reddit: We are nuclear fusion researchers, ask us anything

[KitemanSA]

In any event, the core question is when are we worried about Bremsstrahlung?

Any time we have charged particles of substantially different mass (and charge too) in the same place with high collision velocity. Seems the greater the difference, the greater the brem. So most specifically, with p-¹¹B because of the electrons vs. ¹¹B.

[Joseph Chikva]

I never said that. I said it has a MUCH HIGHER CHANCE than Tokamaks to result in an economic reactor within the next 10 years.

Tokamak reactors have already achived Lawson criterion. Unlike all others. Confinement time in TOKAMAKs has seconds order, number density 10^20. The problem is in not effective heating of plasma. All other approaches are only in conceptual stage.

Still no excuse for not sending Slough a quick email with your concerns. The time you wasted discussing this with me here, would have been enough to do that. If you can formulate your exact question and the equation that you mentioned earlier here, I can try forwarding it to John Slough in your place, but that would be silly, no?

I have commented and many people have read. All they are free to do anything. Including you. But I am sure that Dr. Slough knows better than me the problems with merging of his plasmoids.

[Skipjack]

Joseph, can you please make a single post with the calculations that you mentioned earlier and your point of critizism. I will then take it upon me to forward your question to John Slough and we will both see how he responds. Is that a deal?

[Joseph Chikva]

Joseph, can you please make a single post with the calculations that you mentioned earlier and your point of critizism. I will then take it upon me to forward your question to John Slough and we will both see how he responds. Is that a deal?

Somewhere were numbers: number density of plasmoids, there linear dimension, initial velocity. Now I do net remember where are they. Estimation is easy if not considering that theta-pinch reduces radial diameter increasing the axial. But by order of magnitude I've got 1 ton-force. One assumption: I meant that deceleration rate is constant.
a=v^2/2s
F=ma
Mass of plasmoid is calculated as product of number density, sphere volume and mass of nucleons. If you have eqvimolar DT fuel (2+3)/2*1.67E-27 kg
Middle school level.

[D Tibbets]

In any event, the core question is when are we worried about Bremsstrahlung?

Any time we have charged particles of substantially different mass (and charge too) in the same place with high collision velocity. Seems the greater the difference, the greater the brem. So most specifically, with p-¹¹B because of the electrons vs. ¹¹B.

I admit some uncertainty about the dynamics, but I have the impression that the electron speed at a given radius is the most important component. In the ideal Polywell the electrons are fast on the edge and cool in the center (the top of their potential well is the center). The ions velocity as a funtion of radius is reversed. The 'Purity ' of this relationship depends on the degree of velocity and angular momentum thermalization. Another very important consideration is the near spherical geometry. If there is some confluence of the electrons and/ or the ions, the density increases as the center is approached. The central density may increase by as much as several orders of magnitude, so that most of the fusion reactions may be occuring in this core. Bremsstruhlung radiation scales with density also, so if the electrons are hot in the core (~ same KE as ions) the vast majority of Bremmstruhlung radiation will be in this area. But, if the electrons are cool in this area; the density squared relationship would be the same, but the magnitude of each Bremsstruhlung reaction would be less by the T^1.75 scaling. This temp dependent rate presumably makes up for much of the increased density effects in the core.

This basically translates to the ability to increase the fusion rate in the core with out the proportionate increase in Bremsstruhlung radiation. This modifies fusion output vs the Bremsstruhlung loss rates.

Dan Tibbets

[KitemanSA]

Yup, this is a pretty good description of why we SHOULDN'T be too concerned about Brem, but we only need to discuss it much when we are talking p-11B, not so much D-D. When, vs why not.

[ladajo]

The point I was too lazy to type out.
And even running PB&J, it can be managed, as we have discussed before. The knobs are known and exist.

[happyjack27]

he may have considered both thermalized plasma and mono energetic plasma, he did not consider the energy distribution of the electrons.
Dan Tibbets

Dan if I may, in laymans terms, Rider showed us scenarios why Polywell wont work, he did not show us scenarios why Polywell would work...

No?

If i may: Rider did not show us scenarios why Polywell wont work, he showed us scenarios why another machine, which he mistook for a Polywell, but is fundamentally different from one in a crucial way -- namely, it's electrons are hot, whereas polywell's are cold -- wouldn't work. and the reason it wouldn't work is because the electrons are too hot. which is exactly the way it differs from an actual polywell.

[Joseph Chikva]

In any event, the core question is when are we worried about Bremsstrahlung?

Any time we have charged particles of substantially different mass (and charge too) in the same place with high collision velocity. Seems the greater the difference, the greater the brem. So most specifically, with p-¹¹B because of the electrons vs. ¹¹B.

I admit some uncertainty about the dynamics, but I have the impression that the electron speed at a given radius is the most important component. In the ideal Polywell the electrons are fast on the edge and cool in the center (the top of their potential well is the center). The ions velocity as a funtion of radius is reversed. The 'Purity ' of this relationship depends on the degree of velocity and angular momentum thermalization. Another very important consideration is the near spherical geometry. If there is some confluence of the electrons and/ or the ions, the density increases as the center is approached. The central density may increase by as much as several orders of magnitude, so that most of the fusion reactions may be occuring in this core. Bremsstruhlung radiation scales with density also, so if the electrons are hot in the core (~ same KE as ions) the vast majority of Bremmstruhlung radiation will be in this area. But, if the electrons are cool in this area; the density squared relationship would be the same, but the magnitude of each Bremsstruhlung reaction would be less by the T^1.75 scaling. This temp dependent rate presumably makes up for much of the increased density effects in the core.

This basically translates to the ability to increase the fusion rate in the core with out the proportionate increase in Bremsstruhlung radiation. This modifies fusion output vs the Bremsstruhlung loss rates.

Dan Tibbets

Dan, scalling, Bremstrahlung consideration, aneutronic fuels, etc. this is very interesting.
But first you or someone else could not answer on one simple question: does two-stream instablity allow to Polywell the possiblity of running at high beta?
Once you qouted me the article considering simulation of virtual cathode stability, in which beta=0.14 was mentioned. For your reference TOKAMAK has Troyon limit by which beta can not be higher than 0.4 (real numbers is lower than 0.1). But recall that unlike TOKAMAK nobody can name real beta numbers for any generation of Polywells beginning from WB1 to WB8.
Mr. 9....3 mentioned about Landau damping. Yes, theoretically phase mixing can initatate Landau damping mechanism. But also it is well known that non-relativistic but coherent electron beam generates two-stream instability being injected in backgroung plasma.
Despite of multiple mentioning of this problem nobody could show me that someone has investigated this problem. I also would like to recall you and others that real man really building like Polywell device spoke about two-stream and Viebel instability occuerence. And any speculations that this type of instability are not issue for Polywell remain only as a speculation.

[marvin57]

Dan, scalling, Bremstrahlung consideration, aneutronic fuels, etc. this is very interesting.
But first you or someone else could not answer on one simple question: does two-stream instablity allow to Polywell the possiblity of running at high beta?

The latest report from EMC2 on Polywell is here:

http://www.recovery.gov/Transparency/Re ... e=Contract

During 4Q of 2011, EMC2 has modified the electron injectors to increase the plasma heating. The higher plasma density in WB-8 prompted the need for higher heating power. We plan to operate WB-8 in high beta regime with the modified electron injectors during 1Q of 2012.

It would seem from this report that EMC2 are running Polywell at high beta right now. Right now is, after all, 1Q 2012.

[Joseph Chikva]

During 4Q of 2011, EMC2 has modified the electron injectors to increase the plasma heating. The higher plasma density in WB-8 prompted the need for higher heating power. We plan to operate WB-8 in high beta regime with the modified electron injectors during 1Q of 2012.

It would seem from this report that EMC2 are running Polywell at high beta right now. Right now is, after all, 1Q 2012.

Numbers?

The higher plasma density

Higher than what?

high beta

Value? What number was already achieved and what is the goal for WB8?
Without fairy tails that beta=1

Also some theoretical reasonings would be interesting what factors make influence on achievable beta?

[KitemanSA]

Look thru most of the WB models and many of the others. I believe ALL of them except WB8 have been driven to beta=1 and beyond into instability and blowout.

Achieving beta=1 is not that big a deal for a Polywell, IIUTC.

[Joseph Chikva]

Look thru most of the WB models and many of the others. I believe ALL of them except WB8 have been driven to beta=1 and beyond into instability and blowout.

Achieving beta=1 is not that big a deal for a Polywell, IIUTC.

I'd be glad to look. But results were not published.
Regardless to in what you believe.

[KitemanSA]

Look thru most of the WB models and many of the others. I believe ALL of them except WB8 have been driven to beta=1 and beyond into instability and blowout.

Achieving beta=1 is not that big a deal for a Polywell, IIUTC.

I'd be glad to look. But results were not published.
Regardless to in what you believe.

The results have not been published to YOU. But then you are not king of the world. You do not have all data.

Besides, any Polywell magnetic confinement at high drive / deep potential well is by definition at high beta.

Thinking back, I may have been wrong on some of the more lossy machines. They may have been too difficult to drive to a deep well. But WB6 and 7 surely did.

Heck, I suspect even the U.Sydney machine has been run at high beta.

[Joseph Chikva]

Look thru most of the WB models and many of the others. I believe ALL of them except WB8 have been driven to beta=1 and beyond into instability and blowout.

Achieving beta=1 is not that big a deal for a Polywell, IIUTC.

I'd be glad to look. But results were not published.
Regardless to in what you believe.

The results have not been published to YOU. But then you are not king of the world. You do not have all data.

Besides, any Polywell magnetic confinement at high drive / deep potential well is by definition at high beta.

Thinking back, I may have been wrong on some of the more lossy machines. They may have been too difficult to drive to a deep well. But WB6 and 7 surely did.

Heck, I suspect even the U.Sydney machine has been run at high beta.

This is only an advertizing twitter. Give the link if all mentioned by you is published, instead of gossips of people still yesterday were assured that Rossi is genius. Rossi has died, and gossips will be forgotten. While publications can be read after 100 years too. Where are they?