the problem of pumping electrons into the Polywell

[KitemanSA]

i've been trying to re-acquaint myself with the whole cusp geometry discussion again - naming conventions (eg: funny cusps, corner cusps, etc still a tad confusing).

i referred to our present truncated cube config corner cusp as 'Y-cusp' few posts ago - cause i couldn't think what else to call it - seems a logical name to me, - is it?

Bussard called it the line like cusp. I kind of like your "Y" cusp designation. The vertical part of the "Y" is the line like part.

By the way, what some folks call the corner cusp is just the virtual magnet's point cusp.

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Sometimes I really hate these auto correct features!!!

[D Tibbets]

"Y" cusp is not unreasonable given the shape. I would use a triangular shape with squeezed tails. These tails are narrow, but do extent to the midpoint where the magnets approach each other most closely. The term- point like cusp, is ok when describing loss magnitude, but if taken literally, it ignores the three tails deviating from the corner (of the truncated cube geometry). These tails will intercept metal, or if the the magnets are held completely separated, it will continue into the next corner cusp. I sometimes consider these cusps as the remnants of the line like equatorial cusps, deformed by the 3 D geometry of the polyhedral magnets with their close approach to each other on the sides.

I usually think of them as corner cusps, but that is my preference.

I struggled with "funny cusps" till I realized this was an artificial idea that applied only when in mathematical models, the magnets were considered as lines- which are infinitely thin. Thus the approach between the magnets was fanatically thin and thus an infinitely thin cusp. This made it difficult to describe a cusp as it was infinitely thin in the magnetic mathematic models. This would imply that any cusp leakage in this area , while not zero, was infinitely small and could be considered as zero. I interpret this to be the practical consideration for these cusp areas, but Bussard felt uncomfortable with this state of affairs, and thus coined the term "Funny cusp". It is interesting that it took so long for Bussard, etel, to "recognize the falicy of this concept and it was not till WB5-6 that the real world non zero thickness magnets were recognized as significant- thus the standoffs and conformal magnet cans.
This eureka moment allowed for design of the machine that reached promising results and predictions.

Note that even with WB 6 changes these tails of the corner cusps still allowed significant electron losses to the nubs/ interconnects, as reveled by Nebel's comments about nub heating in WB7. Weather improving to this situation (eg:by going to wall standoffs) adds up to a small improvement or a large one is unknown.

Dan Tibbets

[rcain]

i believe the solution to this problem might be found through dimensional analysis.

like the sound of that. cant think of a problem it couldn't/doesn't solve, almost.

fusion reaction volume is determined by the volume of the wiffleball, whereas electron loss through the wiffleball is determined by the surface area of the wiffleball divided by the field strength (roughly). so if you increase the size of the machine without increasing the magnetic field strength, you increase the ratio of the fusion volume to the rate at which you have to pump in electrons, without increasing the difficulty of injecting electrons.

i have vague recollection of Bussard saying something along the lines of 'make it big enough and it can't fail to work' - words to that effect at the Google pitch.

think he was referring to the B^4*R^3 scaling law, but just as well encompasses/(convolves?) your observation above.

...
however, if the electrons have to travel a greater distance to get in the wiffleball - that might reduce injection efficency enough to obviate the scaling advantage.

... which is of dimension 1, 1/d^2 ? (complicated?)

(by 'obviate' you mean 'eliminate' i think?)

like your thinking though - dimensional coupling, bigger, better.

what if you shoot a helical beam of electrons in counter-rotation to the magnetic field?

similar to my first thought also - spiral them in - but then i got stuck and couldn't figure out whether it actually achieved anything. (since they can also spiral out as easily (or can they?)).

but, two other factors:

1) electrons injected in have advantage over electrons trying to escape, since we can 'direct' former quite precisely and consistently Vs (almost entirely) 'probabilistic' loss cone the other side.
(seems obvious i know - but maybe it'll lead somewhere)

2) i've just forgotten. (something to do with cones / space charge ). maybe after some sleep.

[KitemanSA]

Sorry Dan, your mental image is incorrect.

A "cusp" is formed when a magnetic field in a particular direction reaches maximum compression (strength). Point cusps occur in the middle of areas of unidirectional field, lines between adjacent areas of same direction fields. The point in both cases, they are where the fields reach MAXIMUM strength but are all pointed "out" (or in). A funny cusp is where an even number (4+) fields meet at a point and cancel out. Zero, not maximum. The X-Cusp is merely a funny cusp with no metal in the middle. The fact that DrB's early math used 2D models of the magnets doesn't really effect the existence of funny cusps.

[happyjack27]

what if you shoot a helical beam of electrons in counter-rotation to the magnetic field?

similar to my first thought also - spiral them in - but then i got stuck and couldn't figure out whether it actually achieved anything. (since they can also spiral out as easily (or can they?)).

well i'm thinking it could achieve 2 things:

1.) help the electrons travel in a speed and direction such that the cross-product with the magnetic field produces a minimal outward component (ala lorentz force) - thus cutting down on the "magnetic tornado" effect.

2.) the spiraling would create a magnetic field for the electrons that would maybe help keep them together - to counter-act their mutual repulsion, sort of a weak plasma-pinching effect.

in both cases you are trying to take advantage of the lorentz force. in the first case by trying to inject the electrons at an angle/path that minimzes exposure to outward lorentz force. in the second you are trying to create an inward lorentz force to counteract the electrons mutual repulsion and any outward lorentz force from the magrid mag field X electron motion.

[bennmann]

Dear Charles,

The Sydney experiment and your opinion? So many people forget that one.

Electron losses DO get smaller with size. See quotes from WB7 and 8 in Alan Boyle's articles on MSN Cosmic log, also see quarterly reports previous on recovery.gov.

It's all there in plain English, but no specific numbers.

Good luck, best wishes.

[hanelyp]

A "cusp" is formed when a magnetic field in a particular direction reaches maximum compression (strength).

A cusp is where the magnetic field parallel to the containment surface reaches a minimum, at or near zero. In a baseline polywell that's happens when the magnetic field is directed radially. Maximum magnetic field intensity is near the coils, not at face centers.

[KitemanSA]

Point cusp are at the center, lines between. In the WB6, 7, & 8; there are 14 point cusps and 12 line like cusps that, if the magnets were square platform, would approximate funny cusps.

On re-reading your post, your basic definition is correct. Your implication about none in the center is incorrect.

[hanelyp]

I said nothing about where cusps were, but what they were.

[emiller]

This is an interesting technical discussion, but I think Charles's original question has not been adequately addressed.

... This has been the sad history of fusion to date -- attempting to correct a failed (not positive energy) system by boosting temperature and/or boosting particle density as though supercharging a car.

What happened to Bussard's claim in his speech on Google Talks that he *already* licked the problem of energy loss -- something he determined just as his funding ran out when reviewing data after the fact from the run that burned out his last device?

Two points: 1. Comparing the Polywell to the sad history of "mainstream" fusion is not quite fair. The crucial difference is that the traditional approaches use high temperatures to achieve the necessary cross-section to make fusion "go", and are thus faced with the problem of containing these high-temperature plasmas. Because the relevant energy is in the tail of the energy-verses temperature curve, most of the energy going into the machine is not useful to the fusion process. The huge amount of equipment also makes a lot more places for problems to occur. Because the Polywell uses direct acceleration, all of the energy in the electric field (provided by the "well" of the electron cloud) is effective in the fusion process. The Polywell thus does not need all the ancillary equipment that, for instance, Tokomak-type machines need. (The laser approach also uses a breathtaking amount of hardware.) In addition and very importantly, this direct approach gives it a leg up on making the almost waste-free pB11 reaction much more in the realm of possibility. Bussard abandoned the ITER because he thought the neutron-induced radioactivity would require frequent replacement of much of the machine with resultant costs and downtime.
2. The problem Bussard claimed to have solved was the energy sink caused by the magnetic field lines intersecting the metal surfaces of the the coil housing. They had not understood that problem until the last generation, WB6, and it had demonstrated much lower losses than the previous models. The Polywell was supposed to solve the loss problems of the grids of the Farnsworth-type machines, but the magnetic-interception problem had been a barrier to success prior to WB6.

I think the fact that the Navy wants the electron injection issues resolved at the current scale instead of depending on the expectation that scaling will resolve it, suggests that they are not motivated by the huge global potential of a successful device, but only their own provincial interests, and are pinching their pennies. I think we need a national effort motivated by the spectre of global warming.

[CharlesKramer]

I am fascinated by the idea beryllium is transparent to x-rays (at some frequencies, anyway). For dense plasma focus that might give an incentive to use beryllium in the electrodes. Wouldn't a beryllium Polywell also behave differently than one made of the usual materials?

The Polywell doesn't use materials for its cathode. In the Poly, the cathode is virtual, it's a magnetic mirror, so point in fact, it is transparent to X-rays of all frequencies.

I get it.

Nice!

[CharlesKramer]

I think the fact that the Navy wants the electron injection issues resolved at the current scale instead of depending on the expectation that scaling will resolve it, suggests that they are not motivated by the huge global potential of a successful device, but only their own provincial interests

Power for an electric Navy?

Of course, that is the Navy's job...

I think we need a national effort motivated by the spectre of global warming.

Sadly, funding for fusion appears to be in decline in the USA.

I don't get it either.

[CharlesKramer]

The Sydney experiment and your opinion? So many people forget that one.

Link?

I confess I don't know that one.

[emiller]

Sadly, funding for fusion appears to be in decline in the USA.

I don't get it either.

I am still hopeful that, if its potential for addressing global warming and the lack of any significant nuclear waste can be moved somehow into the national consciousness, it can move out from the shadow of disappointment from the decades of fusion promises unfulfilled. I wrote to the President a couple of years ago, and will again. He may be able to think more about global warming now. I would also like to get publications like Popular Science to pay attention.

[CharlesKramer]

I am still hopeful that, if its potential for addressing global warming and the lack of any significant nuclear waste can be moved somehow into the national consciousness, it can move out from the shadow of disappointment from the decades of fusion promises unfulfilled. I wrote to the President a couple of years ago, and will again. He may be able to think more about global warming now. I would also like to get publications like Popular Science to pay attention.

Humanity does what's expedient.

Prosperity is energy per capita.

Just about EVERYTHING derives from that most basic of equations. Democracy depends on energy being cheap and distributed, and industrial dominance is a function of cheap energy. Cultural flourishing depends on prosperity. And it's always been so. 5000 years ago "cheap energy" meant slaves; in 19th century Britain it meant coal and wage slaves.

All the other factors -- unionization, political system, national work ethic, economic policy, government regulation, you name it -- are all nearly irrelevant. If energy is cheap there is prosperity -- especially (in a petroleum and natural gas economy) when the same stuff used for energy is also an industrial feedstock and requirement for fertilizer and insecticide.

Fracking oil may briefly -- if it doesn't poison us -- restore the USA to it's historic role as world's larger producer, but fracked oil is a lot more expensive per barrel than the old Texas crude a bubblin' out of the ground with natural pressure, which means it won't bring with it the USA's former industrial dominance.

Fracked natural gas -- if we learn to use it as a oil substitute, for which it is suited for many applications -- may do even more to advance prosperity, because it's cheap. Again, if it doesn't poison us.

I'm not denying the dangers of global warming denial, only pointing out that humans are really really good at denying things -- especially when there are more immediate crises at hand.

I never expect Obama (and Steven Chu) to be responsible for curtailment of expenditures on fusion -- or for private capital (which could easily fund 10,000 Polywells, or 100,000 Dense Plasma Focus machines) also to drop the ball. And reverse field, and Tri-alphas or whatevers.

But neither of them are idiots.

Makes me wonder what they know, and what they're thinking. The understandable) pessimism about fusion is not likely the only answer.

Unless it is.