non-local effects on ion-electron energy transfer

[evaitl]

I'm not exactly a fan of neutrons, but the problems they cause have technical solutions These solutions may not satisfy you, but for p-B11 no solutions are in sight, so why obsess about it?

Different folks have different reasons. For me, the main reason is because radioactivity can't be discussed rationally with the general populace.

The current crop of greenies are obsessed with the global warming religion. I was a reactor operator when TMI and Hanoi Jane hit. The greenies back then were obsessed with nuclear power. They destroyed the industry. In the US it is almost impossible to deal with or reprocess nuclear waste. DT reactions are going to generate a lot of it. Dollars to donuts, if nuclear power comes back the old time anti-nuke greenies will see a resurgence.

Another problem with DT is that plant lifetimes and plant safety is significantly compromised.

Nuclear power plants (current fission plants) have something called the BFPL (Brittle Fracture Prevention Limit) curve. For a given temperature, there is a minimum temperature to prevent drawing a bubble in the core or main coolant pumps and a maximum pressure that avoids cracking the containment vessel. The upper BFPL limit depends on how much neutron flux the vessel has been exposed to. For some older plants, the BFPL curves have had to be reset and tested multiple times. The operators on these plants need to maintain something like a 50psi window during a cold startup/shutdown. As an ex-RO, those curves scare the hell out of me. We are playing games by trying to anneal in place old containment vessels, but it is very difficult work.

Has anybody really looked at what high-neutron flux will do to some of the proposed high-temperature superconductors? Bi, Y, Ba, Cu, Ti? Even if the radiological issues aren't horrible, doesn't the same flux that embrittles steel totally mess up the superconductivity properties of the windings? You almost certainly can't anneal those things in place.

The scaffolding holding the windings in place are going to be under tremendous stress and will quite likely be cold (there will be liquid nitrogen for the superconductors). Just what kind of material can be used for that and will also stand up to high-level neutron flux?

If you told me that the future of the human race depended not just on energy, or even fusion energy, but on p-B11 energy (a really silly idea), I still wouldn't waste time with the polywell. I'd give it a shot with ICF:

I'll certainly chime in like everybody else and say I appreciate you spending time here. If it turns out you are wrong, your field is going to explode for a few years. That should assuage any wounded feelings.

[MSimon]

Has anybody really looked at what high-neutron flux will do to some of the proposed high-temperature superconductors? Bi, Y, Ba, Cu, Ti? Even if the radiological issues aren't horrible, doesn't the same flux that embrittles steel totally mess up the superconductivity properties of the windings? You almost certainly can't anneal those things in place.

Yes. I have looked into it. MgB should withstand a total dose of 2E18/sq cm. Meaning a reasonable lifetime is around 1E18.

The rest of the SCs currently available are worse.

MgB made with three or four nines B11 should increase allowable dose by a factor of 1,000.

I have discussed it at IEC Fusion Technology.

BTW the numbers above are from memory. IECFT has the straight skinny.

[D Tibbets]

The baseline number (assuming Maxwellian distributions) for P_fusion/P_Bremsstrahlung is 0.57. ... For p-B11, to keep P_brem/P_fus < 0.5, you need P_recirc/P_fus > 42.

Let those number sink in. When I see them, I become faint of heart and go running for D-T, or solar power, or freezing in the dark - anything but trying to make a p-B11 reactor work. Some of you ex-Navy, can-do guys may feel up to the task. What I want to ask is, if you really want a shot at success, would you choose the route that is too hard by half, or the route that is too hard by a factor of 40? Why does Rick Nebel think the way to overcome the bremsstrahlung problem with p-B11 is to fiddle with the distribution function? We don't know how to do this, and it is a hard thing to do. Why not just try to live with the bremsstrahlung like a man? You could reach breakeven (in a perfect world) if you could absorb the bremsstrahlung at high temperature and convert it to electricity at 50% efficiency. Maybe you can make a sort of x-ray photvoltaic cell. At least it's easier to brainstorm ideas for using bremsstrahlung radiation than it is for digging a hole in the center of the electron energy distribution. And you only need a factor of 2 or so, not 40.

"... a hole in the center of the electron energy distribution..."
Would not the formation of a central vertual anode be equivalent to that (at least to a degree)?

Dan Tibbets

[chrismb]

We live in a universe full of stars that happily burn B11.

That's news to me. I can't see it lasting very long in a stellar environment.

[evaitl]

We live in a universe full of stars that happily burn B11.

That's news to me. I can't see it lasting very long in a stellar environment.

My point. It would burn out quickly.

[Art Carlson]

"... a hole in the center of the electron energy distribution..."
Would not the formation of a central vertual anode be equivalent to that (at least to a degree)?

Aren't you confusing physical space with velocity space? I'm not talking about a deficit of electrons in the core of the machine, I am talking about a deficit of electrons with low velocities. (More precisely, with speeds on the order of the ion speeds.)

[D Tibbets]

"... a hole in the center of the electron energy distribution..."
Would not the formation of a central vertual anode be equivalent to that (at least to a degree)?

Aren't you confusing physical space with velocity space? I'm not talking about a deficit of electrons in the core of the machine, I am talking about a deficit of electrons with low velocities. (More precisely, with speeds on the order of the ion speeds.)

Am I confused about velocity space? Well, that is a distint possibility. Am I confused over all? Well, I admit I am uncertain.

After breifly looking at Google, I didn't see much, except position space is to velocity space as the Earth- moon motion in isolation is to the Earth and Moon's orbits around the Sun. Velocity space is an expanded frame of refrence taking in more variables. I'm not sure how this would change the interaction between the Earth and Moon in isolation on the gross scale. I can see how it would modify tidal forces and perturbations. But, these would generally be insignificant effects within that limited frame of reference. The local effects of an ion and electron would not be modified by the distant fields much(?), but also, these local effect would not nessisarily change the overal behavior in 'velocity space'. It would all depend upon the realitive strength of the effects. In otherwords I'm assuming the local interactions between the ions and the electrons is minor compared to the cumulative effects of the potential well, at least over the lifetimes of the electrons. I don't know the magnitude of these local versus global effects except that they will be an energy drain if any energy is radiated away. The spherical nature of the machine otherwise adds and subtracts the ion and electron velocities symetrically- presumably like your example of a perpetual motion machine.
Despite my rambling answer I think it boils down to the realitive balance between these local and nonlocal effects. The continous dynamic injection of high energy electrons must dominate the distributions and velocities of all of the charged particles. As Rider stated this requires far too much energy input. This must be overcome by cheating or stacking the deck against Rider's rules. Enter recirculation, annealing, etc. My simple understanding of bremstrahlung radiation being controlled by cool electrons in the core may be impeaded by your arguments, but again, it is all a matter of degree.
Or, another way of looking at it - am I confused to a small or to a large degree?


Dan Tibbets

[ladajo]

The Original(TM) argument between the good doctors, Art and Rick...

Seems circular, however, Dr. Nebel seemed pretty durn sure he could beat Bremstrahlung...anyone else remember the exchange below, took me a minute to find it again...I have to wonder what they learned from WB-7 after this was posted.

One of the key points seems to be that electron and ion distributions were not what were thought and/or clear from original ideas. We can see some of that in the contracting where there are specific specifications to clarify that piece.

In any event, the stroll down memory lane...

From: COSMIC Log
http://cosmiclog.msnbc.msn.com/archive/ ... 36887.aspx

"It's fun to daydream, isn't it? And it's easy, too, as long as you don't know too much.

There's more reasons than you can shake a stick at that this won't work. For starters, you can forget about aneutronic fusion. It's not just the temperature, Bremstrahlung is almost to certain radiate more energy than you produce by fusion no matter how good your confinement is. Even if you somehow manage to get a decent power balance, for a given plasma pressure and fusion power, a p-B11 reactor would have to be about 1000 times bigger (and more expensive) than a corresponding D-T reactor.

The next thing to worry about is the electrons. The magnetic configuration has not only lines of radial field from the center to the edge, which is bad enough judging from the experience with mirror machines, it also has lines of *zero* field along which the electrons will gush out. The idea of recycling electrons lost through the cusps won't work because they will come out almost parallel to the field but hit the return cusp with a large perpendicular velocity component they picked up going around the bend.

And the ions? The device is conceived to utilize a bi-modal velocity distribution, which will be destroyed very quickly by the two-stream instability. The anisotropy of the velocity distribution is also know to be a big problem, again from experience in the mirror program.

We haven't even started to talk about energy loss to the grids, the consequences of tiny field misalignments, charge-exchange ion losses, energy coupling between electrons and ions, and whether the potential distribution envisioned is even possible at a non-trivial ion density.

Since they managed to sweet talk somebody into giving them money, let them finish and publish their results, but let's not stop looking for ways to save energy and trying to develop other, less sexy but more reliable energy sources."
Art Carlson, Munich, Germany (Sent Friday, June 13, 2008 1:17 PM)



"Just a few comments for Mr. Carlson

1. The theory says that you can beat Bremstrahlung, but it's a challenge. The key is to keep the Boron concentration low compared the proton concentration so Z isn’t too bad. You pay for it in power density, but there is an optimum which works. You also gain because the electron energies are low in the high density regions.

2. The size arguments apply for machines where confinement is limited by cross-field diffusion like Tokamaks. They don't apply for electrostatic machines.

3. The Polywell doesn't have any lines of zero field. Take a look at the original papers on the configuration. See :
Bussard R.W., FusionTechnology, Vol. 19, 273, (1991) .
or
Krall N.A., Fusion Technology. Vol. 22, 42 (1992).

Furthermore, one expects adiabatic behavior along the field lines external to the device. Thus, what goes out comes back in. Phase space scattering is small because the density is small external to the device.

4. 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.

5. The system doesn’t have grids. It has magnetically insulated coil cases to provide the electrostatic acceleration. That’s what keeps the losses tolerable.

6. The electrostatic potential well is an issue. Maintaining it depends on the detailed particle balance. The “knobs” that affect it are the electron confinement time, the ion confinement time, and the electron injection current. There are methods of controlling all of these knobs."

rnebel (Sent Friday, June 13, 2008 6:17 PM)

[chrismb]

We live in a universe full of stars that happily burn B11.

That's news to me. I can't see it lasting very long in a stellar environment.

My point. It would burn out quickly.

I don't know any stellar processes that would cause 11B to be generated, except in big neutron emitting events (that 11B would obligingly soak up).

[evaitl]

It's been a couple of decades since I last took E&M, so I have a couple of dumb questions for somebody who has taken it more recently:

1) What is the formula for the charge distribution for a charged conducting spherical solid?
2) What is the potential for the charged conducting sphere in spherical coordinates with the center as 0?

(I think in terms of classical E&M instead of folker-plank, dirac equation, and all that other cool graduate level physics stuff. I have various other graduate degrees, but I stopped at a BS in physics. )

Most of Dr. Rider's analysis has to do with quasi-neutral plasmas, so Dr. Rider has a few assumptions in section 1.2. Among these are:

1) Spatial variations of temperature and energy may be neglected in regions of significant sigma d^3x[n(x)^2)]^2
2)In calculating bremsstrahlung rates, the plasma is assumed to be quasineutral...

If electrons are rare near the center of the sphere, but that is where the ion energy is the highest, bremsstrahlung may be significantly less in a working polywell than would be predicted by the quasi-neutral plasma analysis.

To toss out the idea, I think we would need to show that bremsstrahlung losses are excessive even at a minimal bound (ion energies at the edge, but electron densities at the center) or do a computer model.

Also, I'm not sure the Spitzer rate calculation (eq 1.2 on pp25) is appropriate for the polywell. Ion kinetic isn't maxwellian, but is dependent on how far down the potential well the ion is. Can someone comment on whether the equation is correct for mono-energetic ion populations?

Another notion: Because IEC attempts to trap electrons for a fairly long time, could there be an "evaporative effect" in that higher energy electrons escape leaving a lower effective temperature for electrons? I see from eq 1.2, Pbrem seems to depend on Te^(5/2). Cooling down electrons seems to be a great way to drop the losses.

Sorry Art, I know you are tired of us optimists... I imagine that Lord Kelvin would have been annoyed at the Wright brothers if he knew what they were working on in 1902.

[D Tibbets]

The Original(TM) argument between the good doctors, Art and Rick...

Seems circular, however, Dr. Nebel seemed pretty durn sure he could beat Bremstrahlung...anyone else remember the exchange below, took me a minute to find it again...I have to wonder what they learned from WB-7 after this was posted.

One of the key points seems to be that electron and ion distributions were not what were thought and/or clear from original ideas. We can see some of that in the contracting where there are specific specifications to clarify that piece.

In any event, the stroll down memory lane...

From: COSMIC Log
http://cosmiclog.msnbc.msn.com/archive/ ... 36887.aspx

"It's fun to daydream, isn't it? And it's easy, too, as long as you don't know too much.

There's more reasons than you can shake a stick at that this won't work. For starters, you can forget about aneutronic fusion. It's not just the temperature, Bremstrahlung is almost to certain radiate more energy than you produce by fusion no matter how good your confinement is. Even if you somehow manage to get a decent power balance, for a given plasma pressure and fusion power, a p-B11 reactor would have to be about 1000 times bigger (and more expensive) than a corresponding D-T reactor.

The next thing to worry about is the electrons. The magnetic configuration has not only lines of radial field from the center to the edge, which is bad enough judging from the experience with mirror machines, it also has lines of *zero* field along which the electrons will gush out. The idea of recycling electrons lost through the cusps won't work because they will come out almost parallel to the field but hit the return cusp with a large perpendicular velocity component they picked up going around the bend.
.........

A couple of modifying arguments to you first two points:

From a theoretical viewpoint, granting your volume scaling, the volume (I assume this is what you ment) needed for P-B11 fusion in a Polywell is 1000 X larger than D-T. If you consider the diameter being 10X larger it sounds less intimidating.
This also assumes that a D-T Polywell reactor is the same as P-B11, or D-D for that matter. D-T reactors need to handle the neutron thermal loads on the magnets and walls. There has to be a complcated method for generating tritium in situ. Also, from a materials engeenering standpoint the wall heating loads may force a D-T reactor to opporate well below the theoretical capacity. These and possibly other conciderations (like steam cycle vs direct conversion) would introduce confounding factors that could significantly change the cost estimates for construction and opperation.
Comparing to D-T Tokamak's, I have heard that the energy density of the Polywell is ~ 60,000 times as high (at least in the core) compared to Tokamaks, so a P-B11 Polywell could still be smaller than a D-T burnig Tokamak at similar power outputs.
I have heard estimates that a 3 meter diameter Polywell could produce ~ 100 MW from D-D burning. A P-B11 of the same output ~1.5 times greater diameter. A D-T burning Polywell might require only a 1 meter diameter (~1/30th the volume). M. Simon and others have talked about the tolorable heat loading on the walls being just manageble with a 3 meter diameter 100 MW Polywell burning D-D. So comparing reactor sizes should start with the volume performance of a D-D Polywell. A D-T Polywell might be easier, but not significantly smaller per unit of power out (and that is without taking tritium production into account).

I believe that when an electron escapes, it is confined to the weak magnetic fields equal distance between magnets. Deviations to the sides would push against rapidly increasing magnetic fields. Also, the electric charges on the magrids are symetrical around the cusps weather they are on the faces or the corners, so the acceleration of the electrons back into the machine would be twoards the center- just like virgin electrons from low voltage electron guns.



Dan Tibbets

[Aero]

A question. Are the cusps the same size, larger or smaller for a polywell as the radius and B field strength are increased?

And, does size matter?

[D Tibbets]

That's news to me. I can't see it lasting very long in a stellar environment.

My point. It would burn out quickly.

I don't know any stellar processes that would cause 11B to be generated, except in big neutron emitting events (that 11B would obligingly soak up).

Any boron 11 in the core of a second generation star would presumably be consumed early on in the stars formation and evolution. As the core condenses the deuterium would burn first, then any trace amounts of helium 3, and then boron 11 and lithium isotopes, buryllium(?), etc. Probably all of the trace amounts of these various isotopes would be consumed before the star reached the main sequence.* These reactions might provide some heating in larger brown dwarf stars also.
Once a large star leaves the main sequence and heavier elements are being sequentially generated and burned I assume there are some reactions producing and subsequently burning various isotopes of boron and all the other products from various pathways untill the dead end of iron is reached. Production of elements and possible decay (like in nickel) from supernova is a different topic.

* In a red dwarf star- due to mixing between the core and outer layers, some of these easily fused (compared to hydrogen) trace elements could continue to contribute realitively tiny amounts of heat throughout the stars life. The exception to this is carbon 12. Because it is recycled it can contribute hugely to the larger stars' output.

Dan Tibbets

[ankovacs]

The baseline number (assuming Maxwellian distributions) for P_fusion/P_Bremsstrahlung is 0.57. ... For p-B11, to keep P_brem/P_fus < 0.5, you need P_recirc/P_fus > 42.

Let those number sink in. When I see them, I become faint of heart and go running for D-T, or solar power, or freezing in the dark - anything but trying to make a p-B11 reactor work. Some of you ex-Navy, can-do guys may feel up to the task. What I want to ask is, if you really want a shot at success, would you choose the route that is too hard by half, or the route that is too hard by a factor of 40? Why does Rick Nebel think the way to overcome the bremsstrahlung problem with p-B11 is to fiddle with the distribution function? We don't know how to do this, and it is a hard thing to do. Why not just try to live with the bremsstrahlung like a man? You could reach breakeven (in a perfect world) if you could absorb the bremsstrahlung at high temperature and convert it to electricity at 50% efficiency. Maybe you can make a sort of x-ray photvoltaic cell. At least it's easier to brainstorm ideas for using bremsstrahlung radiation than it is for digging a hole in the center of the electron energy distribution. And you only need a factor of 2 or so, not 40.

I can't see how 5eV band-gap limit in diamond-based photovoltaics could be surpassed. That is still just UV.

I suggest starting with a different picture, which would be low-power, but perhaps higher efficiency. Suppose you start with only electrons and no ions -> electrons are all on the wiffle-ball surface. As some ions are shot in, electrons are dragged in the middle in such way that the middle-dip potential is created (see Onishi paper) -> there is focusing of ions to the center. So the calculation to make is whether the positive effect of ion focusing can overcome bremsstrahlung losses.
If the answer is yes, the question is at what ion concentrations would this be still true. I suspect the weak point could be low ion concentration / machine size.

As I wrote in an earlier post, even if it turns out to be disappointing for energy production, it may be useful for space propulsion. In my view polywell is interesting mainly in finding out whether it can become one day a space propulsion engine.

[D Tibbets]

ankovacs, I think your discription of the electrons all being at the Wiffleball border is wrong. The electrons are injected straight towards
the center and bounce back and forth between the (near) center and the Wiffleball border, slowing as they approach the center and speeding up as they aproach the Wiffleball border, then turning and starting over. This is because of the mutual repulsion of the electrons that are always more prevelent than the ions (ie- not a net neutral plasma). Assuming the Wiffleball magnetic border collisions are elastic, this will continue untill scattering collisions with other electrons and ions thermalizes the distribution. The only time I would expect to see the electrons clustered near the Wiffleball border would be when all of their radial velocities have been dampened, and the ions are also clustered in this area- there is no potential well left. This would be the end stage of the charged particle distribution (I think) untill they eventually leaked out of the magrid. It is the continous injection of new electrons (and restored recycled electrons, and annealed ions) competing against this tendency that maintains the potential well for long enough and cheeply enough to allow net power- or not, depenting on interpratation and valididity of the claimed processes.

Dan Tibbets