Farnsworth Questions

Reflections on fusion history, current events, and predictions for the 'fusion powered future.
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Richard Hull
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Re: Farnsworth Questions

Post by Richard Hull » Thu Mar 26, 2020 4:56 pm

Frank may also speak to this question.

A linac accelerator is intrinsically a drift tube in which many hollow cylinders of increasing length are aligned and fed with a high power, high frequency signal such that electrons or ions entering the drift tubes are accelerated. A concept of spheres within spheres with multiple orifices might also create the same result, but with far more energetic net beam convergence at a single point. This is a nice idea, but has yet to see any physical embodiment. Such a system was loosely looked at by the original Farnsworth team with their single internal dynode systems. The issue of importance is how many spheres within spheres are possible and or desirable due to the complexity that each must be fully isolated and insulated from the neighboring two spheres all of which require separate external connections.

Far less complex and easier to implement would be a spherical arrangement of linacs arranged to focus to a single point ala the NIF laser system. The spherical drift system is a great concept, but again, just an idea and theoretical musing. If possible to realistically implement, such a spherical system might prove an interesting construct. Multiple linacs can be made simply and inexpensively if made in quantity at fusion voltages under 100 keV. A 100 keV linac is a snap to make compared to the 5-10 meV linacs of research. An initial spherical drift system for experiment would be complex and, at the early stages of investigation, would involve far fewer spheres than found in linac drift tubes. This would allow for less costly, ($1 million) implementation, but might be a proof of concept system. That very cheap million dollar figure would not begin to pay for lab, staffing and related equipment to investigate and assemble the cheap million dollar device. Such a device would certainly never do usable fusion, but might move the per watt input versus fusion energy output several orders of magnitude forward.

The beauty of the spherical drift system would be that if you could implement this into a physical reality, the number of "effective linacs" in the system might be limited only to the number of concentric precisely aligned holes you bore into the multiple spheres!

Near microscopic ion drift tube sources at the very low power 3-5kv ion source level are currently used that can plug into an IC socket!! Most are used in small mass spectrometers.

Like Frank says 60 years have passed since the ITT effort. Modern miniaturization is amazing and some firm which might seek to use this concept in the vein of IECF might bear the expense of developing something quite compact as a proof of concept, linearly or perhaps even spherically. Anything along the original Farnsworth musing back then would have been larger and far more tough to implement requiring far more money than ITT could supply and as regards PTFA, a bridge light years beyond their grasp. As with many landing here with ideas, they have no money or the "doing" interest or ability. Ideas are a dime a dozen. Implementations of those ideas are always another matter.

Check this out

https://link.springer.com/article/10.10 ... 19-00248-w


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

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Frank Sanns
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Re: Farnsworth Questions

Post by Frank Sanns » Fri Mar 27, 2020 8:39 pm

Thanks for the info Richard.

To continue and clarify my earlier statement, there were some design elements of the Cave Fusor that were interesting. The ion gun side arm was fed through a small hole in a larger otherwise solid inner cathode. There were six such ion guns and six small holes in the inner cathode. The entire cathode was at operation voltage which in at least one photograph, was set at 62KV.

The ion gun then spewed out fusible ions (deuterium and or deuterium/tritium mixtures) which only had a small acceleration gap until it entered via the orifice into the large solid cathode. Inside the cathode, there would be no work done on the ions since there is an absence of a field inside of a conductor. The focus set outside would not be distorted by our current fusor inner grid wires. It would stay focused except for the normal defocusing that happens when you have a bunch of similar charges repelling each other.

So the Cave fusor had a single drift sphere. That is an interesting arrangement but what I had envisioned was an extension of that using multiple spherical drift chambers with acceleration going on between strategically placed ones. To further what Richard has said, a linac for example uses an RF field and a series of hollow cylinders along its axis. A charge is accelerated towards the leading edge of the cylinder by the RF field. During one cycle, the RF is negative so a positive ion would see acceleration. The the RF cycle would go into opposite phase. At this time the positive ions would be traversing the inside of the cylinder and not feel any RF potential. They are "timed" to emerge from the cylinder to just at the moment the field goes negative again. They would be accelerated to the next cylinder picking up energy as they were accelerated. Again they traverse the gap between the tubes and at the right time enter the next tube to continue along their axial path unimpeded by any external RF fields. As the ions gain energy (speed), each successive drift tube is longer and longer to shield the ions as they move through the system. This is a simplified version of what happens as both phases are actually used to push and pull the ions but it complicates the concept for this post.

What I imagined then was to take what Farnsworth had done and jump it up to a set of nested spheres that would use the same RF principle to accelerate the ions from the ion guns. This would allow for much higher collisional energies without the need for ultra high voltages to be applied. This has significant advantages for efficiencies in a fusor.

No I have not yet build this yet but it is an intriguing variation on an initial idea that was present in that cave fusor.

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Richard Hull
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Re: Farnsworth Questions

Post by Richard Hull » Fri Mar 27, 2020 10:15 pm

As Frank notes, timing is everything and a lot of careful calculations force the mechanical design say even in the three nested sphere system. The RF power would still be very high even if the applied vlotage is rather low. This is the same with a the cyclotron. You can't get something potent in output without a far superior potency on the input. Make no mistake about this. In the end, nested spheres is a great idea.

The other aspect is will we need and ion gun for every hole?? Not necessarily. Think on this aspect. See if you might noodle this statement out.

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

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Nicolas Krause
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Re: Farnsworth Questions

Post by Nicolas Krause » Tue Mar 31, 2020 3:51 pm

Hi Frank,

If I recall correctly, Paul noted that the Farnsworth cathode had a number of einzel lenses on it, but I believe he was referring to the large ports. Were the small ports for the ion streams einzel lenses as well, or were they simply passive elements? In addition are there any engineering drawings of the actual assembly of the cathode? I've been pondering the design for a bit, and have some ideas about how I might go about implementing an einzel lens on a sphere, but I'd love to see how the problem was solved originally. I'm also operating under the assumption all the einzel lenses on the cathode were operating at exactly the same voltage levels, was there ever any consideration given to the ability to "tune" different lenses via some sort of variable voltage control?

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