We Will Know In Two Years

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

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vankirkc
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Postby vankirkc » Wed May 20, 2009 3:20 am

rnebel wrote:The present projected Q values for p-11B vary from about 1.7 to about 12, depending on how the physics breaks. The details of how you do that are surprisingly subtle and coupled, and I'm not going to go into that in this forum. I view this as an "optimistic problem". There are a lot more serious issues that need to be dealt with than this one.


This begs the obvious question...what do you consider to be the "more serious issues?"

gblaze42
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Postby gblaze42 » Wed May 20, 2009 4:12 am

93143 wrote:Of course. But the fuel mixing ratio is a separate parameter from the overall plasma density. Changing the density, without altering other parameters, doesn't get you anywhere. That's all.



From my understanding the typical issue with Bremsstrahlung is that the fusion by-products transfer their energy to the electron's before they can transfer the energy to the surrounding plasma, this would mean the fusion rate would be less than the Bremsstrahlung rate, unless the electrons are at a relatively low energy levels already before the fusion products can do their work.

Art Carlson
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Postby Art Carlson » Wed May 20, 2009 7:36 am

rnebel wrote:The present projected Q values for p-11B vary from about 1.7 to about 12, depending on how the physics breaks. The details of how you do that are surprisingly subtle and coupled, and I'm not going to go into that in this forum.

Is that published anywhere?
rnebel wrote:I view this as an "optimistic problem". There are a lot more serious issues that need to be dealt with than this one.

We certainly agree on that. I see 3 serious issues: transport, transport, and transport.

Art Carlson
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Postby Art Carlson » Wed May 20, 2009 7:47 am

gblaze42 wrote:
93143 wrote:Of course. But the fuel mixing ratio is a separate parameter from the overall plasma density. Changing the density, without altering other parameters, doesn't get you anywhere. That's all.

From my understanding the typical issue with Bremsstrahlung is that the fusion by-products transfer their energy to the electron's before they can transfer the energy to the surrounding plasma, this would mean the fusion rate would be less than the Bremsstrahlung rate, unless the electrons are at a relatively low energy levels already before the fusion products can do their work.

Not exactly. The fuel ions always lose energy to the electrons, The electrons always lose energy to bremsstrahlung. If the electron temperature is high, then the bremsstrahlung power is high, and that has to be replaced somehow. If the electron temperature is low, then the power transfered from the ions to the electrons is high, and that has to be replaced somehow. The best you can do is when the only terms in the power balance are the transfer from ions to electrons and the radiation from the electrons. This implies some particular temperature for the electrons, significantly but not dramatically lower than the temperature of the ions. If you run with any other electron temperature, you can only lose.

Getting the optimum fuel mix is also an easy problem, already included in every serious calculation, so I don't know why it keeps coming up. Remember, if you run in pure hydrogen, you don't have any fusion but you still have bremsstrahlung, so it is obvious that running hydrogen rich only helps up to a point.

Barry Kirk
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Postby Barry Kirk » Wed May 20, 2009 12:11 pm

Art Carlson wrote:Not exactly. The fuel ions always lose energy to the electrons, The electrons always lose energy to bremsstrahlung. If the electron temperature is high, then the bremsstrahlung power is high, and that has to be replaced somehow. If the electron temperature is low, then the power transfered from the ions to the electrons is high, and that has to be replaced somehow. The best you can do is when the only terms in the power balance are the transfer from ions to electrons and the radiation from the electrons. This implies some particular temperature for the electrons, significantly but not dramatically lower than the temperature of the ions. If you run with any other electron temperature, you can only lose.


Art,

Are you assuming that the ions and the electrons form a homogeneous mixture? Is there any way to set up standing waves in the reactor that would tend to layer and separate the electrons from the ions?

I know that is probably difficult because the reactor is essentially symmetric in 3 dimensions.

It's probably a little late in the discussion to discuss alternate geometries, but what would be the implications of taking a polywell like WB-7 and squashing it in the Z direction for example?

Would that provide a handle to introduce an asymmetry to introduce a layering to keep the electrons and the ions separated?

gblaze42
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Postby gblaze42 » Wed May 20, 2009 1:13 pm

Art Carlson wrote:
gblaze42 wrote:
93143 wrote:Of course. But the fuel mixing ratio is a separate parameter from the overall plasma density. Changing the density, without altering other parameters, doesn't get you anywhere. That's all.

From my understanding the typical issue with Bremsstrahlung is that the fusion by-products transfer their energy to the electron's before they can transfer the energy to the surrounding plasma, this would mean the fusion rate would be less than the Bremsstrahlung rate, unless the electrons are at a relatively low energy levels already before the fusion products can do their work.

Not exactly. The fuel ions always lose energy to the electrons, The electrons always lose energy to bremsstrahlung. If the electron temperature is high, then the bremsstrahlung power is high, and that has to be replaced somehow. If the electron temperature is low, then the power transfered from the ions to the electrons is high, and that has to be replaced somehow. The best you can do is when the only terms in the power balance are the transfer from ions to electrons and the radiation from the electrons. This implies some particular temperature for the electrons, significantly but not dramatically lower than the temperature of the ions. If you run with any other electron temperature, you can only lose.

Getting the optimum fuel mix is also an easy problem, already included in every serious calculation, so I don't know why it keeps coming up. Remember, if you run in pure hydrogen, you don't have any fusion but you still have bremsstrahlung, so it is obvious that running hydrogen rich only helps up to a point.


Okay, Now I see what your saying.

Art Carlson
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Postby Art Carlson » Wed May 20, 2009 2:10 pm

Barry Kirk wrote:Are you assuming that the ions and the electrons form a homogeneous mixture? Is there any way to set up standing waves in the reactor that would tend to layer and separate the electrons from the ions?

The standard calculation, of course, starts out with a number of assumptions, in particular that the plasma is homogeneous and the ion and electron velocity distributions are Maxwellian, and that the ion energy loss is at least as great as the classical collision loss to the electrons and the electron energy loss is at least as great as the classical bremsstrahlung loss. Within these assumptions, the fuel mix and the temperatures are allowed to vary to maximize the ratio of fusion power to bremsstrahlung power. This derivation is rigorous and not controversial.

If Rick disagrees with this result, then he must believe that one of these assumptions is not justified. I would like to hear which one. You are asking whether the assumption of quasi-neutrality might be relaxed by creating structures smaller than the Debye length. It's an ingenious idea, but I cannot see how it could be done, and if it were, I would expect plasma oscillations that would entail the electrons and ions passing through each other periodically, which presumably would result in the energy transfer you are trying to avoid. Relaxing any of the other assumptions will lead in a similar way to additional effects that could negate any benefit. That is why I would like to hear the details. Maybe Rick is forgetting to take some effect into account. It would be very exciting for me if he presented a detailed model and I could not poke any holes in it.

Barry Kirk
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Postby Barry Kirk » Wed May 20, 2009 2:26 pm

Art, That is no problem if the ions and electrons pass through each other periodically.

Right now, they are mixed together all of the time. If they pass through each other, and spend only a small portion of their time in "contact", then that is a small amount of time for energy transport to occur.

The Analogy I would think of is taking two wires carrying comm signals and crossing them at right angles to minimize the cross talk.

Art Carlson
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Postby Art Carlson » Wed May 20, 2009 2:46 pm

Barry Kirk wrote:Art, That is no problem if the ions and electrons pass through each other periodically.

Right now, they are mixed together all of the time. If they pass through each other, and spend only a small portion of their time in "contact", then that is a small amount of time for energy transport to occur.

The Analogy I would think of is taking two wires carrying comm signals and crossing them at right angles to minimize the cross talk.

Why "only a small portion of their time"? I am thinking of a sinusoidal variation in n_i and n_e, so that they are half mixed anyway at the optimum point in time, and spend half their time passing through each other. I figure the energy transfer will be greater while they are passing through each other than when they are uniformly mixed because of the additional relative velocity. But come back when you can write down an equation.

gblaze42
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Postby gblaze42 » Wed May 20, 2009 2:58 pm

Barry Kirk wrote:Art, That is no problem if the ions and electrons pass through each other periodically.

Right now, they are mixed together all of the time. If they pass through each other, and spend only a small portion of their time in "contact", then that is a small amount of time for energy transport to occur.

The Analogy I would think of is taking two wires carrying comm signals and crossing them at right angles to minimize the cross talk.


Let's put it this way, the length of time they are in "contact" could be long enough, even microseconds is long enough to transfer energy from one to the other. And it's their electric fields that make "contact" which tend to be much larger than the particle themselves.

bcglorf
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Stupid question

Postby bcglorf » Wed May 20, 2009 3:03 pm

Art Carlson wrote:
Barry Kirk wrote:Art, That is no problem if the ions and electrons pass through each other periodically.

Right now, they are mixed together all of the time. If they pass through each other, and spend only a small portion of their time in "contact", then that is a small amount of time for energy transport to occur.

The Analogy I would think of is taking two wires carrying comm signals and crossing them at right angles to minimize the cross talk.

Why "only a small portion of their time"? I am thinking of a sinusoidal variation in n_i and n_e, so that they are half mixed anyway at the optimum point in time, and spend half their time passing through each other. I figure the energy transfer will be greater while they are passing through each other than when they are uniformly mixed because of the additional relative velocity. But come back when you can write down an equation.


Sorry, I'm always asking the really stupid questions it seems but I figure it's the only way I'll learn anything, particularly coming from a very limited physics background.

Art, what do you believe the ion density distribution is within the plasma? I initially understood the view of Bussard and presumably Nebel to be that ion density peaked in a very small region in the core of the plasma and was much lower through the rest of the plasma.

As for the standing waves suggestions, what effect would virtual anodes/cathodes have on ion and electron densities? Could they form layered variations in electron/ion densities relative to the center of the plasma?

I'm sure these questions are self-evident to most, but I think an explanation that a layperson like me can at least grasp would be helpful to more than just ignorant little me :).

Barry Kirk
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Postby Barry Kirk » Wed May 20, 2009 3:06 pm

Art,

I couldn't write down an equation for this.... Not my skill set.

However, does the cross component of the velocity have to be that high?

Also, and again, i'm just asking questions which I probably have the physical model wrong.

Is it possible to set this up, with the ions relatively stationary and it is the electrons that travel in the cross direction.

If the amplitude of the travel by the electrons, is much larger than the thickness of the ion layer, then the electrons would only spend a small fraction of their time in the ion layer.

Again, I'm trying to visualize this with a squashed polywell, where 1 or possibly 2 of the dimensions are smaller than the other one(s).

I have NO reason to believe that any of this would work or makes sense. Just trying to throw out ideas hoping one of them might make sense.

rnebel
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Postby rnebel » Wed May 20, 2009 8:04 pm

The major focus for the next generation Polywell is transport. We will be trying to take the machine from "OK" confinement to "good" confinement. Historically, this is a step that has been difficult for fusion machines. The next couple of years are going to be interesting.

I also notice that a number of people are trying to make Polywell arguments using classical collision models. The dominant mechanisms for transferring energy between the ions and the electrons are collective mechanisms, not classical binary collisions. Our experience is that you have to do full-up kinetic simulations if you want to understand these mechanisms and their effects. We've been doing that for the past 1.5 years, and we plan to be doing a lot more simulations over the next 2 years.

Art Carlson
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Re: Stupid question

Postby Art Carlson » Wed May 20, 2009 8:18 pm

bcglorf wrote:Art, what do you believe the ion density distribution is within the plasma? I initially understood the view of Bussard and presumably Nebel to be that ion density peaked in a very small region in the core of the plasma and was much lower through the rest of the plasma.
I have reason to believe the density will be almost flat within the plasma.
bcglorf wrote:As for the standing waves suggestions, what effect would virtual anodes/cathodes have on ion and electron densities? Could they form layered variations in electron/ion densities relative to the center of the plasma?
I haven't been able to come up with any picture of this that makes the least bit of sense.

chrismb
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Postby chrismb » Thu May 21, 2009 5:53 am

I still don't understand (and have never had an answer when posting the suggestion) why a toroidal solenoid would not do the whole job better. If the whole way Polywell works is some 'multi-species' collective behaviour of ions and electrons, rather than the flat density of both that Art and I expect, then;

why not confine electrons around the central major radius of a toroidal solenoid, then inject the ions into it along poloidal radii? In poloidal cross-section, this would look exactly like any of the diagrams of Polywell, just without the cusps!! If the annealing process works as advertised, then this will deal with ions scattered 'toroidally', just as Polywell should do for those ions scattered at some radius away from the centre.


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