Does the Gyroradius of an Alpha fit in a cusp?

[Joseph Chikva]

i _did_ a number of simulations that showed this. the alphas are well contained electro-statically.

I'm assuming you meant the fuel ions were contained, as opposed to the alpha fusion products, which should definitely not be contained

Did you ever simulate alphas from the p-B11 reaction? It would be interesting to know if you came up with a similar number of passes and similar expectations re their energy when they exit.

I do not know about simulations done by Mr. happyjack and also about basic difference of alphas from other charged particles. But here is the link of simulation of electron beam injection into background plasma: http://www.netlib.org/utk/lsi/pcwLSI/text/node180.html

Electron Beam Plasma Instability

...This initial configuration is unstable to an electrostatic plasma wave which grows by tapping the free energy of the electron beam. At early times, the unstable waves grow exponentially...

• Do you think that in Polywell electron beam isn't injected into background plasma?
• Or that the potential well is so deep that can confine even the areas revolted by instabilities?
• And how correctly is to calculate the projected number density in Polywell from a condition beta=1 in instabilities case?

[D Tibbets]

I've been talking about looses- charged particles with with their associated KE reaching the magnet surfaces or exiting through a cusp.
Thermalization and maintainence of a deep potential well is a separate issue. The early Bussard papers give some results from early work, The potential well duration was measured in milliseconds. Subsequent work has improved on this significantly. My limited understanding is that important issues is the magnitude of the electron current- both input current and recirculating current. The higher the current, the higher the ratio of monoenergetic electrons to thermalizing electrons. This could lead to unacceptable input electron costs, but the recirculation of the electrons mitigates this considerably (by a factor of ~ 10-100). Also, the electrons are near monoenergetic upon injection, and thus the starting plasma conditions are monoenergetic. This is very different from the picture represented in your link. They start with an already thermalized plasma and inject electrons into it. This changes the dynamics. Admittedly I don't know how significant this is over time, but at least for short initial time frames it changes things.

Dan Tibbets

[Joseph Chikva]

I've been talking about looses- charged particles with with their associated KE reaching the magnet surfaces or exiting through a cusp.

And I am talking about that may be that some of those charged particles will have KE more than you estimate.
And by this reason reachable number density will be much less than calculated for condition beta=1.

[TallDave]

Dan -- FWIW, it's been strongly implied WB-8 was going to handle continuous operation. So the EMC2 guys probably have a better handle on that by now.

I wonder what they're using for the inputs on that PID loop. It sounded like on WB-6/7, they were just blowing the cusps out every time. I'm scratching my head thinking what metric they might have used to control the e-current to keep things at or near beta=1 in WB-8. Density interferometry? Just a wild guess...

You know what would be great? If we had another spreadsheet for WB-8 like the one someone dug up for the WB-7 project. Anyone remember how we got that, or know if it's available for WB-8? Might tell us some things.

[Joseph Chikva]

I'm scratching my head thinking...

Virtual reality?

[happyjack27]

As far as sputtering, the elecrtrons have relativelty low momentum, aso even though the may hit the magnets due to cross field trasnport while at their higest energies the effect will be minor compared to ions.

I'd never heard of electron sputtering raised as an issue, but I think they're expecting the Magrid to absorb something like 5-10MW of them. Not sure what if any materials issue that raises, or if this sort of question comes up in any other sort of device (I can't think of any offhand). Maybe Simon or Tom knows?

i _did_ a number of simulations that showed this. the alphas are well contained electro-statically.

I'm assuming you meant the fuel ions were contained, as opposed to the alpha fusion products, which should definitely not be contained

Did you ever simulate alphas from the p-B11 reaction? It would be interesting to know if you came up with a similar number of passes and similar expectations re their energy when they exit.

i never got around to simulating the actual fusion events. though i do have it calculate cross sections, i'd have to calculate spatial densities to get the actual fusion rates. and that'd be much more complicated than any of the other mods i did. alternatively i suppose i could have each particle calculate a fusion probability with every other pair given relative velocities and distances, but time is discrete in the sim so i'd have to integrate over a sample period, and i'm having 1 particle represent many thousands so that eliminates that possibility. long story short, no, 'cause of density. wouldn't have any interesting results, anyways, because i'm not actually simulating the atoms in the magrid so one couldn't learn anything about whether the product ions would effectively pass through or not.

so i'd say the default scenario is you lose as much of their energy to collisions to the thickest part of a the magrid covers a sphere centered at the origin

Apparently the collision cross-section at their energy is too low for that according to the EMC2 guys. But I think that can be calculated, given that we know their energy spread and roughly the operating density of a reactor. Or, hell, we could probably calculate it for WB-8.1.

i remember hearing something like that but wasn't confident. so the product particles shoot right through without imparting much KE to what they pass through? well then how do you get energy from them? i presume your plates for direct energy conversion would have to have a big charge on them to slow them down enough then. i suppose.

[happyjack27]

Now I have read about instabilities on the Sun with deeper potential well and am asking again: are they possible in Polywell?
Where Dr. Nebel says that no?

strictly speaking everything is possible, esp. when you take quantum physics into account. the question is whether the likilihood is substantial enough to merit design or control changes.

Instabilities are not a subject of quantum physics.

nor are they a subject of _classical_ physics. they are a subject of _mathematics_. what's your point?

[TallDave]

remember hearing something like that but wasn't confident. so the product particles shoot right through without imparting much KE to what they pass through? well then how do you get energy from them? i presume your plates for direct energy conversion would have to have a big charge on them to slow them down enough then. i suppose.

Right, I think what they're looking at is basically a reverse particle accelerator. I'd sure like to see exactly what they look like; Rick said EMC2 had detailed reactor designs. But I think the question of the collision cross-section for the alphas can be calculated, and since we know (roughly) the (average) densities and energies for a reactor and WB-8, we ought to be able to find a mean distance to collision, right? I might give this a whirl sometime this week, if no one else does it first.

I'm still skeptical the direct conversion can be as efficient as some proponents are hoping. Though it probably doesn't have be a whole lot better than thermal to be worth the trouble, I'm wondering if the larger savings might be in avoiding the neutronicity effects on materials.

[Joseph Chikva]

strictly speaking everything is possible, esp. when you take quantum physics into account. the question is whether the likilihood is substantial enough to merit design or control changes.

Instabilities are not a subject of quantum physics.

nor are they a subject of _classical_ physics. they are a subject of _mathematics_. what's your point?

My point is that TallDave wrong saying that instabilities like solar flares or solar wind impossible for Polywell. As there are not any external revolting factors in the Sun vs. very strong revolting factor for Polywell - externally injected electron beam. I have provided link confirming my words and also quote of man of FPGeneration who really watched two-stream instability in real experiment.
Now when people of EMC2 are going to increase the number density on a few orders of magnitude till 10^22 m^-3 instabilities will become much stronger.

So, projected number density in Polywell 10^22 is not reachable even if induction of magnetic field will be equal to 10T. Because due to instability significant part of particles will have kinetic energy exceeding projected number.

[ladajo]

Joseph,
The Polywell plasma is not a sphere, it is a spikey ball. And being unstable is not a problem like you think. In my mind it is helpful from the aspect of preventing thermalization.
I, like others, also think you can not compare a polywell plasma directly to a solar plasma. The solar reaction rates per unit volume are magnitudes lower, as well as gravity playing a much higher role, and there are also no external fields or massive particle sources for a sun.
They are apples and oranges.

You, like most, can only guess at the actual conditions in a polywell, and while sometimes insightful, your thoughts are only guesses, with no math or experimental observations. I do like that you are thinking it over hard, and that is good and healthy. But as many a person before has found, do not be so inclined to fall in love with your own ideas. There can always be alternatives.

[Joseph Chikva]

You are absolutely right saying that I can only guess and have not any experimental experience.
But person who has experimental experience namely in Polywell type device said about occurrence of two-stream instability.
And with all difference between apples and oranges they have many similarities as well.

The solar reaction rates per unit volume are magnitudes lower,...

That's correct and in result we may have more vigorous processes per volume unit. Including more vigorously shown instabilities.

...as well as gravity playing a much higher role...

Potential well of gravity or potential well of virtual cathode. In principle not a big difference.

...and there are also no external fields or massive particle sources for a sun....

I do not see any external field in Polywell but think that massive particle sources may become as revolting factor. See the link "electron beam into background plasma".

Etc., etc., etc.

[rcain]

my own understanding has been that whilst various 'stabilities' and 'instabilities' may obtain within the Polywell, so long as the 'overall structure' of velocity space is conserved, the (fusion) process may proceed.

i'm sure two stream (multi-stream) phenomena occur inside the Polywell. But they should average out as so much thermal, rather than structural. I understand Rick Nebel and his team have done a fair amount of experimentation with with ion gun positioning/orientation, etc, and must conclude there do exist some optimal and some less optimal configurations, but few critical ones under the geometry, and those are not so easily/often reached.

also, lets face it, the sun is at a wholly different scale, and it hasn't blown out due to instabilities yet either (though arguably it will eventually).

ps:

....I do not see any external field in Polywell...

- i think perhaps you should look again then.

[Joseph Chikva]

my own understanding has been that whilst various 'stabilities' and 'instabilities' may obtain within the Polywell, so long as the 'overall structure' of velocity space is conserved, the (fusion) process may proceed.

i'm sure two stream (multi-stream) phenomena occur inside the Polywell. But they should average out as so much thermal, rather than structural. I understand Rick Nebel and his team have done a fair amount of experimentation with with ion gun positioning/orientation, etc, and must conclude there do exist some optimal and some less optimal configurations, but few critical ones under the geometry, and those are not easily/often breached.

also, lets face it, the sun is at a wholly different scale, and it hasn't blown out due to instabilities yet either (though arguably it will eventually).

ps:

....I do not see any external field in Polywell...

- i think perhaps you should look again then.

• I am only saying that estimation of number density of future Polywell referring beta=1 is too optimistic.
• And besides cusp losses inevitably will occur also losses at the expense of instabilities.
• I can not understand how people can avoid thermalization as nothing urges scattered particle to return to the right radial direction. As I understand Polywell developers have not reached significant fusion rate yet. Because if not so, fusion events also will participate in thermalization process.
• I do not understand the nature of annealing. And nobody can explain. Something without term "annealing" I have read in the book written in 70-s. If that I have read and "annealing" are the same, so no any novelty. But excluding Polywell now no any other mirror machine are in consideration.

[Joseph Chikva]

ps:

....I do not see any external field in Polywell...

- i think perhaps you should look again then.

And?
• Electrostatic field between virtual cathode and MaGrid
• magnetic field created by Magrid
Any other?

[rcain]

ps:

....I do not see any external field in Polywell...

- i think perhaps you should look again then.

And?
• Electrostatic field between virtual cathode and MaGrid
• magnetic field created by Magrid
Any other?

.. and the electrostatic and magnetic fields produced by(/reflected/translated by) the 2-species inertial plasma itself.

why do you say 'no external field', then say there is one. i can only assume we have some trivial misunderstanding since you know what a Polywell looks like.

and sure 'number density' is a factor set out to maximize - current scaling experiments should yield the answer to your concern - but so far, it works well enough that it is worth the cost of such experiments. it may be less (scale-efficient) than anticipated (unlikely ever to be more) but results and theory so far suggest it should not hit any critical failure.

re: thermalisation - it has always been a major concern - theory suggests, and design dictates, that particles are on average lost from the system, faster than they can heat it up substantially (/critically). fusion products are rare and explosive by comparison and shoot straight out (or into something solid).

cusp losses are what the thread is all about ( i think) and have been hotly debated here before. (see 'ambi/bipolar flow' debates between Art Carlson and others). obviously, they are to be minimized/tightly controlled and scaling effects are expected to be tolerable. we shall see.

and 'annealing' ... nobody knows. you are not alone. i think there is a reasonable theoretical basis for it. it will be interesting to see some actual data when (/if) other labs start repeating the experiments . i doubt we'll learn much on that score from the current US Navy contract.

so we just have to hope that other experimentalists build Polywell's soon, so that we have a chance to see real data. i think we will. eventually.