FAQ-Operating a fusor cannot be oversimplifed

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Brian McDermott
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FAQ-Operating a fusor cannot be oversimplifed

Post by Brian McDermott »

There seems to be a common misconception that operating an amateur fusor is as easy as flipping a switch and cranking a few knobs. While there are probably exceptions, I have found that this is almost never the case. This is not meant to discourage anybody, but to make you aware of all the little things that make operating a fusor less than a walk in the park. Whether you are young like me or older and working, take my advice as you would from any of your peers.

To paraphrase Richard Hull: "...Umpty-umpf [kilovolts] just rolls gently off the tongue like warm butter sliding down the back of your throat..." but how many people come into this hobby having actually handled 20,000+ volts at considerable current? I know I didn't, yet I still committed the sin of coming into this thinking it was as easy as plugging in a string of christmas lights, only to learn later that it was not the case.

Electricity does weird things above 10,000 volts, and all sorts of precautions need to be taken. Insulators get really thick, really fast and the price goes up accordingly.

The x-ray hazard cannot be overstated. At 50kV, I drape a lead blanket over the manned side of the fusor and let that be the end of it. The wall's side is open to the x-rays. One time, I put my ion chamber meter over there and the reading was in the R/hr range. Behind the lead it was normal background. This is at 50kV, remember. If you're thinking about 100kV, you better have a concrete pit ready.

When the fusor is actually fusing, there's no guarantee that it will continue to do so for very long. While I think Carl Willis once mentioned a run time of 40 minutes, my longest run was about 10. More often, they are under 5. The chamber heats up considerably, driving all sorts of crap off the walls. This messes with the discharge structure, the voltage falls and fusion ceases.

The deuterium flow must be monitored and tweaked at all times. It is not as simple as shutting the vacuum valve and injecting the gas. Natural outgassing causes the pressure in the chamber to rise, ruining your mean free path. The vacuum valve must always be open to remove these gases, and the deuterium flow must balance the vacuum pump in order to maintain a constant pressure.

Just because you set the control knob to 30kV dosen't mean that your voltage will stay there, especially in a current-limited power supply such as those made by Glassman. A minute increase in pressure can make the voltage fall considerably, and send the current through the roof. When you go to turn the knob on the deuterium control valve, you had better not turn it too far, or the discharge will extinguish.

This is but a taste of all the things that can annoy you to no end. Others include EM interference, instrumentation problems and vacuum issues.

It is so easy to say, "why don't you try this in your fusor..." but it is not so easy to go and implement it. You need to build the machine and experience it in order to properly gauge what is and what isn't practical.
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by Frank Sanns »

Nice post Brian.

I will go a step more and say that if you are just starting with a fusor or a demo fusor you would be nearly a fool to start with more than 10kv-15kv and 10 ma current. Even if you have all conflats and great pumps and valves and deuterium and everything that the best fusor here on this site has, you will not get rountine stable runs for months even if you work at it.

There are many pieces of the fusor that you just need to develop technique to run and to run safely. I have had my fusor for years but running at 40 kv is not something that I do. Most of my runs are 20 kv and 15 ma. If your run higher you have all of the extra electrical hazards of something flashing over or arcing to you or to some other equipment, you have the huge radiation hazard that starts as low as 12 kv. , and you have melted grids and possible catastrophic oscillations that can occur with larger power inputs. All of this and you have not balanced input and output flows, measured neutrons safely and reliably, measured x-rays (some that won't show up on certain detectors but can still be radiating you). In a month, you may even have not even found all of your leaks.

If I were starting a fusor demo or otherwise. I think 12 kv and 10 ma will get you going nicely. With proper instrumentation you will detect neutrons with this setup. At that point, trade up to an x-ray transformer and go for it. You may find though that it was far simpler and enjoyable to run at the lower powers. Higher voltage will give you more fusion and so will more current. The higher voltages and currents will also give you exponentially more hazard and difficulty with control. If you haven't heard yet, you don't just crank the voltage up to get more fusion, you need to aquire the skill to operated the fusor in a more rarified condition to ever get the voltages to rise in the first place.

Frank Sanns
Achiever's madness; when enough is still not enough. ---FS
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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Richard Hull
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by Richard Hull »

Wow!!! I was beginning to think no one else knew enough to leave a true FAQ! I was also worried that folks thought that only I could write a FAQ by some edict or other. T'aint so. Witness, Joe Mosters recent superb FAQ primer on gas measurement.

THANK YOU BRIAN!

It is good to see those who have done the work create useful and valuable FAQs. (which this one sure is)

Between your and Frank's comments, I think we have this FAQ's subject covered fairly well.

The fusor is indeed an investment in time and money in addition to being an intensive, crash, self-teaching course in practical plasma physics, materials science, engineering, welding and machining, gas handling. electronics, vacuum technology, nuclear metrology, nuclear physics and the fine are of scrounging.

The experience leaves one appreciative of what it takes to have "the right stuff" and how it has made one knowledgable to a base level in many disciplines one didn't know they would have to dive into and proud to have made it out the other side with a fusing device.

The neutron club is a very special place with very special people in it. Placed there not by their peers, but by themselves due to the sweat of their brow and forfiture of time and treasure. Their peers can only welcome them into the fold. This is a group who openly welcomes others and exhorts them to the "doing" yet never looks down on those not yet inducted by their own efforts.

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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Brian McDermott
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by Brian McDermott »

Some people are quick to point fingers at us and say "all you're doing is replicating an experiment done long ago. What progress is there in reinventing the wheel? Can't you guys try new ideas?"

The answer is simple. It takes time and money to even get the infrastructure in place before any futher experimentation can be done past the bare-bones fusor. It took me 3 years and over $3000 to get where I am now, and even then I have not really surpassed Hirsch's original machine. I'm sure the individuals here can all speak to a similar situation.

For most of us, this is not a full time activity. We are hobbyists with limited funds and resources, and we all have lives outside of amateur fusion. We do not have the time or money to try all sorts of crazy ideas that any one person may dream up, no matter how good it may sound. There are lots of good fusor design ideas out there, but the burden is on YOU to test and validate them.
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by HighVoltageFox »

Thank you for that FAQ. You anwsered a good many of my questions. But you said that you get crap coming off the fusor walls and messing with the discharge structure. Are you saying that contamination from the walls of the fusor hit the grid? And I am planning on making my grid out of tungsten, should I not use W because particals will fuse to it, and W being expensive would be to costly? I didn't even realize this factor untill you mentioned it.

Thanks,
Andrew Hahn
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Brian McDermott
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by Brian McDermott »

No, most of the outgassed stuff doesn't even get ionized. It just creates instabilities in the pressure and contaminates the gases in the chamber. If the pressure is unstable, the discharge becomes unstable. You'll get a lot of electrical noise that will screw with your instruments and the current will drop to only a few milliamps. The poissor itself will grow dim and appear to flicker very rapidly.

A lot of the outgassing is water, nitrogen, hydrocarbon vapors and even excess deuterium that got buried in pores in the walls. The heat causes it to desorb and enter the atmosphere in the chamber.

Tungsten is really hard to fabricate, but it makes a pretty good grid material. There is one catch. Material is ALWAYS sputtered off the grid (no matter what material you use) and deposited on everything in the chamber, including insulators and viewports. This is normal and cannot be prevented. In some industries, this is how mirrors are made, but for a fusor it is not good. This metal film will cause viewports to become opaque and insulators to become conductive. It must be cleaned off after maybe 100 hours of operation time. With a stainless steel grid, a little bid of hardware store acid will do the trick. With tungsten or tantalum, you must use molten lye or a sandblaster to clean the deposits off.
sean burke42
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by sean burke42 »

hello brian and richard this is Sean Burke i thank you for your FAQ you guys have taught me valuable information through these i know the required tools for a fusor build and know i know what to do once my fusor reaches 50kv drape the great insulator lead over the operators side you have taught me much i look forward to seeing more FAQs from you two keep up the good work

Sean
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by scinut »

Brian,
Thank you for your postings. It is amazing to realize how many different areas of science, engineering and all around common sense is involved with a project like this. Many areas, I have found, require solutions that are not necessarily self-evident, or intuitive. Although I have been fortunate to have worked on many amazing projects, few of them required the knowledge of attaining high voltages/current. Beside the dangers of high voltage, you mentioned radiation. There is also high vacuum science. I have some experience with high vacuum, but not enough that I am at all confident about it. These are the kinds of subjects that must be reiterated as much as possible, not so someone can create fusion faster and easier, but so they will be around to enjoy it with all of their digits and eyes intact, or to be around at all. I count on seeing posted results about how to do this safely.
May I ask, what events or event inspired you to post this at this time?
Again, I thank you and others for your postings,
Jerry
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Carl Willis
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by Carl Willis »

This is a good FAQ and remains useful, but Brian didn't post it recently. It's dated 2005. Someone just responded to the thread today. A post's date can be seen in its title, for example

2005-09-20 16:54 FAQ-Operating a fusor cannot be oversimplifed (Brian McDermott)

-Carl
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Re: FAQ-Operating a fusor cannot be oversimplifed

Post by scinut »

Thank you!
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