FAQ - Demo fusor feed through under $20

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Richard Hull
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FAQ - Demo fusor feed through under $20

Post by Richard Hull » Thu Jul 15, 2021 7:33 pm

Yes, repurposing a spark plug is a great simple demo fusor high voltage feed through!
The example in the images should be self explanitory.
You will have to put forth some effort....Real sorry you have to put yourself out like this.
you will have to carefully saw through threads or better, still, machine it off in a lathe. Saw off as much of the thread as you are comfortable with.
You will have to buy a tap for the plug and bore and tap a hole in your demo fusor's top
You will have to silver solder or spot weld on a rod to make your grid mounting post.

You can see if it holds vacuum. (plugs are made to withstand high pressure not a vacuum) You might want to or have to put a small ring of high vac epoxy at the base of the insulator.
Modify and gasket or O ring as needed to seal.

Plug found here

https://www.carid.com/ngk/laser-iridium ... 94201.html

A good result will give you a top notch insulator.

Richard Hull
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Joe Gayo
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Re: FAQ - Demo fusor feed through under $20

Post by Joe Gayo » Thu Jul 15, 2021 7:46 pm

Why not Champion FI21501?

https://shop.advanceautoparts.com/wcsst ... i_larg.jpg

M14 x 1.25 Thread

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Re: FAQ - Demo fusor feed through under $20

Post by Richard Hull » Thu Jul 15, 2021 7:56 pm

Looks fine, but the insulator is a bit short and stubby. Probably will work OK if voltage is kept low. The plug I note has a bit deeper reach in atmosphere and into the vacuum with its insulator. Let the user do the work and report back.

I used a stock, old, deep reach Honda civic plug back in the 90's and it arc'd at about 16kv in air. The top insulator wasn't very long, but the vacuum end was nice and deep, once I sawed off 3/4 of the threaded part.

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.

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Re: FAQ - Demo fusor feed through under $20

Post by Joe Gayo » Thu Jul 15, 2021 8:30 pm

Using the air-side "boot" made for spark plugs and some dielectric grease (silicone) should allow for demo fusor operation. We'll see what users say.

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Re: FAQ - Demo fusor feed through under $20

Post by John Futter » Sat Jul 17, 2021 11:04 pm

I published how to make sparkplug feedthroughs 11 years ago
I have put this link up here a few times
I might repost the whole article here for the future before Coultersmithing disappears

these feedthroughs work fine down to 10^-9 millibar

http://www.coultersmithing.com/forums/v ... p=516#p516

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Re: FAQ - Demo fusor feed through under $20

Post by Maxwell_Epstein » Sat Sep 04, 2021 1:23 am

Sorry to post a question in the FAQ, feel free to remove (of course). The link in the original post leads to a spark plug with a resistor of 5kohm while every other source I've seen (including the coultersmithing one linked to in the replies) stress that plugs with resistors will not work. Is the resistor somehow removed in the original instructions and I missed it? Thanks!

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Re: FAQ - Demo fusor feed through under $20

Post by Richard Hull » Sat Sep 04, 2021 3:00 am

5K ohm is nothing.... at a typical 10 ma draw in a demo fusor with 5,000 volts input that's a 50 volt drop. No problem. The simplest electrical math should be helpful through the effort.

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.

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Re: FAQ - Demo fusor feed through under $20

Post by Maxwell_Epstein » Sat Sep 04, 2021 3:47 am

Thanks for the clarification! I still need a lot more intuitive familiarity with high voltage: I'm so used to seeing 5K ohms as a lot that I didn't even consider the relative magnitude of (demo) fusor voltage. Sorry for asking such a foundational question here

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Re: FAQ - Demo fusor feed through under $20

Post by Richard Hull » Sat Sep 04, 2021 3:57 am

As we live, we must learn. Never stop learning.

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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.

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Re: FAQ - Demo fusor feed through under $20

Post by John Futter » Sun Sep 05, 2021 8:44 am

Maxwell
The sparkplugs mentioned in my tutorial do not have series resistors they are way less than an ohm in fact i use them for heater inputs on some of our vacuum systems @ work ie 24 volts at 10 amps

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