Using an inside positive grid

This forum is for other possible methods for fusion such as Sonolumenescense, Cold Fusion, CANR/LENR or accelerator fusion. It should contain all theory, discussions and even construction and URLs related to "other than fusor, fusion".
Post Reply
Garett Goodale
Posts: 5
Joined: Thu Nov 28, 2013 12:13 pm
Real name: Garett Goodale

Using an inside positive grid

Post by Garett Goodale »

Hey, I have a potentially stupid question to ask, but it has been bothering me.

I was wondering I one could charge the outer grid of a fusor (if you had one) with a positive charge, and the inner grid with a negative charge and still achieve fusion. It seems to me that this design would be more efficient than using a ground as positive (particularly if you have a multiplier where your ground is at some 3,000 volts). Additionally I would think that you could use a lower voltage (15kv or so) on both grids because the potential would still be between the two. It seems that this would have been done before, but I was unable to find information on it. Any replies would be greatly appreciated :)
User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 7:05 am
Real name:

Re: Using an inside positive grid

Post by Chris Bradley »

There has to be an earthed shell somewhere, and if you have positive charged wells inside that, then, once a discharge plasma is formed, net electrons fly off the earth shell and enter the melee. Electrons are real party-poopers. With a negative electrode only, the electrons make a single pass out to the ground shell and don't hang about too long.

You might also want to look up Farnsworth's original fusor design. The one practised here is the 'Hirsch' type, which differs to Farnsworth's original* (which used +ve electrodes) which tried to actually create a region of multipacting electrons, which was a behaviour he noticed in his TVs and which he tried to use to create a virtual -ve electrode out of electrons. This was the original concept, which he didn't get to work.

*[and also to the ETW device]
Dan Tibbets
Posts: 578
Joined: Thu Apr 17, 2008 1:29 am
Real name:

Re: Using an inside positive grid

Post by Dan Tibbets »

ETW = Elmore Tuck Watson fusor variation. This consisted of a positively charged grid near the vacuum chamber shell. Electrons are introduced outside of the grid and accelerated inward, and then circulate back and forth limited by the grid transparency and scattering interactions. It is essentially the reverse of the typical fusor. Electrons are accelerated by the grid, not the ions (provided the ions stay inside the grid). A virtual central cathode forms and this accelerates the ions. Essentially the loss picture is similar.
Magnetically shielding the anode grid may help some, but I am not aware of this being done effectively, except of course for the Polywell. One fusioneer (or more?) played with this idea, but I am unaware of any actual experiments. magnetic shielding alone is not enough for net positive energy balance, this has been demonstrated to be insufficient with the Polywell. Some other tricks need to be utilized. In the Polywell the magnetic shielding essentially improved the transparency to ~ 98% or more depending on your definitions. This compares with ~ 90% transparency for fusors. Other considerations upped the effective transperency to perhaps 99.9% . So called Wiffleball effect improves this to a claimed ~ 99.999%. In a fusor this still would be a short fall of at least a thousand fold. But density considerations that can be obtained with these electron losses pushes the ratio over the top, or at least that is the claim.

Multiple grids biased at increasing voltage would increase the final acceleration, but this can be done fairly easily with just one grid. And with multiple grids the overall grid transparency can suffer. Some have attempted multiple grids, not so much to increase acceleration, but to better guide the ions and improve the effective transparency of the grids. The results have been marginal. I am reminded of PhD work at the University of Missouri at Rollo, and some work by MIT Doctors.

EDIT- I don't think I am remembering my references correctly. In any case, some links:


http://www.enu.kz/repository/2010/AIAA-2010-6605.pdf

http://ssl.mit.edu/publications/theses/ ... chCarl.pdf

http://dspace.mit.edu/handle/1721.1/38527





Dan Tibbets
prestonbarrows
Posts: 211
Joined: Sun Jun 24, 2012 1:27 am
Real name:

Re: Using an inside positive grid

Post by prestonbarrows »

Dan Tibbets wrote:...Elmore Tuck Watson...
Are there any actual experimental examples of an Elmore-Tuck-Watson machine producing fusion? The literature often mentions it in passing (often in reference to the polywell) but I have not been able to find any solid examples of it actually working. The most I can find are patents and the original theoretical paper from 1959...
User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 7:05 am
Real name:

Re: Using an inside positive grid

Post by Chris Bradley »

None. No examples of fusion because of a 'virtual electron electrode'. The reason seems obvious to me, which is that the ions that the electrons are supposed to pull in are much heavier than the electrons, so instead the ions rip the 'virtual electron electrode' apart. I'm aware that multipacting electrons do, indeed, create an electrostatic well, but I can't see how that well survives for any significant length of time in the presence of ions that it is supposed to be accelerating.
prestonbarrows
Posts: 211
Joined: Sun Jun 24, 2012 1:27 am
Real name:

Re: Using an inside positive grid

Post by prestonbarrows »

Chris Bradley wrote:... the ions that the electrons are supposed to pull in are much heavier than the electrons...
The different masses of the particles should not affect formation of potential wells; at least not directly.

To first order, all that matters is the charge density. Basically, Poissons equation,
Image
relates charge density (rho) to electric potential (phi).

If you have a more negative electrons in a region than positive ions, there will be an electric potential that results. This acts to repel other electrons and attract ions resulting in a restoring force. So, the electrons must be pushed together actively to achieve any significant charge density (and associated potential). That is where the electrostatic accelerating grid comes in; if an electron is given more kinetic energy than the potential energy height of the well, it can make it into the core before it is reflected.

At high vacuum, there should be negligible sources of ions that would counteract the electrons' charge density. This might be difficult experimentally; too much background gas and any electron potential well is washed out by positive charges from ionized neutrals, not enough background and there is no significant fusion.

Hall effect thrusters rely on virtual cathodes. Certain magnetron systems use them too if I recall. There should not be any magic here. Again, it seems strange that they are referenced so often but never actually built...
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Using an inside positive grid

Post by Richard Hull »

I think they, ETW, are referenced often due to their early efforts, abortive or not, with IECF work, much like Lagmuir and Blodgett's much earlier diode work. Not much is really new under the sun, just modifications and different directions on past similar systems and for varying purposes. Certainly, Farnsworth went down this blind alley for about 4 years!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 7:05 am
Real name:

Re: Using an inside positive grid

Post by Chris Bradley »

prestonbarrows wrote:The different masses of the particles should not affect formation of potential wells; at least not directly.
... In theory, theory will correctly predict everything. In practice, it is a somewhat different matter.
prestonbarrows wrote:Hall effect thrusters rely on virtual cathodes. Certain magnetron systems use them too if I recall. There should not be any magic here. Again, it seems strange that they are referenced so often but never actually built...
Well, they don't work like that, that's why. You have to go study ambipolar behaviour to really get a grip on this. The virtual electrode is really a plasma at a potential that matches the equilibrium conditions within the electrostatic potentials it finds itself in. The plasmoid, the 'poissor', in the centre of the fusor is an example of a virtual negative electrode - it must be at the potential of the grid else it wouldn't be there! Folks often seem to be un-persuaded when I mention that, in actual fact, that central plasmoid is probably acting as a more critical electrode in the fusor mechanism than the grid itself. The grid, IMHO, allows the formation of the poissor, but it is the poissor that is the actual virtual electrode towards which the ions accelerate, and is why the supposed 'grid transparency' issue is a red-herring in the fusor because 'active' ions are following the paths through the grid to the -ve poissor.
John Futter
Posts: 1848
Joined: Wed Apr 21, 2004 10:29 pm
Real name: John Futter
Contact:

Re: Using an inside positive grid

Post by John Futter »

Just an aside

Hall thrusters do not have virtual cathodes
they have a virtual anode suspended between the cathodes by the magnetic field lines above the actual anode hence their other name of "anode layer source"
Another example of a cross field device like the penning gauge
Post Reply

Return to “Other Forms of Fusion - Theory, Construction, Discussion, URLs”