(Non-vacuum?) BeO feedthrough

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Greg Courville
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(Non-vacuum?) BeO feedthrough

Post by Greg Courville » Wed May 24, 2006 10:43 pm

Another impulsive Ebay buy!
This is some sort of HV feedthrough, supposedly made of BeO ceramic, with a base roughly 2.75" in diameter. However, looking at the base, it doesn't seem to have been made for any kind of high vacuum application, as the contact face of the flange seems to have a rather rough finish (including a serial number stamped into it) and I don't see any obvious means of establishing a seal.
Assuming the metal-to-ceramic interfaces don't leak too badly (perhaps an inapt assumption?), might I be able to use this feedthrough in my system by bolting it down over a hole, then sealing the inner edge of the flange to the chamber with some sort of LVP epoxy compound? Any and all insights, admonishments, and/or ideas are appreciated. Thanks!
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Re: (Non-vacuum?) BeO feedthrough

Post by bwsparxz » Wed May 24, 2006 11:52 pm

I believe Richard Hull is using the same feedthrough on Fusor IV. He removed the non vacuum base and replaced it with a conflat. Richard posted on this somewhere on the board but have unable to find it. Maybe he can explain better. I am thinking about using this as well to start.

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Richard Hull
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Re: (Non-vacuum?) BeO feedthrough

Post by Richard Hull » Thu May 25, 2006 3:06 pm

This is a surplus USN shipboard, bulkhead antenna feedthrough and has a federal stock number.

It is definitely BeO. The item was NEVER intended for vacuum use!!! I do use a highly modified version of this on Fusor IV. I have posted on it as noted by Brian. The item is part naval bronze ( a high brass) and steel. It is chrome plated.

It is a worthy effort to modify it, but does require some tools and skills. It is of little value over about 40kv, though I have had it arc over externally on a damp or dirty insulator at about 35 kv.

These were commonly available at hamfests up and down the East Coast about 5 years ago as, apparently, a large mass of them went surplus. Prices ranged from $10.00 each to over $25.00 each depending on what yahoo your ran up on selling them. I bought about 10 of them from one guy for $100.00 and have since sold the bulk of them.

I would put the genuine surplus value on these at about $50.00 each, but I am sure they were hundreds of bucks when new.

Interestingly, the beryllia in the insulator is worth over $100.00 as a chemical!

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.

Greg Courville
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Re: (Non-vacuum?) BeO feedthrough

Post by Greg Courville » Fri May 26, 2006 12:03 am

Ah, this is encouraging.
I think 35-40kV leaves plenty of room for experimentation, especially for a first-timer with a primitive experimental setup, simply excited to see some plasma. It should certainly be sufficient for a "demo" mode fusor.
I do have a tabletop milling machine and a few spare 2.75" CF blanks, but no lathe around and no rotary table on the mill, so removing the existing flange by means of cutting could prove to be quite a challenge. I was unable to find your original posting (perhaps I didn't look hard enough!), but I did find some threads discussing the matter. Desoldering the flange seems like a cleaner way of detaching it than trying my luck with a hacksaw. I do not have access to an oxy-acetylene torch with which to desolder the original flange, but will a propane torch be sufficient? If not, I suppose I will have to cut the flange off somehow. As for preparing the blank flange, I think I will have to invest in a boring tool for my mill. I should be able to use this both to bore a suitable through hole and to cut a lip to solder the skirt edge into.
I am currently saving my money for a visit to the upcoming swapmeet in Livermore, as I have seen vacuum components at such events in the past, so equipment purchases will have to wait until after then in order to ensure that I have a few bucks available if I happen to stumble on any exceptional finds. However, if I don't end up spending much there (which will probably be the case, unless the "laser guy" shows up... I have a weakness for lasers), I will probably go ahead and order a boring tool or a rotary table. The latter would probably cost at least twice as much as the former, so I'll have to decide whether the extra utility is worth it. As always, insights are appreciated!

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Re: (Non-vacuum?) BeO feedthrough

Post by Jon Rosenstiel » Fri May 26, 2006 2:45 am

Greg,
Try this link:
http://rhull.home.infionline.net/highenergy019.htm
And this thread:
viewtopic.php?f=10&t=3441#p20849

I also have one of these feedthroughs. I used a hacksaw to cut the flange off (the metal is only 0.022" thick) but didn't get any further that than as another, "proper" feedthrough appeared on eBay.

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Re: (Non-vacuum?) BeO feedthrough

Post by Greg Courville » Fri May 26, 2006 3:59 am

Yes, I found the thread you mentioned while looking around for information on the subject after Brian mentioned that Richard had adapted the same feedthrough for Fusor IV.
I guess removing the original flange isn't the biggest issue, as there seem to be plenty of ways to do that depending on what equipment one has. The real trick will be attaching it to a flange in a vacuum-tight and mechanically sound manner. Silver solder looks like the way to go, but being able to do the necessary machining operations on the blank flange will require an equipment purchase or two. Who knows? Perhaps I'll find a rotary table for my mill at the swapmeet... :-)
I'm still interested to find out whether a propane torch will be sufficient to desolder the stock flange, as an oxy-acetylene rig is currently out of my budgetary range (not to mention the parental objections).

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Richard Hull
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Re: (Non-vacuum?) BeO feedthrough

Post by Richard Hull » Fri May 26, 2006 3:18 pm

Thanks Jon for finding all the info and putting it up.

Silver soldering is about the only way to secure the cut off flange skirt to the conflat fitting. The machining is pretty easy. It is important, of course, to pre-silver solder both surfaces then just mate them and heat applying only a small amount of silver brazing rod. The big worry is messing up the skirt's ceramic to metal seal with too much heat. This is where the fancy dancing comes in.

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: (Non-vacuum?) BeO feedthrough

Post by Greg Courville » Fri May 26, 2006 11:22 pm

Thanks for all the advice. As soon as I get my hands on the necessary brazing rod, a new torch (we once had a propane torch, but it as I now recall, it was stolen during the renovation of our house -- along with a two electric drills, a shovel, and a laptop computer!), and some means of making clean circular cuts in steel, I shall "give it a go"!
I will update when I've made progress.

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Carl Willis
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Re: (Non-vacuum?) BeO feedthrough

Post by Carl Willis » Sun Jul 02, 2006 5:34 pm

Here's a very explicit description of what I did with such a feedthrough.

You need:
-low-temperature ("soft") silver solder, 3-4% Ag in tin
-aggressive acid-based stainless steel flux (e.g. LaCo N-3)
-2.75" 304SS ConFlat flange, with 1.625" bore (largest sold by Lesker)
-lathe. A hacksaw would work, at the expense of time and cleanliness
-very small gas torch

The skirt on the feedthough is apparently stainless steel that has been plated, along with the flange, with some other metal. Chuck up the feedthough by its flange and slowly, gently cut off the flange with a parting-off tool, leaving 5 mm of the skirt behind. Be ready to catch the liberated feedthrough. The remaining skirt should fit somewhat loosely into the back of the 2.75" CF. Use sandpaper to roughen the outside of the skirt piece and the inside of the CF; heat with a small torch and tin the pieces. Tinning seems easiest if the part is turned while heating and applying flux and solder--let the solder flow by gravity around the surface. File or sand if necessary to make them fit together properly again. Mount the feedthrough "upside-down" by its terminal in a vise and put on the CF (knife edge up). Heat the flange by its underside while applying flux on the underside and inside of the joint with a syringe w / very fine needle to protect the solder from oxidation and help it flow. When the solder is hot and fully molten in a smooth bright bead around the entirety of the joint, it's done. The metal will look very ugly on account of the flux residues. Gently cool the part SLOWLY by spraying with a water mister bottle. When the water starts to stay on the surface without immediately boiling, transfer the hot part to a pot of boiling water and get rid of as much of the flux residue as is feasible. A toothbrush or Scotch-Brite pad helps. That's it. Not hard to do. A hard silver braze such as Stay-Silv 15 would entail the same steps, but you'd use the fluoride-based paste flux and necessarily an enormous amount of heat to get the flange hot enough. A somewhat tighter fit between skirt and CF would be advisable as well. I did not go this route because I was concerned about overheating the ceramic-to-metal joints on the feedthrough.

(Caution when using acid-based flux. This is made from concentrated HCl and zinc chloride. It will soften your fingernails and flat-out eat your clothes, and it will certainly splatter and boil when being added to a hot part. Wear goggles by all means. Don't do this job near anything metallic that you value, as the vapors will eat it alive. Such jobs are best done outside. Wash hands and all other tools involved with the job very thoroughly. This stuff initiates corrosion like nothing else on earth with perhaps the exception of aqua regia, so take cleanup seriously!)

-Carl
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Re: (Non-vacuum?) BeO feedthrough

Post by UG! » Sun Jul 02, 2006 9:46 pm

concentrated phosphoric acid is also a rather good flux for stainless and the fumes seem to be slightly less damageing to adjacent bits of metal. i was amused to discover however that hot conc phosphoric will happily turn paper towls and cloth to black carbonized gunk so caution is still advised :)

Oliver

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