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

Pierre_Thourault
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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.
“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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Richard Hull
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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.

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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