Matt Gibson Fusor

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Dennis P Brown
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Re: Matt Gibson Fusor

Post by Dennis P Brown » Tue Nov 23, 2021 10:00 am

As for the lack of ceramic extension into the vacuum system, that isn't as bad as you might suspect - the plasma is both very conductive (over a volume) but has limits in any given small volume at low pressure (under 100 microns); so it is partly self limiting in the normal vacuum range that a fusor operates: 5 -20c microns. All bets off if you go a good bit above that pressure. Then the plasma can focus and support a considerable current. and at near torr range, support an arc.

If your worried, adding a glass tube extension that fits inside the ceramic tube and affixed with some furnace grade ceramic adhesive should work to insulate the rod.

Your turbo had better zero out a micron gauge for your system; an ionization or cold cathode will be required to determine the lowest value. Anything in the mid 10^-5 torr is good for a fusor as "clean" and vacuum tight.

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Re: Matt Gibson Fusor

Post by Matt_Gibson » Tue Nov 23, 2021 4:39 pm

So I think that I might have a leak…The new bspp to Kf25 flange didn’t come with an o-ring, so I took one off another fitting. This definitely helped things as before (no o-ring) I was pulling in the Torrs. After this improvised o-ring, it pulls into the mTorr, but now only gets down to 45mTorr after 10mins.

I’m guessing that while the o-ring helped, it’s needing something better. What else can I do? Better o-ring? Teflon tape on the bspp threads?

As of now, I’ve added a 8.5in length of steel tubing (Kf25 ends) and the turbo. I know that I shouldn’t expect the same performance as before these additions, but I expected better.

I have yet to hook up the chamber, so I’m expecting worse after that.

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Richard Hull
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Re: Matt Gibson Fusor

Post by Richard Hull » Tue Nov 23, 2021 4:57 pm

Every addition to a piecemeal tested vacuum system will suffer a reduction in performance due to any number of factors. Most often it is something stupid, poor sealing, ill suited additions, etc. The number one issue is that you brought you entire system up to air. Even with valves to close off a tested, partially assembled vacuum system, the new part will be loaded with all manner of water vapor and other vaporous materials that will either require many hours of system pumping or a tedious bake out to bring to a suitable vacuum level.

You are wise in following my vacuum system advice to assemble and test a system step by step. In this manner, you are confident in all the work done up to a point. (Seals are good and the system thus far assembled is good.) Each new addition imposes a trial period of sealing and pumping.

If you get a great idea and go back into the already tested system to add something great like a valve or a new fitting, you fight the battle all over again. Without a whole bunch of heat tape to bake out with, you are subject to long pumping periods and a lot of leak testing. Even technical vacuums can be frustrating to achieve. Scientific grade vacuum levels demand a special level of cleanliness. A good scientific level vacuum system, especially a complex one, is just never brought back up to air unless it is absolutely unavoidable. Such systems are loaded with top grade and expensive valving all along the vacuum path and all pumps are left on 24-7.

A good fusor needs a minimum of three vacuum valves, though two bellows seal valves will do in some cases. The fusor is trapped between a technical and scientific vacuum. Technical vacuum levels, (10 microns), just will not cut it and scientific level vacuums, (10e-6 torr), while nice, are just ridiculous over kill.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Matt_Gibson
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Re: Matt Gibson Fusor

Post by Matt_Gibson » Tue Nov 23, 2021 5:39 pm

Hi Richard,

I have a valve meant to isolate the turbo from the foreline pump, just waiting on more Kf25 clamps :-)

I removed the turbo from the system and found that I can quickly (20 seconds or so) hit 12 microns at the end of the 8.5in steel tubing. I gave the bspp to Kf25 fitting a tighter twist and then reconnected the turbo to the system. It’s better (38 microns after 10 mins) but still not great.

Would this point you towards a better bspp to Kf25 seal?

All in all, the turbo is still able to rapidly drop to “zero” microns, I just can’t stand knowing something can be improved :-)

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Dennis P Brown
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Re: Matt Gibson Fusor

Post by Dennis P Brown » Tue Nov 23, 2021 6:54 pm

Don't over react to out gassing. What you describe sounds a lot like it. Don't assume a slow leak automatically. Just a minor aside, we tend to use microns here to be consentient.

When you can read below 10^-3 torr (using a vacuum gauge for those lower ranges), you can do real testing for small leaks and certainly determine if it is just out gassing or not.

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Re: Matt Gibson Fusor

Post by Matt_Gibson » Tue Nov 23, 2021 7:05 pm

Hi Dennis,

Got my units mixed up after looking at the Pirani so much lately…

I looked back at MTI site and see they sell a gasket meant to go with this converter. They didn’t include it, or make any mention of it. Despite it weighing like a gram, they want $23 to ship it. I’ll see what McMaster has.

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