In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Re: Fusion reaction rates
Date: Dec 21, 10:18 pm
Poster: Pierce Nichols
On Dec 21, 10:18 pm, Pierce Nichols wrote:
>Our fusor is ultra simple and for that we trade a lot of stuff off in the quest for quick and cheap.
Yeah... I'm holding off for a year or so until I have a bit more time (as in enough to block our my weekends for this project) and a lot more money, so I can go to a higher vacuum than the experiments we're discussing here. I think that's where the real possibilities are. Requires some more sophisticated tricks tho.
>1. We are making deuterons in the simple fusor over the entire volume. Thus, only some very small fraction are are created near the walls where they can accelerate through the full potential and reach fusion energy.
This is something I think is a pretty serious problem. How much energy are we wasting this way? I'm looking at a couple of ideas for plasma generators that work primarily at the edges of the chamber and at higher vacuums. This will allow me to effectively decouple current and voltage -- I can set the current by setting the plasma generation rate. One design that I've thought of (and would all those more experienced than I please comment!) consisting of two grids at the edge and a number of electron filaments. The outer of these two grids would be grounded and the inner held at a positive potential just above the ionization potential for deuterons. The filaments would be located on the outer grid. In operation, this would cause a flow of electrons from the outer to the inner grid, where they would ionize deuterons. Careful setting of the inner grid voltage would cause most of the ionization to occur in a narrow shell near that grid. They would then be accelerated in the normal fashion.
>We might look at the fusor as a layered shell of deuteron energies. If we supply ever more voltage, then more of the chamber volume from the wall inward can now support generation of potentially fusion energetic deuterons.
That just seems so wasteful of energy.
>2. Not all of the deuterons are hitting head on. With just barely 20KV on a fusor, only head on collisions are going to do fusion. AND THEN...only those at 1/2 fusion energy...or in other words those few that were created at the outer shell volume.
Making the energy loss problems worse not better.
>3. Our mean free path is just about the radius of the chamber (3-4 inches) so, in theory, only half of the these perfectly created deuterons will make it to the center and then only a tiny fraction of those will hit head on... and then only a tiny fraction of those will hit another deuteron of equal energy (remember, they can hit head on with a deuteron created in the middle of the chamber and nothing will happen.
Which seems to me to be a call for higher vacuum, tighter ion optics, and higher voltages. With those kinds of losses, grid losses are the least of your concerns!
>In summary, We are forced to operate at elevated pressures where the density is high to assure ourselves of any fusion at all. Given a volume x of deuterons created/second, we might look at a simple equation
Unless we find another way to create plasma at low pressures, near the edge. See my earlier suggestions in this post.
>((((x/#at wall)/#surviving to center)/#hitting head-on). Or....a tiny fraction of a tiny fraction of a tiny fraction
Which is unacceptable. The solutions seem to be to improve the generation of ions and increase their lifetime. If we make our grids decently, restrict ion generation to the outer shell, and jack up the voltage, we take care of most of that. Oh, and jack up the vacuum too.
>What we can do is...
>1. use very high voltages.
>This assures us that more of the fusor volume will produce, 1/2Ek fusion, deuterions. Now we can have a deuteron hit from the wall area and one from the middle and get fusion too! It is a win, win situation. Also, the cross section improves and thus fusion increases.
But as has been pointed out to me when I was thinking along even less practical lines, that has its own set of serious problems. If you can jack up the vacuum and properly restrict the ion generation, you don't need to go as high.
>2. We can also use higher currents.
>This determines the actual number of ionizations and thus the deuterons produced. remember that 1 amp is about 10e19 deuterons/second! The difference between 1 ma and 10ma is ten times the deuteron count and thus 10 times the fusion rate.
>A tiny fraction of a tiny fraction of a tiny fraction of 10e19 can still be millions!
This is easier if you can control the number of ions available, and therefore manipulate current and voltage seperately.
>But, as Jim notes all this slides slopilly about on an ever shakey mathematical table.
This is correct. This is why seperating current and voltage control is such a win from a scientific standpoint -- you can step off that table to a certain extent.
>Also a key thing which Jim pointed out...Remember, you won't detect but a tiny fraction of the neutrons with any counter and thus you must be intimate with the neutron detector. As I have noted before many times, neutron counting is a black art.
This is a kicker -- I don't know the black art. I may end up on your doorstep Richard, begging for a dispensation of neutron clue.
>Where does this leave you and the origianl question? There is little math, that will assist you. If you compute the theoretical fusion rate t using the best math, you will find a tremndous shortfall in production. You will have to build it and try, with good instrumentation, to discover the constants for the given system. These operational constants are only found by the doing.
Of course :). This fact is a source of analysis paralysis for a lot of ppl, me included, to a certain extent.
>Finally, once you discover the neutron rate, you will also have the rate of production of Tritium gas in your system. For the rate equals the neutron rate.
To a rough order of magnitude. Remember that trits fuse almost immediately on production, yielding yet more neuts, so if you want to get a better handle on the rates, you need to solve a pretty simple system of equations with the trit fusions.
- Re: Fusion reaction rates - Richard Hull Dec 22, 2:34 am
- Re: Fusion reaction rates - Tom Ligon Dec 22, 7:02 pm
- Re: Fusion reaction rates - Pierce Nichols Dec 22, 9:58 am
- Re: Fusion reaction rates - Richard Hull Dec 22, 11:35 pm
- Using Higher voltages - Jim Lux Dec 22, 11:26 am