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Ideas for a triggered spark gap

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PanosB_GR

Thu Jul 29 2010, 10:43AM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

Hello everyone.

I have built this marx bank, with spark gaps made out of aluminum sphere-likes (machined) that are about 40mm (1,5") in diameter. So far i charge the bank and wait for it to go off, which is kinda mentality-stressing.

In order to make the bank triggered, i was thinking of making a hole and putting some kind of hydraulic or pneumatic actuator, that pops a small needle when pushed on the other end.

I was thinking of a small plastic tubing, in order to avoid galvanic contact. Filling the tube with oil would be a good way of transmitting pressure from a little piston to the tap that holds the needle.

Has anyone seen a construction like this?

Would any other way be easier to build a reliable triggered spark gap?

Regards

Proud Mary

Thu Jul 29 2010, 12:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A mechanical trigger may seem fast to you, but it will cause bad jitter, and ruin the wave front.

This paper has a lot to say about Marx triggering, and how to improve it:

SUB-NANOSECOND JITTER OPERATION OF MARX GENERATORS
J. R. Mayes, W. J. Carey, W. C. Nunnally [1], L. Altgilbers [2]
Applied Physical Electronics, L.C.
Austin, Texas 78734.

Thu Jul 29 2010, 01:14PM

Registered Member #2372 Joined:
Location:
Posts: 62

The easiest way to trigger it is to use a pressurized switch that will hold off peak charge voltage and then you can drop the pressure to trigger it. You only need to do this on the 1st stage of the marx and the rest will self break if you have them spaced properly. If you arent trying to syncronize the firing of the marx with other things that are happening (lasers firing, other pulsed power systems firing) the timing isnt important so something like a mechanical or the above mentioned method will work just fine.

quicksilver

Thu Jul 29 2010, 07:03PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

PM:
A bit on the technical side but worth the time to bone up on that which I am not familiar with.
Excellent paper; thank you for posting.

PanosB_GR

Sat Jul 31 2010, 03:56PM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

would a contraption that uses a piezoelectric igniter (the ones used for gas stoves) work by any chance?

Proud Mary

Sat Jul 31 2010, 04:51PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

PanosB_GR wrote ...

would a contraption that uses a piezoelectric igniter (the ones used for gas stoves) work by any chance?

Perhaps a very good idea, Panos.

You would have to be sure that the charge on the main gap did not misbehave, and take a short cut down the trigger wiring to your finger, or anywhere else you do not want it to be -

- but this is a problem that has to be fixed whatever sort of trigger system you use (except the simple mechanical kind)

radiotech

Sat Jul 31 2010, 05:32PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"and take a short cut down the trigger wiring to your finger"

Following a loud burst of expletives-----"You were told to keep away from that !"

how common is that?

Proud Mary

Sat Jul 31 2010, 06:07PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radiotech wrote ...

"and take a short cut down the trigger wiring to your finger"

Following a loud burst of expletives-----"You were told to keep away from that !"

how common is that?

Are we not in the business of attempting to impose our will upon electrons, to make them come and go by our design, and at our command, and how often do they not outwit us by finding some back alley, or loophole, or short cut, that our imperfect knowledge had left open for them?

Pinky's Brain

Sat Jul 31 2010, 08:59PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Looking around a small igniter gives ~5 kV, so as long as the gap doesn't need to hold off much more than that it could work with a field distortion trigger (ie. a needle in between the two alu balls). Of course you do need a reasonably well regulated HV supply, if voltage just keeps building until spontaneous breakdown any way having a trigger won't help your nerves.

I just remembered ... there was a youtube video from someone who did exactly this :

PS. actually this is not a field distortion gap. He seems to just have two spark electrodes relying on the plasma generated to bridge the gap, where the actual idea for a field distortion gap is to charge needle with a voltage of the opposite polarity of the HV source, with the other end simply grounded, to create streamers from both electrodes. I guess I didn't consider whether the piezo crystal itself could hold off the high voltage.

PanosB_GR

Sun Aug 01 2010, 10:19AM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

I am not sure how these devices are built, I have not taken one apart yet. I guess that they have the piezo element with a push button attached and wiring that drives the high voltage output to the nose of the igniter.
I'm thinking, a good idea would be to build an acrylic glass box, with a handy plastic button on it, and put the piezo element in there. Then take the HV output (probably with the means of an RG cable) and drill a hole through the first spark gap, on the "grounded" sphere of that first gap. Carefully inserting the igniter wire (core of the RG cable) in there and leaving the tip of the wire facing the surface of the sphere would provide a nice triggering plasma source for the first gap.

Apparently I am lucky to have a really stable HV source with controlled HV output. So, leaving the HV on wont be a problem.

Another issue that i am thinking of,l is that inserting a third electrode between the gap is not as a good idea as carefully making a hole trough the sphere and inserting the trigger electrode in there. Less chance of having HV leak to your igniter mechanism with the second method, or not?

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