3 ms·

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 t

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.

KyleBerezin 3y ago
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. edit: changed language about the divertors.
- waterheater 3y ago
  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.
golem14 3y ago
I suppose that any number of twists would be OK, but the more twists, the less efficient ? Is that what you are saying ? Or are there other constraints on the number of twists (e.g. must be odd, ...)
- KyleBerezin 3y ago
  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.

KyleBerezin 3y ago

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. edit: changed language about the divertors.

waterheater 3y ago
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.

golem14 3y ago

I suppose that any number of twists would be OK, but the more twists, the less efficient ? Is that what you are saying ? Or are there other constraints on the number of twists (e.g. must be odd, ...)

KyleBerezin 3y ago
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.