#7 FAQ- mean free path

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Re: FAQ- mean free path

Post by Richard Hull »

I guess I am getting the image that some believe that once an ES field is established, particles move at no energy cost.

By my thinking, charged mass is being accelerated, but, unfortunately, it has that matter part associated with it. Moving matter requires energy. Likewise, ionizing matter requires energy and for every fusion, another two ionizations must take place along with two more accelerations of matter. If not then the system starves for fuel. Regardless of idealisms, the fusor is just a big gas load resistor hung on the HV supply. Obviously, improvements can be made. RF especially if handled as a mechanical resonance of ions (multipaction) might be the lowest cost energy solution.

Question.... who will be doing this?

I would love to see an attempt made at this.

Oh, I agree Dave, all of this has been at a rolling discussion level as we just don't have hard data in hand.

Based on acutal realized fusion and the massive losses I would tend to think the average gas temps are fairly low keeping the MFP low for neutrals and colder ions allowing for more collisions in the bulk of the devices volume. If we really had 100,000 pass recirculations on fusion energy ions, or even 100 recirculations, I think the possibility of really large fusion numbers would be seen.

Regardless of recirculation, I really think the hold back is the paucity of full, fusion energy ions actually making it to the center, compared to the supply current/ion production rate.

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Re: FAQ- mean free path

Post by Todd Massure »

A few thoughts I've had as to collecting hard data about what's going on in the fusor.
perhaps we can get a good estimate of the rate of ionization / recombination by looking at the amount of photons being thrown off by the glow from recombination(cross section of lumens in the spectral range of deuterium?). I have only the vaugest ideas of how to do this at the moment but I think it is in the realm of possibility.
Once we have an estimate of the ionization rate the power required to accelerate the ions will be a function of current for a given voltage across the fusor as they are charge carriers just as electrons are. Of course losses due to Ohmic heating, collisions etc. may be hard to estimate.
Also the last idea and possibly the most useless, but the easiest to do would be to measure the average temperature of the gas in the fusor by running it up to temp. then closing off valves directly adjacent to the sphere but with a pressure guage reading on the sphere, the change in pressure between the "hot" fusor and a cooled off one should be able to be plugged into the ideal gas law to come up with a difference in temperature. Yes, yes I know it's a mix of all different energies and maybe it's useless, but maybe not.

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Re: FAQ- mean free path

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Yes, work is done by separating charge. There is an acceleration so there is a force and there is a distance so there is work. In an ideal fusor, this would be only the input work minus the work needed to ionize the neutrals which would equal the number of collisions time 13.6 eV. I have always invisioned a larger fusor where wall collisions would be negligible. As much of the input energy should be used for fusion or ionization by otherwise unproductive collisions. If the MFP is large then this would be an extra bonus because it would give the opportunity for multiple passes through the inner grid. I think there will be a unique pressure where ionization collisions and fusion would be at an optimum. It is probably close to where we operate now or a little lower in pressure.

Now for the inner drift grid thought. Late last summer, I was working on the RF work with my 1500 watt HF amplifier. I had started at 10 Mhz and was up to the mid 11 Mhz range before I fried my exciter. I was using air core transformers and tuned them with my amateur radio antenna tuner. It was a lot of work with the multiple step up transformers and I was expecting something good at around 13.5 Mhz (kind of like the 88 mph in Back to the Future). That was the number that I calculated for my fusor and the voltage that I was applying. If there was ion recycle there would have been a jump in fusion at this resonant frequency. My exciter went south before I could complete the RF frequency scan. I have not picked it back up yet for two reasons: I wanted to improve my neutron detection equipment so I could see subtle changes in fusion, and secondly, I was tired of winding air core step up transformers that were only resonant over a very narrow RF range.

I now have great neutron detecting equipment and I have somewhat solved the resonant coil problem by varying VOLTAGE and not the frequency all of the time. This effectively is the same thing even though cross section changes. If and ion is moving slower becasue it is accelerated via a lower voltage then the resonant frequency is lower. It is the opposite for higher voltages so I need no make all of the coils again. I will make a 13.5 Mhz coil and vary the voltage, this will be the equivilent of varying the frequency over more than a MHz. I have no idea why this shortcut did not occur to me the 1st time. Maybe the next time Carl visits I will be ready for the resonance/ion recyle experiment. A second set of eyes, ears, and grey matter is always a plus.

Frank Sanns
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We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: FAQ- mean free path

Post by Todd Massure »

To Clarify I was wrong in my previous post to say that the acceleration of ions was a function of current it is not directly a result of current but rather an indirect effect of the ionization produced by the current, it is the voltage potential that accelerates the ions.
In resonant RF systems the positive ions may be considered as part of the total current more than we do in a DC current fusor.
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Re: FAQ- mean free path

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Frank I look forward to you continued work here. Was the exciter solid state? If so, sorry 'bout that. With a lot of tube gear you get a nice friendly warning as the plates go white hot and if this is observed you have time to halt the effort and regroup. A pellet of silicon is pretty whimpy and unforgiving.

Todd. Your Ideas are all good and could be a place to start getting a handle on some quantitative data. Thanks.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: FAQ- mean free path

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Yes, it was a pellet of silicon that went poof. No warning. Specifically, the exceiter was a solid state Yaesu FT-100. It put out 100 watts and was modified to be able to transmit from 1.8 Mhz to 30 Mhz. It was not cheap but was portable and I used it to transmit from a few foriegn countries over the past few years. I hated to see it go south but it had served me well. I ended up selling it to another ham that put new finals in it and he is still enjoying the radio. I have a lead on a tube oscillator that I will most likely get for little or nothing and that is what I hope to use for future experiments.

The amplifier is a tube amp but no glass tubes with a view. It uses a ceramic tube that is unbelievably tiny for its output. No doubt a by-product of the military industry.

I'll keep you posted.

Frank S.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: FAQ- mean free path

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Yep........No doubt about it........them 'bubs' can take it on the chin the way no silly-con toy can. In our line of work the 'fire bottle' or 'glow fet' is a sturdy workhouse. Good space heaters this winter too! If you really kick 'em in the nads they can actually vibrate, throw molten chunks of filament and plate material and, usually, let you get to the variac or the off switch just in the nicotine. They survive, of course, only with a little more metallic waste product in the bottom of the envelope........no matter.....fire that joker up again.

I had an old engineer friend that said if you kill a tube,...................... "that's just wrong".

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ- mean free path

Post by htmagic »

Richard,

I agree with you that tubes can handle a lot of power and they are more forgiving than solid state.

But steering back to the main thread here, thank you for the link for mean free path (MFP). That brought back memories of college... Playing around with the values, by raising the pressure one lowers the MFP. So aren't we going at this the wrong way under vacuum? Sure there's a vacuum in outer space but even that isn't a true vacuum as there are all sorts of particles out there. Sonoluminescence is not creation of vacuum but pressure induced by acoustical waves that induce the matter to impact in the center.

So what am I missing here? Why do we pump down to such a low vacuum if that will increase the mean free path? Don’t we want the MFP to be as short as possible?
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Re: FAQ- mean free path

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In my earlier suggestion regarding the "mean" energy of the deuterons in the fusor. Richard has followed the trail and observed some rather surprising implications. This has raised a couple more interesting physical issues to address.

First just to restate the mean energy estimate: The 79.3% of applied KeV figure is simply the result of assuming that ions can be formed anywhere inside the fusor when KeV are applied to the center grid at internal pressures of a few microns.

The potential distribution from the central grid outward is approximately a 1/r function. This means that the energy available to accelerate an ion is roughly proportional to the radial distance from the center. The volume of a sphere is proportional to the cube of its radius times the (4/3*pi) factor. If we ask at what point do we find half the ions having a larger energy and half a lesser energy, it is the point where the sphere's volume is half its total volume. this occurs at a point 79.3% of the radius. Hence at this point we find the "mean" energy of ions in the fusor.

Now there is a big assumption, here which was mentioned before: We have assumed ions are formed in equal quantities per unit volume eveywhere in the fusor. This is obviously a simplification, but knowing almost nothing about the details of local fields, the ionization rate, the recombination rate and so on, it is at least a place to start.

I think with regard to the ion energies, this is more or less going the right direction.

Now, as to what temperature is found in the fusor, after it has been operating, we must be clear, that that the equilibrium temperature of the gas in the fusor will not be 79.3% of the ion energy, but rather that it would head upward in that direction, at some rate which depended on the actual energy exchange.

What temperature would actually be reached if the fusor operated continuously for hrs or days... who knows? But if the fusor remained physically intact, it would only be slightly "warm", in comparison to the ion temperatures.

But here's the basic point: Even at low ion energies, the neutral gas is heated...How? we have a couple of choices as to process.

(1) Indirectly via collision with hot electrodes and other parts of the shell. Since all the materials of a fusor evaporate before 1.0 eV is reached, the indirect process is not capable of heating the neturals to any significant amount.

2.) Momentum exchange between the deuteron and neutrals.
At the moment, I am a little uncomfortable about appyling the MFP formula to deuteron- deuterium collisions. If the atoms were anything other than hydrogen like... (single electron shell), the ions would have more or less the same interaction diameter as the neutrals, the MFP calculation would probably be good, and unquestioned.

But when the solitary electron is gone, the entirely unscreened nucleus is left. This will be rather agressive to find an electron, whether a free electron or one attached to a neutral. I don't know yet, whether the deuteron would have a longer or shorter MFP, (at the same temperature), than the neutral atom.

If anyone has data to suggest either way, it would great to have , here.

Hope this helps clear up any fog I created there.

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Re: FAQ- mean free path

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It is apparent that Dave continues to muse very deeply over these issues, as do I. The effort here is worth it as we need to understand the mechanism a bit better with some confidence. My recent FAQ posting in the theory section suggests the obvious ideal which we stray very far from in normal operations.

As regards MFP and pressures...... In hot fusion the Lawson criteria tends to rule. Nature has arranged things such that the universe will not gobble up its nuclear fusion fuels at a prodigious rate. This means conditions need to be correct to fuse. Even in nature, there is no such thing as efficient fusion there is only free fusion at no cost. As has been stated before, man can't do nature's fusion by her process so he attempts a work around.

We may find it impossible to achieve that workaround.

With higher pressures, you achieve the ultimate ideal..........Lots of fuel density to fuse and burn. If fusion is successful, you have a very high power denisty of fusion. At lesser and lesser pressures, the system ultimately starves for fuel and those fusions that do occur are rare even if efficiently done.

Thus far, no scenario at any pressure or any ion temperature has proven even remotely workable in our hands. Yet, the beat goes on.

I am a "high pressure guy" as regards fusion. Whispy vacuum plasmas, to me, seem the most shabby of excuses at power ready fusion. Yet, for all this, it is the one area that we have had some success in, albeit grossly inefficient.

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
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Re: FAQ- mean free path

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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.

Frank Sanns
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We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: FAQ- mean free path

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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.

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Re: FAQ- mean free path

Post by Richard Hull »

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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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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ- mean free path

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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 »

There are explosive driven plasma pinch experiments at livermore.
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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.
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Re: FAQ- mean free path

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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 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.
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Re: FAQ- mean free path

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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 »

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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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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