Demo fusor first light

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ian_krase
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Re: Demo fusor first light

Post by ian_krase » Tue Mar 21, 2017 6:38 am

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 » Tue Mar 21, 2017 2:21 pm

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 » Sun Apr 02, 2017 7:30 pm

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 » Sun Apr 02, 2017 7:38 pm

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 » Mon Apr 03, 2017 9:04 am

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 » Wed May 24, 2017 1:34 am

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 » Wed May 24, 2017 2:12 am

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

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