Building a Fusor with University Help

For posts specifically relating to fusor design, construction, and operation.
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Connor McCreesh
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Building a Fusor with University Help

Post by Connor McCreesh »

Hey Everyone,

I have started my fusor construction journey and wanted to document it here.

I'm a British student studying a Masters in Nanoscale physics. I am currently in the process of gathering components together for a fusor project which I hope to have completed within about 4-6 months.

At the moment I should have pumps secured (we're just checking the pressure they can reach - needs to be 10^-9 atm if I'm correct?), and I can get the vacuum chamber ordered or created (so I need to draft up some specs). The main problem I have is the voltage supply (not that the University couldn't buy one, but it would be much easier if I could use one they have currently, and they don't have a 30kV supply to hand). So I have a couple of questions I just wanted to run past the community:

* The university has very accurate neutron detectors, I am currently finding out the lowest neutron flux that they can detect. I wanted to know if there is some sort of relationship relating neutron flux and input voltage? I will put up the minimum flux I can detect early next week, but if I could get an idea of the lower end of voltages I would be able to apply, to detect fusion, that would be great.

* On a similar note, as it is a project with the help of my university, safety standards have to be higher then they might otherwise be, so is there a similar relationship between the neutron flux and the thickness of the stainless steel vacuum chamber needed to shield against the neutrons?

* What is the suggested diameter for a viewing port (keeping in mind that a camcorder will be doing the viewing)?

Thanks for the time guys,
Kindest Regards,
Connor
Last edited by Connor McCreesh on Sun Dec 01, 2013 7:00 am, edited 1 time in total.
Julian Kang
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Re: Building a Fusor with University Help

Post by Julian Kang »

Hi Conner,

You may find this FAQ useful viewtopic.php?f=24&t=3162 as it covers the safety of view ports in regards to your last 2 questions. I assume you're using a 30kV transformer, which means X-rays will be coming out of your view port in a wide cone. The FAQ recommends a 2.75" sized conflat due to the cost and several other reasons. Stainless steel chambers which are normally 0.60 thick are generally safe up to 25kV. As you are going into the 30kV and above, you may need a thin lead shield to reduce the x-ray exposure or operate it remotely (at least 10 feet between you and your fusor.

More information can be found here viewtopic.php?f=24&t=3189

Julian
Connor McCreesh
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Re: Building a Fusor with University Help

Post by Connor McCreesh »

Thanks for the link Julian. At the moment the University doesn't have a 30kV supply, and the problem would seem to be that as they are a University, they are unlikely to buy an old piece of equipment for me to strip, due to safety reasons. The solution I am hopefully coming to is that because they have access to sensitive neutron detectors, I can lower the supply voltage to something they have, and still detect fusion neutrons.
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Richard Hull
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Re: Building a Fusor with University Help

Post by Richard Hull »

At 30kv you will be making easy neutron detection if you have a good detector like a BF3 or 3He tube. You might expect up to 200,000 n/s (isotropic) at this applied voltage. The flux external to the fusor can be more or less calculated based on its size.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Connor McCreesh
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Re: Building a Fusor with University Help

Post by Connor McCreesh »

Thanks Richard. The fusor is likely to be a 6" diameter sphere. Is there a minimum voltage you would expect to need to get neutrons?
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Richard Hull
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Re: Building a Fusor with University Help

Post by Richard Hull »

It depends on the operator's skill and current, but more on the detector.

In general, a well constructed and operated fusor of 6" diameter can give a positive, real time, neutron count out to a decent sized high pressure BF3 or 3He tube and counter that will not require any counting statistics at about 15-20kv with aboutt 10ma of current.

With long background counts and long operational counts 10kv @ 15 ma is often enough to detect neutrons at a relatively easy statistical level. The counting becomes non-statistical and obvious at about 25kv.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Connor McCreesh
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Joined: Sun Nov 10, 2013 2:41 pm
Real name: Connor McCreesh

Re: Building a Fusor with University Help

Post by Connor McCreesh »

Currently have a vacuum set up with a two stage mechanical pump and diffusion pump, and also have access to BF3 and 3He detectors, but could also get access to more precise detectors if needed.

I have been given a room in the University but I need to make a risk assessment for the fusor project. A main point is that the neutrons exiting the chamber will be of high energy and thus should not be stopped by the thickness of the steel chamber. From information I have gathered on this forum, am I right to believe that because there is such a low intensity of neutron flux (at 10-20kV), that even though they penetrate the steel chamber, they are below background levels and so not a problem?
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Richard Hull
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Re: Building a Fusor with University Help

Post by Richard Hull »

Even a poorly designed fusor operating at 25kv or above will have neutron outputs well above background. If it did not, how would you detect fusion? We are blessed, indeed, that they zip right through the fusor chamber walls as if they were non-extant. It allows all the other fusion debris particles in the chamber to be absorbed in the chamber walls allowing only the desired, countable neutrons to signal the fusion process in our neutron counters.

X-rays become the dominant and major issue at about 35-40kv applied as they will start to "shine through" a 2mm steel shell at those levels. Thin lead shielding, (3mm), will provide full protection against these x-rays, however.

I don't know how anyone might consider a nuclear reactor project without realizing some radiation would be present.

No amateur fusor represents any genuine radiation danger to any operator if operated for limited periods, (demonstrations or student experiments), and operated under 40kv. Regardless, the operator and any observers should not stand any closer than 2 meters from the device while in operation. This is as much for high voltage electrical safety as for any perceived radiation issue.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Connor McCreesh
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Joined: Sun Nov 10, 2013 2:41 pm
Real name: Connor McCreesh

Re: Building a Fusor with University Help

Post by Connor McCreesh »

I thought as much, thank you Richard. Will keep the thread updated as I progress.
Connor McCreesh
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Re: Building a Fusor with University Help

Post by Connor McCreesh »

At the moment I am struggling with the power supply. Would rectified AC work? It would be one directional, but I assume it wouldn't work because you want to slowly vary the input voltage without it fluctuating between 0 and your value. Just wanted some clarification on this though as rectified AC would likely be easier to get hold of and implement.
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Richard Hull
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Re: Building a Fusor with University Help

Post by Richard Hull »

100% of all power supplies that do fusion in our fusors are AC rectified power supplies, which makes them "DC power supplies". The key is the voltage never reverses and is always only one single polarity. That makes it a DC supply even if it is going from 0 volts to -50,000 volts and then back to 0volts, 50 or 60 times a second. It will do fusion!

A fully filtered supply offers unvarying DC with little more advange and often distinct disadvantages. (Control and runaway issues if not current limited.)

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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