FAQ: Obtaining that perfect sphere....

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Richard Hull
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FAQ: Obtaining that perfect sphere....

Post by Richard Hull »

This is a carry over from another forum where the topic should have been in this forum.

We have come to view the spherical fusor as the "gold standard" here probably mostly due to the logic of our perception of IECF and the central acceleratory, electrostatic focus touted.

The question arose concerning machining off the standard weldment lip found on all conflat rings often used by those seeking to build a fusor to the highest vacuum standards.

The amateur typically purchases two 304 stainless steel hemispheres and attaches each to one of two mating conflat 304 stainless steel rings. (i.e. 8 inch rings are demanded for a 6" hemisphere, the rings having a 6" hole in them.) These rings contain a convenient weld lip or band often near the center of the thickness of the conflat ring's hole or opening. This allows the hemisphere to slip into the ring neatly and set upon this lip. This lip is often the TIG weld's source of filler metal, flowing into and fusing with the hemisphere's melting edge lip at weld time. For a 6" diameter fusor, this practice will add about a 1" long straight section in the center of the assembled sphere. (An effective flat "belly band") This effective cylindrical section is exacerbated in larger fusor diameters as the conflat rings get thicker in cross section as their diameter increases.

Some people, feeling that the sphere is critical have, in the past, removed this weldment/retainer lip by machining it off and welding the hemisphere at the very bottom edge of the ring using filler metal rod such that when assembled, the spherical chamber shape will be more or less perfect. This effort creates a bit of extra machine work and a bit more daunting weld issue for the amateur.

There has been no side-by-side, controlled study to show that maintaining a perfect sphere adds one iota of extra fusion to the process in the finished device. In fact, cylinders have been used as fusion chambers to excellent advantage as well as conflat "crosses" and "Tees" to obviate the extra effort of constructing a spherical system. This is often done at a savings provided the cross and needed "blank offs" are found surplus or used. Indeed, research has shown that only between 10-20% of the fusion in a fusor actually occurs at or even near what might be referred to as the "central focus area".

Thus, for the purposes of the amateur and the fusion process, there appears to be no real penalty for NOT machining off the weldment lip on conflat rings, but using it as intended, for a weldment lip even though this will leave the finished chamber as a slight oblate. Conversely, there appears to be no quantified advantage to obtaining the perfect spherical chamber by the removal of the weld lip.

Personally, I feel that for an amateur seeking to make a spherical fusor, doing all the machining and welding himself, leaving and using the weld lip, as intended, will save a lot of time and possible heartache. For those paying to have a chamber assembled, removing the lip by an extra maching step and requiring the more complicated weld might add to the cost of the finished item to little or no demonstrable advantage.

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: Obtaining that perfect sphere....

Post by DaveC »

Good thoughts, Richard.

To add a bit more to the issue of field uniformity, one can estimate the effects of local aberrations in geometry, using the concept of treating such geometry variations as introducing small regions havaing a radius of curvature equal to the size of the feature.

Different radii on an electrode's surface will have a 1/R effect on the potential distribution. (R is the local radius of the defect or geometry variation.) This means that at a distance of 10 times the radius of the local "aberation", the potential distributions are distorted from their general shape by only 10%. Etc. and etc. for other sizes.

Since all geometries can be approximated by an assortment of curves of appropriate radius, (even straight surfaces...) the concept works for nearly everything. It should then be clear that small features will not have powerful effects at reasonable distances away from the feature.

Thus, as Richard has suggested, minor variations in the interior shape of the spherical vacuum container for the IEC will have small effects on the "sphericity" of its electrical field. Adding to this relaxed effect is the fact that the spherical outer shell is at ground potential, and local enhancements of a low potential, will result in very small steering effects on particles.

Of much greater importance, however is the effects of the inner grid structure. Since it generally is composed of small diameter wires in some sort of more or less regular intersecting ring geometry, there are many local regions of electric field enhancement, which because of the high potentials on the grid itself, have the capability to steer electrons and ions more strongly.

But, all this being said, whatever the shape of the inner electrode, it will have an effective "focus", somewhere within. This can be seen by the shapes of glowing regions, in many of the great "image-de jour" photos on file.

As with all types of focus issues , the quality of the "focus" depends on bringing all the incoming rays, (or ions, electrons, etc). A well executed design addresses each of the various effects one, by one to refine the final result.

Simulations that I have done using Simion verify these conclusions.

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Re: FAQ: Obtaining that perfect sphere....

Post by Richard Hull »

As Dave notes, the inner grid geometry is rather important and is easy to control by careful design, assembly and placement. The observation of star mode is a reasonalbe indicator of how well this is done.

Still, we are led by experiments at the universities to believe that even the best efforts in geometry control limit the fusion in the vessel to being produced for the most part outside any real or idealized electrostatic focus point. This doesn't mean sloppy is OK, but more that hyper-precision might not be justified or show up in returns.

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: Obtaining that perfect sphere....

Post by DaveC »

I certainly agree with that, 100 %. That some studies indicate a significant amount of fusion occurs outside of the central "focus", supports Richard's point that the quality of focus within a spherical fusor may have a somewhat limited benefit.

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Re: FAQ: Obtaining that perfect sphere....

Post by swiederhold »

When constructing my chambers, I machine the Conflat flanges from virgin stainless stock. This allows me to dimension the weld lip to accommodate for the hemisphere. For a 6” sphere, I trim the hemisphere’s by .250”.

The need for a more “perfect” sphere really only seems to come into play when dealing with port alignment, and then only in situations where ideal port alignment is beneficial. A fusor related example would be plasma density measurement using laser interferometry.

I agree, though, that the need for a perfect sphere is not necessary for most amateur applications.

Scott Wiederhold
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Re: FAQ: Obtaining that perfect sphere....

Post by Richard Hull »

Great to hear that you are doing all the scratch machining. That certainly will allow for a perfect spherical fitup.

If I were doing it I would make a snug, near interference fit on the hemisphere in the ring and then machine out a .100" square channel in the face of the ring about .100 inch in from the lip to supply the weldment material.

There would be many ways to skin this cat, of course. The process would depend on how anal one would get in the attainmnet of a spherical shape in the final assembly.

Some of the hemispheres that amateurs obtain are hydro-formed and may have "lip issues such that flawless spherical assemblies would not be possible without filler weld material and careful machining at that weld joint that is quite meticulous.

Joe Zambelli came the closest to a perfect spherical system that I personally have examined. He burned off a lot of hours doing it, his dad noted.

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
David Nagy
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Re: FAQ: Obtaining that perfect sphere....

Post by David Nagy »

For my teams fusor we are getting help from Dartmouth colleges engineering department so if we had access to all their equipment would it be worth it to make a spherical chamber?
In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin; and this is a knowledge which they cannot lose.

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Richard Hull
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Re: FAQ: Obtaining that perfect sphere....

Post by Richard Hull »

FAQs are stand alones and are not really discussion forums. you should have posted this in the construction forum and not in its FAQ forum.

As to your Question..... Is it worth it to make a spherical fusor?......I do not know. Do you like the spherical form? It is not demanded. Your choice.

Ricahrd 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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