All that can go wrong with recirculation
@scareduck: yeah, I think it was kinda understood. lol.
@D_Tibbets:
you ought to read the actual paper, its *only* 55 pages. Actually, I wish we'd found this thing earlier, it would have saved a lot of misunderstanding & arguements here on the forum.
"I'm guessing that by 'electrostatic cusp blocking' he is referring to an opposing electric field at the cusps that stop most of the electrons traveling through the cusps."
Nope: the electrostatic field at the cusp throats is a high positive potential, which stops the ions. It is created by electrodes that serve the same function as the charged magrid. The repelling feild for electrons is created by electrodes outside the cusps; in the polywell the walls serve as the repellers b/c they are negatively biased, relative to the grid.
"Also, I'm under the impression that the wiffle ball effect had been demonstrated (at least to Busard's satisfaction) since the 1990's."
Yes, but not to anyone else's satisfaction. That's the whole point of the work Dr. Nebel is doing: his sponsors told him that none of Dr. Bussard's data was to be trusted.
Dr. Bussard used the term recirculation to mean basically the confinement time of the electrons, but measured in "passes through the system." We've taken to use the term to describe the electrostatic containment of the electrons that occurs outside the cusps.
@D_Tibbets:
you ought to read the actual paper, its *only* 55 pages. Actually, I wish we'd found this thing earlier, it would have saved a lot of misunderstanding & arguements here on the forum.
"I'm guessing that by 'electrostatic cusp blocking' he is referring to an opposing electric field at the cusps that stop most of the electrons traveling through the cusps."
Nope: the electrostatic field at the cusp throats is a high positive potential, which stops the ions. It is created by electrodes that serve the same function as the charged magrid. The repelling feild for electrons is created by electrodes outside the cusps; in the polywell the walls serve as the repellers b/c they are negatively biased, relative to the grid.
"Also, I'm under the impression that the wiffle ball effect had been demonstrated (at least to Busard's satisfaction) since the 1990's."
Yes, but not to anyone else's satisfaction. That's the whole point of the work Dr. Nebel is doing: his sponsors told him that none of Dr. Bussard's data was to be trusted.
Dr. Bussard used the term recirculation to mean basically the confinement time of the electrons, but measured in "passes through the system." We've taken to use the term to describe the electrostatic containment of the electrons that occurs outside the cusps.
Yeah. that's a good point, thanks for the reminder of that comment.Yes, but not to anyone else's satisfaction. That's the whole point of the work Dr. Nebel is doing: his sponsors told him that none of Dr. Bussard's data was to be trusted.
From that perspective, one might regard the whole WB-7 experiment as an exercise in peer review.
Bussard seems to have mixed that recirculating term himself, with his allusions to "open recirculating machines." I remember this causing confusion with people citing Rider's objctions about "recirculating power."
That is an interesting paper.
One of them is the Polywell concept itself, for which Bussard gave him the data. Nebel is cited as well.
Heh:
I've been reading Dolan's paper on electrostatically plugged cusps. (Apparently Dr. Bussard didn't do his homework, there is plenty of research on magnetic/intertial/electrostatic confinement. There are dozens of references to the production of potential wells in cusp and mirror machines.)
One of them is the Polywell concept itself, for which Bussard gave him the data. Nebel is cited as well.
Heh:
Can't argue with that!ii) The Polywell'" and modified Penning trap concepts are just beginning to be studied. They need more research to determine the attainable degree of focusing and to explore related concepts that may lead to attractive fusion reactors (sections (viii)
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...
I'm sure Dr. Nebel et al are going to milk this cow for all it's worth, not just the whiffleball effect, but convergence if they can, power balance info, well depth, etc. But definitely in order to put it through the proper channels. I think they will get a better understanding of the machine that way; I think Dr. B perhaps took off with the thing w/o stopping to understand some of the underlying effects well enough.
~the paper: Dolan mentions the issue of contaminant confinement. That's the first I'd heard of it. Although it's only a problem for power balance in an actual reactor-sized machine, it definitely needs looking it.
One thing that bothers me about the way Dr. B discusssed conformal coil casings: I can see how it would be good for preventing arcs (nice smooth surfaces), but the problem is that the electrons are going to diffuse across the B-field, and the flux surfaces outside the beta=1 sheath have to be swept clean of low-energy electrons to prevent sheilding of the coils and low well depth. Perhaps the difference is in making sure that the actual flux surfaces of the sheath aren't intercepted, so that the trapped electrons can be electrostatically reflected outside the cusps, rather than ramming into the grid coils.
Dr. Nebel said that if there are low energy electrons trapped in the positive potential in the cusp, causing sheilding of the grid, then "we'll get rid of them." I don't know how he's planning on doing that. A peice of metal would suffice perhaps, but Dolan mentions this diocotron instability that slings the low-energy electrons out to the grid. I wonder if Dr. Nebel was refering to that mechanism.
~the paper: Dolan mentions the issue of contaminant confinement. That's the first I'd heard of it. Although it's only a problem for power balance in an actual reactor-sized machine, it definitely needs looking it.
One thing that bothers me about the way Dr. B discusssed conformal coil casings: I can see how it would be good for preventing arcs (nice smooth surfaces), but the problem is that the electrons are going to diffuse across the B-field, and the flux surfaces outside the beta=1 sheath have to be swept clean of low-energy electrons to prevent sheilding of the coils and low well depth. Perhaps the difference is in making sure that the actual flux surfaces of the sheath aren't intercepted, so that the trapped electrons can be electrostatically reflected outside the cusps, rather than ramming into the grid coils.
Dr. Nebel said that if there are low energy electrons trapped in the positive potential in the cusp, causing sheilding of the grid, then "we'll get rid of them." I don't know how he's planning on doing that. A peice of metal would suffice perhaps, but Dolan mentions this diocotron instability that slings the low-energy electrons out to the grid. I wonder if Dr. Nebel was refering to that mechanism.
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KeithChard
- Posts: 41
- Joined: Tue Jul 08, 2008 9:16 pm
- Location: Banff,Scotland
The point Dr Bussard made was that electrons had to be prevented from
hitting the Magrid casings which are at a positive potential. The chamber
walls are not a problem because they are at the same potential as the well.
However the picture is very different if a pB11 machine is required because the collector(s) will attract the electrons so that any electrons with
enough kinetic energy to pass the Magrid will be lost. This is where a trap grid at negative potential w.r.t. the Magrid plays it's part. The trouble with a trap grid is that, unless it is located entirely within the shadows of the Magrid, it will intercept the alphas causing power loss and further cooling and protection requirements.
Experments (small scale) need to be done to investigate the effectiveness of such a trap grid, particularly regarding electrons coming out of the
point (face) cusps. If that geometry does not work I suggest two modifications that may help.
Firstly the trap grid could be made magnetic (extra complication
) with polarity opposing the Magrid. A half coil of a coarse winding could be omitted from one side of the magnets so that the central cusp of the B field miss-alligns with the Magrid point cusp, thus helping to block the path
of electrons escaping that way.
Secodly a sparse grid of X form could be added with the centre of the X
blocking the path of the electrons coming through the central pont cusp.
This modification would, of course, cause loss of power from alpha impacts
and would require protecion and cooling. Hopefully the power loss would be small (less than 5%, possibly less than 3%).
hitting the Magrid casings which are at a positive potential. The chamber
walls are not a problem because they are at the same potential as the well.
However the picture is very different if a pB11 machine is required because the collector(s) will attract the electrons so that any electrons with
enough kinetic energy to pass the Magrid will be lost. This is where a trap grid at negative potential w.r.t. the Magrid plays it's part. The trouble with a trap grid is that, unless it is located entirely within the shadows of the Magrid, it will intercept the alphas causing power loss and further cooling and protection requirements.
Experments (small scale) need to be done to investigate the effectiveness of such a trap grid, particularly regarding electrons coming out of the
point (face) cusps. If that geometry does not work I suggest two modifications that may help.
Firstly the trap grid could be made magnetic (extra complication
) with polarity opposing the Magrid. A half coil of a coarse winding could be omitted from one side of the magnets so that the central cusp of the B field miss-alligns with the Magrid point cusp, thus helping to block the pathof electrons escaping that way.
Secodly a sparse grid of X form could be added with the centre of the X
blocking the path of the electrons coming through the central pont cusp.
This modification would, of course, cause loss of power from alpha impacts
and would require protecion and cooling. Hopefully the power loss would be small (less than 5%, possibly less than 3%).
Last edited by KeithChard on Mon Aug 25, 2008 7:25 pm, edited 1 time in total.
Keith
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Barry Kirk
- Posts: 69
- Joined: Wed Sep 19, 2007 9:33 pm
- Location: York, PA
- Contact:
After reading this thread for a while, I wonder if it would be possible to adjust the confinement of the cusps in the following manner.
If the electrons near the cusps and therefore candidates for escaping are all low energy, what about beaming microwaves into those electrons to provide energy and kick them away from the cusps?
If the electrons near the cusps and therefore candidates for escaping are all low energy, what about beaming microwaves into those electrons to provide energy and kick them away from the cusps?
Sorry, it wasn't meant to be confusing. I'm just saying if you have a well, you have a gradient from the center of the well up to the positively-charged Magrid, and an ion has to get all the way up to the Magrid to get to the electrons up at the cusps, which may never happen, and even if it does get there the gradient still points the wrong way, esp if the Magrid is a Faraday cage that blocks the ion from seeing the 1/1000th of the electrons that are outside.Art Carlson wrote:I don't understand your comment. Not even grammatically.TallDave wrote:Hmmm? I guess I'm not sure why that matters, if there are far more electrons on the inside and the Magrid between the ions and the outside. Are you saying you think they're energetic enough that they can climb all the way out, despite the gradient?
I am assuming T_ion ~ T_electron ~ |electric potential at the center compared to the magrid radius| ~ |electric potential of the magrid relative to the walls| ~ 100 keV. My estimate refers to what happens around the magrid radius. The factor of (R/lambda_D) trumps everything.
If that were happening, the thing would be hemorrhaging ions, because once they get out there's no well holding them and they'd fly off away from the Magrid.
We know there is a well thanks to the LF measurements I've cited before. Hopefully we've got more empirical data on the ion distribution now as well, and will get to see it soon (*crosses fingers*).
Hey, I like equations too. Just not sure how applicable that one is in that situation, in terms of proving your conclusion.I presented a quantitative estimate that showed the importance of the electric potential resulting from the sheet of electrons. We are past the point of "think", "doubt", and "seem".
EDIT: Oops, I guess this isn't that relevant since we already decided this is a non-LTE situation, which was the main point of confusion.
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...