#7 FAQ- mean free path

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Richard Hull
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Re: FAQ- mean free path

Post by Richard Hull » Wed Mar 09, 2005 8:37 pm

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.

RIchard Hull
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Re: FAQ- mean free path

Post by winterhaven » Wed Mar 09, 2005 11:22 pm

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.

Todd Massure

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Re: FAQ- mean free path

Post by Frank Sanns » Thu Mar 10, 2005 3:45 am

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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Re: FAQ- mean free path

Post by winterhaven » Thu Mar 10, 2005 5:13 pm

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

Post by Richard Hull » Thu Mar 10, 2005 5:47 pm

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
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 Frank Sanns » Thu Mar 10, 2005 8:57 pm

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.

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Re: FAQ- mean free path

Post by Richard Hull » Thu Mar 10, 2005 10:02 pm

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
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 htmagic » Fri Mar 11, 2005 3:04 am

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

Post by DaveC » Fri Mar 11, 2005 6:58 am

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.

Dave Cooper

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Re: FAQ- mean free path

Post by Richard Hull » Fri Mar 11, 2005 3:53 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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