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"Emulation" of Bubble Fusion in a Farnsworth Fusor via Reversed Polarity

Posted: Sun Jan 15, 2012 9:09 pm
by Miras Absar
A couple of days back, I was taking a look at the schematics of a Farnsworth Fusor. Then, an idea came to mind.

As we all know, one of the major downfalls of the Fusor is that energy is lost when an ionized fuel comes in contact with the negative grid. This is why the Bussard Reactor (Polywell) is favored. When a fuel is ionized in the fusion chamber, the electrons stick to the positive grid (outermost) while the nuclei stick to the negative grid (innermost). Because energy is lost to the negative grid, achieving fusion is difficult, and in some cases, impossible.

The idea that came to mind was simple. Would the reverse of the grid polarities affect the odds of fusion? Instead of the innermost grid being negative and the outermost grid being positive, the innermost grid would be positive and the outermost grid would be negative. This way, the nuclei (having a positive charge) would be contained within the innermost (positive) grid. The higher the electrostatic potential (voltage), the more pressure would be put on the nuclei, getting the fuel mass compressed to the point that the nuclear force between the nuclei would overcome the polar force and achieve fusion. Immediately, there is an issue with this theory. If the innermost grid is positive, how would the nuclei (having a positive charge) get into the grid?

This is where the Bubble Fusion concept comes into play. Instead of having 2 grids, the modified Fusor would have 3. In the first stage of fusion, the first (innermost) grid is negative, the second (middle) grid is positive, the third (outermost) grid is neutral. Once the electrons have taken “residence” on the second (postive) grid, and the nuclei have taken residence on the first (negative) grid. This is when the second stage of fusion begins. The polarities are switched so that the first (innermost) grid is now positive, the second (middle) grid remains positive, and the third (outermost) grid is negative. The electrons would stay put, but the nuclei, which have settled on the first grid are now pushed inwards with tremendous force (because the first grid is now positive). This inward motion, or compression is the Bubble Fusion concept. If the electrostatic potential (voltage) is great enough, the nuclei would be pushed to the point that the nuclear force would overcome the polar force, and cause fusion.

Its kind of complex, but what do you guys think?

Re: "Emulation" of Bubble Fusion in a Farnsworth Fusor via Reversed Polarity

Posted: Tue Jan 17, 2012 10:27 am
by Steven Sesselmann

You might need to draw a diagram, showing how the particles in your polarity reversed fusor move. I fail to see how the positive nuclei can be confined within a positively charged grid. Remember there is no field gradient inside a Faraday cage.

In a fusor, the ions move around all over the place in elliptical orbits, most traveling through the grid, so as you see, there is no real confinement, just a space region within the fusor volume with a relatively higher particle density.

Pretty hard to come up with something that hasn't been tried, but keep trying ..


Re: "Emulation" of Bubble Fusion in a Farnsworth Fusor via Reversed Polarity

Posted: Tue Jan 17, 2012 8:15 pm
by Dan Tibbets
Several considerations you must persue. First familiarize yourself with
Gauss's Law. The grids do not contain the charged particles of like or opposite charges, they recapture and reverse opposite charged particles that travel beyond the radius of the grid. The charged particle inside of the grid does not see the charge on the grid due to Gauss Law. The grid is not a perfect sphere or even an irregular sphere. But it is close enough. There can be some leakage of effect depending on the frequency of the plasma (?), but the effect is apparently dominate enough even with a few wire grid to ... er ... dominate.
What you suggest is somewhat like the Elmore Tuck Watson variation on the fusor. Additional grids have been tried in multiple experiments (see the Thesis by a Univ. Mo student). Thesehfave focused on mostly trying to ... focus the charged particles so that they hit the accelerating grids less often. A positive ion is accellerated inwards by a cathode, but once inside the cathode it is not accelerated or decellerated by the grid, only the inertia of the ions (and interactions with other charged particles nearby) effect the ion till it either hits the grids, or it passes beyond it, where the process is repeated. The picture would be somewhat different if you used a needle cathode in the center, as there is no 'inside'. You can accelerate electrons towards the center or more likely near the center, or you can accelerate ions towards the center, but not both with the same electrode. Thus all of the games played with virtual electrodes, space charge effects, magnetic fields, grid focusing, etc. Thus all of the mirror machines, Loffle (sp?) bars, multiple grids, frequency variations, etc, etc. There was a group from MIT that recently finished a study that tried to address these issues They were not successful, though they have some additional ideas that they would like to pursue if they got the money. The FRC efforts by Tri Alpha, may be combining some plasma polarity (similar to a virtual electrode) to combat some of the shortcomings of magnetic containment. And of course there is the Polywell.

You always have at least two electrodes. One may be ground, or virtual, but there is always at least two in any electrostatic device. The only sort of exception might be a grid in space- the 'ground' electrode may be so far away that it's effect is minimal. Of course you then have concerns about the charge buildup (That is why an ion gun must also have a means of disposing of the electrons).
Not only is grid collisions a concern, but Coulomb scattering is a concern, because of the second electrode, some ions/ electrons will always lose energy on one or the other electrode. Shielding (or virtualizing, or avoiding) only one is not enough. The ion must make many passes/ orbits (greater than 10,000?) before losing it's energy for any chance for breakeven.

The Polywell is interesting and possibly successful (if you are a fan boy), because it accounts for all of these often competing processes, not just one or a couple of them. It fails miserably as a magnetic containment device for ions. But, due to the momentum (gyro radii) difference for electrons it is almost adequate for electrons. Thus the idea of excess electrons forming a central virtual cathode which serves to contain/ recapture the ions.

Note that the electron magnetic containment is still not good enough. Only after Bussard realized the importance of low energy cost electron recirculation outside of the magnetic containment was the goal (hopefully) reached. This would seem to solve the problems, but due to the extreme complexities of plasma physics, there may be any number of problems. As you solve one problem, you may be introducing two other problems. That is why the ongoing experiments are so fascinating (and educational). If only they were more open...

Dan Tibbets

Re: "Emulation" of Bubble Fusion in a Farnsworth Fusor via Reversed Polarity

Posted: Tue Jan 17, 2012 8:48 pm
by Dan Tibbets
And, just to throw out more excess considerations...
When you mention bubble fusion, I assume you refer to a system where the ions have a high degree of central focus/ confluence as the bubble collapses . For a single bubble that applies, but an actual bubble fusion device consists of many tiny bubbles, not one large bubble, which would be closer to a Fusor. The net concentrating effect will be washed out. The only way to compensate would to have extremely many bubbles simultaneously collapsing- each a microscopic one pass fusor in a sense.

Bubble fusion looks promising on first glance, but the devil is in the details. Ideally as the bubble collapses the fusion fuel is accelerated to useful speeds, and it concentrates into a tiny dense focus. But as the bubble collapses to smaller and smaller sizes, any inconsistency is multiplied. It is very difficult to maintain near perfect symmetry and thus focus. Gravity variations, sound waves, temperature variations, eddies, etc would all tend to deform the shrinking bubbles, and the effects become increasingly significant as the diameter decreases. Also, no mixture is absolutely homogenous, tiny variations in the chemical composition on opposite sides of the bubbble may have significant effects In the final stages, and even quantum mechanical effects may be limiting the focus. Think of imploding a plutonium bomb. At a baseball size, it is very difficult to maintain the spherical symmetry within limits. As the size decreases the the problems increase exponentially. The surface tension may may be precise enough, but only if there are no excessive external influences as mentioned above.

[EDIT] Some may think that when central focus, confluence is mentioned, it implies a near perfect focus where all of the ions pass through a commen central point. Actually, even good focus may imply a much different picture. I'm sure almost any researcher would be thrilled if he could obtain a central focus where 90% of the ions passed through a central region 10% of the radius of the machine on each pass. Because of the density squared rate of fusion (at least in beam- beam interactions) this is enough to greatly effect the outcomes.
Consider some numbers- ion passing within 10% of the machine radius results in ~ 1000X increase increase in local density(speed also needs to be considered), and thus a 1,000,000 fold increase in the fusion rate. This 'core may only make ip 1/1000th of the tolal volume of the machine, but it would account for ~ 99.9% of the fusion in this oversimplified consideration.
The Japnese expirements which showed ~ 10% to as much as 50% of the D-D fusions occuring within the radius of the central cathode grid, implies there was some confluence, but I suspect the concentrating magnitude is actually very modest in this example in part due to neutrals (it was a glow discharge, not ion injected, fusor, but still the effect was significant. Why this was not also seen with the D-He3 tests is interesting.

Dan Tibbets

Re: "Emulation" of Bubble Fusion in a Farnsworth Fusor via Reversed Polarity

Posted: Tue Jan 17, 2012 11:10 pm
by Miras Absar
Thank you for you replies. They have showed me where I have fallen short in my description of what I'm proposing. I'll prepare an image and a detailed description to better describe what I'm saying. Thanks again!