Density of a plasma

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Pierre_Thourault
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Density of a plasma

Post by Pierre_Thourault » Tue Mar 24, 2020 11:37 am

Hello everyone,

I was thinking to improve the chance of fusion we can adjust 3 parameters : velocity, density and time. Lets consider that in a fusor we cannot change the time. Now we have velocity / energy / temperatures which are all linked together that we can act on by the power supply and the density which we don’t.
So I was wondering what would happen if we fill the fusor with more deuterium?

It will increase the current needed and we could make a smaller fusor since the distance where a spark occurs between the 2 electrodes is larger because of the higher density. The fusor would also be smaller and more performant because of the increase of density so what is the problem with this design ?

Peter

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Maciek Szymanski
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Re: Density of a plasma

Post by Maciek Szymanski » Tue Mar 24, 2020 1:23 pm

Not so easy. Increasing density (pressure) leads to shortening the particle mean free path. This means, that deuterons will be colliding with other deuterons and neutrals (most of the collisions) losing their energy or recombining back into neutrals. There will be practically no chance for any deuteron to be accelerated by full anode-cathode potential difference and you will get a standard glow discharge. Ions will be traveling with quite low drift speed, recombining and ionizing again in different zones of the discharge.

Here you have quite good description of the processes in the glow discharge:
https://mysite.du.edu/~jcalvert/phys/dischg.htm
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Maciek Szymanski
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Re: Density of a plasma

Post by Maciek Szymanski » Tue Mar 24, 2020 6:28 pm

And a side note: The fusors operate in pressure range in which MFP is comparable with the dimensions of the chamber. So smaller fusors can operate at higher pressures/lower voltages. It is described in the FAQ: viewtopic.php?f=24&t=12033.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Re: Density of a plasma

Post by Pierre_Thourault » Tue Mar 24, 2020 8:20 pm

Thank you for the link

( I will read it carefully probably tomorrow because I’m tired now so I’m sorry if I’m asking a question whose answer is in the link )

What is the resistance in Ohm of a plasma ? Is the Ohm law even applicable for the fusor ?

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Maciek Szymanski
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Re: Density of a plasma

Post by Maciek Szymanski » Tue Mar 24, 2020 9:46 pm

You can get the resistance of the fusor by dividing the voltage by the current. But it's not a constant. It will depend on pressure, voltage, geometry, grid temperature. The processes in the fusor are in fact very complex. By the strict definition what you have inside the fusor even is not a plasma. Plasma as described by most of the MHD theory is a mixture of ions and electrons with is globally neutral. In the fusor charges are not uniformly distributed.
There are two separate currents - electron current and ion current which are not equal. Electrons an ions are created in various regions of the chamber by various phenomena and lost due to different phenomena in other regions. There are no easy and complete models of this mess.
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Re: Density of a plasma

Post by Richard Hull » Tue Mar 24, 2020 10:02 pm

Smaller chambers retain the MFP at higher pressures, but the losses will increase per unit watt input. The increased current/wattage required and smaller chamber with associated reduced surface area will really add to the waste heat and this is being seen as a serious issue. You can't outrun the physics here, as Maciek points out. We now have to provide heat sinks, fans, and adopt a more material science selection prospective during construction, if this rush to smaller systems continues.

As I finish my own cross effort, I muse more and more on the outcome. I have always seen the value of the smaller systems. But!!! I fear they are more and more for DIY win seekers. This is due to the fact that you can get decent and easily detectable neutrons on the cheap, at lower voltages and then walk off stage with your trophy.

I have seen masters like Jon Rosenstiel hit and exceed the mega mark in his very heat-sinkable aluminum cube, and have also viewed a highly respected person in Jim Kovalchik fight a heat problem and somewhat lagging numbers in his cross, I have a little pause.

Sure, I am on a path and will continue to make my own determinations related to the cross system. My quest is not to do fusion, for I have already done that. My quest is not to make a breakthrough in fusion. I am in search of improving fusion for only one reason..... I want neutrons.....Lot of 'em!

This thread is great in that Pierre was thinking fusion based on good sound theory which, through self-direct learning, he had a good grasp. Maciek provided immediate and thoughtful realities. It is these realities that hamper not only us, but all the efforts in fusion and why it is an ever dimming dream. In the real world, where the doing is key, we are physics, funding and materials limited here at fusor.net.

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Fusion is the energy of the future....and it always will be
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Re: Density of a plasma

Post by Dennis P Brown » Thu Apr 09, 2020 2:17 pm

I just posted on this topic in the "Fusor construction" forum. I'll simply repost this main part and add a bit:

Getting a greater neutron count for identical plasma energy isn't a function of chamber size itself but rather the greater operating pressure one can then create using the identical wattage (voltage and current.) The goal people need to go after is greater deuterium pressure not just a simple smaller volume. If one can raise the chamber pressure for the same energy (voltage/current) than the neutron production will be increased.

The key in getting greater neutron production under any chamber size is getting a higher pressure (for the same plasma power.) My system (same chamber/grid) went from operating at 5 microns to 15 mircons (both had stable plasma's.) My neutron count increased by 50%. This was done simply by using a smaller ground plane system (a wire cage) within the existing fusor chamber. No small chamber build was required!

Maybe consider adding a ground cage within your original larger chamber. You'll see a significant increase in maximum neutron yield for identical powers.

My original experimental results are in this thread: viewtopic.php?f=13&t=12048

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Re: Density of a plasma

Post by Pierre_Thourault » Sat Apr 11, 2020 10:33 am

Hello Dennis,

I don’t understand why a smaller plane ground is better. Is it because of the more uniform electric field that is created ?
In your thread, you have said that increase the anode size is also a good move and the smaller the volum between the 2 electrodes the better, but then there will be more neutral D2 inside the anode. Perhaps quantum tuneling can be made with a neutral and an ion so it doesn’t matter.

Also I don’t understand how can we make the voltage stay at for instance 40kV if the resistance varies all the time ?

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Maciek Szymanski
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Re: Density of a plasma

Post by Maciek Szymanski » Sat Apr 11, 2020 8:26 pm

In a typical self-ionizing fusor you are trying to put some contradicting ideas to work:

1. You want to have the MFP long enough to accelerate your deuterons to highest possible energy before they collide with other deuteron or neutral. That means you need a LOW pressure. The rate of collisions depends on chamber size to MFP ratio.

2. But at the same time you want to ionize as much deuterium molecules as possible. For this a HIGH pressure is needed. The pressures for point 1. are much below the typical glow discharge regime. Thus you have to apply very high voltage to get any ionization. A typical glow discharge happens at pressures of 1Tr and voltages of 300V. In a typical fusor you have 0.005Tr and 30kV. This means that the ionization is very ineffective.

3. For other hand you need a tens of kilovolts to accelerate your deuterons to fusion energy (just few of them). If you apply that high voltage at higher pressure you will get not a townsend or glow discharge but an arc, which means that the resistance of the fusor will dramatically drop causing power supply overload or if you have supply powerful enough immediate annihilation of the grid.

4. There is no definite answer what is the dominant mechanism of ionization in the fusor, but it seems that all of them depend on the electrons emitted form the grid. The higher density of electrons means more chances for collision with neutral, more chances for knocking out and ionizing molecules from outer shell and higher x-ray flux density. The electron density is proportional to the current and inversely proportional to the area. So smaller fusor will have the higher electron density at the same current than a big one.

5. There are quite sound indications, that very important mechanism is deuterium desorption from the chamber walls under electron bombardment. Here smaller chamber means less deuterium deposited in the walls.

6. All the fusion in fusor (and in the Sun too) happens by tunneling. So there may be reactions between deuterons with deuterons, deuterons with neutrals and even neutrals with neutrals (a fast deuteron may recombine with a fast electron giving a fast neutral). All this reactions may occur anywhere in the chamber. There are reputable measurements showing that most of the fusion is done in the space between the grid and the shell not inside the grid. For other hand in the Hirsch article the neutron distribution scan indicates that the most of the fusion happens inside the cathode (but that was the fusor with ion guns).

There is probably some ideal size of the chamber and grid to chamber diameter ratio. An probably for the moment no theoretical indication of it. All the fusors build for the moment are so different in design, shape, grid geometry and power supply design that there is no way to compare the experimental results.
Last edited by Maciek Szymanski on Sun Apr 12, 2020 9:10 am, edited 1 time in total.
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Re: Density of a plasma

Post by Richard Hull » Sun Apr 12, 2020 4:48 am

Maciek makes all good points. This is why I stress the point of high field emission at or near the shell via local ionization perhaps through the use of small sharp points on the shell or from electron bombarded wall embedded gas molecules, (stopped fast neutrals). This could keep the amateur fusor simple, yet effective.

Sharp points will be bombarded and heated by super fast electrons quite capable of ionizing local deuterium molecules just in the wall zone where they can undergo full acceleratory effects by the central grid. Such embedded points would allow formal, physical ionization with but a single high voltage applied via high field emission. Such point need not be long, just enough to grossly disturb the local wall field area due to their sharp points disrupting the gradient.

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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