Fusor Forums

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

Why not Champion FI21501?

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

M14 x 1.25 Thread

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

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.

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

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!

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

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

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

Richard Hull

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

This is a demo fusor FAQ. Demo fusors never have current flow in them. Thus, the feed through need only handle 10kv or a maximum of 15kv. Most demo fusors use a neon transformer and are themselves self current limiting and voltage limiting. A zero resistance feedthrough might be great for use in ion implanters, etc. where ampere curent levels at low voltages of 5kv at 1 ampere are demanded. A 5k ohm resistor at 15kv would only offer voltage loss issues at above 30ma. This current is well outside of the demo fusor range.

Never use spark plug wiring unless it is pure copper wire! (racing wire) All stock spark plug wire is highly resistive being carbon on silk or polymer fibers. (Reduces RFI for car radios and electronics.)

Finally, if you make an SS fusor body for your demo and plan on moving onto real fusion, do not consider a spark plug for a feed through at all!! You will just have to discard it to do fusion. We deliberately throw resistors in the power line of our fusors to limit current in some instances. Why worry about 5k resistance in a demo spark plug insulator when you have a 10k ohm DC resistance in a neon transformer secondary of huge inductance and a shunted magnetic path already there to limit current?

Only demand a zero resistance plug when you are going to demand amps to flow through it. Worry far more about the air insulating capacity of the plug and the behavior in a vacuum of the base electrode/insulator combo.

Joe's comment about a spark plug boot to avoid sparking is wise. I used one on my early iteration of fusor III in 1989 and did limited fusion with it. I cut it short to just lock over the top of the plug, but first slathered the top with silicone HV putty and let the boot encase it to avoid air paths to the base. Ultimately, above 25kv, it had to be replaced with a real vacuum rated HV feed through.

Richard Hull