recirculation - acceleration

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
teslapark
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Re: recirculation - acceleration -How good is it really?

Post by teslapark » Fri Aug 03, 2001 1:07 am

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 » Fri Aug 03, 2001 3:51 am

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

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Re: recirculation - acceleration

Post by quinnrisch » Mon Sep 17, 2001 7:39 pm

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 » Tue Sep 18, 2001 7:18 am

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 » Wed Sep 19, 2001 2:21 pm

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

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Re: recirculation - acceleration

Post by ijv » Thu Sep 20, 2001 1:52 am

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 » Thu Sep 20, 2001 7:35 pm

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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