recirculation - acceleration

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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recirculation - acceleration

Post by Richard Hull »

The discussion of gas mixtures and such has some thinking of increasing the density of the core in the fusor by reflecting deuterons back into the core with argon gas atoms.

It must be remembered that we are electrostatically accelerating the deuterons. A deuteron decelerated and turned or reflected at the core would waste its energy in creating a fast argon neutral or a singley or multiple ionized argon ion. the deuteron could not now fall through the full gradient and be at nearly zero KE on arrival at the core.

The beauty of the fusor is that the recirculation is ideally completed over a full circuit (the gradient being more or less evenly distributed over the radius of the device.)

We are colliding pairs of high speed deuterons and not attempting to bombard a mass of core trapped deuterons. The former requires less energy per fusion per pair or, restated, the cross section for interaction increases with the former over the latter at any given voltage gradient applied to the device.

I will try and restate my fusor definitional verbage which was long ago posted on the songs BBS.....

The fusor is an inertially confined, electrostatically accelerated, deuteron collider where fusion occurs in velocity space, NOT by maxwellian heating of a plasma.

We are not bombarding a target. A straight tubed linear accelerator can do that much better, but with no benefit and higher voltage requirements.

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: recirculation - acceleration

Post by guest »

All I have to go by is the old material.
Frenckel 1-5
I guess I'll just have to build one and see.
Has 123 Bang gone by the way side also?
Is the hydrogen Thyratron just for amperage or high speed
switching?

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Re: recirculation - acceleration

Post by Richard Hull »

The hydrogen thyratron can handle huge currents (hundreds to thousands of amps). It also switches same in the sub-micro second time frame.

It is a super high current, fast switch.

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
guest

Re: recirculation - acceleration

Post by guest »

Greetings,

Does the poissor it act as a true blackbody?

One would think not due to nuclear resonances;
this might suggest measurable realtime-diagnostic
to locate band-gaps where recirculation vs power levels
fall in a preferred nuclear-mode vs radiative mode.

Is there any published information available on the
light/power vs spectral characteristics of the poissor?
guest

Re: recirculation - acceleration

Post by guest »

I differ in the sense you do have a target area - a shell
of incoming ions - thick at the center - thin else where.
the problem is the ions are still thermal in order.
One could add more order to the stream of ions so you have a
better chance of hitting something the first time around.
The one mode of operation of the original RF fusor may been that in dense ion cloud a empty space was cleaned out at low electric field strength giving ion bundles and accelerated when the field strength was highest. Could a fusor be adapted to this idea permiting a higher density operation at the cost of adding more grids.
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Re: recirculation - acceleration

Post by guest »

I am unaware of any publications dealing with the visual aspects of the central poissor region. The Fartnsworth ITT team came to a genral consensus that the visual appearance gave no operating clues.

I do now that lasers have been used to determine density within the central region and spectrophotometers have been used to determine the velocities of the deuterons entering and leaving the area due to doppler broadening.

Richard Hull
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Re: recirculation - acceleration

Post by guest »

This concept was Farnsworth original idea which saw a real life tube called the multipactor based on the idea of RF packing and nodal group accelerations.

Unfortunately, the idea was seen to not be viable at the time and after spending about 2 years on it, the thought was abandoned. This doesn't mean, of course, that someone couldn't pick up the mantle and try and make it work.

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Re: recirculation - acceleration

Post by DaveC »

What type of fusor are we discussing here? I would expect the ion gun type of fusor to have characteristics that differ significantly from the simpler spherical grid (or cage) electrode systems.

As long as the mean free path of the ions is much greater than chamber dimensions, multiple passes in and out of the cathode potential well will occur without further "real" ion currents being required. The system ion current simply oscillates between the potentials.

The actual irreducible loss ("grid loss" of a related, earlier question) is approximated by the projected area of the cathode and anode cages (2*pi*Dcage*dwire*nwires)as a ratio of the area of the potential shells they define (pi*Dcage^2).

A 1.25" diameter cathode cage of 3 rings of .020" dia wire (my design at the moment) has a ratio of (.2356 /4.9087) = 0.04799 or about 4.8% opacity. Thus I would expect about 4.8% loss or expressed another way.. the upper limit to circulating currents would be something like 20 times (~ 1/.048) the input current.

Thus the system looks somwhat like a lossy capacitor.

However, I can't at the moment think of a way to measure the circulation at any non-zero pressure. We can of course only guess at the mean free path of an accelerated ion, since its actual temperature and hence velocity enter into the MFP calculation and differ everwhere along its trajectory. Fun stuff.

An electron trajectory plotting program ( I have one) for relatively low vacuums, uses a Monte Carlo method to plot trajectories, and generally assumes the ions are static relative to the electron velocity. For electrons at kev energies and ions at room temps, this works great.
For gas ions at kev energies, the problem is not so "easy".


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Re: recirculation - acceleration

Post by Richard Hull »

We were talking about fusors in general, but I got the impression we were discussing the simple fusor working at 5-10 microns. Here the mean free path is real short, maybe a chamber radius or so, and re-circulation is just about non-existant. Only gunned systems working at 10e-4 torr or lower or special systems with re-entrant schemes can reduce losses of the ion current to collisional neutralization.

A simple wire gridded, non-gunned system can, indeed, be operated to 10e-6 with carefully designed electron emitters which are a form of defacto gunning, albeit shotgunned systems.

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: recirculation - acceleration

Post by DaveC »

Thanks Richard for the additional clarification.

I have been thinking some more about the mean free path of ions. The way it is usually calculated involves the avg velocity of the gas molecule, its effective radius and number density. Since the avg velocity derives from the mean temperature, when we are dealing with accelerated (and also acclerating) ions, they are effectively at much higher temperatures, although not in equilbrium with the remainder of neutral gas molecules. Since the velocity increases as the square root of voltage for an ion, the mean free path should increase in the same manner. Thus for the keV ions, their mean free path should be much larger than the neutral gas molecules at nearly room temp.

Then, the more dense core (poissor?) of the fusor with decelerating ions adds more fun to the whole picture.

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Re: recirculation - acceleration -How good is it really?

Post by teslapark »

To play devil's advocate here.
How much can recirculation really add to the neutron counts of most fusors, especially of the typical amateur variety? Not too much I'd think. Even if a two grid system were pumped down to real low vacuum and had alot of acceleration voltage, how important a role would ion recirculation play in the total production of fusion in the machine? It seems like recirculating ions would be "played out", so to speak after passing around a few times, falling through less and less of the potential well, lowering probability for collision at fusion energy, I could be very well wrong about this though. Now since we are colliding dueterons head on it is perfectly possible that a low energy particle could smack head on with one that came all the way from the chamber wall and still fuse, but fusion rate should always be higher if the mean energy of the particles are higher.

With this said, I'd think the thing to do would be to obviate the need for super recirculation and get a greater percentage of deuterons to fuse on the first pass. The ion gunned fusors seem to be an attempt to do this. A good ion gunned system would seem to solve alot of problems that most regular fusors have. Since you are injecting the ions, grid transparency can virtually be anything you want it to since you can direct the ion stream right through a hole. A similar affect may take place to a lesser degree when a regular fusor enters star mode and the rays provide a path for ions to follow, helping transparency and recirulation a bit. I'd suppose that the effective transparency of a grid is quite a bit less that the geometric transparency, since ions that come too close to the wire probabky curve right in and hit 'em.

The ion gunned unit of the 1960's also had another thing in common, horrendous voltages across the chamber. To me, this is the real value of a good vacuum, and not recirculation. I think one of the machines at ITT went up to about 150kV? That is way above threshold, and alot more ions are gonna fuse one the first pass, head on, or glancing. It wouldn't be too hard to find the limit how glancing the collision can be and still reach threshold at that mean drive energy.

Recirculation is a really neat phenomenon that can theoretically increase neutron count, unlike most folks on this list, I just don't see it as a big practical player, and I don't think the ITT people really did either.

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Re: recirculation - acceleration -How good is it really?

Post by Richard Hull »

Devil's advocates are always welcome as they usually make us all think more deeply on issues. Recirculation does indeed help increase fusion chances especially at lower energies and in well designed systems made to take advantage of the process.

Gunned systems and very applied high voltages always have an edge. However, the X radiation from a fusor even in the 70kv range is a lethal matter.

Short of building a pit or a cave with proper shielding, the amateur must strive to do what he can at lower voltages, This is a good challenge to get the ball rolling and the head working on better solutions within a limited framework.

Really high voltages are not even used by the big boys (Miley and others) Again, the X-ray issue.

At large voltages the neutron issue compared to the x-ray issue is like a load of wasps compared to a load of
rifle bullets in the air.

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: recirculation - acceleration

Post by quinnrisch »

Dave Cooper,

Classically the mean free path is only related to cross section and number density.

Quantum mechanically the mean free path would be related to the impact parameter which turns out to be quantized, and related only to relative angular momentum, which the case of the fusor makes scattering collisions with neighorboring ions more likely, especailly with sphereical hot cathode sources for the ions.

The mean free path is also a very odd thing to talk about in a system with a density gradient like what we have with the fusor.
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Re: recirculation - acceleration

Post by DaveC »

Thanks for the comments on mean free path.

Regarding the classical MFP, it actually depends on velocity, cross section and number density. The volumes swept by each species, times the number densities gives total swept volume from which we derive the path length between collision...ie the mean free path.

The swept volume depends on ion velocity, hence path length is governed by the temperature (velocity) of the particular species.

In the very low density gasses used in fusor experiments, where even at 5- 10 microns pressure we are expecting MFP's of some 10's of cms, thermal equilibrium is unlikely. This implies to me that ion velocities will tend to be quite different throughout the device.

And I certainly agree that considering the radial ion density gradient, MFPs of ions is probably not a very useful concept at least in the fusor center.

I don't think I follow your comments about Quantum Mechanical issue of MFP, unless you are referring to the problems of doing trajectories caused by quantized momenta. I expect at these ion energies, most QM issues are far below the gross effects of velocity and incoming trajectory.

The whole issue is no doubt quite complex.

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Re: recirculation - acceleration

Post by Richard Hull »

Dave is, of course, correct. The fusor MFP is subject to variations not seen in a cool laboratory vacuum gas and ideal gas laws. We are never doing the classic thermal ion fusion in a uniformly heated plasma in the fusor. The fusor has no real maxwellian form factor associated with bulk heater type fusion systems. However, it is just upset enough in its operation so that idealized fusion will not occur in the simplest variants. (actually no variant to date)

I would agree with Dave in that quantum mechanics is just not in play in our systems at any realistically analizable level. We are ostensibly operating an electrostatcially focused deuteron accelerator-collider and at 20-30 kv we are working on a rotten part of the cross sectional curve. But, alas, we are doing real honest to god fusion and for the power input level, exceeding anything most so-called fusion systems operated by the big boys are doing.

Some of you might say that the big boys have claimed break even for milliseconds........well, that is all slight of hand. They figure only on the input power to the plasma which is incredibly low average power for the size of the systems. If you add in every watt of energy in the support center around their device, they are still out of the ball park by a huge margin.

Again, our systems are a few parsecs short of the mark, too, but we are using a total of about 500 watts with pumps, and instruments included.

The only thing we can brag about is doing fusion with a one man staff from machinist to technician to engineer to theoretician. We are probably not proficient at any single task, but we are doing the job! Sadly, it is a pocket draining experience.

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: recirculation - acceleration

Post by ijv »

On the positive side.....
The $/fusion neutron is a couple of orders of magnitude cheaper than the big boys as well.

And to my way of thinking that just has to be a good thing!.
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Re: recirculation - acceleration

Post by quinnrisch »

My comment on the likely hood of scattering from high energy particle was this:

Since angular momentum is quantized, the smallest angualr momentum number is 1. The higher the energy of the incoming duetron the larger the impact parameter gets for angular momentum # 1. ONce the energy is high enough the impact parameter is larger than the duetrons diameter and thus NO SCATTERING OCCURS, EXCEPT FOR DIRECT IMPACTS, (head on collision). HOWEVER, angular momentum is relative and even though at higher energies NO scattering occurs, the duetrons can bump the duetron next to its self on the way into the center.

Catch my drift?
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