Bell Jar Viable for Fusion?

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Jeff Robertson
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Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Hi all,

I've been passively researching fusion/contemplating designs for a fusion reactor for the past year or so, and now that the school year is over I'm going to start putting some serious effort into making this happen. Lately I've been scouring the forums for any and all information on bell jar fusion chambers, as I think an entirely clear chamber would be a lot more visually appealing than, say, an opaque spherical one with a small viewing hole. I see a lot of people have successfully created plasma using glass/pyrex bell jars, but I couldn't find any threads in which someone claimed to have made actual fusion in a bell jar. Is there anyone out there who has successfully done so, and can attest to the viability of such a chamber?

I realize there are a couple challenges that would probably arise with a glass bell jar. The first one is "browning" of the glass due to plasma discharges and overall heat exposure. Most people who use bell jars seem to remedy this with a secondary casing inside the bell jar, such as an additional glass jar or even a metal cover. In one thread I was reading, a student at the university of utah used a "sacrificial" beaker to protect the outer glass chamber from staining. The second challenge is making a vacuum high enough to allow fusion. How daunting are these challenges to overcome (for someone new to fusion)? I'm trying to figure out if a bell jar would even be worth it in terms of the effort required, as opposed to a more conventional container.

From what I've gathered, spherical chambers (usually composed of two stainless steel hemispheres) are generally the way to go. If most of the veterans here seem to be against the idea of bell jars for serious fusion attempts then I have no problem abandoning the idea, however I wanted to get some input first.

Sorry if there's already a thread with this information. I did a lot of searching/reading on this topic before posting, but wanted to make a thread specifically for assessing the overall viability of fusion in a glass chamber.

Cheers,
Jeff Robertson

Side note: I'm aware of the added dangers that come with glass chambers (such as excess radiation, xrays, etc), and would adjust precautionary measures accordingly. If anyone has any additional warnings or hazards which I may not be anticipating, however, I welcome any feedback regarding safety measures. Safety first!
Jeff Robertson
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Looking further into that thread from the University of Utah, I see that they did in fact get a small amount of fusion. I'm still curious to hear about the overall viability of this approach though.

Here is the thread I'm referring to:
viewtopic.php?f=18&t=7838#p55797
Chris Trent
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Re: Bell Jar Viable for Fusion?

Post by Chris Trent »

Can you do fusion in a glass vessel? Sure, if you wish. It's been done before.

The reason it's not popular among the old hands is because it adds danger, cost and complexity.

Danger from X-rays and potential implosion.
Complexity from extra shielding both inside to protect the glass vessel, and outside to protect you.
Cost from the added design complexity and shielding requirements.


I don't know about you, but I lost interest in glass at "Danger".
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

My thoughts for a design were that I'd have a total of 3 "casings" of glass. The first would be a solid (probably pyrex) glass container that completely surrounds the fusion reaction (doesn't have to be airtight) and can withstand some heat abuse. This container would be replaceable and is intended to protect the actual bell jar from heat damage. The second layer would be the glass bell jar (probably also a borosilicate glass) which would be the airtight container. The third and outermost container would be leaded glass (shape doesn't matter, although I'd probably make it a box just for simplicity) to block radiation.

So in total, the overall container would be shielded from heat damage and block nearly all xrays emitted. sounds reasonably safe to me.

While the container design is a little more complex this way, I'm not worried about it. A little added complexity is worth it for the added visual performance.

And cost isn't a huge issue for me (as long as the price is within reason). But even so, if I'm a smart shopper I could see this casing even being cheaper than, say, a stainless steel shell. The inner container can just be a cheap beaker (I would probably get one for free from the physics department). The bell jar chamber would be a little more expensive when considering the modifications that would go into it, but it would be a permanent chamber. And then, if I made the leaded glass into a box, I could make it just from 5 square panels. Looking at the prices online, it wouldn't be too bad.

One thing, you mentioned was risk of implosion. Would this be from stress due to atmospheric pressure?
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Re: Bell Jar Viable for Fusion?

Post by Chris Trent »

Yes,

The potential for an implosion exists because of the pressure differential across the bell jar. 15psi times the surface area of a decent sized bell jar turns out to be an awful lot of stored energy. The hazard exists because glass breaks suddenly into fragments instead of bending or crumpling like metal.

If you are demonstrating for an audience you will always want a "blast shield" between them and any glass vessel under either vacuum or pressure. If the vessel breaks, for any reason, you don't want chunks of glass sailing into the audience.


When you're dealing with high voltage and vacuum you also have to be careful of a phenomenon called localized heating. Ion or electron beams hitting a surface can heat up a single small spot very quickly, and cause a failure. It can sometimes happen even if there is something in the way if the conditions are right. Should that surface be your bell jar then you have the setup for a sudden and catastrophic failure

Fortunately, a sheet of polycarbonate, or perforated metal is usually sufficient protection. Really, anything that would stop large chunks of glass hurling your way would do. One of the more popular options is a fully enclosed implosion shield or cage, but just a sheet of Lexan in addition to your lead glass panels would be a vast improvement.
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Thanks for the fast response and feedback.

Exploding glass would definitely be something worth preparing against hah. Lexan seems reasonably cheap and would barely impede any visibility, which is good. Does prolonged x ray exposure have any negative effects on polycarbonates, such as discoloration or deformation? Sorry if that's a really dumb/obvious question, I couldn't find any information on it haha.
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Re: Bell Jar Viable for Fusion?

Post by Chris Trent »

Unfortunately I don't have any specifics either. It's an organic polymer so I suspect it will eventually degrade under X-Ray and UV exposure, probably either yellowing or crazing.

I don't recall any reports of it happening, and I know a number of folks here have Lexan shields for one purpose or another, so I expect it would last a good while.
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Doug Coulter
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Re: Bell Jar Viable for Fusion?

Post by Doug Coulter »

The trouble with a glass bell jar is it can be shattered almost instantly by a stray beam of ions or electrons -- spot heating can happen quick. With Lexan (or other plastic) it will melt, vaporize, out-gas.

Either needs a huge, flexible seal making good high vacuum hard to get to, for pure conditions when you let in the desired gas. Either are quickly ruined by sputtering of grid or other metal onto them, which is also a problem with windows in a metal tank -- generally I have to use a sacrificial piece of glass to preserve the more expensive sealed in window. (my 8" CF window/door cost nearly a grand - but gives a better view than any old bell jar does -- optical quality). Bell jars are kind of problematic to out-gas via heating as well. Anyone who has done a lot of vacuum work would be thinking about that real hard.

You can of course do all kinds of fusion and other fun stuff in a bell jar, the main thing they are great for is being taken apart to give total access to the innards of the system - so if you want to do something where reaching in through a smaller port or door is hard, they're ideal.

For something where you want to have an outer electrode to create a uniform field, all the advantages are kind of negated -- fine mesh screen isn't too easy to see through, and the loose outer grids you sometimes see -- well, there are reasons that while fusion can be done in a bell jar, no one has ever set a record at it, or even come near. Most efforts get to "detectable" but never to "loads of neutrons and activation studies".

There are plenty of pretty good reasons those of us who have a choice, and especially those who've tried both (me included), don't go with bell jars. If you desire the extra challenge, go for it, but understand that you're probably making it harder, not easier for yourself.
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Chris Roberts
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Re: Bell Jar Viable for Fusion?

Post by Chris Roberts »

Hello Jeff,

I thought I would mention some things we noticed from our attempts at the University of Utah, perhaps it can help you make a decision. You are right that we did get a small amount of fusion from the university's bell jar fusor. However, the bell jar does limit you in a number of ways for shooting for big fusion numbers, and does make it a fair bit more work.

Probably the biggest liability is that x-rays become a real issue when trying to reach voltages that metal chamber operators consider to barely be the starting point for fusion. We have to cap our voltage levels at around 16-18kv. As you mentioned you can get some leaded glass or acrylic to place around our fusor, which would let you crank the voltage up higher, but that can get very expensive very quickly. It kinda depends on if you want to go either low-voltage or high-budget.

If you do manage to cover the thing in x-ray protection and thereby do not have to worry about high voltage x-rays, I would not worry much about localized heating so long as you have a sacrificial container inside as we did with the pyrex beaker. When we were first testing the fusor we made sure to keep tabs on the bell jar's temperatures, and it never got that terribly warm. After turning the system off and opening it up, the inner beaker was definitely too hot to touch, so it was obviously doing its job very well. Also as you mentioned the beaker prevents the bell jar from getting browned from sputtering, and when the beaker gets coated to the point of being opaque you can take it to a glass worker and they can wash it off with acid, and good as new!

The outermost "blast shield" is also a must. We have a lexan box around ours, you can see it in the third post with pictures on the thread you linked. So your idea of having three layers surrounding the plasma is spot on. If you add the x-ray protection so as to allow going to higher voltages, that might make a total of four layers.

The other big liability we noticed is that one cannot maintain quite as pure of a vacuum environment as a stainless chamber. Since you have to prevent the bell jar from being heated by the plasma, there is never the chance to really bake the chamber out and drive away all the obnoxious stuff such as water, pump oil, stray fingerprints, etc out of the chamber. You have to make sure your bell jar and inner beaker are really really clean when installing them, and even then you will never really get rid of all the water and other outgassy stuff. The flat rubber seal is not nearly as leak-tight as conflat or iso/quick-flange stuff, so every time we leave the machine off for a long period of time it goes back up to atmosphere, re-introducing all the ambient humidity back in the system.

We try to counteract these impurities during our runs by valving open our diffusion pump a tiny bit more, and giving the chamber a slightly more robust feed of deuterium, that way we don't give the outgassing stuff a chance to accumulate, keeping the deuterium levels purer. So the purity issue is indeed manageable, just be prepared to use more gas and accept slightly less-than-ideal conditions.

Now, with all those disadvantages mentioned, the one thing that I have to say with a bell jar fusor is that it indeed looks absolutely BEAUTIFUL when running. Once you manage to hit star mode, you can walk around the thing and see all the little rays in three dimensions, and everybody can see it at the same time, rather than all crowding around a viewport. This is the primary reason we made the fusor at the university a bell jar, is that we can show the thing to an entire class all at once, rather than having to rely on a video camera, which cannot capture everything as well as the human eye can.

So to sum things up, there are a number of limitations that a bell jar has which a metal chamber does not, so if you are either hoping to achieve decent fusion numbers, or have the peace of mind with a more reliable, "tried and tested" approach, I might go with a stainless chamber. The arguments for a bell jar are if you are placing a great deal of emphasis on the visual aspect, or if perhaps you already have a bell jar. I would even be cautious to say that last reason, because if you again are trying to make a real fusion performer, you might still be better off building the stainless chamber than using a bell jar you already have. But it can be done, and boy is the bell jar pretty...

Hope this helps.

-Chris
Jeff Robertson
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Chris and Doug,

Thanks for the informative replies, it's nice to hear from two experienced fusioneers, including the person who more or less inspired me to go for a bell jar design. I'm not trying to break any records with this design, like you said I'm interested in the visual aspect of it.

Ideally I would like to exceed voltages of 18kv eventually, so I have plans to implement a leaded glass container. On the inside surface of the leaded glass I will probably put a layer of lexan, as a blast shield. Additionally, I'm not too worried about the glass failing due to localized heating. The sacrificial glass container (the one taking most of the abuse) is within the vacuum itself, so i wouldn't have to worry about it imploding due to atmospheric pressure. I'm not sure if there's any risk of implosion due to other factors, but I figured atmospheric pressure was the main culprit.

The one thing I am concerned about, as you said, is the quality of the vacuum. I'm still researching in this area so I'm not sure exactly how I'll seal the container, but any advice in this area is welcomed and appreciated. I also lack any experience with outgassing so I'm not sure how problematic that'll be, but I'll deal with it when the time comes.

So as you said, Chris, I'm interested in a bell jar because I do place a great deal of emphasis on the visual aspect. I know it'll be a challenge but I think I'm up for it, and the additional cost doesn't bother me too much as long as it's not completely ridiculous.

One last thing, Chris, you said you didn't want to exceed 16-18kv because of xray exposure. Additionally, I remember from your original thread that you guys barely crossed the threshold for fusion. From what I've gathered reading these forums, isn't around 15kv the bare minimum voltage required for fusion (give or take a few kv's of course)? So if one were to introduce leaded glass into the system, thus mitigating the threat of xrays, couldn't you pump the voltage up further and thereby increase the rate of fusion? I mean if the only thing holding you back is the risk of xray exposure, I feel like that could be easily remedied. Unless of course there's some other complexity that goes along with upping the voltage.

Cheers,
Jeff
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Doug Coulter
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Re: Bell Jar Viable for Fusion?

Post by Doug Coulter »

Jus' tryin' to help, and if you're going for max visual, a bell jar is hard to beat - go for it. I'm kind of spoiled with my huge SS tank (the fusor lives in a 6" sidearm) that has a bunch of windows so I can get the 3-d view anyway -- and yes, it's very cool and beautiful. Don't forget to put a big mirror someplace so you can see more than one side at a time in that case.

Somewhere around 16kv seems to be the onset of detectable fusion. It goes up very quickly with voltage above that when other things are right - much faster than linear, and here it's still rising fast at my current limit of 53kv. Seems like most of us found that number in our early work. Just getting to about 30 something kv makes a lot more neutrons, easing your detection problems a lot.

If you take care of X ray exposure, you should be fine going up in volts to get more fusion. They'll eventually darken your glass as well, and have a couple of my sacrificial pieces I put on the inside of my viewports, as well as making them slightly craze/crack. I have managed to shatter pyrex in the tank from flash heating in a beam, even though under no particular strain, but if you're not using a turbo pump without a screen, it's no disaster if it's your sacrificial piece, just a mess. It does take a lot of runtime to darken glass with X rays, at least here - it's not a huge deal.

Any elastomeric gasket is going to have some diffusion into the tank, effectively a leak. Viton seems best, but allows some water through. Others stop the water, but let in some air. For things that are semi-permanent, I've used additional sealant (glyptal) over the Viton and had things improve that way, and you can still take things apart later. PVA glue (elmers, white) also works well for that. I've also had good luck with blue silicone gasket maker (NOT the clear stuff from the hardware store that gives off ammonia, the expensive stuff from the auto parts place that is odorless and cures better).

You may not have much problem with contamination, it depends on the run mode you are gong for. Most let gas flow in all the time, and then try to balance the pressure by controlling the flow of the gas, and the pump. If you do that, it will tend to swamp any little bit of outgassing or water, but it's one heck of a job to "fly" that thing stably -- like trying to keep a precise water level in a leaky barrel by pouring in water just right -- you'll see some talk here about gas control, it's an issue.

I just got tired of that -- fusors are really picky about pressure -- and since I could, I went to a "batch mode" which requires much better tank setup -- no skin off my nose, as I had that already due to some super scrounging by BillF. Your mileage will vary.
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Doug,

A previous student at my school Loyola Marymount, Brandon Sorbom (i believe he posted a couple times on this site), was working on a spherical steel chamber fusion reactor a year ago. Sadly he graduated and left before he could produce fusion, but i believe his reactor was able to create plasma. His reactor is currently in the physics department, probably gathering dust. If this bell jar model turns out to be a total failure (which I definitely haven't ruled out as a possibility haha), I'm sure I could get permission from him/the department to integrate some of his components into my reactor. I plan on making this an independent project for the most part, but if this glass jar design doesn't work I may bring my design to the physics department. We'll see when the time comes.

So you're saying the pyrex shattered due to the heat alone? That's surprising, I'd expect it to melt or collapse at high enough temperatures but not shatter. I am by no means no means a master of high energy physics, though, so I'm sure there's several factors I'm not considering.

I still need to research into gaskets and how I'm gonna create the seal between the lip of the jar and the base of the chamber. Ideally, what I wanted to do was attach a metal ring to the lip of the glass jar so you could clamp, screw, etc the metal part to the base of the machine. I dunno if this would be more or less practical though for creating a high vacuum. Thank you for your ideas for multilayered vacuum seals though, I'll definitely refer to this post again.
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Re: Bell Jar Viable for Fusion?

Post by Doug Coulter »

The pyrex didn't shatter due to heat alone, but due to *differential* heat, suddenly applied, as in one side getting very much hotter than the other before the heat could conduct and equalize. You can easily shatter pyrex this way -- just dump it hot into cold water, which gives the same effect, still hot inside, cold outside.

I think you'll have "issues" trying to bond glass to metal over that size, due to differing tempcos. Pyrex matches tungsten pretty well for tiny seals, but not for large, and everything else is just out unless you can let something have a good bit of "flex". The motions due to this are small, but the forces only limited by the strength of things...oops.

Here's a pic of something "known to work". In this case I was using 3 liter wine jugs for the bell jar. I cut the bottom out in the normal way that's done, and sanded the glass smooth on a plate of steel with sandpaper on it. The baseplate is 1/4" steel, which I copper plated for lower outgassing. The critical feature here is that small ring, about 1/2" high just inside the jug base, with a rubber band around it to keep the jug in place, and keep it from collapsing at the base now that it has no bottom to stiffen it. This is a tight slip fit when not under vacuum, so the base of the jug can't collapse inward.

The actual vacuum seal is that o ring under the jug, which I'd painted with glyptal once or twice, but I don't usually do that (this thing is dirty as it's been on my junk pile, and as you can see, it's also got oil on the baseplate because I didn't turn off the valve to the mech pump one time). I made some high current feedthroughs for the base in the manner of John Strong's book, and any HV I do by making a teflon plug to go where the jug cap was, or simply glue a bolt through the existing cap.

This system never did do fusion, but it worked fine for evap and sputtering, which is what I built it to do, and hey, you get to drink the wine!

These are obviously not pyrex, and so much easier to shatter. I used a quarter inch mesh screen around the thing, but never did break one, as I was careful, and not doing things that make fast particle beams anyway. They do go bad from deposited metal, but hey, these only cost about $12.50 a jug...and can be cleaned a few times before tossing them in the trash.
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Chris Bradley
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Re: Bell Jar Viable for Fusion?

Post by Chris Bradley »

Jeff, I am experimenting with a device which is not a fusor so I cannot give any 'knowing' answers with specific regard to belljar fusors, but I think I can address your specific questions;

>.. I still need to research into gaskets and how I'm gonna create the seal between the lip of the jar and the base of the chamber. Ideally, what I wanted to do was attach a metal ring to the lip of the glass jar ...

Sounds like work for nothing, to me!...

I use a 250mm OD, 300mm high glass cylinder, with a ~10mm ground edge at both ends. That's getting on for 2 metres of glass edges sealed with hand-cut-gaskets. I have cut my own gaskets from various 'trial' rubbers and none show any particular advantages simple because I've never had a problem from them. So long as you are not hunting for 'scientific' vacuums lower than e-6 (that's where I bottom out, and it looks like I can't get lower for reasons not related to the gaskets) then the gasket is likely going to be the least of your concerns.

I've always found the clamping force resultant from the vacuum is enough to keep the gasket in place, but my preferred way to cut the gasket is to cut a hole in a square of gasket, such that there is extra width outside of the bell jar edge. (As the vacuum takes hold, there can be a little slippage until the glass edge bites into the gasket, but after that it is well-bitten into that it won't move.)

I have tried with and without silicone grease and pump oil to 'help' the seal. I have found no point whatsoever in doing this, all you end up with is a bit of a mess to no advantage.

I've had my chamber apart dozens of times, and the rubber seals just keep on doing their thing. You forget how good a job they do after a while, so I suggest you don't worry much about that.
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Re: Bell Jar Viable for Fusion?

Post by Chris Bradley »

>..Does prolonged x ray exposure have any negative effects on polycarbonates, such as discoloration or deformation?..

This is one of the reasons I don't get much below e-5. I use a number of sheets of polycarbonate to protect the glass, and polycarb presents certain problems in vacuo.

It is not so much that polycarbonate outgasses - it's quite a stable molecule and is rated as 'medium for outgassing' on most of the websites you might find on the subject.

But I have found polycarbonate is a p!g in regards sucking up water and contaminants, then holding on to them and releasing them slowly. If you use the stuff, be prepared for outgassing times of weeks. If I tear down my chamber, I run it for a few hours every day for a week or two to work it off, and even then you can still measure it outgassing water remnants. People who work with it regard it as a bad 'outgasser', I guess because it just seems to go on and on bleeding out contamination, but it looks like you can get close to the bottom of all its outgassing eventually if you have enough patience.

I have recently found that transparent PET-P (aka PETG) is now available in transparent sheets (here in UK). PETP is rated as vacuum compatible on some of the internet lists of such materials. I did not find any when I first built by chamber. So, next time I do a major refit I am intending to replace all the polycarb for PETP.
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Sorry I've been busy with work, haven't had time to reply.

Oh wow, I didn't know you had tried anything like this! It looks pretty good, albeit a bit dusty haha. I'm impressed that you were able to use a wine jug to get results, even if it didn't fuse.

I'm trying to see what's going on inside the jug but there's a lot of reflection/dust. Like Chris said in his post, I plan on using atmospheric pressure to create a firm "clamp" between the glass jar and the base. Could you describe any additional efforts you made to make the chamber more "leak-proof"? Were you able to get this chamber down to relatively low pressures (near fusion-acceptable levels)?

Also, how were you able to feed all the necessary apparatuses into the jug (without screwing up the quality of the vacuum)? Since I don't have an opening in the top (like you did with the jug), I plan on feeding all the equipment through the bottom. This includes the pump, voltage source, and deuterium supply. I plan on using alumina and teflon for thermal/electrical insulation where needed, but I'm wondering if there'll be any unforeseen complications by having the feedthroughs somewhat clumped together.
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson »

Chris,

Thanks for the informative response.

I'm only looking to achieve a vacuum high enough to allow fusion. But, on the other hand, I don't want to just *barely* reach the threshold pressure level. I want to be confident in my pressure so that I know it's not my limiting factor in producing fusion.

I'm surprised that you were able to just use atmospheric pressure alone to sustain an acceptable vacuum level. I've done a lot of reading in the vacuum forum, and I was under the impression that sustaining a reasonably high vacuum was more challenging than that. Although I suppose atmospheric leakage isn't the only culprit, maybe I've underestimated other factors such as outgassing.

Do you think your method of using atmospheric pressure to "snap" the chamber into the gasket will be enough to create a vacuum high enough for fusion?

As for your second post...

Sorry, I should have been more clear. I'm applying polycarbonate layers outside the chamber as a "blast shield", should something unexpected happen to the glass jar. The polycarbonate will be outside of the vacuum and safe from any high temperatures, so its sole purpose is to protect the viewers from glass shards. It'll be on the inside of the leaded glass though, so I was curious if it would warp or discolor due to xray exposure. It was more a curiosity than anything else, as I have no problem occasionally replacing the polycarbonate sheets if needed.
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Re: Bell Jar Viable for Fusion?

Post by Chris Bradley »

Jeff Robertson wrote:
> I'm only looking to achieve a vacuum high enough to allow fusion. But, on the other hand, I don't want to just *barely* reach the threshold pressure level. I want to be confident in my pressure so that I know it's not my limiting factor in producing fusion.
> Do you think your method of using atmospheric pressure to "snap" the chamber into the gasket will be enough to create a vacuum high enough for fusion?

As per my post, I get to e-6 with 2m length of hand-cut gaskets, and it doesn't appear to be the gaskets that stop me going lower. With all the pipework, etc., to my nominal 60 l/s turbo, I have a calculated total flow conductance of around 16 l/s from the port of my 10 litre chamber, and an outgas rate that I tend to be able to get down to just under 0.01sccm after several days pumping (a virtual leak rate of around 1 micron/1-2 mins).

[My chamber contains 4 x A4 sizes pieces of 1mm polycarbonate, 2 x 150mm square mild steel, 400 mm length of 20 mm diameter teflon, 750mm of kapton insulated silver wire, and around 1 m of 25mm wide acrylic adhesive kapton tape, 8 spark plugs and nitrile o-rings, and an assortment of smaller SS, teflon, PE and nylon pieces, in a chamber of 1,000sqcm of Al, and 2,500sqcm of glass..... Whereas you'll have - a feedthrough and a grid of metal wire! Therefore, one can presume your outgassing rates will be somewhat lower!!]

You can work out my expected base pressure from those figures [pumping 1e6 cc/min with 0.01sccm outgassing == 0.01sccm/1e6 = 1e-5mbar]

So you can see with this figure it depends on your outgassing rate (which should be much lower than mine) and the speed at which you can pump. Tiny pump = worse vacuum. Enormous pump = you can pull even a dirty leaky chamber down low enough!

If you are aiming for a 10 micron working pressure, then getting a 1e-5mbar base pressure in your chamber means, theoretically, you get a 99.9% pure environment after backfill to 10 microns. Lower than 1e-5mbar when aiming for a 10 micron operating pressure looks like overkill. You may choose to aim for lower pressures so as to perfect your vacuum skills, but I do not see how it is necessary.

.
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