5 ms·

Any chance for an ELI5 of what a stellarator and tokamak are?

by mynewtb 10y ago

germinalphrase 10y ago
Yes - I would appreciate the ELI5
ars 10y ago
Sure. If you squish hydrogen atoms together hard enough, and hot enough, they release energy. But it needs to be really really hot - so hot that anything you made it out of would melt. So what do you do? You use a magnet. The magnet squishes the really hot hydrogen without actually touching it. But if you squish one side, the hydrogen will want to go out of the other side. So you have to squish all sides exactly the same amount. It turns out, it's impossible to make a magnet in the shape of a ball that squishes on all sides equally. But, it is possible to make one in the shape of a doughnut! That's a tokamak. They are complicated because you also have to use the hydrogen inside the tokamak to help make the magnet work and keep the hydrogen inside. Another shape that works is a kind of twisted doughnut, this is called a stellarator, if you do that, you don't need to also use the hydrogen inside as a magnet, and this makes it easier. But the twisted shape means it's harder to build because you have to put the magnets in exactly the right place. The name "tokamak" comes from a sentence in Russian describing the machine, and the name "stellarator" comes from a word that means "sun".
- deleted 10y ago
  [deleted]
- sametmax 10y ago
  This is a wonderful ELI5. The doughnut makes it all.
- stevep98 10y ago
  > It's impossible to make a magnet in the shape of a ball that squishes on all sides equally... Ah, my favorite theorem, The Hairy Ball Theorem! https://en.m.wikipedia.org/wiki/Hairy_ball_theorem https://en.m.wikipedia.org/wiki/Hairy_ball_theorem
  - vecter 10y ago
    Can you explain how the Hairy Ball Theorem applies to the spherical magnetic field? My first intuition is that the magnetic field would not be a tangent field because you'd want all of the force lines to point inwards to the center of the sphere. However, based on what you said, it sounds like my intuition is wrong. To phrase it another way, could you explain why the magnetic field is tangent to the sphere and not normal to it?
    - evanb 10y ago
      There are two things here. The force magnetic fields apply is always perpendicular to the field: F ~ v × B (right hand rule, and all that). So if you want F to point inward, the one component of B that doesn't matter is its radial component---ie. the tangent component is all that counts. But it's not clear to me that you can't compensate for the restrictions of the hairy ball theorem by having a nonuniform velocity distribution. Of course, there's no "hairy torus" theorem (because it's not true!) and this immediately suggests the tokamak design. It's also not possible to take a sphere and have the radial magnetic field point inward everywhere (or outward everywhere), because Maxwell's equations prohibit magnetic monopoles.
      - vecter 10y ago
        It's been too long since E&M. I forgot about magnetic fields being normal doh. And of course, the last point about no magnetic monopoles should be sufficient to make a spherical field impossible.
    - CamperBob2 10y ago
      I'd guess that it has something to do with eddy currents induced within the contained plasma. The HBT probably implies that these can't be perfectly symmetrical, and if they aren't symmetrical the whole thing will fall apart.
    - wtallis 10y ago
      Charged particles will spiral along magnetic field lines. Thus, you want to have the field lines only be parallel to the confinement surface, so that the particles will migrate around the surface while doing their smaller-scale spiraling, instead of migrating outwards along field lines that exit the confinement.
- mozumder 10y ago
  What's different about the stellerator magnetic field compared to the donut? And how does that shape help contain fusion better?
  - snarfy 10y ago
    The main difference is the plasma circulates. With the donut, the problem is similar to trying to inflate a balloon that has a weak point in it. Blowing causes the weak point to inflate instead of the balloon. Plasmas are electrically charged and have their own magnetic field. Trying to squeeze down on the plasma inside the donut shape to cause fusion causes the plasma's own magnetic strength to increase and counter the field being applied to it. The stellarator works with the plasma's magnetic field instead of against it by spiraling it around in circles.
- proteinbased 10y ago
  great eli5. I attended a lecture by one of the Wendelstein-7X engineers a month ago, and the main reason why they would construct such a thing (took 10 years), from what I figured, was that a tokamak is operated in short pulses, whereas the stellarator runs smoothly. Wiki: However, stellarators, unlike tokamaks, do not require a toroidal current, so that the expense and complexity of current drive and/or the loss of availability and periodic stresses of pulsed operation can be avoided, and there is no risk of toroidal current disruptions. It might be possible to use these additional degrees of design freedom to optimize a stellarator in ways that are not possible with tokamaks.
- kavrick 10y ago
  Thanks so much. That was a really great explanation.
- mrfusion 10y ago
  Say why could t you use an electric field instead of a magnetic field?
  - _tvei 10y ago
    Apologies if this is a stupid question but aren't they the same thing?
    - gliese1337 10y ago
      Nope. They are both part of the same thing - the joint electromagnetic field - and they transform into each other in different reference frames, but in a given reference frame, the electric field and magnetic field components behave quite differently.
      - DennisP 10y ago
        It blew my mind when I learned how magnetism is a consequence of special relativity.
  - gliese1337 10y ago
    You can. That's what a Farnsworth fusor does (there are other related designs, but that's the original, and the kind you can build in your garage). But for complicated reasons, it just isn't efficient enough to actually produce net power output. Of course, neither is any other design right now, but magnetic confinement appears to be the more promising path forward.
    - mrfusion 10y ago
      Thanks for the explanation. I looked up some of those designs. Now I'm really puzzled why no one has tried a more naive approach. I'm picturing a hollow metal sphere with a high positive charge with positive ions inside of it. Wouldn't it push them all together and with a high enough charge get them hot enough to fuse?
loblollyboy 10y ago
Fission is getting energy by having heavy atoms break up into lighter ones, fusion is getting energy by having light atoms, well, fuse together. Fusion reaction has pros over fission in that a fusion reaction produces a lot more energy than a fission - and that it can't 'melt down' - unlike fission it doesn't have to be kept in check. If Homer Simpson messes up at the fusion plant, the reaction will just stop. Why? Because for fusion to happen, the atoms (hydrogens) need to overcome their electrostatic repulsion of eachother - the Coulomb barrier generated by the protons in the nuclei (at fusion temps the atoms have all become ionized - electrons have shot off from the atoms, so you have electrons and positively charged nuclei flying around - aka a charged gas aka a plasma). Anyway that coulomb force has the same dependence on distance as gravity (1/r^2) - but unlike gravity - is a repulsive force. If you can force the atoms close enough together (by putting them under high temperature and pressure) then they will be close enough that the nuclei of the atoms will be pulled together by a stronger attractive force - the nuclear force - which only works at a very short range. As mentioned, you have to get the atoms going really fast and really close together for this to happen (put the gas under high temp and pressure, which incidentally will ionize the gas atoms into electrons and positively charged ions). The fact that it is now 'electric' is the key to both approaches (tokamak and stellarator) at containing the plasma. These are just two approaches at Magnetically Confined Fusion, which operates on the simple principle that charged particles (both electrons and the ions that you are trying to fuse w other ions) travel along magnetic field lines. They spin in tight little circles around magnetic field lines, the stronger the magnetic field, the tighter the circle. Tokamak, which is simpler than a stellarator, is basically a solenoid https://en.m.wikipedia.org/wiki/Solenoid https://en.m.wikipedia.org/wiki/Solenoid that wraps around into a donut shape. Just as the magnetic field lines in a solenoid go along the length of the solenoid, the magnetic field lines in a tokamak will go around the donut (toroidal field). Just imagine that the tokamak magnets are really powerful so the magnetic field lines (not really lines) are really strong - then your ions will gyrate around and bump into other ions that are gyrating around B-lines. There are problems though. It's been a while since I studied but basically, in addition to ions and electrons very fast giration around the magnetic field 'line' and it's slower movement along the field line (around the donut) - it also 'drifts' https://en.m.wikipedia.org/wiki/Guiding_center https://en.m.wikipedia.org/wiki/Guiding_center - this is the slowest movement. There are a few mechanisms that cause this drift, but to the great misfortune of mankind, these drifts are 'out' - the ions drift away from the plasma core, and towards the wall of the tokamak. To stop this drift from happening, you need the toroidal (around the donut) B field lines to also twist, so they look like a twizzler that has been wrapped around on itself. In a tokamak this is generally done by running a current through the plasma around the donut. Plasmas are highly conductive, so you can do this. It is like a big coil of wire. Just like the coils on a solenoid generate a straight B field through the center of a solenoid, and the coils of a tokamak (donut shaped solenoid) generate a donut shaped B field, running a current through the donut shaped plasma will generate a B field from the ceiling to the floor thru the donut hole (the poloidal field). The poloidal and toroidal fields vector add to make your twirly twizzler shaped field that prevents drifts. This running a current thru the plasma is how you 'twist' the field in tokamaks. Stellerators, on the other hand, generate the field with complex (very) magnetic configuration (look up images of WX-7). There is no need to run current thru the plasma, and this in theory, and I think now in practice, with WX-7, leads to a more stable plasma. This stability issue is very important. Scientists have known about drifts since year 0 of fusion research, but there are many instabilities that cause the plasma to break down. As mentioned in the article - there is a triple product that basically describes the success of the plasma - plasma density, plasma temperature and confinement time. This is more or less equivalent to 'are we getting more energy out than we are putting in' and I believe the advantage of stellerator would be confinement time. Hopefully these brainiacs can get it happening.
deleted 10y ago
[deleted]
deleted 10y ago
[deleted]

Any chance for an ELI5 of what a stellarator and tokamak are?

by mynewtb 10y ago

Any chance for an ELI5 of what a stellarator and tokamak are?

germinalphrase 10y ago

Yes - I would appreciate the ELI5

ars 10y ago

Sure. If you squish hydrogen atoms together hard enough, and hot enough, they release energy. But it needs to be really really hot - so hot that anything you made it out of would melt. So what do you do? You use a magnet. The magnet squishes the really hot hydrogen without actually touching it. But if you squish one side, the hydrogen will want to go out of the other side. So you have to squish all sides exactly the same amount. It turns out, it's impossible to make a magnet in the shape of a ball that squishes on all sides equally. But, it is possible to make one in the shape of a doughnut! That's a tokamak. They are complicated because you also have to use the hydrogen inside the tokamak to help make the magnet work and keep the hydrogen inside. Another shape that works is a kind of twisted doughnut, this is called a stellarator, if you do that, you don't need to also use the hydrogen inside as a magnet, and this makes it easier. But the twisted shape means it's harder to build because you have to put the magnets in exactly the right place. The name "tokamak" comes from a sentence in Russian describing the machine, and the name "stellarator" comes from a word that means "sun".

deleted 10y ago
[deleted]
sametmax 10y ago
This is a wonderful ELI5. The doughnut makes it all.
stevep98 10y ago
> It's impossible to make a magnet in the shape of a ball that squishes on all sides equally... Ah, my favorite theorem, The Hairy Ball Theorem! https://en.m.wikipedia.org/wiki/Hairy_ball_theorem https://en.m.wikipedia.org/wiki/Hairy_ball_theorem
- vecter 10y ago
  Can you explain how the Hairy Ball Theorem applies to the spherical magnetic field? My first intuition is that the magnetic field would not be a tangent field because you'd want all of the force lines to point inwards to the center of the sphere. However, based on what you said, it sounds like my intuition is wrong. To phrase it another way, could you explain why the magnetic field is tangent to the sphere and not normal to it?
  - evanb 10y ago
    There are two things here. The force magnetic fields apply is always perpendicular to the field: F ~ v × B (right hand rule, and all that). So if you want F to point inward, the one component of B that doesn't matter is its radial component---ie. the tangent component is all that counts. But it's not clear to me that you can't compensate for the restrictions of the hairy ball theorem by having a nonuniform velocity distribution. Of course, there's no "hairy torus" theorem (because it's not true!) and this immediately suggests the tokamak design. It's also not possible to take a sphere and have the radial magnetic field point inward everywhere (or outward everywhere), because Maxwell's equations prohibit magnetic monopoles.
    - vecter 10y ago
      It's been too long since E&M. I forgot about magnetic fields being normal doh. And of course, the last point about no magnetic monopoles should be sufficient to make a spherical field impossible.
  - CamperBob2 10y ago
    I'd guess that it has something to do with eddy currents induced within the contained plasma. The HBT probably implies that these can't be perfectly symmetrical, and if they aren't symmetrical the whole thing will fall apart.
  - wtallis 10y ago
    Charged particles will spiral along magnetic field lines. Thus, you want to have the field lines only be parallel to the confinement surface, so that the particles will migrate around the surface while doing their smaller-scale spiraling, instead of migrating outwards along field lines that exit the confinement.
mozumder 10y ago
What's different about the stellerator magnetic field compared to the donut? And how does that shape help contain fusion better?
- snarfy 10y ago
  The main difference is the plasma circulates. With the donut, the problem is similar to trying to inflate a balloon that has a weak point in it. Blowing causes the weak point to inflate instead of the balloon. Plasmas are electrically charged and have their own magnetic field. Trying to squeeze down on the plasma inside the donut shape to cause fusion causes the plasma's own magnetic strength to increase and counter the field being applied to it. The stellarator works with the plasma's magnetic field instead of against it by spiraling it around in circles.
proteinbased 10y ago
great eli5. I attended a lecture by one of the Wendelstein-7X engineers a month ago, and the main reason why they would construct such a thing (took 10 years), from what I figured, was that a tokamak is operated in short pulses, whereas the stellarator runs smoothly. Wiki: However, stellarators, unlike tokamaks, do not require a toroidal current, so that the expense and complexity of current drive and/or the loss of availability and periodic stresses of pulsed operation can be avoided, and there is no risk of toroidal current disruptions. It might be possible to use these additional degrees of design freedom to optimize a stellarator in ways that are not possible with tokamaks.
kavrick 10y ago
Thanks so much. That was a really great explanation.
mrfusion 10y ago
Say why could t you use an electric field instead of a magnetic field?
- _tvei 10y ago
  Apologies if this is a stupid question but aren't they the same thing?
  - gliese1337 10y ago
    Nope. They are both part of the same thing - the joint electromagnetic field - and they transform into each other in different reference frames, but in a given reference frame, the electric field and magnetic field components behave quite differently.
    - DennisP 10y ago
      It blew my mind when I learned how magnetism is a consequence of special relativity.
- gliese1337 10y ago
  You can. That's what a Farnsworth fusor does (there are other related designs, but that's the original, and the kind you can build in your garage). But for complicated reasons, it just isn't efficient enough to actually produce net power output. Of course, neither is any other design right now, but magnetic confinement appears to be the more promising path forward.
  - mrfusion 10y ago
    Thanks for the explanation. I looked up some of those designs. Now I'm really puzzled why no one has tried a more naive approach. I'm picturing a hollow metal sphere with a high positive charge with positive ions inside of it. Wouldn't it push them all together and with a high enough charge get them hot enough to fuse?

loblollyboy 10y ago

Fission is getting energy by having heavy atoms break up into lighter ones, fusion is getting energy by having light atoms, well, fuse together. Fusion reaction has pros over fission in that a fusion reaction produces a lot more energy than a fission - and that it can't 'melt down' - unlike fission it doesn't have to be kept in check. If Homer Simpson messes up at the fusion plant, the reaction will just stop. Why? Because for fusion to happen, the atoms (hydrogens) need to overcome their electrostatic repulsion of eachother - the Coulomb barrier generated by the protons in the nuclei (at fusion temps the atoms have all become ionized - electrons have shot off from the atoms, so you have electrons and positively charged nuclei flying around - aka a charged gas aka a plasma). Anyway that coulomb force has the same dependence on distance as gravity (1/r^2) - but unlike gravity - is a repulsive force. If you can force the atoms close enough together (by putting them under high temperature and pressure) then they will be close enough that the nuclei of the atoms will be pulled together by a stronger attractive force - the nuclear force - which only works at a very short range. As mentioned, you have to get the atoms going really fast and really close together for this to happen (put the gas under high temp and pressure, which incidentally will ionize the gas atoms into electrons and positively charged ions). The fact that it is now 'electric' is the key to both approaches (tokamak and stellarator) at containing the plasma. These are just two approaches at Magnetically Confined Fusion, which operates on the simple principle that charged particles (both electrons and the ions that you are trying to fuse w other ions) travel along magnetic field lines. They spin in tight little circles around magnetic field lines, the stronger the magnetic field, the tighter the circle. Tokamak, which is simpler than a stellarator, is basically a solenoid https://en.m.wikipedia.org/wiki/Solenoid https://en.m.wikipedia.org/wiki/Solenoid that wraps around into a donut shape. Just as the magnetic field lines in a solenoid go along the length of the solenoid, the magnetic field lines in a tokamak will go around the donut (toroidal field). Just imagine that the tokamak magnets are really powerful so the magnetic field lines (not really lines) are really strong - then your ions will gyrate around and bump into other ions that are gyrating around B-lines. There are problems though. It's been a while since I studied but basically, in addition to ions and electrons very fast giration around the magnetic field 'line' and it's slower movement along the field line (around the donut) - it also 'drifts' https://en.m.wikipedia.org/wiki/Guiding_center https://en.m.wikipedia.org/wiki/Guiding_center - this is the slowest movement. There are a few mechanisms that cause this drift, but to the great misfortune of mankind, these drifts are 'out' - the ions drift away from the plasma core, and towards the wall of the tokamak. To stop this drift from happening, you need the toroidal (around the donut) B field lines to also twist, so they look like a twizzler that has been wrapped around on itself. In a tokamak this is generally done by running a current through the plasma around the donut. Plasmas are highly conductive, so you can do this. It is like a big coil of wire. Just like the coils on a solenoid generate a straight B field through the center of a solenoid, and the coils of a tokamak (donut shaped solenoid) generate a donut shaped B field, running a current through the donut shaped plasma will generate a B field from the ceiling to the floor thru the donut hole (the poloidal field). The poloidal and toroidal fields vector add to make your twirly twizzler shaped field that prevents drifts. This running a current thru the plasma is how you 'twist' the field in tokamaks. Stellerators, on the other hand, generate the field with complex (very) magnetic configuration (look up images of WX-7). There is no need to run current thru the plasma, and this in theory, and I think now in practice, with WX-7, leads to a more stable plasma. This stability issue is very important. Scientists have known about drifts since year 0 of fusion research, but there are many instabilities that cause the plasma to break down. As mentioned in the article - there is a triple product that basically describes the success of the plasma - plasma density, plasma temperature and confinement time. This is more or less equivalent to 'are we getting more energy out than we are putting in' and I believe the advantage of stellerator would be confinement time. Hopefully these brainiacs can get it happening.

deleted 10y ago

[deleted]

deleted 10y ago

[deleted]