reddit: We are nuclear fusion researchers, ask us anything

[KitemanSA]

Try using the internet.

ASKMAR is a good place to start. So is the Polywell FAQ wiki. EMC2FDC has a bit. That is all the search work I choose to do for you. I read many of the reports, you try it too.

But even so, you probably won't find the WB6 Final Report. Too bad. It is instructive.

[Joseph Chikva]

Try using the internet.

ASKMAR is a good place to start. So is the Polywell FAQ wiki. EMC2FDC has a bit. That is all the search work I choose to do for you. I read many of the reports, you try it too.

But even so, you probably won't find the WB6 Final Report. Too bad. It is instructive.

I know how to us internet as well as library. FAQ wiki is not publication. Because any person for example like who has nothing to do with real research you can write there.
Where publications of real researchers? 6 not equal to 7 and also not equal to 8.

[krenshala]

Try using the internet.

ASKMAR is a good place to start. So is the Polywell FAQ wiki. EMC2FDC has a bit. That is all the search work I choose to do for you. I read many of the reports, you try it too.

But even so, you probably won't find the WB6 Final Report. Too bad. It is instructive.

I know how to us internet as well as library. FAQ wiki is not publication. Because any person for example like who has nothing to do with real research you can write there.
Where publications of real researchers? 6 not equal to 7 and also not equal to 8.

WB7 was intended to duplicate the results of WB6, so they are pretty close to equal. You are correct that WB8 is different than WB7. If I remember correctly, Dr Bussard included numbers for WB6 in one of the articles I read (linked to from the design or theory board of this site, I believe).

Out of curiosity, what do you consider to be "high beta", Joseph?

[ladajo]

two-stream instablity allow to Polywell the possiblity of running at high beta?

I have seen this discussed somewhere why it is not a problem for Polywell, as I recall it is because of distribution and 3D construct, and the annealing. But I will have to look around again.

Too bad you have not read the WB6 report.

[KitemanSA]

I know how to us internet as well as library. FAQ wiki is not publication.

No, a library is not a publication. But it CONTAINS publications. I listed three locations that are like libraries where you can find "publications". If you know how to use a library, use the sites to find and read the publications.

Duhh!

[KitemanSA]

Jo,

Define what you mean by "beta" please.

Are we all talking the same language?

[D Tibbets]

Joseph Chikva, you can complain about data transparency, but Beta = 1 condition is straight forward in the Polywell and any other electrostatic machine that has reasonable confinement. Beta = ~ density * temp/ B field.
EMC2 ran WB6, etc at Beta = 1 through the simple method of maintaining an input current while ramping the B field. It is inevitable that Beta= 1 will be reached. This is fairly simple. The only requirement is that you have an adiquate poewer supply which is easy with capacitors. You can reach Beta= 1 with 100 volts, mAmps of current and a low B field. The real questions is how much current is actually required for a given voltage and B field strength. This reflects the efficiency of confinement, which is related to the magnitude of the Wiffleball effect. Beta=1 says nothing about the balance of fusion power out / input power. The confinement efficiency does. The question is not can Beta= 1 be obtained- that is trivial in these machines. The real question is if Beta= 1 can be reached with acceptable input current and at B fields of such magnitude that useful fusion can be achieved. Even a Bi Conic opposing Mirror machine can reach Beta = 1. You might need to pour in as much as one thousand times as much current (or have a B field one thousand times weaker) and of course this would preclude profitable fusion, but it is straight forward.

I should add that profitable fusion means Q >1 AND densities that results in useful fusion rates both for end use but also to overcome the baseline loads that are not generally considered when talking about plasma temperature and containment. Things like power for the vacuum pumps, and other systems.

As for two stream instability. I have previously referred you to an article that addresses this issue. It is not that two streams instabilities do not occur, it is that this is tolerable under certain conditions. In fact, I think that if there was not a degree of two stream instability, there would be no (or little)electron confinement in the Polywell. Any electron injected in one cusp would immediately exit through the opposite cusp. There has to be some electron scattering on the first pass. How much of this is due to collisional scattering and how much is due to magnetic effects is unknown to me.
Two electron beams on opposite sides would scatter along their entire lengths equally (I think). But consider three or more electron beams or two beams that are not 180 degrees opposed. Now the greatest density is where the beams intersect at the center, thus most of the initial scattering will be here. And in this near spherical geometry scattering from near the center is nearly radial. In other words, at least initially, the inevitable scattering would result in little angular momentum change. There are other considerations, but this is enough for now.

Dan Tibbets

[Joseph Chikva]

Out of curiosity, what do you consider to be "high beta", Joseph?

High beta is beta close to 1.
Beta is ratio between magnetic pressure B^2/2*(mu) and kinetic pressure (gas pressure) nkT.
If both two these pressures equal each other beta=1

[Joseph Chikva]

two-stream instablity allow to Polywell the possiblity of running at high beta?

I have seen this discussed somewhere why it is not a problem for Polywell, as I recall it is because of distribution and 3D construct, and the annealing. But I will have to look around again.

Too bad you have not read the WB6 report.

Yes, we discussed this earlier.
And all Polywellers are repeating the words of Dr. Nebel that two-stream instability is not issue for Polywell.
I would like to know why not?
Because as I know and it is well known (at least all plasma physicist today knows this as an axiom) that non-relativistic electron beam suffers two-strem instability being injected into background plasma.
Density of electron virtual catod is critical for Polywell as on it the depth of Potential well is dependent. See for example Stanley Humphries, Jr.'s book "Charged Particle Beams" in which on page 675 is written the following:

Two-stream instabilities may occur when beams of particles with different velocities flow through each other. Free energy is available to cause axial bunching of the beam particles – the space-charge fields of the bunched beams convert part of the directed kinetic energy to an axial velocity dispersion. A mild two-stream instability causes longitudinal emittance growth – a strong instability leads to destruction of the beam. Two-stream instabilities are a major concern
when beams propagate through plasmas.

This book is available in the web.
I saw also the thought here that there is not plasma in Polywell in classical understanding of plasma and definition "beam into background plasma" is not applicable for Polywell.
This is discussion with AlexK - the man who really built Polywell like reactor in as I understand UK: viewtopic.php?p=59785&highlight=#59785

Generally, once beam particles are above ~50 kV, thermalization happens at a rate that causes ion/energy losses which are smaller than the energy production rates due to fusion, unless there are instabilities (such as the 2-stream or Weibel).

I am declaring that even in non-catastrophic scenario (weak 2-stream instability) this will not allow to Polywell running at beta=1
This is in the best case.
And, so, I do not believe people when they speak "10 T magrid, beta=1 Polywell will run with 10^22 number density"
That's all.
Thanks.

[Joseph Chikva]

Joseph Chikva, you can complain about data transparency, but Beta = 1 condition is straight forward in the Polywell and any other electrostatic machine that has reasonable confinement. Beta = ~ density * temp/ B field.

I hope that it is only mechanical mistake as beta is inversely proportional to square of B and not to the first degree.

[KitemanSA]

Because as I know and it is well known (at least all plasma physicist today knows this as an axiom) that non-relativistic electron beam suffers two-strem instability being injected into background plasma.
Density of electron virtual catod is critical for Polywell as on it the depth of Potential well is dependent. See for example Stanley Humphries, Jr.'s book "Charged Particle Beams" in which on page 675 is written the following:

Two-stream instabilities may occur when beams of particles with different velocities flow through each other.

This book is available in the web.

What makes you think that there are two beams with different velocities? Where do you think this applies in a Polywell? What is the issue?

[Joseph Chikva]

What makes you think that there are two beams with different velocities? Where do you think this applies in a Polywell? What is the issue?

It's easy. The first is injected beam with non zero coherent velocity, while the second is plasma which can be considered in this case as the beam with zero coherent velocity.

Especially for you once again I am quoting the Stanley Humphries, Jr.'s "Charged Particle Beams"

Two-stream instabilities are a major concern when beams propagate through plasmas.

For two-stream instability different spices (pair) are considered. And the most vulnerable is electron-electron interaction (I think due to their in thousands times lower mass). Because as Ladajo says distribution in velocities inside the streams weakens this type of instability. But positive effect of veloscities' spread is very weak for non-relativistic electron-electron interaction. High relativism also gives some immunity against 2-stream. And the third way how to fight with 2-stream is strong longitudinal mag field.

[ladajo]

Joseph,
In the cae of Polywell, once the electrons are inside the shell, why do we care if the beam is destoyed? Once the electrons gain entry (which seems to be to current issue being explored, "electron injection anomalies", they do not need nor want to remain in a beam. We only want them to join the general populace of the potential well cycle. As we know, the two heavy concentrations of electrons are the shell (the turn around zone), and around the core area. We can assume that there is no actual 'center' point potential, but is is an area effect around the center, probably in effect existing as some kind of shell like average distribution around the geometric center, but shaped by the external drivers. So in this case, as the electron beam enters the device, the electrons encounter high Delta-V collisoins with the outer (turn around) shell, then join the lower density transit zone (less collison opportunity), to then hit the higher densities surrounding the core geometirc center zone (the deep well). What we do not want is to drive the electrons in a coherent beam into and through the core. We want electrons to join the distribution, the more collisions the merrier, if it were.

The critic I recall seeing about Rider may have been by George Miley.

[Joseph Chikva]

In the cae of Polywell, once the electrons are inside the shell, why do we care if the beam is destoyed?

We only want them to join the general populace of the potential well

The second quote is the answer on the first your question. Bunching of electron beams means that six electron beams do not intersect each other in the center. And may be you will not have the core (or waffleball) in the center. And even all will not develop according this catastrophic scenario, instability will not allow you to reach high beta.

Destroyed beam (if destroyed) will be splitted on separate bunches and all of them will be the low depth potential wells for ions instead of one and deep well as desired.

[D Tibbets]

Joseph Chikva, you can complain about data transparency, but Beta = 1 condition is straight forward in the Polywell and any other electrostatic machine that has reasonable confinement. Beta = ~ density * temp/ B field.

I hope that it is only mechanical mistake as beta is inversely proportional to square of B and not to the first degree.

Actually I was too lazy to look up the formula again. The important point is that the B field is in the denominator. This inverse relationship means that at weak B's the contained charged particlesc can be small to extremely small, thus reaching Beta= one is trivial. And, as mentioned reaching Beta= one at a desired relatively high density and KE with corresponding B field may be another matter.

In WB6 at ~ 10,000 volts and density of ~ 10^19 charged particles per meter cubed(?) and B field of ~ 0.1 Tesla required a current of ~ 4.5 million Amps. With Wiffleball trapping of ~ 10,000 X and and recirculation of ~ 10X (net recurculating current multiplier of ~ 100,000 X), the actual input current was ~ 45 Amps. I leave it to you to determine if this is ~ Beta = 1.

PS: Bussard used PMT measurements of peak plasma brightness which peaked as Beta= one was reached and passed. Nebel said that this measurement may have been questionable. With WB7, they used another measure for plasma density and they got the same results, confirming earlier conclusions.

Dan Tibbets