Spark plugs for feed throughs

MitchellJames · Post by **MitchellJames** » Sat May 31, 2008 4:04 am

I just spent some time tracking down the smallest cheapest spark plug that I could find for a vacuum chamber feed through. The smallest cheapest one I could find is a Denso U20M-U sold by www.sparkplugs.com. 10mm dia at the threads. The cost was $2.24 each with shipping and handling of $10. I bought 20 for one test system. I need a lot of feed throughs. The vacuum chamber wall for this test doubles has a magnetic field conductor so the size of holes in it matters.

Tom Ligon · Post by **Tom Ligon** » Sat May 31, 2008 4:18 am

Long time no see! Sounds like you're seriously building something!

drmike · Post by **drmike** » Sat May 31, 2008 1:24 pm

Excellent, thanks! I've just been looking at ways to build a vacuum chamber with my spare scrap, and figured spark plugs would be perfect for feedthrus.

MSimon · Post by **MSimon** » Sat May 31, 2008 1:40 pm

Spark plugs are supposed to be good for 30 to 35KV. Their nominal rating is around 15KV with the spark gap in place so that makes sense.

You might also want to get a plug that is available in resistive/nonresistive styles as many of the fusor folks are using a ballast resistor of around 400K ohms. The resistors in the plug are around 5K.

MitchellJames · Post by **MitchellJames** » Sat May 31, 2008 3:26 pm

Tom Ligon wrote:Long time no see! Sounds like you're seriously building something!

I haven't had anything to report. The current test set-up is the simplest possible thing I could think of for investigating extending electron entrapment time. However it still requires a lot of stuff some of which I haven't figured out how to obtain yet at a price I can afford. For something like these spark plugs it took days to figure out the right search term to get a lead on the type of spark plug that I was after. I expect that other people could use the results without all the searching.

MitchellJames · Post by **MitchellJames** » Sat May 31, 2008 3:31 pm

MSimon wrote:Spark plugs are supposed to be good for 30 to 35KV. Their nominal rating is around 15KV with the spark gap in place so that makes sense.
You might also want to get a plug that is available in resistive/nonresistive styles as many of the fusor folks are using a ballast resistor of around 400K ohms. The resistors in the plug are around 5K.

These plugs are an extremely old non-resistive style. I intend to cut off the spark gap electrode (the bent piece) and just use some non-lead solder for internal connections.

MitchellJames · Post by **MitchellJames** » Sat May 31, 2008 3:39 pm

drmike wrote:Excellent, thanks! I've just been looking at ways to build a vacuum chamber with my spare scrap, and figured spark plugs would be perfect for feedthrus.

I didn't realize that spark plugs could be used as feedthroughs until I came across a reference to the practice in "Vacuum Equipment and Techniques" which is a book about practices and equipment used at the Radiation Laboratory, University of California during the Manhattan Project. Modern off-the-shelf feedthroughs cost more than a garage experiment can afford.

tombo · Post by **tombo** » Sat May 31, 2008 10:49 pm

Cool.
How hard a vacuum are they good for?

MitchellJames · Post by **MitchellJames** » Sat May 31, 2008 11:19 pm

tombo wrote:Cool.
How hard a vacuum are they good for?

The experiments they were being used in went down to 10-8. I tend to think that other parts of the vacuum system would fail first. Mostly depends on how good the metal seal is. Might take a little bit of experimenting. But at $2.50 for a feed through I am willing to do quite a bit of experimenting.

tombo · Post by **tombo** » Sat May 31, 2008 11:47 pm

MitchellJames wrote:
tombo wrote:Cool.
How hard a vacuum are they good for?
The experiments they were being used in went down to 10-8. I tend to think that other parts of the vacuum system would fail first. Mostly depends on how good the metal seal is. Might take a little bit of experimenting. But at $2.50 for a feed through I am willing to do quite a bit of experimenting.

Nice.
That book sounds like a really good resource.
Does it have any water cooled high voltage power feed-throughs?
I'm looking for something with a 1/2" to 1", or more, copper tube through it.

MSimon · Post by **MSimon** » Sun Jun 01, 2008 12:00 am

MitchellJames wrote:
tombo wrote:Cool.
How hard a vacuum are they good for?
The experiments they were being used in went down to 10-8. I tend to think that other parts of the vacuum system would fail first. Mostly depends on how good the metal seal is. Might take a little bit of experimenting. But at $2.50 for a feed through I am willing to do quite a bit of experimenting.

Copper gaskets are supposed to be especially good. Can you get those and buy extras?