FAQ - Feedthroughs - What are they?... What stress is on them?

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Richard Hull
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FAQ - Feedthroughs - What are they?... What stress is on them?

Post by Richard Hull »

A feedthrough is classically, as used in the fusor, a piece of ceramic material. This is most often porcelin or beryllium oxide. Contained within a classic vacuum rated feedthrough is a large, electrical and thermally conductive terminal. This terminal is often as large as 1/4" in diameter and most often of pure copper. This terminal is sealed such that electrical energy can pass from atmospheric conditions to a vacuum where the electrical energy is transfered in or out of the vacuum.

The body of the feedthrough ceramic has a second critical vacuum seal near its base where it is attached to a vacuum rated metal base, often of stainless steel. The "conflat" fitting is virtually the industry standard base found on all high voltage feedthroughs.

In a formal vacuum rated feedthrough the seal is effectively a metal surface to ceramic "glass seal". In most fusors, the heavy central copper rod is threaded such that the inner grid and a somewhat smaller copper "stalk" can sink deep to the center of the fusor anode or shell.

There is, in a 1,000 watt fusor, virtually zero mechanical stress on a feedthrough and also virtually no thermal stress transmitted to the feed through through the often red hot grid and stalk structure. (all of the foregoing assumes a good vacuum rated feedthrough is used).

To thermally stress or risk the critical seal near the atmosphere-vacuum connection one would have to heat the seal to over 1000 degrees F. This would never happen in any fusor at 1,000 watts input. Most fusors even mega-neutron jobs rarely operate at 500 watts. The bulk of all the applied energy goes into heating of the shell anode of the fusor and little winds up heating the inner grid. The thermal volume of the grid metal is small and it often glows red or even orange hot in a well run fusor. All of this heat goes into the more volumetric stalk and ultimately to the far more thermally massive and super heat conducting main electrode. Properly designed stalks never get red hot. The large feedthrough copper terminal where the critcal vacuum seal is located, right at the distant outside tip of the feedthrough, rarely reaches even the boiling point of water. The ceramic feedthrough, itself is also barely warm to the touch once power is removed following an operating session.

Far more deleterious to the feedthrough is metallic deposition during fusor operation and the subsequent degrading of the effectiveness of the ceramic insulator within the vessel to hold off the high voltage appled and do its job of preventing internal flashovers. This buildup on the insulator can be cleaned off chemically with acids or alkali compounds or abraided off via sand blasting with fine sand.

Homemade or cobbled up feedthroughs range from "forlorn hopes" to very creditable efforts on the part of the amateur. However, no amateur effort can rival that of a properly selected feedthrough, manufactured for the job and sold by a vacuum supply house.

In all honesty, only a demo fusor might allow for a homemade feedthrough. Once large neutron numbers are sought at elevated voltages over 30kv, a professionally made Feedthrough is almost a must.

While the feedthrough is critical to the maintenance of the internal vacuum and transport of high voltage, this discussion is placed in the construction forum as they are part of the physical and bodily construction of the fusor chamber, itself.
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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Doug Coulter
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Re: FAQ - Feedthroughs - What are they?... What stress is on them?

Post by Doug Coulter »

Not to start an argument here, but I ditched commercial feedthroughs in favor of my homebrew ones that are better designed for the conditions we actually have in a fusor, which isn't high vacuum.
I just got tired of buying more and more of the commercial ones, only to have them fail due to being chemically reduced by hot hydrogen and then punching through. Thermal stress starts happening at any temperature differential, it doesn't magically hold off till some number, but the commercial FT's are designed with both close tempco match, and some flex part of the seal in the better ones. But letting plasma up inside that cone kills them quick -- some fusor designs may not have this issue, but it's really "the devil is in the details" and it's hard to make sweeping statements like that.

Here's more on the topic. It's not that simple, to keep it short here.
http://www.coultersmithing.com/forums/v ... ough#p1849
Why guess when you can know? Measure!
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Re: FAQ - Feedthroughs - What are they?... What stress is on them?

Post by Jeff Robertson »

Richard,

Good read! I always enjoy reading your posts, I can't tell you how much I've learned from your various threads throughout these forums. I have a couple questions, if you don't mind.

First, what are the advantages of beryllium oxide over aluminum oxide as a feedthrough? In all the posts I've read, alumina has seemed to be the electrical insulator of choice for a lot of fusors. Is there any significant difference between them, or am I just splitting hairs at this point?

Second, what are some common market prices for high voltage feedthroughs (ones capable of withstanding fusion)? From the research I've done online, it looks like you have to be ready to put down at least a few hundreds dollars for a decent one. Is this the price I should expect to pay, or are there cheaper options out there? I'm not looking to achieve monstrous voltages, just ones high enough to easily allow fusion (probably around 25-30 kV).

Thanks again for the informative post!

Cheers,
Jeff
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JonathanH13
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Re: FAQ - Feedthroughs - What are they?... What stress is on them?

Post by JonathanH13 »

Having had experience with both commercial and homemade feedthroughs, I can testify to the advantages of a well-constructed homemade one. Doug and I put one together while I was visiting him recently, which I now use, and it's a beauty. It has not given me a moment's trouble (which is quite the opposite of my commercial one), and it has some nifty advantages such as allowing me to easily rotate the grid, and move it closer or further from the tank wall. The downside is the viton gasket, but my chamber is full of them anyway. It was not trivial to make though, and took many hours on the lathe, patiently grinding quartz and pyrex tubing. We also anodised the central conductor, which can be a tricky process. Although a suitable manufactured one may also work well, they don't come cheap.
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Re: FAQ - Feedthroughs - What are they?... What stress is on them?

Post by Richard Hull »

I only mention Beryillia as there is/was a huge number of really sweet Beryllia based insulators on e-bay and at hamfests over the last few years. These were NOT vacuum rated, but were easily converted to conflat bases by a person with "good hands". In spite of this, they were vacuum tight and served well. In air, they are limited to about 35kv if kept scrupulously clean. With field control they can go to over 40kv.

A lot of folks here have bought and used these naval "bulkhead insulators". I have picked 4 up at two different hamfests at $10.00 each! They still show up on e-bay or at hamfests every now and then. Some here have sunk these in oil to achieve much higher standoffs.

Before any of the usual "nervous nellies" chime in, Beryllia is a big no-no to the greenies, OSHA freaks, MSDS bible thumpers and catalog readers, safety whiners, ISO, RoHS and "not in my home" types. For me, it is just another very useful material that I am not afraid of any more than I worry about cadmium or lead.

Bottom line, there is virtually no big shake for Beryllia over Alumina or even porcelin if vacuum rated or sealed sufficiently to work in a vacuum. Some folks can cobble well and others can't, as I noted in my FAQ originally.

$500.00 is not too a high price in the vacuum biz for a vacuum rated, conflat based insulator rated at 60kv. Thus, the natural impetus to cobble and jury rig.

In the end, everyone is on their own in this biz as, for most here, much of it is an art and engineering process, rather than some scientific advancement effort.

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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