Recovery.Gov Project Tracker

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

Moderators: tonybarry, MSimon

TallDave
Posts: 3140
Joined: Wed Jul 25, 2007 7:12 pm
Contact:

Post by TallDave »

At this point I think this is my major concern:
rnebel wrote:For present generation machines the electron confinement time is less than the electron collision time so thermalization isn't an issue. For reactors, the electron collision time and the confinement time become comparable. Electron distributions are expected to be isotropic, but not thermal. Thermalization is a global process because electron orbits cover the entire interior. If electrons lose their energy (kinetic + potential), they will accumulate near the coil cases until they leave the system.
Since I don't think cusp-plugging is very well understood, I worry whether and how changes in the electron distribution will affect it.

I wonder if EMC2 has a better model of the WB effect now. That would be pretty interesting to see.
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...

ladajo
Posts: 6258
Joined: Thu Sep 17, 2009 11:18 pm
Location: North East Coast

Post by ladajo »

I think that in this case the question is "why would they leave the system?"

Going back to Joeseph's system analogy,
Here I would like to say that beta=1 is like the pool filled to edges.
And if water in the pool is absolutely quiet even one drop will not overflow. But if there the waves appear, the quantity of lost water will depend on intensity of those waves.
Increasing number density you will also increase intensity of instabilities. And I am not sure that dependence will be linear.
I think that it is an interesting way to look at it, however I would posit the consideration that a pool does not have something as quickly responsive pushing the water back in, whereas beta=1 and approaching would see the B- field as more invovled. There is some merit I think in considering the robustness and responsiveness of the push back, just like the damping and spring for an automobile shock absorber. The dynamics for the plasma are sure to be different to the B-field in this regard, and in that, we would see some sort of chaotic interaction between them through the Beta=1 regime. How much is the question I guess.

In reagrd to your other question Joeseph, regarding the 2 stream instabilities, I am not sure how reliable the construct is for an all direction polywell. Yes at its heart, it can be said to be an opposing beam device, but that is to simplify it, and n ot be valid once one expands the two beams to an <sic> infinite number meeting from all points of a sphere. Plus the fact that the e- actually oscillate back an forth, and are focused to the center of the core as a volumetric target point, vice a thermal population on holiday, and in this regard so goes the Ions from all directions, I think makes a difference, especially when considering the idea of offset collisions, and the fact that the plasma is not a perfect sphere, but possibly a dynamic spikey ball. I think that there will be volumetric concentrations of ions oscillating along the conic centers of the spikes, and in some cases we may even see some of your theorized events where deffent speed ions chase each other towards the long arm of a spike from volumetric center of the overall plasma. It all makes for a pretty picture in my head. (if it works).

Tom Ligon
Posts: 1871
Joined: Wed Aug 22, 2007 1:23 am
Location: Northern Virginia
Contact:

Post by Tom Ligon »

I personally watched one of the machines cusp-plug, and have always believed it did so with low energy electrons. I don't consider it a problem, except that I never got WB3 to do that trick. I wanted it to ... it seemed to prolong the life of high energy electrons.

I don't think the low energy ones had necessarily lost energy, but may have been shed ionizing the gas, in the case of this particular machine near the walls. They settled at cusps where leaks would otherwise occur and seemed to put up a roadblock.

TallDave
Posts: 3140
Joined: Wed Jul 25, 2007 7:12 pm
Contact:

Post by TallDave »

Tom,

That's right, I'd forgotten your PZL-1 story (if I'm remebering the name right) where it was unexpectedly still plugged after being turned off. Made a nice pop iirc.

Okay, I'll admit it: I'm secretly entertaining the notion the effect actually might work better at larger scales, as the volume of cold electrons rises faster than the area of the cusps. Well, one can hope.

ladajo,

Yep, I wouldn't worry about two-stream. Art brought that up way back on the first MSNBC article, and Rick had this to say:
Rick Nebel wrote:The machine does not use a bi-modal velocity distribution. We have looked at two-stream in detail, and it is not an issue for this machine. The most definitive treatise on the ions is : L. Chacon, G. H. Miley, D. C. Barnes, D. A. Knoll, Phys. Plasmas 7, 4547 (2000) which concluded partially relaxed ion distributions work just fine. Furthermore, the Polywell doesn’t even require ion convergence to work (unlike most other electrostatic devices). It helps, but it isn’t a requirement.
Art never raised it again afaik. It can be an issue for other IECs, but shouldn't be for Polywells.
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...

happyjack27
Posts: 1439
Joined: Wed Jul 14, 2010 5:27 pm

Post by happyjack27 »

TallDave wrote:At this point I think this is my major concern:
rnebel wrote:For present generation machines the electron confinement time is less than the electron collision time so thermalization isn't an issue. For reactors, the electron collision time and the confinement time become comparable. Electron distributions are expected to be isotropic, but not thermal. Thermalization is a global process because electron orbits cover the entire interior. If electrons lose their energy (kinetic + potential), they will accumulate near the coil cases until they leave the system.
Since I don't think cusp-plugging is very well understood, I worry whether and how changes in the electron distribution will affect it.

I wonder if EMC2 has a better model of the WB effect now. That would be pretty interesting to see.
eh? kinetic and potential energies for electrons are opposite in polarity in this system; their go opposite directions. they are both greatest near the coil casings and least in the center. the lowest energy electrons in terms of abs(potential)+abs(kinetic) are the ones in the center. but that's a good thing.

for ions it's kinda the reverse - ions experience pendulum-like motion cause they're just ruled by the electrostatic charge in the center ("gravity" in this case) so for ions abs(kinetic) + abs(potential) = constant. and the question is what that constant is. ions that oscillate further from the center have a higher ke+pe constant. this all showed very clearly in my sims.

happyjack27
Posts: 1439
Joined: Wed Jul 14, 2010 5:27 pm

Post by happyjack27 »

Tom Ligon wrote:I personally watched one of the machines cusp-plug, and have always believed it did so with low energy electrons. I don't consider it a problem, except that I never got WB3 to do that trick. I wanted it to ... it seemed to prolong the life of high energy electrons.
re cusp plugging and the speed of said cusp plugging electrons. i was going to show this: http://www.youtube.com/watch?v=ULfi1fr3JqU as an example. at this point it's pretty leaky so it's probably a bit past b=1, and i think the coil charge is off and that probably would help to plug it up. anyways i was going to show it as an example that any cusp plugging electrons are fast and they plug it more or less statistically, but then i though i don't actual have single electrons in the sim, each orange dot represents at least a thousand. and it could concievably operate considerably different w/single electrons.

but ya what it looks like to me from that sim is that the mutual electrostatic repulsion of the electrons pushes them a little extra hard against the magnetic fields so they deflect sooner than they otherwise would and thus most of them "miss" the tiny cusp hole.

a simple analogy might be shoot a bullet down a funnel and it goes right through the whole at the end. now make it explode when its in the middle of the funnel and most of its shards are gonna be deflected back. the mutual electromagnetic repulsion is said "explosion".

now it's a little more tricky than that, of course, cause the fields are "soft", rather than hard matter like a metal funnel, and it can get pushed back, etc. all in all i can see from this analogy how cusp-plugging and wiffleball mode can happen.

TallDave
Posts: 3140
Joined: Wed Jul 25, 2007 7:12 pm
Contact:

Post by TallDave »

happyjack wrote:eh? kinetic and potential energies for electrons are opposite in polarity in this system; their go opposite directions. they are both greatest near the coil casings and least in the center. the lowest energy electrons in terms of abs(potential)+abs(kinetic) are the ones in the center. but that's a good thing.


Remember, the well the ions see is an upside-down well for electrons -- at the center they have high potential, but tend to be slow, while electrons at the edge tend to have velocity but there's relatively little potential there as compared to the "top" of their well at the center. Slow electrons at the edge (which will develop through thermalization) have neither velocity nor potential, and can only go the casings/cusps, is what Rick is saying there.
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...

Joseph Chikva
Posts: 2039
Joined: Sat Apr 02, 2011 4:30 am

Post by Joseph Chikva »

ladajo wrote:I think that in this case the question is "why would they leave the system?"

Going back to Joeseph's system analogy,
Here I would like to say that beta=1 is like the pool filled to edges.
And if water in the pool is absolutely quiet even one drop will not overflow. But if there the waves appear, the quantity of lost water will depend on intensity of those waves.
Increasing number density you will also increase intensity of instabilities. And I am not sure that dependence will be linear.
I think that it is an interesting way to look at it, however I would posit the consideration that a pool does not have something as quickly responsive pushing the water back in, whereas beta=1 and approaching would see the B- field as more invovled. There is some merit I think in considering the robustness and responsiveness of the push back, just like the damping and spring for an automobile shock absorber. The dynamics for the plasma are sure to be different to the B-field in this regard, and in that, we would see some sort of chaotic interaction between them through the Beta=1 regime. How much is the question I guess.
Any magnetic trap can be compared with reservoir filled with very mobile and instable liquid. For example, with a bucket.
The volume of reservoir determines with B – higher B so, higher quantity of plasma can be confined in given space (volume).

Beta < 1 can be compared with not totally filled reservoir. If waves (instabilities) generate, the parity between amplitude of waves and filling degree will define liquid losses.

Also, here at this site I see everybody knows about the second way of losses - cusp losses. Those are common for all mirror machines. So, mirror machines can be considered with not conventional but with holed bucket.
And yes, here scaling would play positive role. As the more the bucket volume at the same or comparable area of holes (cusps), so, we’ve get less liquid losses in a unit of time.

About spring and shock absorber.
Yes. I would too imagine the behavior of separate particle as oscillation of point weight suspended on a spring together with the shock-absorber. But this is only ideal model, by which particle’s motion is “strongly radial” (Dr. Nebel in correspondence with Patent Office of USA).
As, random in direction and value force periodically acts on the particle as result of scattering. And, so, particle oscillates not only radially but it is also still shaken here and there.as well.
In fact we will have addition of two motions: “right” harmonic oscillation and random (thermal). And if amplitude of harmonious oscillations’ velocities would be comparable with average velocity of randome we have thermal plasma. And by strict definition the "temperature" is only the measure of that thermal motion and not KE of particle.

Also
We should not consider plasma as set of separate particles.
By definition of Langmuir I have read in Arcimovich’s book, plasma is a set of oppositely charged particles in which Debye length is much smaller than linear dimension.
As in such system also so called “collective” phenomena are observed.

Joseph Chikva
Posts: 2039
Joined: Sat Apr 02, 2011 4:30 am

Post by Joseph Chikva »

TallDave wrote:ladajo,
Rick Nebel wrote:The machine does not use a bi-modal velocity distribution...
How we should call the following?
Electron gun produces non-zero velocity electron beam that is injected in zero electron velocities environment.
No "bi-modal velocity distribution"?

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Post by D Tibbets »

Some comments from reading the last couple of pages. Beta= one is easy to achieve. It is the result of something like KE of charged particles * density / B field. You can play with all three numbers and achieve B=1 with very weak B fields, etc. To achieve Beta =1 conditions with KE and densities useful for practical fusion energy production is another matter. My impression has been that the achievement of Beta= one is not contested. What is contested is if this condition leads to the cusp surface area / total surface area of the plasma 'sphere' decreasing to create a Wiffleball.

As for the swimming pool filled to the brim, where any wave or instabilities could lead to water (plasma) slopping over the side. I would think that this would refer to MHD instabilities due to magnetic surfaces concave towards the center of the plasma. These macro instabilities are supposed to be avoided in the Polywell due to the universal convex magnetic surfaces towards the center. This sloshing is apparently so bad that systems with this instability (like Tokamaks) have to stay below ~ 0.3 Beta, to prevent these waves from sloping over the side to much .

Cusp plugging in some of EMC2's machine was a major effort. like in WB5. I'm not sure of the interactive dynamics in this process, but Bussard, etel apparently abandoned these efforts. Bussard claimed he could not plug the exits to electrons without opening them to ions at the same time. My impression is that cusp plugging consists of holding a collection of like charged particles in the cusp to repel electrons (how would Gauss's law effect this with this quasi spherical machine?). The problem is that if this negative space charge in the cusp repels electrons entering the cusps, it would also attract the ions- or at least lessen the net negative space charge behind the ions and this would allow more up scattered ions to escape without a subsequent opportunity to be edged annealed on the next pass. Instead, he went with electron recirculation instead. I'm not sure if the mechanics is the only difference while the end results are the same, or if recirculation entails significantly different physics. Irregardless, this recirculation approach seems to be the key. Does that mean trying to supplement this process with more mundane cusp plugging have merit? Does it mean there is some inherent cusp plugging, but increasing it further is a disadvantage? I have no idea!


Concerning Coulomb colisionality, thermalizing influences in a larger machine. It is a complex relationship. My impression that in the ~ 10 KeV, 0.3 meter WB6, with its ~ 10^18-19 particles/ M^3 the MFP was somewhere around 100 meters. That would be ~ 300 passes through WB6 before much thermalization occurred. As the lifetime was ~ 200 microseconds, or ~ 10,000 passes or less- this would be the confined passes without consideration for electron recirculation. With an electron average speed of perhaps 5,000,000 M/s (1/2 of speed at 10 KeV) this would be ~ 1000 M traveled/ 200 microseconds. This is within an order of magnitude, and considering my loose approximations of MFP and density, it is reasonable to state that the electrons would not fully thermalize. It might take several multiples of the MFP to fully thermalize.

With a 3 meter WB100 operating at ~ 100 KeV. The distance per pass would increase 10X. The confinement time as a function of passes would not change, but the distance traveled per pass is 10 times greater, so the confinement time would be 10X greater on a time scale. This is why the losses scale at the 2nd power as the volume scales as the 3rd power. And this condition requires that the B field is increased as the square of the radius in order to maintain the same cusp hole surface area in relation to the entire surface area of the Wiffleball. This B vs volume scaling is convient for calculation, though it is not preclude different ratios in an actual machine.

In this larger machine the KE of the electron at 100 KeV would result in a speed increase of the square root of the delta change in the KE, or ~ 3X. The average speed would be ~ 15,000,000 M/s. At 15,000,000 M/s / 3 meter diameter the electron would complete one pass in ~ 1/5,000,000 of a second or ~ 0.2 microseconds. This times ~10,000 transits =~ 2 ms confinement time.

This is ~ 10 times longer than in WB6. The MFP lengthens by a factor of delta energy (KeV) of the electrons squared. This would be 10 squared or ~ 100X. This means that so long as you increased the drive potential 10 X, the MFP will increase so that the net effect is null, or actually negative. That is the thermalization concerns becomes smaller.

I actually think the drop off in Coulomb collisionality with temperature is closer to ~ exponent of 1.75 rather than 2. (I should look it up and store the formula, but that is too much work). But this still illustrates that the problem is not compounded as much as might seam apparent on first inspection, concerns may actually be relaxed within limits.

Note that the coulomb collision rates in this example for D-D fusion still exceeds the fusion rate, but the difference is less. And, since we are talking about electron lifetimes and thermalizing rates, the fusion rate of the ions are mostly irrelavent in this discussion.


Of course if you stayed at 10 KeV electron energies in the larger machine, the thermalization concerns would be magnified. I'm guessing this is why Bussard mentioned 80 KeV for a 3 meter diameter D-D burning demonstrator. It is the best compromise between electron thermalization concerns, fusion power yield and D-D fusion crossection slope. It is not the ideal drive energy from a pure crossection slope viewpoint where Q is maximized (this is ~ 15KeV), nor is it the ideal point if you are considering fusion rate to thermalization rates. Even higher driv energies increase bremsstrulung more, decreases fusion fusion yield per reaction, and increases engineering problems. As in Goldilocks.. it is just right. 8)

I have not considered the effects of increased density on the thermalization times of the electrons. This would throw the thermalization rate/ confinement time issue back into the problem side (the MFP would become much shorter). But the magnitude is mitigated somewhat by the above considerations.

Dan Tibbets
To error is human... and I'm very human.

Joseph Chikva
Posts: 2039
Joined: Sat Apr 02, 2011 4:30 am

Post by Joseph Chikva »

D Tibbets wrote:Beta= one is easy to achieve. It is the result of something like KE of charged particles * density / B field. You can play with all three numbers and achieve B=1 with very weak B fields, etc.

These macro instabilities are supposed to be avoided in the Polywell due to the universal convex magnetic surfaces towards the center.
Yes, you can play with numbers. But those numbers can be too far from reality.
Yes, there is not any problem to pump and pump electrons and ions into machine till particles losses will not equalize to their input. But that equalization will not occur when plasma pressure will equalize to magnetic pressure (beta will become equal to 1). But much earlier.

Yes, magnetic lines in Polywell are convex. But nobody has proved that it can suppress for example two-stream instability.
I am afraid that it is only your speculation.

93143
Posts: 1142
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

Joseph Chikva wrote:Are you still sure that beta=1 is possible?
Of course it's possible. All the fusion runs I mentioned were pulsed runs - they went through beta=1 on the way to a blowout. They didn't need to sustain it; just hit it.

As for steady-state, there's not enough data. It could be a relatively simple controllable minimum-loss point, or it could be a catastrophic blowout point; IIRC happyjack's simulations indicate the second, but Tom Ligon's experiments seem to indicate the first. In the second case you'd have to operate a bit below beta=1, but it would still be much closer to beta=1 than a tokamak. Or you could pulse - but I suspect the losses would be high in pulse mode...
By definition of Langmuir I have read in Arcimovich’s book, plasma is a set of oppositely charged particles in which Debye length is much smaller than linear dimension.
I believe I determined a while back that Polywell breaks the assumptions underlying Debye theory, because of the way the plasma is formed and fed - the kinetic energy of the electrons is due almost entirely to the applied potential and is on the same order as it, and the ion kinetic energy is similar but out of phase, caused by the electrons penetrating the plasma and forming a well. A Debye sheath in the traditional sense can't form under those circumstances.

In a Polywell plasma, the shielding length should be on the same order as the plasma dimensions. Smaller Langmuir waves might form, but they wouldn't be able to fully shield the potential. (Or I could be out to lunch...)

...

Yes, collective phenomena need to be considered. But continuum fluid dynamics is not a good technique, because it makes unwarranted assumptions about such stuff as particle temperatures and mean free paths...

That doesn't mean the pool analogy is bad - it sounds good to me...
But nobody has proved that it can suppress for example two-stream instability.
Rick Nebel said he had, as far as theory and the limited experiments done up to that point can be said to prove something. Of course, he didn't prove it to you... or me...

It is indeed frustrating working with such little information. But what information we do have is quite promising IMO, and thankfully the project's ultimate success does not seem to depend on how much we on this board know about the details...

Joseph Chikva
Posts: 2039
Joined: Sat Apr 02, 2011 4:30 am

Post by Joseph Chikva »

93143 wrote:
Joseph Chikva wrote:Are you still sure that beta=1 is possible?
Of course it's possible. All the fusion runs I mentioned were pulsed runs - they went through beta=1 on the way to a blowout. They didn't need to sustain it; just hit it.
Very well. But on which beta we talk about?
As, as I understand, pressure at the edges initially in created only by electrons, but then by the thermalization growth ions pressure increases as well. And if it’s so, and ions’ pressure by the order of magnitude would reach to electrons’, we will have initial beta only 0.5 and not 1. And that is if device runs steadily.
Am I right?
93143 wrote:As for steady-state, there's not enough data. It could be a relatively simple controllable minimum-loss point, or it could be a catastrophic blowout point; IIRC happyjack's simulations indicate the second, but Tom Ligon's experiments seem to indicate the first. In the second case you'd have to operate a bit below beta=1, but it would still be much closer to beta=1 than a tokamak. Or you could pulse - but I suspect the losses would be high in pulse mode...
Why not enough data after 7 generations of experiment? No any doubts?
By the definition in competition of simulation and real experiment the last always wins.
Poloidal beta in TOKAMAKs also is close to 1. As kinetic pressure of plasma calculated as nk(Te+Ti) and n defines with that how high is the current flowing in plasma and creating the poloidal mag field namely responsible on confinement.
Toroidal field there is much stronger than poloidal and that responsible mostly on stabilizing. And yes, toroidal beta is 20-50 times less than 1.
93143 wrote:
By definition of Langmuir I have read in Arcimovich’s book, plasma is a set of oppositely charged particles in which Debye length is much smaller than linear dimension.
I believe I determined a while back that Polywell breaks the assumptions underlying Debye theory, because of the way the plasma is formed and fed - the kinetic energy of the electrons is due entirely to the applied potential and is on the same order as it, and the ion kinetic energy is similar but out of phase, caused by the electrons penetrating the plasma and forming a well. A Debye sheath in the traditional sense can't form under those circumstances.
I have mentioned Debye length only for purpose to say that if Debye length is less than dimensions, we should consider not only the single particle’s behavior in the given field but also plasma collective effects as well. Such as inevitably occurring instabilities. Regardless to creation history of plasma: injection of oppositely charged particles into a common space (in Polywell), gas discharge (gas-puff Z-pinch, etc.) or somewhat else.
93143 wrote:
But nobody has proved that it can suppress for example two-stream instability.
Rick Nebel said he had, as far as theory and the limited experiments done up to that point can be said to prove something. Of course, he didn't prove it to you... or me...
It would be very interesting to see what he did.

Thanks.

93143
Posts: 1142
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

Joseph Chikva wrote:Very well. But on which beta we talk about?
Beta=1 would be the point at which the core plasma completely excludes the applied magnetic field, becoming a diamagnetic volume. But with nominally-convex fields, knowing nothing else about the system behaviour, this description could cover a range of pressures. Bussard's description of beta=1 has it as the point at which plasma pressure squeezes the cusps almost shut (the "wiffleball" condition). Beyond that point, the cusps start to open up again and blow out.
As, as I understand, pressure at the edges initially in created only by electrons, but then by the thermalization growth ions pressure increases as well. And if it’s so, and ions’ pressure by the order of magnitude would reach to electrons’, we will have initial beta only 0.5 and not 1. And that is if device runs steadily.
The wiffleball probably doesn't form properly at beta=0.5, so if the ion pressure did increase with time without the electron pressure decreasing, you'd just have to bleed off some plasma so as to maintain the same pressure.

That's also if the current experimenters are wrong about thermalization not proceeding fast enough to destroy the specific non-Maxwellian plasma profile that supposedly makes the device work. (IIRC, annealing is supposed to slow down thermalization enough that the electron loss rate and the fusion rate are faster than the involved populations' respective thermalization rates.)

Also, it strikes me (without having done the math) that even a fully-thermalized ion population wouldn't match the electron pressure, since the ion energy is due to the potential well. You'd have to get the ion-electron collisional energy exchange rate involved, and that's a pretty slow process...
Why not enough data after 7 generations of experiment? No any doubts?
I mean there's not enough publicly-available data. I'm sure EMC2 knows more than enough about this issue to answer your questions with a reasonable level of confidence, but I don't...
By the definition in competition of simulation and real experiment the last always wins.
True, but I didn't say Tom Ligon's experiments definitely showed a particular mode of operation; I said it seems that they did. That is, it seems to me that they did.

The PXL-1 test wasn't exactly instrumented to get that sort of data. When it apparently entered the low-loss plugged-cusp mode, all Tom saw was a deep current sag. He thought the cathode had been poisoned, so he shut it down. He found out otherwise when the test article suddenly recharged the battery bank he'd been using to power it, violently destroying a fist-sized diode in the process...
It would be very interesting to see what he did.
Yes, it would.

Joseph Chikva
Posts: 2039
Joined: Sat Apr 02, 2011 4:30 am

Post by Joseph Chikva »

93143 wrote:Also, it strikes me (without having done the math) that even a fully-thermalized ion population wouldn't match the electron pressure, since the ion energy is due to the potential well. You'd have to get the ion-electron collisional energy exchange rate involved, and that's a pretty slow process...
Also without having done the math it strikes me that not ion-electron but ion-ion collisions would make more significant contribution in thermalization of ions' population.
As in each collision (scattering) ion might transfer much more momentum to another ion and scattering cross-section is very big at the edge (but rare plasma), lower in the core but have significant value and particles permanently oscillate there-here (edge-core, core-edge).
And if so, that process may be not so slow as you think.

Also we should recall that in case of significant fusion rate alphas also will take very significant part in thermalization.

Post Reply