#7 FAQ- mean free path

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Frank Sanns
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Re: FAQ- mean free path

Post by Frank Sanns » Fri Mar 11, 2005 8:42 pm

High pressure is a point often overlooked on this forum. I think it is worth stating that there are two ways to potentially get fusion. One is the rarified plasmas that exist at 0.015 torr and the other is the high temperature plasmas that can exist from around 1 torr up to atmospheric pressure.

The mean free path goes down with pressure but up with temperature. Higher pressures give orders of magnitude more atomic density for fusion to occur. It should be entirely possible to do fusion with a small gap (ie <1 inch) between electrodes. Once an arc is struck the temperature of the plasma will alow the arc to be pulled out to some distance that fusion potentials could be applied and fusion should result. I have tried a few times to run higher pressures in my fusor but you really nead high current to keep the arc going. My electrodes are too far apart and they would melt under such conditions. It would be a good experiment if somebody has a higher power supply and some high temperature electrodes. A carbon arc setup would be ideal as it can maintain a constant current by automatically moving the electrodes in an out to control the power.

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Re: FAQ- mean free path

Post by DaveC » Wed Mar 16, 2005 3:03 am

Frank - I think your point about higher pressure (or gas density) is valid and well taken. We have speculated in earlier discussions on this subject, that if the density of neutrals and/or ions was higher, all other things being equal, the likelihood of fusion would increase.

I have thought about the point you make here regarding getting an arc going in a higher pressure deuterium atmosphere, and then somehow pumping energy into the arc to get the temperature to the fusing range. (I think I have paraphrased more or less correctly).

What I see as a hurdle to making this work, is the enormous and growing conductivity of the arc as it has more energy pumped into it. The voltage drop across the arc has to reach the KeV levels before there is sufficient energy to consider fusion.

But as the arc heats, more and more ions are formed and the electrical conductivity increases enormously. This was the downfall of the old ohmic heating schemes for some the magnetic pinch/tokamak/etc. devices if memory serves correctly, here.

For a given current density in the plasma, as the temperature increases, the conductivity increases, and the resistance decreases.. lowering the power absorbed by the plasma... since that is something like I^2*R. As R decreases, the plasma absorbs less and reaches some maximum temperature.

But... if we were to consider a deuteron (ion) beam to collide with a higher pressure neutral deuterium gas, then the collision probability is more less proportional to absolute pressure and thus more fusion event should occur... it says here in the fine print.

I am thinking of a thin window to separate "high pressure" still below atmosphere, region from a low pressure region through which the ions first fly. After acquiring the needed kinetic energy, they pass through the "window" into the denser gas region and collide till they stop. In a one atmosphere gas this would take a few inches, and proportionately longer at reduced pressures. This would not be a recirculating device, but would have energetic ion-neutral collisions.

Building this would not actually too tough.

More later, comments welcome, of course.

Dave Cooper

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Re: FAQ- mean free path

Post by Richard Hull » Wed Mar 16, 2005 2:44 pm

Indeed, we have discussed the high pressure issues in other posts.

Dave's comments about ohmic heating are noted to have lead down a blind alley back in the 50's and 60's fusion efforts.

We seem to just not have the "right stuff" to do fusion to advantage. Of course, this won't stop us from musing and hopefully experimenting.

Fusion via the sneak around of pulse fusion is functional, but just fempto-H bomb technology. As such, any useful pulsed energy scenario so savages the material science that containers fail instead of fusion efforts.

I am still planning, at some point, on assembling the little 1 inch heavy water filled water arc system mentioned elsewhere.

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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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Re: FAQ- mean free path

Post by winterhaven » Wed Mar 16, 2005 8:16 pm

In response to Dave's last post, I've had some ideas about using reflected resonant sound waves in a fusor where the acceleration of particles could take place in the rarefied (low pressure) areas and slam into the wall of high compression in an effort to get the best of both worlds. I was thinking that it also might be possible to have a resonant voltage to so that the wall of compression might gain a potential relative to the inner grid so that the particles could be accelerated over a shorter distance. My original idea was more of a disk shape than a spherical so that the inner grid would actually be a flush electrode which would not interfere with the sound waves, or a single wire with a high magnetic field to shield it. Not sure how the compression wave would be created.

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Re: FAQ- mean free path

Post by Edward Miller » Thu Mar 17, 2005 7:12 am

There are explosive driven plasma pinch experiments at livermore.

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Re: FAQ- mean free path

Post by winterhaven » Fri Mar 18, 2005 8:44 pm

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.

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Re: FAQ- mean free path

Post by DaveC » Sun Mar 20, 2005 8:36 am

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.

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Re: FAQ- mean free path

Post by winterhaven » Mon Mar 21, 2005 9:48 pm

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.

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Re: FAQ- mean free path

Post by DaveC » Tue Mar 22, 2005 8:18 am

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.

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Re: FAQ- mean free path

Post by Justin Nichols » Wed May 21, 2014 10:24 pm

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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