Picture Of WB-7 Fusion Test Reactor Available

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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jmc
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Post by jmc »

[quote="TallDave"][quote] in addition to this, electrons will spend much more time in the core than the edge this compounds the problem still further.[/quote]

My understanding is they do not. Unless the virtual anode is too strong, they tend to see the positive magrid and spend most of their time crowding the edge of the magnetic field trying to get to it. At least, that's how I've always pictured it.

This is one of those things that might only be resolvable by experiment.[/quote]

Now you mention it, if the Polywell is more or less quasineutral, then in most places the electron density should be similar to the ion density, (less so in the edge but maybe still within a factor of two), if we assume this and that the electrons can move about wherever they want then they should spend a lot of their time in the edge as most of the ions (density times volume) are located in the edge.

However, when comparing the recirculation of electron energy to ions with ions annealing themselves to a monoenergetic distribution, there is still one outstanding issue some outstanding problems.

1) In the case of ion annealing the cross section of collision balloons as it approaches the annealing region of the edge, while in the case of electrons it shrinks as the electrons move faster and faster in this edge region.


There is also an annoying paradox that might prevent convergence:

In a maxwellian distribution, the electron density varies as n=n(0)*exp((e*phi)/KT(e)) [Chen p9] if the electron temperature is constant throughtout the polywell then in order to get electron density peaking to neutralize the ions at the core the electron density will have to be greater in the core, then in the edge, this implies the potential will have to be more positive in the core then in the edge, which will further imply that ions dropped in at the edge will not make it to the core let alone have more energy in the core than in the edge.

There are two ways to resolve this paradox:

1) Give the electrons a drifted radially convergent Maxwellian velocity like the ions. There are several problems with this:
a) the electron electron collission time is far smaller that the ion-ion collission time which will make the electrons isotropize far more quickly.
b)In Krall phy fluid B the idea of preserving ion convergence rested on having them trapped in a potential that ensured that they could never touch the magnetic field, if the ions are confined this way by an electric field the electron cannot an so will rapidly lose their convergence by bouncing off the magnetic field which is not properly spherical.

2) Give the electrons in the core a lower temperature to electrons in the edge, this will enable a lower temperature to magnify their density to a greater extent even if the potential is lower.

If the electrons in the core are being heated and the electrons in the edge are being cooled this would infact create the reverse situation, qualitatively one could argue that electron in the high potential region in the core could acquire enough energy in this region to "evaporate" from the potential hump (potential energy well for electron) this would carry them all the way to the edge where they might heat it. This can be imagined, the reverse situation where cool electron from a large 3-dimensional shell in the edge region just happen to 'drift' against a force applied by the E-field in the bulk, towards the single small point in the core, is almost impossible to imagine. So too is the situation where enough hot isotropic electrons from the edge have just the right velocity to carry them into the core where the potential energy they lost going through the bulk causes them to cool the core to a degree that counteracts the heating effect from the ions.

My prognosis:

In a quasineutral polywell electrons will isotropize rapidly due to collissions with the surrounding magnetic field, this will (must to prevent a prohibitive charge buildup in the core) set up some kind of instability that will cause the ions to isotropize aswell.

Bussard said his Polywell WB-6 had reached steady state in a few microseconds, I doubt that qualifies as an ion-electron collission time.

jmc
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Post by jmc »

rnebel wrote: If it is OK with you, I may chime in from time to time on your blog. However, this is your forum and I don't want to stick my nose in too heavily.
I think its anyone's forum who want to participate, I've been thinking alot about the Polywell, if convergence can be achieved then it could the most effecient manner of obtaining fusion with a magnetic field. I think I may start a topic on the ion injection method in the theory section.

MSimon
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Post by MSimon »

jmc,

I could be wrong and you will correct me if I am but, I think your conception of Polywell is incorrect.

The Polywell is an oscillating beam system with the beams naturally phase locked to each other. The ions will oscillate between the grid and the central region. Electrons will oscillate between the central region and the "low density" region outside the reaction space.

However, ions not oscillating will tend to bunch in the central region away from the positive grid. Electrons will tend to form a double layer with the central anode neutralizing the space charge.

In general analyzing such a system is very, very, difficult. Experiments are probably the only way to get an idea of the actual ratios between the different interactions. We will not have definitive proof useful for scale up and engineering usage until we have a continuously operating test reactor.

The first such device need not be much larger than the current WB-7. Cooling if done with LN2 is not the major expense of such a set up. It would be an operating cost and not a capital cost in any case. The big capital expense would be continuous operation power supplies for the grid capable of 80 KV output at 50 to 100A. 4 to 8 MW. The magnet power supplies would be more modest. Under 1 MW. Most likely half that unless you wanted to pulse them for higher field tests. The coils for such a test would be Cu of the Bitter design to provide the cooling and mechanical strength required. Since the actual fusion power out from such a test device will be small compared to the coil dissipation it can be ignored in the design. i.e. re: alpha intercepts.
Engineering is the art of making what you want from what you can get at a profit.

Solo
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Post by Solo »

Definitely an honor to have you onboard at the forum, Dr. Nebel ! :D I'm an inquisitive undergrad physics major hoping that Dr. Bussard's ideas pan out. I really appreciate you giving us the inside story; I think I speak for the rest of us in saying that we're sitting on pins and needles, fingers crossed.

93143
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Post by 93143 »

Why do you say the electron temperature is uniform? The potential well applies to any charged particles, not just ions. Electrons should be hot at the edge and cool in the core.

Also, didn't they confirm experimentally that the electrons don't thermalize before being lost?

rnebel
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Post by rnebel »

to Average Joe:

The people in the fusion program really aren't evil people. You just need to understand the situation that they're in. Most of them have worked their entire careers on Tokamaks. In the 2008 budget, ITER got cut to zero. Right now they are very vulnerable. If this funding isn't, at least, restored in FY 09, the US participation in ITER is done. Consequently, when an idea comes along like the Polywell and is mentioned in the same article as ITER, you get a very emotional response. Remember, most of these people were around in the 80s and got blindsided by cold fusion. They look at the Polywell and see an idea that they think they laid to rest a long time ago and now it's threatening to cut off their funding even though they think it has no merit. I think they are misguided, but most of them aren't malicious.

Tom Ligon
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Post by Tom Ligon »

rnebel,

I trust you guys all got your copies of the Analogs with the fusion article? I sent them to Dolly a couple of months back.

I'm probably going to order another batch, and wondered if you need more.

Askmar.com has a version that will be posted soon, and I'll have a copy to MSimon shortly to post here or at his blogsite.

Tom

rnebel
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Post by rnebel »

Thanks Tom. We appreciated those, particularly since we got autographed copies! If you have a few more available, we could use them. We are sending one to Bob Hirsch.

dnavas
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Post by dnavas »

rnebel wrote:They look at the Polywell and see an idea that they think they laid to rest a long time ago and now it's threatening to cut off their funding even though they think it has no merit.
Sure, though it's hard to see how $2M can be viewed as much of a threat. They may have even been right -- that Polywell won't work. We have our share of skeptics :) It would seem more logical to be concerned that the $2M work will be successful, a follow-on, much more expensive project announced, but that that one (dependent, as it is, on enormous scaling) will encounter the stumbling block that prevents it from being successful.

Is the concern the amount of press, money, time?

scareduck
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Post by scareduck »

I would have to believe the concern would be that the bar for "success" (from the tokamak fusion perspective) would be set so low for Polywell that it starts to sap funding, even if only in a minimal way, from the larger project. Richard Hull, the eternal fusion skeptic, has elsewhere said that the problems besetting Polywell extend far beyond mere electron well recirculation. He may very well be right, as with Todd Rider, and this turns out to be a dead end.

TallDave
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Post by TallDave »

Sure, though it's hard to see how $2M can be viewed as much of a threat. They may have even been right -- that Polywell won't work.
Sure, I still say a 1 in 3 chance of commercial viability, myself. I just don't buy any of the reasons why it can't work given so far.

Even a $2M project is a threat when they've been zeroed, and there's an emotional investment there just like for anyone. Tokamaks have been their whole lives, and are supposed to be the Energy Source of the Future (tm). No one wants to have wasted his life on an unworkable concept.

To put ourselves in their shoes, how will we feel if it turns out the WB-6 results were just noise, or if we try to build WB-100 and find it fundamentally cannot work for some unforeseen reason? And they've been at it a lot longer.

That's probably why we find them patrolling wikipedia, etc.

MSimon
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Post by MSimon »

How would I feel if WB-6 turns out to be just noise?

Great.

We need a lot more $2 million a year fusion experiments that can be scaled up in 5 years (if results warrant) and then accepted or rejected. This 30 years to get an answer crap is stupidity raised to the stupidity power.
Engineering is the art of making what you want from what you can get at a profit.

jmc
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Post by jmc »

I'm not sure how much I agree with that. I'm not sure I believe that a myriad of poorly though out fusion schemes should sap funding from the ones that yield the most success.

I think I said before, nuclear fusion is hard heat likes to flow from hot regions to cold regions and the hotter you make a region the more frantically it tries to escape, its simply the second law of thermodynamics. Another law is that as things collide they become Maxwellian, yet if you want fusion you have to go through a lot of collissions first.

Nature will make fusion hard for us whatever we try and the chances are that any scheme that attempt to achieve fusion will become larger, more expensive and more convoluted than it appears at first sight before success is reached. After success the price may start to come down again due to economies of scale.

Try googling "is nuclear fusion possible?" you will find a host of websites where people actually think it is impossible to get net energy gain from a manmade device on earth and not from lay-people but from technically minded scientists aswell. If ITER can even change that mindset and show that energy gain from fusion is a possibility it will have done the world a service.

When I see a tabletop device that outperforms ITER then I'll scrap ITER, but none exists. Right now I agree with trying out a range off small fusion experiments, but not at the expense of the most successful ones to date.

MSimon
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Post by MSimon »

jmc,

The tick-tokamockers haven't even solved the problems that they are aware of that could be solved in small machines - the first wall problem, ELMs, etc.

I wouldn't halt research along that line. I'd just do it in machines that could give actual knowledge in 5 years or less so we could build a big one with some confidence.

You don't go from mud huts to cathedrals in 3 easy steps.

You don't go from wooden frigates to aircraft carriers in 6 steps.
Engineering is the art of making what you want from what you can get at a profit.

jmc
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Post by jmc »

ELM suppression using coils to induce ergocity has been tested on smaller machines, D-IIID used it to suppress ELMS with great success. Although how to implement it in ITER is still a big question.

The problem is you can't really test everything you want to test on small machines, the point at which neutrons and heat flux becomes a problem only comes once you hit ignition.

The ITER vacuum vessel is only twice the dimension of JET in every dimension (giving it 8 times the volume).

Maybe your right that we should built the material test facuilty first and then get the materials right for ITER and only then try to build a full scale power plant, the only problem is once you do that, you push the time frame beyond the horizon of most politicians. ITER will be risky, but if it works it will at least show that man-made machines can produce net fusion power and will give potentially useful insight into the plasma physics effects of burning plasmas, it also serve as a test bed for materials designed to operate near fusion plasmas.

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