A Thought Experiment on Ideal Conditions

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Dennis P Brown
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Re: A Thought Experiment on Ideal Conditions

Post by Dennis P Brown » Tue Aug 04, 2020 9:15 pm

Using deuterium ions instead of electrons makes the effort even more difficult in energy costs for MeV level ion energies. One would never use electron collisions to accelerate deuterium to such high energies. MeV level deuterium ions are generally achieved by using large accelerating electric fields - like a Van de Graaff. And the x-ray flux from any electrons in such a field would be highly dangerous.

As for your pump, if mechanical, getting down to 5 to 15 microns is possible (sorry, highly unlikely any mechanical pump can do 0.75 microns - if you can reach 5 microns you have an excellent pump.) The issue is often leaks in the system, rather than the pump. To test that, connect a vacuum gauge that can read microns to the pump inlet and measure the pump alone. That tends to give a better idea on if the pump is the issue or leaks.

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Re: A Thought Experiment on Ideal Conditions

Post by S Silvawalker » Tue Aug 04, 2020 10:17 pm

It isn't the electron- ion collisions that are the primary acceleration source that is the accelerating potential between the electron beams and the housing. Ions will be attracted towards the beams and repelled by the positively charged housing and CRT anodes. I saw experiments with pressures around 13 microns getting neutrons so 5 to 15 is reasonable. I don't expect the pump to reach its stated pressure I had to do a double take when I saw what it said it could do as well. I predict that so long as the pressure is low enough for the electron beams to be fired, this setup should be more forgiving towards pressure when compared to a regular fusor.

In an electron beam ion trap the ions are confined radially by their attraction to the electron beam through the centre and along the length of the beam by the positively charged drift tubes. These devices are operating at far higher charge densities within the beams when compared to mine but can also confine heavy elements such as uranium ions so lighter ion confinement should also be possible.

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Dennis P Brown
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Re: A Thought Experiment on Ideal Conditions

Post by Dennis P Brown » Wed Aug 05, 2020 1:58 pm

Yes - 5 to 15 microns (and higher for smaller fusors) is absolutely the ideal range. However, you have indicated you are aiming for long lived ions; and this is only possible at much higher vacuums. Ditto for electrons and them forming a trap. At 5 to 10 microns, most electrons will be scattering off other ions or electrons. To form any type of ion trap, the very high vacuum must be achieved. Ditto for 'drift tubes'. Just calculate free paths lengths for given pressures or just look them up.

The ions in a fusor are traveling at fairly high velocities so even if one has a 'negative' charged region (i.e. as a polywell was supposed to do) the amount of ions then being 'trapped' would extremely small. Plus ions that are trapped, can only fuse via tunneling and that is very difficult for slow ions. So trapping deuterium ions would more likely lower fusion rates. The ion speed does assist in getting the ions close enough that tunneling can be enhanced.

Last, when you say that the ions are accelerated by the potential between the case and an electron beam - sorry but no. First, at 5 mircons you won't have any defined electron beam (again, see free path.) The charge created by the electrons striking deuterium molecules cause a small region near the gun to be filled with positive ions so the net field in that area will be essentially neutral. That is why fusors use very powerful transformers to create the electric field to accelerate ions and not electron beams.

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Re: A Thought Experiment on Ideal Conditions

Post by S Silvawalker » Thu Aug 06, 2020 1:29 am

Firstly, I agree for future builds I will definitely need a higher vacuum pump such as a turbo or diffusion one. However, for testing with ions only traversing the diameter a few hundred times each the ions are long lived enough.

Secondly, the electron beam will be well defined as the electrons travel unscattered enough in a CRT television, at 300 microns far more than I am working at, to form a sharp image at, in the case if the televisions I used, over 25cm away from the accelerator. I had seen the negative region I predicted was not far in front of the anode so for this build the region where the beams cross is only 1 cm in front of the accelerating anodes and the square the beams trace is only 2cm to a side. This minimizes the distance needed for the electrons to travel.


Unfortunately my deuterium is taking longer than expected to arrive and I am still chasing leaks so I still have to work from theory for a bit longer.

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Re: A Thought Experiment on Ideal Conditions

Post by Richard Hull » Thu Aug 06, 2020 3:41 am

All CRTs are pumped to at least 10e-6 torr usually deeper, if possible. 10e-6 torr is .001 microns! We never use micron terms, however for deep vacuums that are in CRTs. 300 microns are terrible junk glow discharge ranges. 300 microns = .3 torr. CRTs are at .000001 torr.

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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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: A Thought Experiment on Ideal Conditions

Post by S Silvawalker » Thu Aug 06, 2020 8:17 pm

My apologies I completely misread the pressure of the tubes I was using, the mean free path with the pressure I hae is too short. My chamber can definitely work for the pressure but there is no way I'm going to be able to get there with this pump. I need to find an oil diffusion pump in the next 2 weeks which is a more difficult challenge than the actual experiment but I will find a way.

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Dennis P Brown
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Re: A Thought Experiment on Ideal Conditions

Post by Dennis P Brown » Fri Aug 07, 2020 8:16 am

Now that you are starting to understand the issues facing your experiment, I would caution you about high vacuum work: it is a very difficult techonolgy to master without proper equipment. Certainly a good vacuum system is required - hence you need to select connector types - either kf connectors (ok down to 10^-6 torr) or conflat (copper gaskets; essential for better than 10^-6 torr).) Building a quality vacuum system is essential for both achieving a leak tight system and one that can be more easily leak tested - which for high vacuum is an essential skill. This means finding good deals on e-bay or paying big $$$ on new.

A diffusion pump (DP) without a cold trap means you will be restricted to 10^-5 torr range - period; also, a dirty system. If you need 10^-6 torr or better, then a cold trap is essential (as well as a super clean and leak proof system.) Further, the vast majority of DP's available require water cooling; so that means plumbing of water lines. Air cool exist but are hard to find and need special adapters (usually must be custom made - this is true for many water cooled DP's, too. So that is a consideration in selecting units.) For these reasons (and others), most people use turbo's for high vacuum work.

Again, glad you are both experimenting and following your ideas. Good luck and do ask about specifics on DP's before buying one so you get both the right DP for your needs and a proper price. If you are going low cost, learning to use a lathe/drill press/mill are essential skills (and for undergrad physics students this use to be part of our suggested lab training - so maybe your future school will allow you access.)

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Re: A Thought Experiment on Ideal Conditions

Post by S Silvawalker » Fri Aug 07, 2020 10:51 pm

I am using all KF 50 pieces and have the system baked for 2 hours at 200°C, should be good for the diffusion pump setup. I have experience working with precise metal fabrication, I learned making art pieces and turbojet when I was younger and have a full shop to work with. I'm employed, with no tuition, and in my second year in aerospace engineering, so I have all the resources I need to pursue this the only resource I don't have at the moment is time. My deuterium gets here on the 19th so I'm moving heaven and earth to get a working diffusion pump and test the electrical before then. I am aiming for 10^-6 torr with this new setup but the next one, likely next year, will need to use a custom housing and cathode ray tube setup with as few openings as possible. This summer I'm rushed but this is definitely a long term project.

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