Bussard's bremsstrahlung calculation

[D Tibbets]

Once the ions get into or are created inside the Wiffle ball, they idealy never see any magnetic field. They are contained by the electrostatic field setup by the electrons. Only the electrons reach the Wiffleball border where they suddenly see the concentrated magnetic field and turn around. Because there are more electrons in the Wiffle ball, and the electrons spend most of thier time deep within the well , the ions will be confined into a smaller ball that does not reach the Wiffleball border- ie: the ions are electrostatically contained, not magnetically contained.

....
This is the only part of your post I have a problem with. You seem to have forgotten that electric fields are conservative. If the ions are electrostatically confined to never even reach the border of the wiffleball, one of three things is true:

1) The ions were formed at low energy inside the wiffleball, significantly below the magnetic boundary. It's possible to run the machine so that this is true for most (not all) of the ions, but it's probably not desirable from an energy distribution standpoint, for reasons I've already mentioned.

2) The ions have lost a substantial chunk of energy to collisions and cyclotron radiation and heating of cold electrons and such. If this is true, the distribution is probably long since thermalized and we're in big trouble.

3) The ions had negative kinetic energy (ie: imaginary velocity) when they were formed.

Note that all three of these options result in only partial utilization of the potential well, so you have to crank the voltage higher for the same result...

I agree that the ion positions at peak potential energies seems to be at or well beyond the wiffleball for many of the injected ions. I'm depending mostly on hand waving and perhaps faulty recollection that Bussard repeatedly said that the ions do not see the magnetic field. Also, did Dr Nebel suggest something similar when he said that the fusion derived alpha particles are not electrostatically confined ( due to their multi MeV speeds) and are thus contained by the magnetic field till they find a cusp after ~ 1,000 passes? Perhaps I'm being too literal. The ions may be reaching these regions, but still the electrostatic containment may be dominate, or at least more thorough.

Does the excess electrons that are slow in the center and fast in the periphery, combined with the ions that are fast in the center and slow in the periphery result in a shift in the ion top of the potential hill range from the center (major hand waving in progress)?

Failing that , the neutral gas puffer would result in more ionizations deeper in the machine since the ionization rate is time dependant. In larger machines the ionization rates could allow deeper ionizations, but because of the increased distance to the center,the area of most ionizations may be a modest percentage of the machine radius. Not mono energetic, but closer, and while keeping the majority of the ions away from the Wiffleball border.

Dan Tibbets

[93143]

I think that's probably it - the ions are supposed to be slow at the edge, so the magnetic effect doesn't really matter all that much unless something is off nominal.

We need either a simulation or some serious results. Something is wrong with the standard picture; the densities and potentials just don't seem to add up...

[TallDave]

I'd sure like to see the EMC2 simulations (someday...). I think Rick has mentioned them once or twice.

There's also the Chacon/etc. paper, which did the bounce-averaged Fokker calculation iirc.

[bcglorf]

My point is, there's ANOTHER density maximum at the edge, and it's more important in a collisional sense than the one in the centre. Not much fusion, but quite a bit of thermalization, and at a reasonably low temperature. Hence annealing.

How much certainty or probability would you put to this?

If it were the case would it not violate one of Rider's main assumptions? In particular: Other regions have low enough densities that collision-related effects are negligible there. Rider later calculates the ion thermalization rate based on that assumption. Would this significantly change that? Enough to get past his objections?

[TheRadicalModerate]

Tibbets, 93143 (or anybody else, for that matter)--

I"ve been staring at your answers for a week now, and I think I finally understand (at least until I stop understanding again...)

Does this picture make sense?



If this is right, I'd think that we'd have a hard time getting enough potential difference between the 0-velocity point for the ions and the center. On the other hand, I can see how annealing works really well--not only do you have clustering from the low speeds, but presumably high-energy ions get stuck in the B-field and lose energy through brem other ions collisions, until they get cold enough to drift back into the oscillating region.

Am I getting close?

[D Tibbets]

Tibbets, 93143 (or anybody else, for that matter)--

I"ve been staring at your answers for a week now, and I think I finally understand (at least until I stop understanding again...)

Does this picture make sense?



If this is right, I'd think that we'd have a hard time getting enough potential difference between the 0-velocity point for the ions and the center. On the other hand, I can see how annealing works really well--not only do you have clustering from the low speeds, but presumably high-energy ions get stuck in the B-field and lose energy through brem other ions collisions, until they get cold enough to drift back into the oscillating region.Am I getting close?

Link didn't work for me.

Keep in mind that alot of what I say is from a layman's perspective and my confused ramblings and the responses to them probably enlighten me more than anyone else. But, once again- I believe that brem. radiation is from lightweight high speed electrons whipping around relitively massive ions, not from the ions themselfs. I suppose ions lose energy through black body radiation, or by hitting other ions or structures. They transfer energy to the potential well as thay are climbing out (no net loss of energy). Of course the ions may pull electrons along with them and thereby homoginize the plasma (thermalize the electrons faster) but the continous resupply of monoenergetic radial electrons to replace losses of the more thermalized electrons serves to maintain the potential well within acceptable time frames, or at least that is my understanding of the basic premise.

Concerning ions reaching the Wiffleball border at the top of thier orbits/ occillations and seeing the magnetic field, these very slow speed ions would have small gyroradii and I wonder how they then escape being traped on a magnetic field line. I suppose the centrally attracting potential well must be strong enough even at these hieghts to not only reverse the inertia of the ions but also pull these ions out of the magnetic field domain and back inside the Wiffleball where they only see the electrostatic field. I don't know how dense the magnetic field lines/ strength gradient is at the Beta=1 border (Wiffleball border) but again I supose that the outbound gyroradii might be compressed relative to the inbound gyroradii so that the orbit is more elliptical than circular. I'm guessing that this might aid the ion in escaping back into the magnetic field free region thanks to the tugging of the potential well. What debye sheaths, etc. might be doing in this enviorment is confounding for me.


Dan Tibbets

[Betruger]

[D Tibbets]

Nice graphic.

Edits that I would recommend based on my understanding-
Since electrons spend most of thier time twards the center of the potential well, I would draw more of them there rather than at the Wiffleball border.
There is no electric field seen within a sphere like the magrid or the direct conversion grid so I would show sharp shoulders of the potential at these borders. At the magrid jump from zero to positive (eg +12,000 V for WB6), and from the positive voltage (drifting twards zero (?)) to the ~ -5 M volts at the conversion grid.

Also, keep in mind that the negative potential well will be ~ 80% (eg negative 10,000 volts) of the positive potential on the magrid. If you are assuming the drive energy comes from the electron guns rather than the a positively charged magrid (magrid grounded) then that part of the graph would match your drawing. In that case indicating the voltage on the electron gun (eg negative 12,000 volts) would serve.

Recirculation of the electrons through the same cusp that they escaped from is the favored model on this site. I have never heard R. Nebel chime in with his opinion. This recirculation through the same cusp is presumably due, in large part, to the positive potential on the magrid that reverses the electron and accelerates it back into the same cusp, rather than causing it to orbit the magnet in an otherwise tighter orbit. You could show this with arrows pointing both directions.

Finally to be nit picky, label the fuel sources as "gas puffers or ion guns".

Hopefully, these details would not add too much clutter to your elegant drawing.


Dan Tibbets

[Art Carlson]

Betruger, you show an inwardly directed electric field over much of the plasma volume. Why do you not expect a radial current to flow, whose divergence would quickly annihilate your excess negative charge?

[Betruger]

I'm only posting TRM's picture that failed to display in his post.

[KitemanSA]

Regarding the image, the MaGrid should be at a positive potential proportional to the well depth wanted for the species in question. Since the pB11 reaction takes (typically) ~500keV and z=+5 on the B11, this means we want a potential of about 120kV to get a well depth of about 100kV to get an energy of about 500keV. No?

I also wonder about the -5MV grid and how it will effect electron recirculation.

[TheRadicalModerate]

Betruger, thanks for re-posting. Do you know what I did wrong?

Edits that I would recommend based on my understanding-
Since electrons spend most of thier time twards the center of the potential well, I would draw more of them there rather than at the Wiffleball border.
There is no electric field seen within a sphere like the magrid or the direct conversion grid so I would show sharp shoulders of the potential at these borders. At the magrid jump from zero to positive (eg +12,000 V for WB6), and from the positive voltage (drifting twards zero (?)) to the ~ -5 M volts at the conversion grid.

I guess in real life, the electrons injected into the center are actually scraped off of the magrid so that the magrid+wiffleball is electrically neutral. I was just arbitrarily setting the magrid to ground, but it's probably clearer to have ground proportional to 1/2 the charge in the wiffleball.

Also, keep in mind that the negative potential well will be ~ 80% (eg negative 10,000 volts) of the positive potential on the magrid. If you are assuming the drive energy comes from the electron guns rather than the a positively charged magrid (magrid grounded) then that part of the graph would match your drawing. In that case indicating the voltage on the electron gun (eg negative 12,000 volts) would serve.

If I set ground to what I proposed above, then the e-gun should inject at a potential of 1/2 of the potential difference between the magrid and the wiffleball.

Recirculation of the electrons through the same cusp that they escaped from is the favored model on this site. I have never heard R. Nebel chime in with his opinion. This recirculation through the same cusp is presumably due, in large part, to the positive potential on the magrid that reverses the electron and accelerates it back into the same cusp, rather than causing it to orbit the magnet in an otherwise tighter orbit. You could show this with arrows pointing both directions.

Well, this is an area where I'm still confused, and Art's comment below also impacts this:

you show an inwardly directed electric field over much of the plasma volume. Why do you not expect a radial current to flow, whose divergence would quickly annihilate your excess negative charge?

If we've truly got the ions oscillating within the wiffle-field, rather than across it, then we're relying on some charge in the sphere as the only force that can drive the ions into the center. It makes perfect sense to me that the charge inside the sphere should be annihilated as electrons run away from each other and get stuck in the edge wiffle-field. But if that's the case, then the sphere has no potential difference across it and the ions won't oscillate.

This was why I originally thought that the ions oscillated from the edge of the magrid, through the wiffle-field, and finally to the center of the machine. That way, we know that the voltage between the wiffle-field and the magrid will drive the ions. The problem with this is that lorenz forces on the ions as they traverse the wiffle-field are going to fling them all over the place, resulting in no focus on the center of the machine.

Either way, I'm not seeing how this can work, unless there's some mechanism that's scattering electrons out of the sheath and back into the sphere. What am I missing here?

Dan, as for which cusp the electrons re-enter and their direction will fall into one of six cases:

1-2) e energy < wiffle potential: exit in either positive or negative direction, then reverse to negative or positive, respectively, and fall back.
3-4) e energy slightly > wiffle potential: exit in positive or negative direction and whip all the way back around, re-entering on the opposite cusp of whichever coil captured them.
5-6) e energy >> wiffle potential: exit in positive or negative direction and escape containment.

I'll be happy to re-post the picture as soon as somebody tells me what I did wrong the first time.

[TheRadicalModerate]

Concerning ions reaching the Wiffleball border at the top of thier orbits/ occillations and seeing the magnetic field, these very slow speed ions would have small gyroradii and I wonder how they then escape being traped on a magnetic field line.

Dan, an ion with a 0 velocity will have a 0 gyroradius and will feel no lorenz force at all. Therefore, ions that are more energetic than average will get trapped in the B-field until they lose a fair amount of their energy, then they'll fall back into the well. In other words, ions that are too energetic will anneal in the B-field.

This doesn't do anything for the ions that have less than average energy. Looks to me like they wind up slowly poisoning the well and will have to be cleared out periodically, somehow.

[MSimon]

TRM:



Without the spaces

[KitemanSA]

TMR

Your shortcut was to a page, not an image. If you use the shortcut alone, folks will be able to see the image but it won't show up directly. Like this:

http://docs.google.com/View?id=dgm57drr_84bb296d2

Looks like Google doesn't let you access the image name directly. Seems Flickr does.