Thursday, March 18, 2010

Pulsed Power Possibilities Pregnant With Promise

I hope you'll forgive me a little alliteration if I promise not to do it too often.

Recently I was playing around with the Rijke Tube and, while looking around my shop for larger pipes to try, I was suddenly seized with the possibility that a couple of constructions I have sitting around - my own distortion-triggered spark gap switch and a stack of doorknob capacitors -- might well fit inside of a few combinations of insulating tubes and conducting shells, to make coaxial geometries. When I tried some, I was surprised to find that a bunch of unrelated parts happened to fit together in very useful ways, all un-planned.

What follows are just idle ideas, with no actual projects or applications in mind - yet.

First, consider the following image:

fast pulser mockup 1 web.jpg

At lower left we have a stack of barium titanate (er, I think) capacitors, 6 x 500 pf @ 20kV = 3nF per section, 3 sections in series, for 1nF @ an absolute maximum of 60kV. However, operating a stack like at its rated voltage in pulsed power duty would start killing caps right away due to voltage reversal. What we want to do is derate the caps about 50% and run them at 10kV / stack for 30kV total. For those following along at home, that's 900 joules. The acrylic insulating tube came from one of six ignitron housings from The Big Pulser.

I still need suggestions for a name for that thing, by the way, if any of you three readers are feeling creative. Large scary devices ought to have names. Or perhaps a ridiculous acronym. But I digress.

At upper right, we have the distortion-triggered spark gap switch which I built. It's a close fit, but i believe there is room for a wrap of mylar or similar film with a good dielectric strength and a low dielectric constant (and er, absorption too, please).

Now, I want you to imagine a very low self inductance, fairly fast pulser with capacitive store and switch in a close-fitting coaxial housing, possibly with a non-linear pulse compression transmission line between the output of that supply and the load. I'm not sure whether I'll ever be competent to try pulse compression techniques, but the first half of that seems a slam dunk, aside from getting the trigger signal to the switch, which is something I'm going to have to think about if I ever want to try this. Clearly, the self-inductance would be ridiculously low if you did the terminations right.

In any case, that pipe is destined to become the micro-Marx generator, so if I ever want to fool with that idea, I'll need another bit of this somewhat expensive stuff.

Next, have a look at this:

holy crap it fits 1 web.jpg

That's the same switch as above, fitting into one of the ignitron housings from The Pulser. Again, there is enough room for an insulating film between switch and housing. This would be a handy geometry to apply to a parallel plate transmission line with minimal inductance used by the switch connection. Again, pure happenstance. I don't have an application in mind, I'm just thinking out loud.

Last but not least, this is just a lash-up enabling me to evaluate an idea. I've promised to build someone a pulsed magnetic field device. I want as much rise time as I can get from first a cap string and later the micro-Marx. I want all the inductance in the one turn work coil of course. This wouldn't be made on that cardboard coil form, it would be enclosed in some resilient fiberglass-reinforced epoxy, possibly a two-part urethane or silicone. A hard epoxy will shatter when the coil expands a little very very suddenly.

H field antenna mockup web.jpg

None of these are projects I'll be starting on any time soon, I have plenty on my plate already.

More news soon in another post.

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