#7 FAQ- mean free path

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Todd Massure
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Re: FAQ- mean free path

Post by Todd Massure »

We've talked a lot in this thread about mean free path regarding actual collisions of particles, which we can do some calculations and estimations based on the particle size and # of particles per unit volume.
I wonder how much we need to take into account non-neutrals, both positive and negative and even high energy electrons affecting the path of deuterons at a distance due to coulombic forces.
In reality there are very few non-neutrals and high energy free electrons relative to the number of low energy neutrals which by far make up the vast majority of particles in the fusor, however they will affect each other at a distance with the force decreasing with an inverse square relationship (good news), but non-neutral particles will increase in number per unit volume closer to the inner grid, and even a small nudge may be enough to make a deuteron miss the poissor. The effect would be greater if it happens further out from the poissor where there should be fewer non-neutrals and high energy electrons per unit volume, so maybe the overall effect is not that great, but I would be interested to hear thoughts on the subject.
DaveC
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Re: FAQ- mean free path

Post by DaveC »

This is correct that the ions will have a mutually repulsive effect that increases dramatically as they approach the exact center of the fusor. The repulsive effect is what we intend to overcome by the kinetic energy of the accelerated ions (deuterons).

There is no real likelihood that they will miss the poissor, however. Whatever the ultimate minimum diameter of this region of confluence, it IS the poissor. As the kinetic energy is increased... (higher KeV) the diameter of the possior will shrink, ultimately reaching some minimum size governed by voltage, and aberrations to the spherical focus. If one were able to observe the trajectories of all the ions going toward ths "poissor" region, one might conclude there is a great increase in ions density.

But the size of the poissor is somewhat misleading, since the ions are not synchronized in velocity and spatial coordinates. By this I mean that a snapshot of the ions, (were it possible to do) would not show a series of spherical shells but some sort of more or less continuous distribution of positions. Thus when ny given ion passes through the poissor geometric center, it will have little company. The instantaneous ion density will be low. But the time averaged ion density will appear to be a number like ion current x 6.25 x10^18 divided by the poissor volume, which is rather impressive.

The more I think about this, the more convinced I become that the there are two key hurdles to overcome: Absolute focus precision, and time synchronization. I would suggest that rather than aim sky high, we should begin work to improve these two parameters by a factor of ten each. This would, if successful, increase instantaneous particle density at the poissor, by about 4 orders of magnitude...three orders from volume reduction and one order for improved time of flight coordination.

Better focus should all concomittantly reduce energy loss at the center grid.

Dave Cooper
Todd Massure
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Re: FAQ- mean free path

Post by Todd Massure »

Well, I agree, there is definitely a higher density and energy at the poissor, and you bring up some other interesting points as well. I just wanted to point out that in our calculations and estimations of mean free path that the particles and ions don't necessarily have to strike each other to affect each other's path and kinetic energy if interacting particles / ions have net charges, I think that for a deuteron for example, this influence may be strong even at a distances greater that that of the radius of the hydrogen/deuterium electron shell. This is something that can't be figured into the classic equations or on line calculators that I have seen.
DaveC
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Re: FAQ- mean free path

Post by DaveC »

Todd - Actually the influence is NOT strong at distances equal to the classical electron shell radius. We actually do know this. The energy (potential) at that distance is about 13.6 eV. Contrast this to the 20,000 eV of the ion's energy toward as it reaches the center grid structure, and you can see that the deviation will be minimal. However, a trajectory out farther away from the center, that brings two ions close together, when they are moving rather slowly, those paths will be quite strongly altered.

Thus, as the ions reach the poissor region, they are most resistant to serious deviation from single ions. Out farther where their kinetic energy is smaller, they are more easily deflected mutual repulsion or attraction.

Some trajectory plotting programs do have the ability to account for these effects.

Dave Cooper
Justin Nichols
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Re: FAQ- mean free path

Post by Justin Nichols »

Okay, I need just a little bit of clarification on the ionization of the neutral deuterium atoms. So previously ionized atoms (now deuterons) crash into the neutrals with enough energy to usually ionize them, which creates more ionization, thus creating more fuel for fusion. But where did the first ionized atoms come from? There must be something else that creates deuterons, right? Is it just the kinetic energy of the atoms that causes electrons to just fall off? And where does that kinetic energy energy come from?

Thanks, I hope it isn't too much of a hassle to clarify this to me.
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Chris Bradley
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Re: FAQ- mean free path

Post by Chris Bradley »

Justin Nichols wrote:But where did the first ionized atoms come from?
This has been discussed ad nauseam before here. Once a free electron gets loose in an electric field, it will be accelerated and crash into the gas atoms. If the electron's KE is above a level sufficient to cause the ionisation of that gas molecule, it will ionise liberating more electrons each of which then do their own accelerating and ionising. Depending on the density and electric field gradient, this process will either cascade or will be suppressed.

Where does that first loose electron come from? Doesn't matter, and you'd never be able to find out. Background radiation, a field emission process at a sharp point, thermionic emission, just a fluctuating background electric field? Doesn't matter. Point is, you'll usually find a loose electron in a mass of gas and it only takes one to begin the cascade.
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