Tuesday, September 15, 2009

micro-Marx work update

The Micro-Marx Generator design is coming along slowly. Because I am trying to use as many off-the-shelf parts as possible, machining them or modifying them as little as possible, the process is somewhat tedious and slow. If all goes well (cough) it is possible I will have the entire design finished by the end of this week. We'll see. I've already started on the housing, although the base/insert assembly - the bit with all the electrical parts - will need some tweaking to fit the housing once I've got the housing design finished. This thing is gonna be slicker than snot on a glass doorknob.

Unfortunately, it is also very likely to be a lot more expensive than I had at first surmised. See, it really needs to go into a housing made of a very good conductor. The proven design from which I am taking my inspiration for this project used copper pipe. It needs to be something I can solder to, which lets out steel and aluminum tubes, which are lousy conductors anyway. Skin effect can come into play with very short rise time pulses, and skin effect increases dramatically as conductivity goes down. So I'm all for using copper pipe myself.

There's just one itty bitty problem: the device can't be made to fit into a piece of pipe smaller than 3".

A liner of a non-conductor with pretty good dielectric withstand (able to hold off say 100kV for 1mS) has to go inside that, so the OD of the liner and the ID of the housing need to be close to each other, and the ID of the liner has to be large enough to accept the smallest practical configuration of the Marx generator assembly. I also need a way to hold the Marx assy firmly when it is inserted into the liner. I have some ideas about that.

I'm hoping to use gray PVC conduit for the liner. PVC isn't the best choice due to its high dielectric constant and loss tangent (both of which degrade rise time), but it's the only plastic I can work with easily in terms of machining shapes and gluing pieces together. I am not gonna go in for custom epoxy castings. I thought I had a line on some styrene tubing that would have been much better than PVC due to its lower loss tangent, but it turned out to be too large in diameter.  Fiberglass & phenolic tubing of the required size can be had, but the price is too dear for my budget.

So, the current plan is to make the housing from 3" Type K copper pipe and the liner from 2.5" gray PVC conduit, plus some additional slabs and pieces of PVC machined and cemented together. It is unfortunate that the dimensions required force me to use the heaviest-wall copper pipe which is Type K. It's more expensive than the others since it uses more copper.

The spot price of which, by the way, is on the rise again, currently at $2.75/lb. Dammit. I should have bought the coil I needed for the primary winding of my Tesla coil when it was still hovering around $1.25 a pound in December of '08!

I'd like to use copper pipe fittings for the output end of the housing, but it remains to be seen how well that will work. The output connector will be designed to use a standard coaxial cable with a modified UHF connector, the insulation extended by 4" - 5", and a banana plug and jack on the end of the center conductor. I might also consider some sort of direct connection to the switch this thing is being built for, but I want to have the option of using a coaxial cable later, as I may want to use this pulse generator with other equipment.

I'd also like to use some off-the-shelf, solder-on brass bolt flanges for the ground/input end, but those look expensive, and frankly, I don't like their dimensions. I am already imagining that I may have to make my own flange and end-cap for the ground/input end. I sure do like that bolt circle feature of the DRO on my mill. Incredibly handy feature, that.

3" copper pipe isn't carried in the usual homeowner stores locally, but I will call around to the professional plumbing / pipe fitter supply houses. Online suppliers sell it for around $24 a foot. That sure beats McMaster-Carr's price, but I hope to find a cheaper source locally, including some of the surplus yards or building material recycling places such as Resource 2000 in Boulder or Bud's Warehouse in Denver.

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