Fusor reactor size question

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Jacob Lord
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Fusor reactor size question

Post by Jacob Lord »

Hello, before I start I would like to say that I am VERY new to fusion theory and have only started to read up about it in detail around about a month ago.

So here comes the question. I am working on some plans of building my own fusor as a long term school project but there has been this niggling question in the back of my head, does the size of the reactor chamber have a difference with the output and would charging the Deuterium before injection have any effect as well. With my reactor I was hoping to have it be run at the power level of around 100Kv (pretty big charge for a first project, but where would the fun be if it were easy), so in the case of this high powered reactor, would having a larger chamber be more beneficial to the rate of neutron reactions, say a 1mtr radius chamber than a 10cm radius chamber?

And the second question is for the fusion fuel. In the reactor i was going to use Deuterium, so I was wondering that if pre-charging the gas before being injected into the chamber would be beneficial as well.

It would be nice to hear your thoughts on this.
Tom McCarthy
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Re: Fusor reactor size question

Post by Tom McCarthy »

Welcome to the forum Jacob.

In relation to your post, yes, chamber size will have an effect on your neutron emission as larger chambers can "support" higher voltages - as chamber size reduces you may reach a level where arcing can take place inside your chamber. Higher voltages have a direct effect on your fusion rates. Current and pressure also affect neutron rates.

Voltage is related to the deuterons' cross sections or likelihood to fuse. The electric potential gives the particles themselves potential. Also, I'm not sure if this is confirmed somewhere, but I believe that a higher voltage creates a larger force acting on the deuterons, making them move faster as they speed toward the grid.

You say that you plan to work with 100KV and this is your first project. If that is the case, I would advise you to try and find yourself a cheap neon sign transformer of about 10KV and play around with that before trying to get at anything larger. Using a smaller supply will help you get an understanding of some sort of what you will be working with. Another thing to think about is how do you plan on getting a beast that produces 100KV? Aside from dental transformers (which require a bit of wiring and are hard to get), you are faced with either acquiring a pre-made supply or constructing your own supply.

Pre-made supplies capable of providing 100KV at 10+mA (over 1,000 watts) are not easy to find and if you find one, it will most likely have an exorbitant price tag attached.

On building a supply: As a first project, building a 100KV switching supply (multiplying would be expensive) is not very smart. The safety issues involved are beyond comprehension of most but a qualified electrical engineer or very experienced amateur who has already built something akin to this. If you go down this path, I would strongly advocate approaching an electrical engineer who would be willing to help you out.

In short, if you are going to be working with 100KV, which has never been done before by anybody on this forum - highest is 43Kv I think, it could be about 60KV though - you will definitely need a larger chamber and good radiation shield to protect you from the huge amounts of x-rays you will be producing.

I find your second question hard to understand, please re-word it and I'll try and help out.

As a final note, here are some very good FAQs to get you started. You'll most likely want to read all the FAQs in the forum to set you straight on the mechanics of Fusors.
viewtopic.php?f=24&t=9174
viewtopic.php?f=24&t=3223#p12733
http://fusor.net/board/viewtopic.php?f=29&t=4843
http://fusor.net/board/viewtopic.php?f=29&t=9171

Tom
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Chris Bradley
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Re: Fusor reactor size question

Post by Chris Bradley »

Tom McCarthy wrote:Welcome to the forum Jacob.

In relation to your post, yes, chamber size will have an effect on your neutron emission as larger chambers can "support" higher voltages - as chamber size reduces you may reach a level where arcing can take place inside your chamber.
This is the wrong way around. At the pressures of an operating fusor you are at the Paschen minimum where breakdown reverses.

If you have ever experimented with lots of different setups, you'll realise that increasing the spacing between cathodes from earthed points leads to 'lightning' right across the chamber. Your best bet is to shield the cathode with a suppressive electrode at some median voltage.

I can't help myself but to picture these forms of cross-chamber breakdowns in the Paschen minimum as 'dragons' because they fly across the chamber and you just can't tame them!

Basically, there is no evidence I have seen to indicate chamber size has a clear effect one way or the other. And there is no point in ionising deuterium before injecting it into a fusor chamber because it would almost immediately be quenched, as the volume of a fusor is mostly neutral gas.
Jacob Lord
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Re: Fusor reactor size question

Post by Jacob Lord »

Thank you tom for your help so far.

As you say, a 100kv fusor is a bit ambitious but I was thinking of using a aircraft transformer to get voltages that high(specifically the ones used in the AWACS, the ones which have a by product of producing cyanide), but also I think that it is best to start with a low power reactor as an entrance into this type of science, lets not dive before we can swim.

With my question about pre-charging the D, the way I was thinking about is that when you inject your fuel into the chamber, it gets charged by the positive outer grid then gets attracted into the inner grid. what I was thinking is that if you pre-charge the D as or just before entry into the chamber to be charged as normal, it would make the process of charging the gas much more efficient and thus could improve reaction rate.

I hope I have worded the pre-charging bit better because I absolutely suck at wording things.
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Dennis P Brown
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Re: Fusor reactor size question

Post by Dennis P Brown »

Hope you realize that voltage alone will not achieve fusion? That is, power needs to be at least 200 watts or higher. A 100 kV power at micro amps will be useless and a transformer that can deliver 2 ma or higher @ 100 kV would be a rather large beast and extremely dangerous. Besides the massive & deadly x-ray production/flux the electrical danger would be extreme, as well.

As Chris pointed out, pre-ionizing a gas in a chamber at that high a pressure is useless - the ions are neutralized almost instantly.
Jacob Lord
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Re: Fusor reactor size question

Post by Jacob Lord »

OK, so ionizing the gas is a no go.

And on the front of the radiation and power usage. the place that I am hoping to be able to construct this will be at my collage/uni where I am hoping to get a room dedicated to the reactor because I'm building it with the intention of then donating it to the collage and uni for its own purposes, with this in mind, I am going to hopefully be able to fit the room with thick shielding and either get the aircraft transformer or a substation dedicated to the reactor, so I would be able if I wanted to(not sure why i would want to) go to 200kva. Everything else is able to be constructed at the school with its facility's. but this is around 5/10 years down the line from now.
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Chris Bradley
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Re: Fusor reactor size question

Post by Chris Bradley »

Aircraft transformers and sub-stations? I think your imagination is getting ahead of reality there.

You have suggested the perfect route already - build yourself a modestly sized ~30kV 30cm fusor first, and then see where that takes you.
John Futter
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Re: Fusor reactor size question

Post by John Futter »

I must say something
Tom
"multiplying would be expensive"

What made you write this??
Nearly 99% of commercial supplies use multipliers on the output to keep the cost of the transformer and rectifier and its construction within reasonable achievable bounds.
An upper limit of 10-15kV ac before the multiplier is the norm except in very large older (note older) 50/60Hz x-ray supplies that are intended for very short duty cycles.

I have said this before SMPS design becomes more and more difficult as the turns ratio goes higher than 1:100. Leakage inductance starts to run away causing large voltage transients that are reflected back into the primary playing merry hell with the switching device, its drive electronics and general EMC issues.

My advice to all on this forum is to seed the ebay search in differing ways for high voltage power supplies. I got a very good reversible Bertan 30kV 1kW supply for US$350.
And yes I do design high voltage supplies for a day job. And I cannot build one for 3 times what I paid for the Bertan
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Re: Fusor reactor size question

Post by Tom McCarthy »

Thanks for amending that Chris.

John,
I came to that conclusion because I guessed that the necessary capacitors for such a multiplier ( 4-stage beginning at 20KV?) would not be light on one's wallet. I was also taking a guess that an experienced "do it yourself" man would be able to source his parts - or have them lying around from previous projects - for much cheaper than your average newbie.
Either way, as you're the one working in the field, you know what you're talking about and have more knowledge of the topic than me, mistakes must be corrected!

Tom
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