it will find itself near the bottom divertor. That is a product of the "mobius-like" shape, so although it isn't really a 'ribbon' and isn't really a mobius, it helps explain the concept concisely. I just don't know WHY that shape helps lol. Maybe it doesn't and it was just a practical design change like you said.
Did a little research to try and understand this better.
The most precise term to describe the "twisted ribbon" flux tube in W7-X is "toroidal helix". The toroidal quality comes from the general torus shape of the stellarator, and the helix quality comes from the twisting of the magnetic field by magnets. (The torus shape is required only topologically; look up the knotatron to see what I mean.)
The "ribbon" we're talking about is properly called the flux tube. The flux tube is the volume created by the flux surface, which is where the magnetic field lines lie. A given volume of plasma contained within a flux tube should remain inside it, causing magnetic confinement of the plasma.
The optimality of the confinement of the flux tube is expressed with the term "omnigeneity". Conceptually, a flux tube has onmigeneity if ideally all of the non-colliding plasma inside the tube stays in the tube. W7-X's flux tube appears to be approaching omnigenity. (Another experiment which approaches omnigenity is HSX. Interestingly, HSX has one set of primary magnets, whereas W7-X has two. That's likely because HSX achieves omnigenity via quasisymmetry, whereas W7-X uses various stellarator optimization techniques.)
With these points, we can call the W7-X "ribbon" a near-omnigenous toroidal helix flux tube, which sounds way cooler. So, all that said, why is a helical property desired? From what I've read, the twist in the flux surface reduces plasma drift inside the flux tube.
I think it makes sense to analogize this stuff as a circular semi-permeable pipe filled with a high-pressure "magic fluid" flowing around-and-around inside. By semi-permeable, it means fluid will leak from the pipe if the internal pressure is too high (remember that this is magic fluid). Trying to understand the helical twist along this analogy, I think the effect is evening of internal pressure across the pipe surface to reduce fluid turbulence and permeation while maximizing laminar flow. At least, that's my best analogous interpretation of "why" the twist helps.
The divertors are useful for long-term reactor operation but have no direct relevance to the magnetic field geometry. I'm guessing there's two divertors for engineering reasons (performance, redundancy, etc.) and not for reasons of basic physics.
Yes I was referring to odd. But I've been reading some papers and I think even 4-fold had the mobius effect. I'll comment here again tomorrow when I have learned a bit more on the topic.
>Also, he3 is plentiful on the surface of the moon, so mining the surface of the moon will be performed to obtain the required fuel, which is the fundamental premise of the movie "Moon".
This is repeated a lot, but the practicalities are... questionable. Here's an article I consider to be the definitive criticism of the concept:
Nuclear fusion occurs at extremely-high temperatures. As you heat your fusion fuel to sufficiently-high temperatures to allow fusion, the matter transitions into a plasma, which is great: plasmas react to electromagnetic fields. As such, a major challenge with achieving viable nuclear fusion is making a vessel capable of holding the fusion reaction. Because we can't create on-demand gravity wells, the next best option for confinement is using electromagnetic fields to hold the plasma in the air.
So, you now have an "electromagnetic bottle" capable of suspending a fusion reaction above the reactor's walls. Now, you have another issue: how do you ensure the fuel will sufficiently mix to sustain a fusion reaction? One approach is to move the plasma in a loop. The topologically-simplest method to accomplish this loop is the torus. Such a plasma-confinement device is called a tokamak. A tokamak uses two magnetic fields, torodial and polodial, to accomplish its task. The torodial field is driven through the plasma to push it forward, while the polodial field pulls the plasma in toward the center. Proper balance of these fields will allow the plasma to circuit the vessel following a helical path, achieving confinement.
However, driving two separate magnetic fields is energy-intensive, and a successful fusion reactor will want to minimize its own power consumption to maximize the amount available for external usage. Enter the stellarator. The stellarator also drives the plasma around in a circle, it but uses a single magnetic field. How? It "tricks" the plasma into "thinking" there's only one magnetic field by using computer-optimized magnets with highly-complex geometries. This provides stellarators with a major engineering advantage over tokamaks and is a primary reason Wendelstein 7-X would have chosen it.
With the confinement vessel topology largely identified, the next main step is to figure out how to build a vessel able to contain a sustained fusion reaction. For context, fusion experiments traditionally only operate on timescales of milliseconds to maybe a second. The reason? Fusion occurs at millions of degrees, and keeping the reaction vessel cool, ensuring a continuous supply of fuel, and dealing with reaction "exhaust" (e.g., alpha particles) and stray high-energy neutrons from the common deuterium-tritium reaction (which irradiate your reactor walls because neutrons don't react with electomagnetic fields) is a major, major engineering challenge. Any operational, net-positive fusion reactor must be able to operate for days, weeks, and months on end.
What Wendelstein 7-X has been attempting to do for years is demonstrate that building such a vessel is even possible. Their overall goal is to sustain a fusion reaction for about 30 minutes. Such a timescale will show a proof-of-concept system which enables sustained fusion reactions to occur.
Currently, the preferred fuel is deuterium-tritium because the fuel is generally available and has an attainable fusion temperature. The stray neutron issue can be mitigated by lining reactor walls with lithium to breed tritium fuel. Even better is to use the helium3-helium3 reaction, which completely annihilate to produce pure energy as the output (welcome to e=mc^2, enjoy your stay). The main holdups are: (1) the reaction occurs at much higher temperatures than deuterium-tritium, and (2) he(lium)3 is quite scarce on Earth. Once Wendelstein 7-X shows how to engineer a proper confinement vessel at a "lower" temperature, you can then work on the higher temperature levels required for he3-he3. Also, he3 is plentiful on the surface of the moon, so mining the surface of the moon will be performed to obtain the required fuel, which is the fundamental premise of the movie "Moon".
Someone asked for information on electromagnetic plasma containment folding. I recommend reading up on magnetohydrodynamics (MHD). It's the mathematical and physical foundation of your interest.