String-in-tube fusor design

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Alexey Khrushchev
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String-in-tube fusor design

Post by Alexey Khrushchev »

Let me state my idea in thesis:

1. A tungsten wire cathode is fixed in along the center axis of the tube.
2. An internal insulator separates the cathode from all sharp corners of the cross to avoid arcing.
3. The end of the wire is protected from electrical breakdown by a metal sphere or insulator.

Why it is necessary:
1. The high transparency of the cathode for deuterons should ensure a high frequency of nuclei collisions with minimal heating of the cathode.
2. By precise positioning of the wire, a uniform electric field can be achieved. Of course perfect is impossible and the current will find the point of least resistance. I need to learn how to work with the FEMM program.
3. The effective volume of the chamber can be easily increased by adding KF50 tubes.

I will be glad to hear your thoughts on this idea.
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Dennis P Brown
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Re: String-in-tube fusor design

Post by Dennis P Brown »

The issue of loss of ionized deuterons by a cathode - which certainly occurs, is not much of a driver for reduction of a fusor's performance - as I've read by others here. Of course, a cathode free fusor was developed and didn't achieved enough results as to be pursued by professional funding organizations (through it did achieve some interesting results.)

The fusor's cathode as you know, dissipates a great deal of energy as heat. I wouldn't think a thin metal rod would last very long due to increased area to volume compared to a conventual design. Also, and significantly, heat would be an extreme issue causing it to bend and make contact with the anode (of course, vertical mounting would fix that issue.) Another issue would be sputtering of a long cathode. That arrangement would experience a lot of sputtering that could quickly erode that wire/rod.

However, as we say here - talk is easy; so build it, test it, and report the results.
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Liam David
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Re: String-in-tube fusor design

Post by Liam David »

A few issues:

Breakdown will be tough, i.e. you will need high pressure, since you're not capitalizing on the hollow cathode effect like most fusors. The "high transparency" of the cathode will not be true due to the high collisionality of the system.

The wire will get extremely hot so beam-target fusion will be nonexistent.

There will be no diverging stream of fast neutrals so there will be no beam-target fusion at the wall.

The hot wire combined with its small radius will cause field and thermionic electron emission to vastly exceed ion current.

You might have significant insulator charge-up issues in the cross since it will still be the ideal breakdown volume (Paschen's Law) and the secondary electron coefficient of insulators can be very high.
Alexey Khrushchev
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Re: String-in-tube fusor design

Post by Alexey Khrushchev »

In fact, the surface area of the cathode with the classical sphere shape is the same as in the proposed variant. In a 5x25 cm tube, the active part of the cathode will be about 20 cm. In a sphere of the same volume (490 cm3), a geodesic cathode of 3 rings with a diameter of 20mm (20% of the sphere diameter of 10cm) will have a total wire length of 18cm. In this regard, I can't really understand why a straight wire will heat up more than a ring cathode. If this is indeed the case, then of course electron emission is a problem. Liam, you mentioned the hollow cathode effect. Can you explain in a nutshell what it is? Just an increased electric field strength in the center of the ring?
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Richard Hull
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Re: String-in-tube fusor design

Post by Richard Hull »

Looks more like a reverse connected GM, D2 gas filled tube. Bizzare!

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Liam David
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Re: String-in-tube fusor design

Post by Liam David »

I should have worded things more carefully: both a typical fusor with a wire cage and your single wire design will get very hot, and so cathode beam-target is nil and thermionic emission will be high. In other words, the problem isn't unique to your design. The hollowness and channels with a hollow wire grid will likely allow for much higher fusion rates for a given power input, so you'll likely have to push more power in for equal performance. But hey, I'm all for you doing the experiment and reporting your results. I hope I'm wrong.

The hollow cathode effect occurs when there are holes or pockets in a cathode. Geometry is very important. Due to a variety of factors like pendular electrons, photoionization, and sheath effects, ionization is greatly enhanced allowing for operation at reduced pressures. The Wikipedia page has limited information and I would suggest reviewing the published literature for most info. Normal gridded fusors, as well as cube fusors, act as hollow cathodes. You can also find them in ion guns and the filaments of fluorescent lights.
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Richard Hull
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Re: String-in-tube fusor design

Post by Richard Hull »

To Liam's point...Good ole Doug Coulter (RIP) ran a longish, hollow cylindrical grid in a long tank (relative) and did great! Hollow grid in a longish, hollow tube.

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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
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Liam David
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Re: String-in-tube fusor design

Post by Liam David »

Doug's graphite and tungsten cathodes were the inspiration for my own, and were what I first did fusion with. I wish I'd talked to him more, he really was a unique guy.

There are many, many published articles describing cylindrical cathode "cages" of all shapes and sizes. They act somewhat like a line source in some cases. I'd encourage you to read through some of them as they may be more in line with what you're looking for.
Alexander Ziemecki
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Re: String-in-tube fusor design

Post by Alexander Ziemecki »

Hello Liam, can you explain what you mean by "cathode beam-target is nil"? My understanding was that ion collisions with the cathode account for the majority of fusion events. Are you saying that the thermionic emissions will result in higher charge-exchange events than normal, thus neutralizing the bottom of the well?

Alexey: You may wish to inject your gas at an angle and with sufficient velocity to achieve an orbit around your cathode, like a satellite. Might help with confinement time problems.
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Liam David
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Re: String-in-tube fusor design

Post by Liam David »

The cathode beam-target fusion rate is dependent on a few things: ion energy, ion flux, and deuterium target density in the cathode. I will assume constant energy and flux and ignore any feedback effects like thermionic electron emission that change with temperature. The deuterium target density goes down with temperature. I would encourage everyone to look at the disassociation pressure of various deuterium-absorbing materials as a function of deuterium/lattice ratio and temperature. For example, from https://link.springer.com/article/10.1007/BF02868888 and https://doi.org/10.1021/acs.jpcc.1c08635:

Screenshot 2024-02-05 185709.png
Screenshot 2024-02-05 190038.png

If your cathode gets hot, much less glowing, the target density of titanium will decrease significantly. At 500C, for example, the equilibrium pressure is ~2 kPa = 15 torr, which is much higher than your chamber pressure. Beam-target fusion will hence cease. For this reason, and to reduce thermionic emission, keep your cathode cold.

Injecting gas so that it's "orbiting" around the cathode will not help things given the high collisionality of the system, especially at the pressures needed to sustain a discharge without a hollow cathode.
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Re: String-in-tube fusor design

Post by Alexey Khrushchev »

Alexander, at a pressure of about 10 microns, the gas does not behave like a liquid, but as independent molecules because the intermolecular interaction is negligible. So I doubt that it would be possible to spin the gas by turbulent mechanism.

Liam, I did not mention the beam-on-target method in this fusor design, moreover I think a hollow ring cathode would be better suited for this method. Correct me if I'm wrong, but I thought the titanium target was placed near the anode (chamber wall) and the plasma beam coming out of the cathode hits the target.

About cathode heating in the proposed fuser design. You assumed that more voltage/current would need to be applied to the cathode to achieve the same performance as in the spherical design. The reason for this is the lower local field strength in a linear cathode, since the charge is ideally evenly distributed along the entire length of the rod. This seems like a very logical assumption to me. On the other hand, it may be possible to operate at higher voltages without arcing, or to increase the pressure in the chamber. In both cases this would probably result in higher n/s/mA.
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Dennis P Brown
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Re: String-in-tube fusor design

Post by Dennis P Brown »

Liam, my take from your post is that one does not want their cathode to get hot - least of all, glowing hot. Is this due to enhanced absorption of D2 into the metal? Also, would a more uniform field an advantage or something to avoid? Much as a cylindrical cathode does via focus plasma spikes.
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Liam David
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Re: String-in-tube fusor design

Post by Liam David »

Titanium anodes help with most glow discharge fusors because they have fast neutrals and the rare negative ion streaming from cathode to anode. I don't think you'll see many, if any such particles with a wire cathode since the main acceleration mechanisms are absent. There's no symmetric hollow structure for fast ions that charge exchange to miss and impact the opposite wall, and there's no virtual anode in a hollow cathode.

My claim that the wire cathode fusion rate will be less for a given input voltage/current is based on a variety of factors, but none of them are the distribution of charge on the cathode. Alexey, I think you misunderstand electrostatics: the voltage on the cathode will be constant on the whole surface, regardless of its shape. A longer wire cathode will not have a lower field strength than a shorter wire cathode since the former's capacitance and hence charge is higher. In a vacuum without plasma, this amounts to solving Poisson's equation with constant voltage boundary conditions. For a given wire diameter and chamber diameter, the electric field on the surface of the wire will be invariant to the wire length (ignoring end effects).

Yes, higher voltage will always be better for fusion rates. I do not think you will be able to sustain higher voltages than a spherical fusor due to field emission. A higher pressure typically helps only by allowing higher discharge currents for a given voltage. If the current and voltage are kept constant and the pressure is increased (which glow discharge fusors can't really do), a variety of effects may increase or decrease fusion rates, e.g. gas target density, mean ion energy, sheath effects, etc.

Dennis: Yes, you want your cathode to stay as cold as possible to limit thermionic current (and hence increase the ion to electron current ratio) and to increase deuterium absorption into the metal. In fact, there are two deuterium loading effects in play: diffusive absorption and beam-loading. Either way, high temperatures will cause embedded deuterium loss. I don't quite understand what you're asking about the field uniformity. To answer what I think you're asking, having a given plasma beam impact a larger surface will reduce the surface temperature and so improve fusion rates at the cost of having a distributed rather than a point-like source.
Last edited by Liam David on Tue Feb 06, 2024 11:04 am, edited 1 time in total.
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Dennis P Brown
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Re: String-in-tube fusor design

Post by Dennis P Brown »

Liam, your explanations are extremely appreciated! :)
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