Advantages of the Dual Hemisphere?

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bk8509a
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Advantages of the Dual Hemisphere?

Post by bk8509a » Wed Jan 13, 2010 5:42 pm

Here's a question I've been wondering about:
What are the advantages of using the dual hemisphere conflat design? Its the predominate chamber that everyone uses. From what I can see, it allows you to use a one grid system because you can use the outer grid as the chamber itself, but its price limits you to a tiny chamber and a usually tiny viewports.

I've been discussing using a 6 way cross system that has 6 6'' CF ports. I would then put in two grids on the inside and ground the outside one. The design is in the picture below.

I also have an alternative design that attaches both grids together using a ceramic stand off and just a grounding wire to ground the outer grid.

Although I've seen other designs such as the cylindrical system, the designer still uses the outer chamber as a ground and puts a cylinder grid on the inside. Can anyone enlighten me?

-BK
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Chris Bradley
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Re: Advantages of the Dual Hemisphere?

Post by Chris Bradley » Wed Jan 13, 2010 5:52 pm

Brian Kelleher wrote:
> From what I can see, it allows you to use a one grid system because you can use the outer grid as the chamber itself, but its price limits you to a tiny chamber and a usually tiny viewports.

What's your point? The question must then be; "how much do you want to see", and "how big a chamber can your pumps efficiently cope with/how much D can you afford to stream through it"?

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Re: Advantages of the Dual Hemisphere?

Post by bk8509a » Wed Jan 13, 2010 5:59 pm

The main question is Why is the dual hemisphere design predominate?

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Doug Coulter
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Re: Advantages of the Dual Hemisphere?

Post by Doug Coulter » Wed Jan 13, 2010 6:36 pm

Hi, I'll toss in a thought or two. I'm the cylinder guy, by the way, and I like them -- no way you can "comb the hair on a sphere" and get actual spherical symmetry anyway -- ask any topologist. With the cylinder, at least the part away from end effects can be perfect (in theory). Heck, if you can't get it in theory, then why assume you can in practice? There's a chance, always, but it's kind of a slim one.

I do run the tank as the outer electrode for now, though I've done a little bit with two grids as well, but not enough to have interesting/good things to report yet, and never did simply ground the outer grid anyway -- I was using it more like a control grid in an inside out vacuum tube design, and letting ions hit the outer grid at lower energy input vs hitting the high voltage one with more wasted energy on them in those attempts, which are ongoing, but the two grid thing is much harder to do mechanically with any decent precision in this set of tests -- and metal sputtering onto any inter-grid insulating supports makes a lot of troubles unless a major design effort is put into that issue.

In any case, even with the outer grid charged to some kv negative, things do get past it and hit the chamber walls anyway, and that's pure loss. If your outer grid is really a solid sphere, you've just built a bowl in a bowl, no gain unless you are getting some other advantage that way, and indeed you may.
For example, testing with various outer to inner grid size ratios would be done this way perhaps.

We are doing one of our cylinder things in a big Tee (6" pipe, about 14" long). The vacuum system connects on the middle arm, and we just put a SS screen over that to keep the field shaped right -- it works fine for us.

Our other cylinder thing is in a side arm of a much larger tank, and it works great, some might say impossibly well, but we have good numbers and much replication to back up our claims.

Frank S posted some pix of a two grid system with the outer grid positive, and all that inside pyrex bowls to keep the ions in and to prevent making an inside out fusor between the outer grid and the tank walls. As far as I know, he never posted any actual neutron outputs from this test though. The pictures were nice anyway.

Personally, I like the big cross-type thing -- if you want a spherical E field in there (neglecting other sources of error for the moment) you can put screens over the port holes to maintain the field spherical, or make cylinders, whatever. I think the dual hemisphere is mainly due a "religion of the sphere" and some copy-catting going on, rather than any real science justification. I do know that it's pretty important for the shapes to match. We tried a nearly exact duplicate of Richard's grid here in a cylinder and the results were terrible, so bad we didn't go to the work of making a spherical outer conductor to test that way, as we were already doing better with cylinders than anyone is doing with spheres.

I like large view ports and a lot of ports to stick in diagnostic sensors myself. I can't see how anyone learns much from just making a copy of someone else's fusor, other than "things well known in the art" already. But that's me, I think there's probably some other opinions here on that.

Kind of had a chuckle over the $$$Newark in your art. Yup, good place to overpay at. I use Digikey a lot, but they are also on the high side -- but the service is great and they have the stuff, kind of like the McMaster of electronics.
Why guess when you can know? Measure!

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Re: Advantages of the Dual Hemisphere?

Post by adrian.f.h » Wed Jan 13, 2010 7:07 pm

Hi

Well I'm not actually fusing anything yet but I don't think my approach is that far away from it. My chamber is a small cross and it works fine so far but it's much too small. It's difficult to get in the inner grid and cleaning is a nightmare. So take care to get a chamber, that has somewhat bigger ports than those KF-25 on my Fusor. But even this should work.

Regardless of this, for me most of those readily build crosses are too expensive. Buying a sphere, the ports and getting this stuff welded together professionally doesn't seem to be a bad idea. Just use at least one large port for the hv-feedthrough.

Adrian

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Chris Bradley
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Re: Advantages of the Dual Hemisphere?

Post by Chris Bradley » Wed Jan 13, 2010 7:12 pm

Brian Kelleher wrote:
> The main question is Why is the dual hemisphere design predominate?

But your question implies it may not be the best solution after you've just pointed out benefits of cost and ease of use. What else would be the best solution?

...like asking why cars don't have 5 wheels.... I'm sure someone would build you a car with 5 wheels if you like.... what's your problem with the 4 wheeled type? Same here - why do you want to make the view bigger and need bigger vac pumps?

If visibility is what you want, do like me!!: viewtopic.php?f=15&t=7222#p49094

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Re: Advantages of the Dual Hemisphere?

Post by bk8509a » Wed Jan 13, 2010 7:23 pm

The reason I've brought this up is that I've found a 4 way cross just lying around. The OD of the flanges is 6", so I would need some reducers.

Adrian, How small is your chamber?

-BK
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Re: Advantages of the Dual Hemisphere?

Post by bk8509a » Wed Jan 13, 2010 7:29 pm

Doug to the rescue.

Always great advice. Really thought provoking. Although I'm super attracted to the aesthetically pleasing and interesting design of the spherical chamber I think you convinced me to use this cross that I found lying around in the department. The knife edges are fine. All it needs is some heavy duty methanol/acetone cleaning. I think I'll get a better view, save money, and save time, but I wont have that cool looking spherical design.

Still I need to think about it a bit more.

Thanks again Doug, you keep giving me prompt and valuable advice.

Doug Coulter wrote:
> Personally, I like the big cross-type thing -- if you want a spherical E field in there (neglecting other sources of error for the moment) you can put screens over the port holes to maintain the field spherical, or make cylinders, whatever. I think the dual hemisphere is mainly due a "religion of the sphere" and some copy-catting going on, rather than any real science justification. I do know that it's pretty important for the shapes to match. We tried a nearly exact duplicate of Richard's grid here in a cylinder and the results were terrible, so bad we didn't go to the work of making a spherical outer conductor to test that way, as we were already doing better with cylinders than anyone is doing with spheres.

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Re: Advantages of the Dual Hemisphere?

Post by Carl Willis » Wed Jan 13, 2010 8:22 pm

Brian,

The appeal of the hemisphere approach to most do-it-yourself users is probably cost. I have built two fusors that way. The hemispheres are sold as architectural ornaments for a few dozen bucks and they have a nearly ideal fit with standard CF bored flanges. A single equatorial flange is all that is necessary to seal a spherical chamber, whereas with pipe stock and blank flanges the complement of premade flange hardware required would double. If money is an object, the tried-and-true RB Wagner flagpole ornament approach has a lot in its favor.

Also in favor of the spherical geometry: good symmetry of the radial electric fields (may or may not be of interest to a particular builder).

Since you have a CF cross, there's no particular reason to avoid using it.

-Carl
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Re: Advantages of the Dual Hemisphere?

Post by adrian.f.h » Wed Jan 13, 2010 10:24 pm

I second Carl's answer. For me the most important reason why not using a cross like this is, that buying one and adding the reducers is that expensive.

May be there are better chamber designs regarding efficiency and such.

I don't see any reason why not using the one you got. At least as long as you don't have something better.
However you should at least check the prizes for all the stuff you still need to make this thing work and then you can still decide if building a new one is the better thing.

That's what I would do.

My chamber has an outer diameter of 7 cm (2,76 inch). That's small.....
I will beginn scrounging the parts for a bigger shell as soon as my current fusor works, although I have another chamber but this one is more suitable as a vacuum testbed.

I'm confident that I get a refill for my wallet soon.

Adrian

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