Spark gaps -- making and triggering

I finally picked up the caps from Fermilab. Each of them weighs about 40 lbs and they look nice, very cute color also.
So... I would like to make some sort of a pulse discharge thingy from readily available materials. Can crushing would be my first choice.
My plan is as follows, to use copper pipe as conductor. I bought some HV diode stack (4 7.5 kV diodes) and a HV probe.
The first thing to make would be a reliable system for discharging the caps safely, and test it at low voltages. I would probably want to use two 12 MEG Victoreen resistors in series a permanently attached bleeder.
With four 1 uF caps, RC would be 2*12,000,000 * 4/1000000 = 96, which is hopefully acceptable, and power dissipation in the resistors will be about 3W per resistor.
My main questions concern the spark gap. How to practically make a decent spark gap, and also how to safely trigger it.
For instance, would 3/4" brass balls such as McMaster item 9617K47, be adequate? If not, I have 3/16" tungsten electrodes. Would they make an adequate spark gap?
If not, can I use sections of, say, 1" copper pipe, placed at some distance with their axes perpendicular?
Also, what is the best way ot safely triggering a spark gap. My understanding is that there is a rule of one inch per 15 kV. Would making the gap 1.5" and injecting some argon into it be a safe and effective triggering method?
I am going to do a lot more reading and I will post my "design" here and get it to pass some consensus before implementing it.
i
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Dry argon is not conductive at STP. Consider making a "trap" above the spark gap that drops a short section of, say, 36awg wire across the gap to initiate the arc.
LLoyd
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On Tue, 04 Apr 2006 17:43:31 GMT, Ignoramus29428

I saw a coin-crusher guy's website a while ago that had a sliding arrangement that put two large brass balls, or maybe a ball and a disk, into contact with each other.

Probably bigger.

I think with HV you want to stick with a sphere.

Slide 'em towards each other until they touch. Do this not with your body parts.

If you can't find that coin shrinking website let me know and I'll dig around until I recognize it.
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Dave Hinz wrote:

When I was playing with nitrogen lasers, hammering the spark gap with a screwdriver handle produced much better pulses than just letting the voltage increase until sparkoer (once every 10 seconds or so).
I suppose part of the charge went into corona discharge. This will matter less with these huge caps.
Thomas
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Don't have any real info for you, but you may want to contact the guy who runs this site: http://www.powerlabs.org / He seems to have some really fun HV stuff
Tom
Ignoramus29428 wrote:

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    --Time to cruise over to sci.physics.electromag which is where the folks you need to gab with hang out. Good idea using pipe for conductors; when I was messing with mass drivers I discovered that floppy, unsecured wires will thrash about when pushed to their limits; it can be a little disconcerting, heh.
--
"Steamboat Ed" Haas : Concave, convex, con
Hacking the Trailing Edge! : carne: all is Zen..
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Thanks, Dave, Surftom, steamer, and Lloyd. I am going to subscribe to that newsgroup now. I have Bert's website in my bookmarks now.
http://205.243.100.155/frames/shrinker.html
i
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OK... I realized that I have two steel bearing balls stolen two decades ago from a Soviet factory. About 2" in diameter. Their downside is that they are very heavy and thus would require some support, with the attendant issues pertaining to HV. McMaster has some balls, but relatively expensive.
i
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message de

Maybe have 3 spheres. 2 to make your spark gap, the 3rd slowly rolling on nylon rails right between the two others...
--
Thanks,
Fred.



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Thanks... I went to farm supply and bought some 1/4" copper tubing, 1" by 1/8" aluminum flat to make a bus connecting several caps, 3/4" copper pipe and corresponding pipe caps. I will start putting a plan together.
i
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96 seconds is a pretty long time for the RC discharge time but much better than nothing. Remember that 3 rc times will be about 4.5 minutes and the caps will still have about a thousand volts on them. Be careful out there.
Dan
Ignoramus29428 wrote:

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True, but at a thousand volts, they would hold about 1/2 joule of energy, which could be discharged with just a crowbar.
i

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OK, what about using "hitch balls" from walmart for spark gaps? They are cheap and I could buy them locally.
i
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Ignoramus29428 wrote:

For your application, keep it simple. You can use heavy flexible cabling (such as welding cable) to make the interconnections. A little bit of cable jumping from magnetic forces won't cause you any problems.
For the spark gap, there's no need for pressurized gases, inert gases, etc. - air at STP works just fine. Copper or brass balls will work great (from McMaster-Carr), and copper end caps on copper pipe should work as long as you're not going to do thousands of shots. Tungsten or copper-tungsten would also work, but because of the high peak currents, I'd recommend using electrodes that were at least 1/2" in diameter. I'd avoid using steel electrodes since steel tends to generate a messy spray of pyrotechnic sparks. For similar reasons, avoid aluminum electrodes. Aluminum would erode rapidly, and the spray of sparks and flash from oxidizing aluminum vapor might make quite an impression. Do think about enclosing the spark gap to help deaden the sharp "BANG!" when it fires.
I previously used a triggered gap (a home-built Trigatron) that used a modified spark plug and 40+kv pulse to fire the gap. However, I've recently converted to a simple solenoid triggered gap with brass electrodes, since it provides much wider operating voltage range and I don't require precise switch timing. Solid brass electrodes show relatively little wear even after thousands of 50-100kA shots. The only downside is a bit of zinc oxide film on nearby objects, apparently from some zinc leaching out and oxidizing during the discharge. After thousands of shots, the surface of the brass looks similar to a dry river bed, with little islands of copper surrounded by small fissures.
A picture of my solenoid gap can be seen here. The Lexan case helps dampen the noise somewhat. It's STILL loud, but not as sharp. You could rig something similar using a (long!) string to pull the electrodes together, or you could swing a third electrode between the main pair to fire it.
http://205.243.100.155/frames/gallery/newgap5a.jpg
http://205.243.100.155/frames/gallery/newgap2a.jpg
As someone suggested, you could also use a pulse-rated ignitron. These use a molybdenum anode (instead of a graphite one) to handle current reversals and 100 kA peaks on an infrequent (1/minute) basis. However, although an ignitron is quite a bit quieter when firing than a spark gap, there's no real performance advantage. In fact, an ignitron may actually be significantly lossier than a well designed spark gap for your application. Unfortunately, pulse-rated ignitrons tend to be scarce and costly even on the surplus market... and you never know how much life remains on a used pull. However, if you do want to pursue this route, some pulse-rated 20 kV ignitrons include the 7703 series, NL1037H, and NL1039.
BTW, if you are near Fermilab, you are also near me. Contact me off Usenet if you'd like to see the coin shrinker in action or discuss some of this in greater detail.
Good luck, and play safely!
Bert
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We specialize in UNIQUE items! Coins shrunk by huge
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On Tue, 04 Apr 2006 19:34:14 -0500, Bert Hickman

Bert, thanks. I am going to make a big Gnumeric worksheet with all values, etc, to calculate stuff. Prior to that though, I would like to get a "feel" of things. When you talk about heavy cables, what size cables would apply to 4 uF charged at 11-13 kV, discharging through a spark?
I think that for paralleling the caps, I will simply use aluminum or steel angle, which I already have.
For connecting paralleled caps to the actual can crushing setup, I would indeed use some cabling. I have some six gauge wire, for instance.

What size balls? They have 3/4" diameter balls pretty cheaply.

Definitely not thousands, so, it would be great to not need anything.
If I can buy copper tubing (what diameter?) and some end caps, and be in business, that would be awesome.
Just what forces act upon the coil? Do I need to wrap it in fiberglass cloth with epoxy to make it safe from exploding?

All I have is 3/16" electrodes.

I like the idea of copper pipe with end caps.

Yes, I saw your solenoid triggered gap, and wondered to myself,
1) does high intensity discharge impact the colenoid 2) how do you switch the solenoid without exposing yourself to possible shock
Otherwise I like the solenoid idea a lot, a push solenoid would not be that expensive. I will start with something simplier though.

That's what I will do, indeed!

the second ipcture does not work.
If you want a free hostname, like coincrushing.algebra.com, let me know.

That's a little above my head at the moment.

Will do! Thanks a lot!
i
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I haven't played with these but I think you can just stop the contacts a few thousanths short of contact. At that voltage, it just doesn't make any difference. Anything between .010" and .060" should work. using some 1" copper pipe with smooth end caps would be fine, they have flat heads and rounded corners.
Ignoramus29428 wrote:

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Sounds like a time for a trip to Ace Hardware!
i

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["Followup-To:" header set to sci.electronics.design.] On Wed, 05 Apr 2006 01:55:09 GMT,

Why not bring them together all the way? To avoid them welding together?
robert
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["Followup-To:" header set to sci.electronics.design.] On Wed, 05 Apr 2006 01:16:10 GMT,

If I wanted to build this I'd look into using a small pneumatic cylinder to bring the electrodes together.
BTW, a setup like this is really more like a relay, not a spark gap. It's a relay that happens to spark a lot ;-)
robert
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wrote:

can crusing coils will try to flatten themselves. They don't explode.

Trying to contain giant bangs in plastic boxes is called making a bomb. Although a bit different, it's the same reason gunpowder is shipped in really flimbsy containers, not tough containers.
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