A 'multi-purpose' chamber with multiple HV HF feedthroughs.

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Chris Bradley
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A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

I thought I might attach a few details of a chamber I'm constructing for the purpose of some flexible experimentation. This is anticipated to be mid-vacuum, not sub-micron, nor will it be high [electron] currents nor neutron producing. But perhaps I might now get up to a few experiments that could help plan further such developments.

I am posting now so that you can see the 'anatomy' of the feedthoughs (see the pdf; viewtopic.php?f=19&t=8007#p57411).

The thing is yet to be tested to see if it is sound or whether it will leak like a sieve! Feel free to make comments (and/or place bets!) as I can make mods now before evacuating it, if good suggestions are offered for improvements.

Any questions, please ask. The first experiments will be employing a magnetic assembly as per viewtopic.php?f=15&t=7189#p51095 , and you can see on the pics the studding that will support this assembly (these are threaded blind into the bulkhead and don't go all the way through). Also to note, the crinkled copper on the edges is soft foam with adhesive copper over it; an EMC/blast screen of plate steel will go around the whole thing and the complant copper edges are intended to press against it and act as an EMC seal. Some HV EMC filters will be added to the top feedthroughs and will be for DC inputs.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

Very radical, Chris. I'm impressed.....
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

I don't seem to be able to reply on the 'other' thread, Chris, so I'll comment here.

Impressive, again.

I was using some ceramic cement/glue/?? at work today. Apparently it's water based. Was just wondering if any-one else has come across it?
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Steven Sesselmann »

Nice work Chris,

It will be interesting to see if you can pull a vacuum in that chamber, I am not sure if I like your chances.

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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Well, that's why I posted it, so you can say what your concerns with it are and I can plan improvements.

I don't really see any issues as all the points of potential leaks have compliance, rather than being stiff and "unforgiving" joints compared with metal parts. If I was after submicron levels, I would be more pessimistic, but my only concern is how long humidity will take to outgas from the phenolic.

If the feedthrough points do leak, then I'd probably start again as the time consumed is far more costly than the materials, hence if you can pinpoint your concerns then that would help me consider this, and future variations.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Doug Coulter »

I made, and still use, a wine jug bell jar system that looks like this for moderate vacuums. With a new good two stage rotary pump it doesn't get to non conductive vacuum, but if you don't mind some air in your gas, you can sputter in it with Ar + leaks, or evaporate the less reactive things fine.

To make the jug live (and me unhurt -- a wire cloth screen cage will still let through some glass), I found I had to make a strip sticking up from the baseplate that was a decent fit into the ID of the wine jug, with a rubber band on it to prevent high local stresses at point contacts.
This was a 1/2" tall by 2/16" thick piece of steel I bent into a circle to fit, and welded to the base plate.
This rubber band was a moderate fit -- you had to tease the jar on there, but the real vacuum seal was an o-ring wrapped around the strip at the baseplate (viton).

Funny thing about unsupported cylinder ends -- they don't crush themselves round, they go oval and then die kind of explosively. The ring is in there to limit the maximum possible crush at that point, it's just there to keep it round,

I can post a picture of my old system if I go clean it up some -- I use it for vac impregnating things (even quick-marinating mushrooms for my cooking sometimes) and it's kind of messy just now.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

It will give at the weakest point, Doug. And there is always a 'weakest' point.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

I believe you should have no concerns form my sake over this cylinder, Doug. It is an industrial piece of annealed stress-relieved kit specifically for the purpose and for much harsher applications than this. It is about 11mm thick. (he says optimistically, as he hears it crack under the first vacuum!.... )

It is also used (I made the presumption that it had handled toxic substances and cleaned it accordingly), which will actually mean the stress-relief is likely to have improved. I bought it, as you might expect, off ebay. Odd how ebay supplies those bits you're just after, sometimes;

I had a belljar that just wasn't big enough for all the feedthroughs I wanted to construct, plus gas ports. So I figured I could do it with two bulkheads instead of one if I could find a vacuum cylinder. I figured I'd need it 250mm diameter and about 300mm high. I look on ebay and type in 'vacuum cylinder' and, blow me, there was this beauty up for 45 quid. 250mm diameter and 305mm high! I was the only bidder.

In the meantime I've found a supplier/manufacturer of these locally who can supply this size (fully annealed for high vacuum) for GBP240, in the event that this one is defective in some way. But for now, I'll go with this one - some believe in fate, others in coincidences; how this came to be on ebay at the precise moment I went looking for one of this exact size I cannot say but the event of coming across this rara avis just as I looked for it did singularly incentivize me with sufficient impetus to progress this particular construction principle, and thus it is....
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by DaveC »

Chris -

Doug's concern about the cylinder (which I presume from your remarks on how you acquired it - is glass/Pyrex....) is mine as well. The Bell Jar pulls itself down onto an "L" shaped gasket. The "L" shape not only seals at the edge, but also provides a substantial frictional force in the plane of the base to withstand the external atmospheric forces.

We have at work a PEI plastic bell jar with a home fabricated aluminum base. It also has an flat elastomer ring seal as you are using, which sits in a groove in the base. This is a low vacuum unit... good for probably not more than 10 - 20 Torr. absolute pressure. It is about 4 or 5 mm in thickness and does show some deformation when under vaccum. It works very well for outgassing molding compound, and similar duties.

Wild thought - if you can find a couple of polarizers, you could check out the cylinder beforehand for any high stress areas.

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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

It'll be all wrapped up behind steel plate. Whatever it does behind there, it does!

Whether 1 millibar or 1e-9torr, the mechanical forces will be essentially the same. I should mention also that my intention is to tighten the bolts on the side supports *upwards* on the upper bulkhead as the vacuum bites so that not all the load is taken by the glass, mainly, actually, to mitigate overcompression of the [homemade] gaskets. But I think this thing will take an elephant in compression! If you were to handle the thing, I'm sure you'd come to that conclusion. Thanks for the thoughts and comments, I will bear it all in mind (when sweeping up the bits?!!).
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

Listen to the advice, Chris, before you pump it down.

Either that, or 'stand well back'.

I'm very interested in how this 'pans out'.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Ash Small wrote:
> Listen to the advice, Chris, before you pump it down.
> Either that, or 'stand well back'.
What do you want me to do, then?

This isn't a regular cylinder, it's a cooling-tower shaped thing and is clearly designed to mitigate a 'flat-collapse'. It is so thick that were it to deflect at all then I think its stiffness would, indeed, cause a fracture and a bit of bracing would be far *less* stiff and have no effect. Were it a commercial glass then Doug has put in place the right mitigations. I think this would be useless for this bit of glass. I'm not saying I know that I'm right, but it seems a moot point. If it goes, it goes.

if you handled this thing, you'd see that the idea of just standing back is funny - this is a half-inch thick glass and is about *3 times* as thick as a UHV bell-jar I have - if this were to actually shatter, the pieces would be enormous and distance alone wouldn't protect you! Solid steel plate is the only practical protection, and this is done. I am guessing that this sat in an industrial process line as a visual check and had to be able to take sudden pulses of high rates of change of pressure.

I was contemplating trying out a polycarbonate cylinder and reinforcing it internally. But it seems a redundant thing to attempt given this bit of kit. Whether it has some inherent weaknesses due to its prior life or not, we'll find out. It's moot - all I can do is pump it down behind plate steel and we'll see what happens. I'm far more concerned about the viability of the feedthroughs!

The one thing I might do is not bother to take up some of the compressive load with the external struts. As Dave says, the actual contact pressure would help to stabilise its location. I'll also forego a smear of vacuum oil I was going to apply and just leave the silicone's frictional forces to do their bit. If it leaks, I can throw some Apiezon 'mud' at it.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Doug Coulter »

Chris,

Knowing it's that thick and now what it's made of, you're probably fine unless you get a stray beam of something hitting it (easier than you'd think). You still need a lip to keep the o-rings from sucking in though. I was using 3 liter Gallo bottles, not known for thickness, uniformity and so forth, and hey, got to drink the contents first. I didn't break enough of them to need to drink too awful much, they actually work well in most cases.

You're going to find I think you didn't make the end-plates thick enough, flat plates don't do as well with that stress level. My books say, make it thicker by about 2x if you can stand a fair amount of flex, more if not. But they probably won't break, just flex and leak if I'm right.

Of course no matter what, the amount of energy you effectively store in a container at vacuum is purely related to volume and about 14.7 psi max, and frankly, I've broken some glass stuff about that size and been kind of disappointed that I didn't get a better show out of the large brand new 25" picture tube I dropped and which came out of my paycheck when that mattered a lot more. Glad I wasn't in the path of the main blast though, those were some sharp bullets -- lousy aerodynamics in flight, so the "Stand well back" isn't a stupid idea. Glass bits stuck into the high ceiling.

Looks like your shield will handle that easy anyway, as you say. It's only so many foot pounds.

Chemical stuff can either add or subtract stress from glass, you can't assume it's the better for it.
This is covered extensively in some references I have. Depends on which side the chemical changed the expansion coefficient on for example and compared to which side the tension is on.

One very good ref is Walter H. Kohl's "Handbook of materials and techniques for vacuum devices", and another is the older RCA tube designer's handbooks --these are not how to design *with* tubes, but how to design the tubes, used to be internal documents to bring new guys up to speed. The former is frankly better and has more of that "guild secret" stuff in it, but is an expensive AIP publication, copyright current, while the other ones can be found free on the web.

You will have the odd beam leak if you have a way to use the mark one eyeball to see whats going on inside your stuff. Do use a sacrificial transparent piece of stuff (glass, quartz, mica) in there as you can shatter pyrex quite easily with a sudden hit of a decent beam at a point. It's good, but won't stand anything. I've easily shattered completely unstressed pyrex by accident. It was enough mess without it having been the containment vessel itself.

The polarizer suggestion is good, and fun besides, even with some fairly cheap film types readily available surplus. A real eye opener for this beginning glass blower-- who then got an annealing oven rigged up fast and a lot of problems went away.

All this may not strictly apply to Chris here -- I suspect other people lurk and need to know this stuff, who won't have Chris' precise setup, which however presents a good example of this class of thing for discussion.

And good to see you getting this thing built at last -- hat's off.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Doug, Thanks for the replies and the things to double-check;

Doug Coulter wrote:
> You still need a lip to keep the o-rings from sucking in though.
The lip is a half-inch so call it 15si for the seal area, assuming ~10" diameter. The total vertical force due to the vacuum will be some 1100lbs, so there will be 75psi on the seal. If the coefficient of friction of the seal is >0.2 then I reckon it'll stay there, and I think it is?

> You're going to find I think you didn't make the end-plates thick enough, flat plates don't do as well with that stress level. My books say, make it thicker by about 2x if you can stand a fair amount of flex, more if not. But they probably won't break, just flex and leak if I'm right.
I did look at this, but worth a check. Bear in mind this is Kite [electrical] tufnol and is the stiffest with 6.9GPa. Compare, for example, aluminium with 69GPa so this is equivalent to a 15mm aluminium plate (as flexure is proportional to 1/t^4). But if there are any issues - including sealing of the feedthroughs - there is a 'plan b': I will add a second layer of tufnol with a sandwich of gasket and the feedthroughs off-set and connected by foil within that sandwich. I could only get Kite to 25mm, but did debate the merits of the mechanical grades that I could get to 50mm. I made a decision figuring it was likely to be close to satisfactory... we'll see!!... In any case, a pretty simple remedy. Now that I know 'what it looks like' and how to make it, and that the standoffs are already made, it will only be a couple of hours to remake them from new tufnol blocks. No risk...all useful findings!

> Chemical stuff can either add or subtract stress from glass, you can't assume it's the better for it.
I was thinking temp rather than chemical duress. You're right. I'll contemplate that when picking the glass shards out of the internal teflon parts! The thing was very clean when I got it and *presumed* chemicals just for safety as various nasties can go through these. It got a fine abrasive clean under a small selection of solvents.

> Do use a sacrificial transparent piece of stuff (glass, quartz, mica) in there as you can shatter pyrex quite easily with a sudden hit of a decent beam at a point.
I'll definitely be doing this where beams are a risk. I would plan to use polycarbonate internally where I am not expecting, but there is a risk of, beams or 10mm glass where I half-expect it.

As you say, it is a different setup to usual so hopefully there will be learning points anyway. It's a materially cheap set-up and, I would suggest, if it all works out, might therefore have useful outcomes for the amateur.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

Chris, Doug is right. You don't know what might happen. You don't know the history of the chamber, stray beams could cause localised heating, etc.

I don't know if lining it with a metal gauze would stop stray beams, I don't even know what beams you will be using.

As I said before.....Stand well back.....

Maybe an external polycarbonate or similar shield would be worth considering.

It would make a very nice plasma light if you keep the voltage below xray levels.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Ash Small wrote:
> You don't know the history of the chamber, stray beams could cause localised heating, etc.
> It would make a very nice plasma light if you keep the voltage below xray levels.
Difficult to know how to respond to your posts, Ash. The support is nice but the insipid commentary is distracting. ....maybe you could spend time to go figure out why there won't be stray beams and high voltage in the first experiments I'm planning.

All that I will be doing I have previously posted about over the last 18 months, I've simply not yet put it all in one place to spoon feed you...and if you can't [be bothered to] figure it out or don't think there's enough information for you, then chill out and just wait for some actual results. I'm sure everyone else has already figured I'll go over that stuff again and in one place when there are actual results to discuss along with it, else I'll just be doing yet more armchair blah-de-blah. I was hoping this thread was just about the actual chamber construction. I'd suggest it'd be better for you to put discussions and speculations of the actual experiments in the threads where I've discussed those potential experiments.


Carl Willis wrote, on 2009-11-05 18:32;
> Do you think we're in nursery school, Ash? I understand this was offered in service of some kind of rhetorical point, but the sheer insipidity makes me wonder if you think your audience here walks around in diapers!
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

What's wrong with the suggestion about metal gauze, Chris?

Plans change and details change, Chris. I doubt you''l do 'everything' you've previously discussed.

Personally, you've got me thinking about glass chambers for some plasma lighting. If I assemble them inside a vacuum chamber, I won't have to pump them down.

We all know the level of mentality of some contributors to this site, Chris, but I'm trying my best not to lower myself to it.

When do you hope to achieve 'first light'?.....I'm looking forward to the pictures.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Ash Small wrote:
> I'm looking forward to the pictures.
I'm rather hoping that you won't have much to see. As I have said before in my posts (have you looked yet), if you can see unit-eV recombination going on then I don't see how it will be efficient enough to be a net fusion device.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

There are some of your posts that I've been planning to read 'in depth' at a later date.

I noticed you don't seem to have any gas feedthroughs, Chris.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Ash Small wrote:
> I noticed you don't seem to have any gas feedthroughs, Chris.
It's the bolt on top with the mudguard washer and the complaint washer underneath. Just 'plugged' for now - atm gas [and whatever else outgasses!] will be the first test gas.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

OK.....Just looks like another HV feedthrough, unless you know what you are looking for.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

The chamber sealed, but initially leaked like a sieve. Spraying a bit of IPA around, I could actually see the solvent being sucked up into the homemade silicone seals, even though I had clamped the top and bottom down quite firmly. So I pulled those out and tested the flatness with vac grease. I guess one advantage of glass enclosures is you can actually see where the surfaces are or are not in close contact, if there is a fluid between them.

With a bead of Apiezon Q around both ends to minimise doubt from leaks there, I was still getting no better than a miserable 1 torr. The culprit looks to have been the ASA flange on the top bulhead, which also had one of my 1mm thick silicone gaskets under it. Once under vacuum I could further clamp it down, presumably meaning the bulkheads had flexed compared with unloaded, and confirms Doug's thoughts, though measuring flatness by eye and a straight edge, there didn't appear to be any flex at all.

On clamping the flange down more firmly, I was in the 100s micron range, with continuous pumping. My conclusion is, essentially, that the silicone wasn't thick enough to accommodate the irreguarities of the tufnol along with the effects of the vacuum load as well. I now intend to obtain some viton sheeting with enough thickness to do the job better. Not sure what thickness is optimum, perhaps I should take regular viton seals as the standard and go for 4 or 5 mm thick. Any opinions?
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Quantum »

Why viton sheet instead of seals?

A cylindrical seal will seal better than a flat one.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Chris Bradley »

Whilst getting an order together for more suitable seal materials, I found some 'bits of old' rubber sheet in the garage that I cut to shape. No idea what the actual material is, but it is very evident that the lack of sealing was, indeed, from the excessively thin silicone bits I had previously made.

I can't vouch for the calibration accuracy of my PKR251, but it seems to pump down within minutes to around the 80 micron range (indicated) and it stays there whilst pumping away (E2M2) and very slowly (hours) pulls maybe a little more, progressively. On isolating the valve it appears to recover within 10's of minutes to around the units of mbar range where it stays, steady, for days.

I am presuming that what I'm looking at is a 'severe' case of water outgassing from the tufnol (bakelite), as was expected. I'll not be trying too hard to get the vacuum lower than this until I have good seals (of at least *known* rubber!) and the first experimental bits are installed. Perhaps I might get rid of most of this with a long pump.

I'll say that the teflon stand-offs appear to have visibly lost their surface 'gloss'. The teflon rod I used appears at least a little bit shiny in strong light, but now they appear quite matt in the chamber. Together with the tufnol surfaces, I guess these all add up to some very significant sources of outgassing humidity. I might consider some alternatives to the teflon standoffs, if these are likely ot contribute significantly. Any suggestions welcome - e.g. I am debating if melting vac wax all over the tufnol surfaces might help out and I'd be heating the surface up as well in that process, which can only help.

The graph is pressure (Pa) wrt time (s) after isloation.
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.

Post by Frank Sanns »

I don't think a sheet of rubber will be any workse than your Bakelite. Melting wax over the Bakelite will not gain you anything. Wax is not a whole lot better than kerosene for vapor pressure/outgassing with linear low density polyethylene being better and HDPE and UHMWPE being the best of the linear alkanes.

Also, you do not really need to bolt the top and bottom halves together as the vacuum will supply far and enough force to keep them in place. Aluminum plate would have worked with feedthroughs especially if the bottom were insulated from the table with the Bakelite.

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We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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