Demo fusor first light

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Pablo Llaguno
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Demo fusor first light

Post by Pablo Llaguno »

About two months ago I started looking for parts and getting ready to build a demo fusor. After some work, we, my team and I, were able to make our demo fusor, which was intended to be used as a science fair project, which was this last weekend and overall it worked very well.

In the video I will link below, you can see how we were able to create a purple plasma, needless to say, this "demo fusor" is not as good as lots I've seen here, but this is just a start and we hope to get better at it and in a few months be able to produce neutrons (I know it will take a lot of redesign). Obviously, this will take a lot of work and research, and I am sure I will find lots of info here.

Some things to note:
·We used a 15kV @ 30mA (rated) NST, we didn't use a voltage meter or current meter, but judging by a consumption chart and what most people say about NSTs, we probably are in the 7kV range or less.
·Our vacuum system wasn't very good, mainly because of our gauge, because we didn't have the funds at the time to get a nice thermocouple gauge, so this is something we will try to improve and hopefully we will be able to give some readings.
·Also is worth noting that in our video the plasma was purple because of possible leaks and not letting the vacuum pump enough time, in our science fair we let it pump for about 5 minutes and instead we got a blue almost white plasma and it was more confined. We will try to record that.
·Talking about our plasma, in our science fair our glass chamber suddenly turned black, possibly because we were producing a lot of heat, so we had to disassemble and clean (which was very difficult) but we managed to do it quickly. After this, we restricted the time our fusor was running.

And that's all! Hope you enjoy and any suggestion/comment/critic would be appreciated.

Link to video: https://www.youtube.com/watch?v=RCsls0o6kHI

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ian_krase
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Re: Demo fusor first light

Post by ian_krase »

My comments on this:

1. Don't forget the possibility of a Mini Convectron or MicroPirani gauge (make sure it's the version with the built in digital display!) which come up on Ebay routinely. These have the nice feature of working all the way from medium vacuum to atmospheric, and the full setup (used!) may be cheaper than a themocouple. It's what I use, anyway.



2. If you were able to get to the point where the plasma turned whitish, but it took a very long time (for a chamber that small), then it sounds like you have fewer leaks than I ever have had, but very slow pumping speed. That long, thin foreline may be the problem. With a chamber this small, you can possibly do what I do and bolt it to the top of your pump (make sure to use some kind of screen or horizontal run so nothing falls in if there's an accident).


I'm curious what you are using as a feedthrough.
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Dennis P Brown
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Re: Demo fusor first light

Post by Dennis P Brown »

If the chamber turned black - that is a sure sign that either pump oil (back flow) or gasket material is being "burned" by the plasma. Until you have some type of working vacuum gauge, you will remain in the dark on possible leaks and performance of your vacuum pump/system. A vacuum gauge is rather critical to move forward in using your demo system.
Pablo Llaguno
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Re: Demo fusor first light

Post by Pablo Llaguno »

Thanks for the comments/suggestions,
Yes we will look into a new gauge, those mini convectron gauges seem to be fairly common on ebay and not expensive, We just have to figure out what thread size/fitting we want. I am not familiar with ConFlat or KF, just with standard milimetric or NPT/UNF fittings, any suggestions or links to where I could learn about them? From what I read, a ConFlat works with an o-ring and an edge making pressure on it, which would mean I will need to machine/buy an adapter for my chamber, and in that case maybe I should probably make a new chamber anyways. What do you say?

Also yes plasma started purple, turned blue and then almost white but didn't took a lot of time. Something like 5 minutes or so. BTW our chamber's volume is 0.9 liter (something like 54 cubic inches) and since the vacuum pump was borrowed, we don't know the specs because the printings on it aren't visible anymore, but it is a robinair and looks like a two stage. In the CFM rating, I can barely see a 6.

We used a lawnmower sparkplug, cut a bit of the threads and then welded a #10 SS screw to it, which is welded to the grid.

As for leaks, we may have some in the seals, we are not to proud of them, and we are not even sure they work all that well. If we had the equipment, we could make a groove in each plate for an o-ring and it will probably seal way better, but in that case maybe we will just make a new chamber.

We did some leak testing with a nitrogen tank and filled the chamber to about 40psi (had to replace vacuum gauge with a pressure gauge) and checked for leaks with soppy water, we fixed the big ones at the fittings but not the ones around the rubber seal...
Pablo Llaguno
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Re: Demo fusor first light

Post by Pablo Llaguno »

Dennis P Brown wrote:If the chamber turned black - that is a sure sign that either pump oil (back flow) or gasket material is being "burned" by the plasma. Until you have some type of working vacuum gauge, you will remain in the dark on possible leaks and performance of your vacuum pump/system. A vacuum gauge is rather critical to move forward in using your demo system.
It could be burning but the weird thing is that it turned black instantly just after a big blue arc appeared, so I think maybe we just run it for a very long time.
ian_krase
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Re: Demo fusor first light

Post by ian_krase »

A sudden "big blue arc" is... something going badly wrong, but I'm not sure what it would be, I'm afraid. Was your grid (and everything else) intact afterwards? What kind of arc was it?




Conflats don't use O-rings, although you can use O-rings or a special rubber gasket with them for quick temporary setups. They are expensive and use a soft copper "crush gasket" -- the knife-edge rings on conflats bite into the copper gasket, and you can generally only use the gasket once.

For a demo fusor, conflats are of questionable utility and I'm not sure if they are absolutely needed for a real fusor (can somebody more knowledgable chime in?). They also are large, bulky, heavy, and expensive. You will probably want to stay away from it for anything like a small gauge port.

VCR fittings are like tiny Conflats. Also very expensive and not really relevant for this.

I'd aim for a KF16, KF25, or NPT fitting. For gauges the 1/8 NPT seems common and is probably a good enough choice.

KF fittings, and their bigger brothers ISO flanges (sometimes ISO LF) are easier to work with since they use O-rings to seal (of course, this means that they are not very heat resistant). A cool thing is that you can buy solderable/brazable brass KF fittings and then use ordinary plumbing solder to connect them to copper tubing, which lets you build small assemblies cheaply.

Personally I wouldn't be too quick to throw much stuff out -- I can't quite see how your chamber is constructed -- but incremental improvements do a lot. Things will suddenly get much better when the last leak is sealed.


Do you have vacuum grease? If so, you can probably improve your chamber seals. Also, with a can of Dust-off you can probably do leak checks using a glow discharge -- it will abruptly change color (to paler and brighter) and get larger when you spray the gas around the leak. No need for a gauge with this trick (though it is nice to have). This will find lots of leaks that are not really visible with pressure and bubbles.
Pablo Llaguno
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Re: Demo fusor first light

Post by Pablo Llaguno »

I am not sure if it was an arc or what was it, but what I do know is that it was like a flash and suddenly my chamber was black, maybe it was the heat from the plasma? Also regarding NPT fittings, do they work well enough even for a real fusor? And lastly, yes I did use vacuum grease, how can I improve that?
John Beutz
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Re: Demo fusor first light

Post by John Beutz »

Simple google search with the keywords "fusor" "NPT" and "vacuum" gives some good results.
viewtopic.php?f=10&t=4246

Your plasma looks a lot like my old demo fusor's. I found that a minimum amount of vacuum grease over the rubber gasket worked better than a slather. The black on the chamber might be burning rubber. Does your gasket extend into the chamber? Hard to tell without pictures.
Pablo Llaguno
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Re: Demo fusor first light

Post by Pablo Llaguno »

So for a real working fusor I would probably want KF fittings, thanks. Here is a picture of my chamber Image
John Beutz
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Re: Demo fusor first light

Post by John Beutz »

From what I've seen on the forums and with my experience with a much inferior pump, I bet with a pump like yours a KF chamber should get down to a couple dozen microns easy, even with a few NPTs. KF crosses and tees can be found on eBay for less than 100$
ian_krase
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Re: Demo fusor first light

Post by ian_krase »

OK then...

1. If your setup is like my setup, that ball valve may be Bad News. Greasing it (if you haven't already) may improve it, but is not that great of a solution.

I think you're using it as vent/airleak valve, right? At least in the USA hardware stores sell small and inexpensive brass needle valves which work well for this purpose -- you can incrementally open them (though they don't compare with a serious micro needle valve). Get the angle type, disassemble, degrease, replace with vacuum grease, and put the male NPT that is in line with the valve stem into the chamber.

2. NPT isn't necessarily too much of a problem, but it looks like you're using the white type of Teflon tape. For vacuum purposes, this tape is usually inferior to the natural gas rated kind which is (IIRC) usually yellow or the milspec type which is also white but stiffer and thicker. An even better (but harder to disassemble) option is either epoxy (Torr-Seal (expensive) or Hysol 1C Epoxi-Patch) or Loctite 290.

3. Make sure your chamber top and bottom plates are flat and smooth, the ends of your glass are flat and smooth, your gaskets are flat, parallel and smooth, and that everything stays that way when you bolt it together -- are the four threaded rods tightened the same amount? Can you get it all sucked together with air pressure and then "shift" it -> either a better or worse seal?

4. Vacuum grease kinda goes two ways. In an ideal world, you would not use any of it. When you have problems, it can be a powerful way to "cheat" on leaks, but it's not actually a *good* solution. So generally you actually want to use a very thin film which cures many scratch-related problems.

5. What are your top and bottom gaskets made of?
John Beutz
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Re: Demo fusor first light

Post by John Beutz »

Epoxy (and 290) is pretty easy to remove with a little heat. It can even be done with a match, I've found. Put one of the two sealed components in a bench vise (parallel to the floor), and have one guy get a wrench on the second component and apply a little pressure (In my experience you shouldn't necessarily over-tighten when you have epoxy on the threads, it just makes it hard to disconnect). Then hold a match on the threads until it starts to give under the force of the wrench. But it's much easier to just take a blowtorch to it until "All zat is alive simply evaporates".
Pablo Llaguno
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Re: Demo fusor first light

Post by Pablo Llaguno »

Thanks for the replies guys, Ian I will most likely follow what you've said, also my gaskets are made of natural rubber, should I consider another material like nitrile? Also, I have some blue Loctite laying around that I could use, what do you think? Do you guys have any tips as of how much time should I let the pump running? In all of my tests, I let it pump for 5 minutes and then turned on the HV.
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Garrett Young
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Re: Demo fusor first light

Post by Garrett Young »

A fluoroelastomer like Viton (FKM) is best in terms of creep resistance, temperature, and outgassing.
- Garrett
ian_krase
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Re: Demo fusor first light

Post by ian_krase »

Yeah, natural rubber has a lot of problems, including the rather serious problem of being ruined by grease. Honestly the main advantage of natural rubber is its mechanical properties.

Viton (also known as fluoroelastomer or FKM) is a bit expensive but very good. It's also very heat resistant. My gaskets are made of viton, at a cost of about 12 $ USD each. Unfortunately, most Viton is a little stiff/hard which makes it worse at sealing large imperfections. Most KF fitting O-rings are made of Viton.

Nitrile, often known as Buna-N is a cheaper option that is also good for vacuum. It is often softer, is much cheaper and easier to get, and while it's not as good for vacuum as viton, it's still pretty nice. A number of my O-rings and stuff are Buna-N just because you can buy Buna O-rings at the hardware or auto parts store sometimes. Also, before I completely standardized my chamber I was using Buna gaskets because they were only a few dollars.

Viton/FKM is not to be confused with Kalrez/FFKM/Perfluroelastomer, which is hyper-expensive and only useful when you have aggressive chemicals.

Silicone isn't so great. It outgasses and permeates much more than Buna-N. In fact, silicone tubing is sometimes used in the biology world as a kind of artificial lung or gill for gas exchange. It's main value in the world of vacuum is for its cold-resistance.


I don't really have any idea what blue loctite is like vacuum-wise. It seems sorta soft even when cured, which isn't a good sign.
Pablo Llaguno
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Re: Demo fusor first light

Post by Pablo Llaguno »

John Beutz wrote:Simple google search with the keywords "fusor" "NPT" and "vacuum" gives some good results.
viewtopic.php?f=10&t=4246

Your plasma looks a lot like my old demo fusor's. I found that a minimum amount of vacuum grease over the rubber gasket worked better than a slather. The black on the chamber might be burning rubber. Does your gasket extend into the chamber? Hard to tell without pictures.
Sorry for the late reply to that, yes my gasket extends about 1/4" inside the chamber, is that a problem?
ian_krase
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Re: Demo fusor first light

Post by ian_krase »

It's probably not terrible. As little as possible is good.
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