For those ions scattered off their radial paths, i.e. with a circumferential component, is there any mechanism that brings them back into a radial path, or do they keep their angluar momentum about the centre?
The term 'annealing' has been mentioned a few times. I have no idea what that is, but what is the mechanism of forces that might stop an ion orbiting indefinitely, viz. with a circumferential velocity component, after such scattering (and then, presumably, is a risk to scatter other ions off their radial paths also)?
best regards,
Chris MB.
Circumferential scattering and edge annealing.
Re: Circumferential scattering and edge annealing.
As I understand it Bussard, the theory was that the angular momentum gained would be largely as much in one direction as in every other, so collisions on the edge would in essence cancel out. I believe a similar effect for up/down scattered ions was expected as well. Of course that enters a level of particle physics way over my head.chrismb wrote:For those ions scattered off their radial paths, i.e. with a circumferential component, is there any mechanism that brings them back into a radial path, or do they keep their angluar momentum about the centre?
The term 'annealing' has been mentioned a few times. I have no idea what that is, but what is the mechanism of forces that might stop an ion orbiting indefinitely, viz. with a circumferential velocity component, after such scattering (and then, presumably, is a risk to scatter other ions off their radial paths also)?
best regards,
Chris MB.
I was thinking of collisions more like quater to half-way out, rather than just the slow stuff right at the edge.
And once ions are scattered half way and heading circumferentially, they would be more likely to then collide with radially moving ions, which would then be displaced circumferentially, which would then collide further, etc...
Some force mechanism must get all these ions back onto radial paths, i.e. must scrub off their angular momentum about the centre, else they'd just cause all the ions to thermalise.
And once ions are scattered half way and heading circumferentially, they would be more likely to then collide with radially moving ions, which would then be displaced circumferentially, which would then collide further, etc...
Some force mechanism must get all these ions back onto radial paths, i.e. must scrub off their angular momentum about the centre, else they'd just cause all the ions to thermalise.
A lot depends on the density gradient. If it goes as r^2 that will help.chrismb wrote:I was thinking of collisions more like quater to half-way out, rather than just the slow stuff right at the edge.
And once ions are scattered half way and heading circumferentially, they would be more likely to then collide with radially moving ions, which would then be displaced circumferentially, which would then collide further, etc...
Some force mechanism must get all these ions back onto radial paths, i.e. must scrub off their angular momentum about the centre, else they'd just cause all the ions to thermalise.
Engineering is the art of making what you want from what you can get at a profit.
Precisely. They thermalize at a VERY low energy because they are near the top of the well. They then get accelerated radially inward and achieve a uniform radial content with a very small thermal distribution. At least that is how I have understood it.chrismb wrote:I was thinking of collisions more like quater to half-way out, rather than just the slow stuff right at the edge.
And once ions are scattered half way and heading circumferentially, they would be more likely to then collide with radially moving ions, which would then be displaced circumferentially, which would then collide further, etc...
Some force mechanism must get all these ions back onto radial paths, i.e. must scrub off their angular momentum about the centre, else they'd just cause all the ions to thermalise.
So unless you postulate a mechanism that introduces a NON-thermal angular momentum, the system should anneal the thermal component and retain the radial. No?
chrismb - you are thinking about individual particle motion where the forces are maximum and speeds are minimal. If it's a smooth potential and the
particles are far apart, that's ok, but at high density I think the idea of "slow moving ions" just isn't going to happen. You also have the fact that electrons
are at max speed there so there is a lot of current due to them as well. The
current flows will be complex and interactive, and most likely turbulent. If they average out to smooth, then we can control the darn thing. If they
don't, we get the same thing as ITER - disruptions.
It will be interesting to read EMC2's report.
particles are far apart, that's ok, but at high density I think the idea of "slow moving ions" just isn't going to happen. You also have the fact that electrons
are at max speed there so there is a lot of current due to them as well. The
current flows will be complex and interactive, and most likely turbulent. If they average out to smooth, then we can control the darn thing. If they
don't, we get the same thing as ITER - disruptions.
It will be interesting to read EMC2's report.
Chris,
Rick Nebel wrote something on this a while back. Essentially, as I recall, the explanation was that when the ions are near the top of their orbit, they tend to have more angular than radial momentum, so the collisons tend to reduce angular momentum. They also have a higher collisionality due their lower energy.
So what you end up with is a mechanism by which ions tend to lose angular momentum.
Rick Nebel wrote something on this a while back. Essentially, as I recall, the explanation was that when the ions are near the top of their orbit, they tend to have more angular than radial momentum, so the collisons tend to reduce angular momentum. They also have a higher collisionality due their lower energy.
So what you end up with is a mechanism by which ions tend to lose angular momentum.
-
Art Carlson
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That's what they say. There was a long thread on The problem with ion convergence in which I detailed (quantitatively) why I don't buy it.TallDave wrote:So what you end up with is a mechanism by which ions tend to lose angular momentum.
blimey!Art Carlson wrote:That's what they say. There was a long thread on The problem with ion convergence in which I detailed (quantitatively) why I don't buy it.TallDave wrote:So what you end up with is a mechanism by which ions tend to lose angular momentum.
I got to page 5 diligently and saw enough that this is much along the same lines. Yes, thanks for that. Will read again later and finish when I have a free evening!!
Perhaps it matters not why off radial motion occurs, an irregular shape of electrons would come to the same thing. I was thinking simply of collisions at r/2 - after all, particles are running back and forth along all radials at all positions, so this will be inevitable.
I'm not sure I follow that logic. What does it thermalise with, and precisely when might it do it? (I thought it was collisionless except at the centre) And if there is a thermalising process it must happen a little before the turning point as well, so one energy may go up, the other would go down. Next time around, the same might happen. This process may explain a 'slow' global thermalisation, but the result will still be thermalisation.R.Nebel:"The reasons edge collisions remove angular momentum is that as the ions reach their radial turning point, their angular velocity exceeds the radial velocity. Consequently, thermalization takes energy from the angular direction and puts it in the radial direction."
The other thing touched on in that long post is the nature of ion convergence itself. Of course, as said before, if the core was 1cm radius and 800MW, this would be a total proton flux entering the surface of the core of ~16GA/m2. Considering that a 4MA tokamak current is a deuteron flux in JET is ~2MA/m2 and they see stability problems then, it would be a fair guess there would be some instabilities at 16GA/m2. OK, I accept the geometry of particle flow is quite particular in a tokamak and that I should be wary of bringing in the comparison, but surely it gives us the best glimpse of the issues in perspective. Understanding the ion flux instability leading to non-radial transport, due to the magnitude of that ion flux convergence, appears to need proper and rigorous understanding. I fear it is likely to lead to the same dragged out timeline to engineering implementation as tokamak, even if that was just the one remaining hurdle. How to proceed on such simulation work?
And if the situation were: most of the motion is tied up in the beams?Perhaps it matters not why off radial motion occurs, an irregular shape of electrons would come to the same thing. I was thinking simply of collisions at r/2 - after all, particles are running back and forth along all radials at all positions, so this will be inevitable.
We will know in time.
Last edited by MSimon on Thu Dec 18, 2008 6:15 pm, edited 1 time in total.
Engineering is the art of making what you want from what you can get at a profit.
I'm referring to the table of data in http://www.askmar.com/Fusion_files/Poly ... oncept.pdfMSimon wrote:if the core was 1cm radius and 800MW, this would be a total proton flux entering the surface of the core of ~16GA/m2.
I think the numbers posited are 10 cm and 100 MW. That would be about 1.6 MA/m2.
Anyhow, 10cm radius x 100MW would come out as 20MA/m2 to my reckoning, but is of no major consequence to the point. The point is made, I think, no need to press it; instabilities should be expected and will not show up experimentally until the higher flux rates are being driven.
My theory - and it is worth what you paid for it - is that the beams are the instability.chrismb wrote:I'm referring to the table of data in http://www.askmar.com/Fusion_files/Poly ... oncept.pdfMSimon wrote:if the core was 1cm radius and 800MW, this would be a total proton flux entering the surface of the core of ~16GA/m2.
I think the numbers posited are 10 cm and 100 MW. That would be about 1.6 MA/m2.
Anyhow, 10cm radius x 100MW would come out as 20MA/m2 to my reckoning, but is of no major consequence to the point. The point is made, I think, no need to press it; instabilities should be expected and will not show up experimentally until the higher flux rates are being driven.
Engineering is the art of making what you want from what you can get at a profit.
chris: where was that point made quantitatively? You made the entirely unfounded leap of faith, "because tokamaks see instabilities at these fluxes, Polywell will see instabilities".The point is made, I think, no need to press it; instabilities should be expected
The more I read of your posts the more I think you are our first, bonafide tokamak evangelist/troll. It's bad enough that any fusion effort from here on has to labour under the bad rap given by the over-promising and under-delivering of the magnetic confinement brigade but to come here and tar Polywell with the same brush is pretty low.
Seems like you know enough about physics/engineering to be dangerous but not enough about people, manners and common decency to be useful.
"because tokamaks see instabilities at these fluxes, Polywell will see instabilities".
That's kinda misquoting me, isn't it? I'm saying that things should be expected if others have had simlar troubles in much the same situation. That is, if you want something to be successful.
If I get stuck one year on a snowy pass and the next year I say to you "you should expect to get stuck if it snows real hard, take something warm".. that seems like a reasonable and helpful comment to me.
I'm sure my wife has said exactly that, word for word!! Sorry - I'm a borderline Asperger's case, or something like that. Just stick with attacking what I say rather than me, and we'll be fine.Seems like you know enough about physics/engineering to be dangerous but not enough about people, manners and common decency to be useful.
That's so cool! Thanks for the compliment! I'll send that to the Press Officer at ITER next time I send her one of my questions! I rather think she may have a considerably different opinion. She probably thinks I am an IEC evangelist!The more I read of your posts the more I think you are our first, bonafide tokamak evangelist/troll.
I think there is rationality in that. It would seem to me that ions *could* perform some form of radial-gyro-radius-swapping until they get into a magnetic lull (there *has* to be something there!) and small fields may seed radial oscillations. That would make sense to me.MSimon wrote: My theory - and it is worth what you paid for it - is that the beams are the instability.