Florida lab to pursue Bussard Polywell and IEC fusion resear

[chrismb]

Each time ions don't fuse when they meet (which is 3-sigma statistically all the time!), they loose energy.

Sure, but that energy doesn't vanish, it gets passed to one or the other ion. Since the ions don't get lost to upscatter to any significant degree...

[my italics] Neither I, nor anyone else, can publically say what is or is not significant in this regard as there is no experimental measures to justify whether that is true or not. It remains guesswork, poss with a mix of simulation (see below!)

except to the electrons, which can upscatter to the wall, but tend to give up all their energy doing so -- Joel's latest simulation suggests this process is very efficient and in fact satisfies Rider's requirement in that respect).

I don't understand what you're saying there. Are you saying electrons give up their energy to the wall, or not?

Chacon et al did the full bounce-averaged Fokker-Planck simulation and it says large Q values are possible with partial thermalization.

... simulations.... hmmm...... the same type that said a JET sized tokamak would break even 30 years ago?

So, what would this same simulation suggest about magnetically confined toroidal plasmas with partial thermalisation? Surely, whatever can be done in a Polywell by such means can be done better in a toroidal plasma (with no cusps)?

No plasma experiment that I have ever heard of has done what was predicted for it. Plasma always does something a bit different to any simulations of it.

[TallDave]

Neither I, nor anyone else, can publically say what is or is not significant in this regard as there is no experimental measures to justify whether that is true or not.

Yes, we can't know with absolute certainty that ion upscatter won't be an issue in a reactor until a reactor is actually built, but that's true for most any potential issue you could name, so it's not a very interesting observation. What we can say is the WB-7 hotspots were not on the wall which suggests there isn't an ion current, the simulations look promising in this regard, Rick seems confident, and Chacon called upscatter a red herring for this kind of machine. I think electron transport has a much bigger potential to derail this concept than ion thermalization or ion upscatter.

I don't understand what you're saying there. Are you saying electrons give up their energy to the wall, or not?

They have to climb up the potential well to get there, so the loss of energy is small. Quoth Joel:

The external electric field decelerates each electron as it approaches the wall. By the time an electron hits the wall it has already transferred 99% of its original kinetic energy into potential energy stored in the external E-field. The lost electron is replaced by a zero-energy electron from an electron gun at the tank wall. Falling toward the center the newborn electron recovers from the E-field all the potential energy stored there by the lost electron. 99% of the original kinetic energy is transferred with no losses from the up-scattered electron to the newborn electron. The minimum recirculation efficiency is therefore 0.99.
The discovery of the WiffleBall avoids the dire prediction of Rider's 1997 paper. An engineering design is needed for the gas cells; the gas-puff ion-source in WB-6 will not work in steady state. Although more design work is needed the future of Polywell is bright.

... simulations.... hmmm...... the same type that said a JET sized tokamak would break even 30 years ago?

Wasn't that mostly due to transport not scaling as predicted?

No plasma experiment that I have ever heard of has done what was predicted for it. Plasma always does something a bit different to any simulations of it.

Well, sure, if simulations were perfect we wouldn't need experiment. They could be wrong about any number of things, but electron transport scaling seems most likely.

Dan -- True, brem is another source of loss, hopefully only a really problematic one in p-B11. The Glasstone text suggests they will be in the gamma range. A bit scary.

[icarus]

How exactly are you using the term "transport" in this context?

Is there a term in the equations you can point to quantify this effect?

[TallDave]

As electron losses. I may be muddling the term "transport" a bit there lumping together cross-field transport with other electron losses.

I've seen the electron power loss equation given here. (p6, top right) There might be something better out there. Joel just took the average energy of the electron times the number of electrons.

Rick has said he's not sure how to calculate cross-field transport in these machines, which stands in some contrast to Bussard's confident statements in Valencia about their "well-developed transport models and equations obtained from 13 years of EMC2 experimental research." It took Bussard quite a while to get to WB-6, I guess we'll see if his solution scales well.

[chrismb]

They have to climb up the potential well to get there, so the loss of energy is small.

If electron energy is small at the wall, in the potential field of the magrid, this means that the wiffleball will be at ~ground potential. Is this correct?

[KitemanSA]

They have to climb up the potential well to get there, so the loss of energy is small.

If electron energy is small at the wall, in the potential field of the magrid, this means that the wiffleball will be at ~ground potential. Is this correct?

How does this equate?

[chrismb]

They have to climb up the potential well to get there, so the loss of energy is small.

If electron energy is small at the wall, in the potential field of the magrid, this means that the wiffleball will be at ~ground potential. Is this correct?

How does this equate?

I don't understand your question. Does what equate?

[hanelyp]

They have to climb up the potential well to get there, so the loss of energy is small.

If electron energy is small at the wall, in the potential field of the magrid, this means that the wiffleball will be at ~ground potential. Is this correct?

Ground reference is wherever you find it convenient to put it. That could be at the wiffleball boundary, the magrid, the outer wall, or elsewhere. The point is that there is a potential difference between the wiffleball boundary and the wall comparable to the electron kinetic energy at the wiffleball boundary.

[chrismb]

there is a potential difference between the wiffleball boundary and the wall

..so each electron lost to the wall, from the wiffleball, will take with it that potential difference, then....

If a cool electron in the wiffleball is accelerated to the Magrid, passes it, then slows to the wall, then for it to still be cool at the wall, so the wiffleball and wall have to be at the same potential.

Now, if the wiffleball and the wall are at the same potential, then there will be an electrostatic cusp as well as a magnetic cusp through the centre of each magrid opening, and stuff will fly through those cusps as soon as anything comes near them (because it is an electrostatic low point in the field).

If they are not significantly at the same potential, then each lost electron will take that potential difference away with it, and that will be a lot of energy exiting into the walls.

So I don't see how a wiffle ball could form, at the same potential as the wall.

[KitemanSA]

there is a potential difference between the wiffleball boundary and the wall

..so each electron lost to the wall, from the wiffleball, will take with it that potential difference, then....

If a cool electron in the wiffleball is accelerated to the Magrid, passes it, then slows to the wall, then for it to still be cool at the wall, so the wiffleball and wall have to be at the same potential.

Now, if the wiffleball and the wall are at the same potential, then there will be an electrostatic cusp as well as a magnetic cusp through the centre of each magrid opening, and stuff will fly through those cusps as soon as anything comes near them (because it is an electrostatic low point in the field).

If they are not significantly at the same potential, then each lost electron will take that potential difference away with it, and that will be a lot of energy exiting into the walls.

So I don't see how a wiffle ball could form, at the same potential as the wall.

I'm sorry, you are not making sense. The wiffle ball is at (or near) the point of greatest positive potential. It is where the electron has the greatest KINETIC energy. It converts that to potential as it approaches the wall. The virtual cathode is where the potential almost equals the potential of the wall, not the wiffleball.

[icarus]

Why doesn't anyone put numbers, graphs, equations up so that these confused discussions do not go on meaninglessly forever ... or is that how you like it?

[chrismb]

The wiffle ball is at (or near) the point of greatest positive potential. It is where the electron has the greatest KINETIC energy.

Isn't the magrid at the greatest positive potential?!? I thought the magrid was high positive, the edge of the wiffleball was roughly neutral and the centre was high negative, making the wiffleball generally negative wrt magrid so that any loose ions accelerate from magrid-wiffleball region into wiffleball.

If this is not so, then I will finally give up any hope of thinking it is possible to comprehend the whole concept of this thing.

[D Tibbets]

The wiffle ball is at (or near) the point of greatest positive potential. It is where the electron has the greatest KINETIC energy.

Isn't the magrid at the greatest positive potential?!? I thought the magrid was high positive, the edge of the wiffleball was roughly neutral and the centre was high negative, making the wiffleball generally negative wrt magrid so that any loose ions accelerate from magrid-wiffleball region into wiffleball.

If this is not so, then I will finally give up any hope of thinking it is possible to comprehend the whole concept of this thing.

This discussion is ignoring Gauss Law. The interior and exterior of the magrid can be considered as two independent systems. The border between is an interesting area. Interactions depends on debye shielding, ion - electron coupling, space charge, etc, etc. My understanding is that the electrons within the magrid has a potential well with the peak near the center and the achievable bottom at the Wiffleball border. This border potential well bottom for the electrons is not due to electrostatic forces by themselves, but due to the relative impenetrability of the magnetic field associated with reversing the kinetic energy directions of the electrons back towards the center (or at least along magnetic field lines). In this system the surface of the magrid is effectively ground. Any electrons that are transported through the magnetic field ground there. Remember though, that so long as the electron remains an infinatly small distance inside of the magrid surface the magrid surface is effectively non existent to the electron (Gauss Law again- at least in a perfect system). But, the electrons that escape through a cusp now see the positive potential on the magrid and are attracted towards it, so it is now the anode in that system. The vacuum vessel walls serve as the ground. The point here is that magnetic effects (and Gauss Law) complicates picturing the system from a purely electrostatic single system view point, so you consider it as two seperate domains. Then you throw in the ions, consider the spherical geometry effects, and the differential dynamics in the various shells of the machine and the complexity only increases.

Ot to put it another way, the magrid is both the anode and the mostly invisible ground relative to the electron, depending on which side the electron is on. But, it is never the cathode from the electrons perspective.

Dan Tibbets

[happyjack27]

Each time ions don't fuse when they meet (which is 3-sigma statistically all the time!), they loose energy.

i'm 3-sigma statistically and let me tell you it is _hard_ to find a suitable partner, even in a dense n^2 environment!

[happyjack27]

in all this talk i think it's important to distinguish whether you're looking at absolute electric potential energy, or whether you're talking about acceleration along a gradient of electric potential energy. (the latter being the first spatial derivative of the former.) e.g. inside a hollow charged sphere, the gradient may be zero but the electric potential energy is not.