Bell Jar Viable for Fusion?

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Jeff Robertson
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Bell Jar Viable for Fusion?

Post by Jeff Robertson » Thu Jul 21, 2011 4:29 pm

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 » Thu Jul 21, 2011 4:44 pm

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 » Thu Jul 21, 2011 10:39 pm

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".

Jeff Robertson
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Re: Bell Jar Viable for Fusion?

Post by Jeff Robertson » Fri Jul 22, 2011 2:40 pm

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 » Fri Jul 22, 2011 4:31 pm

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 » Fri Jul 22, 2011 5:53 pm

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 » Fri Jul 22, 2011 11:23 pm

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 » Sat Jul 23, 2011 1:59 am

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.
Why guess when you can know? Measure!

Chris Roberts
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Re: Bell Jar Viable for Fusion?

Post by Chris Roberts » Sun Jul 24, 2011 8:19 pm

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 » Mon Jul 25, 2011 4:37 pm

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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