Grids of the week

For Farnsworth type designs.

Grids of the week

Postby Doug Coulter » Mon Nov 12, 2012 10:23 am

Here are some grids I'm using or am about to use.
Grids.jpg
Grids I use


All are 1" OD. The one on the left has been my main grid for a long time now and has produced well over 3 m neutrons/second in normal (not one of the pulsed, which can be better) operation. It is 8 tungsten TIG rods, .040" diameter, with graphite endcaps. Note the twist - it was an accident, but it seems to help? Graphite has the lowest secondary electron output when struck of any practical material for this, but note I had to wrap the base with wire to hold this together - electrostatic forces, particularly in a small arc like you get while conditioning, blow the grid apart otherwise, and those rods are an ever looser slip fit into the graphite caps. A little side-torque there holds things together well enough, but that wrapping wire gets good and glowing hot.

In the middle is a grid with .020" TIG rods, 6 of them. Ends are Ti, and the bottom has an extension tapped 10-32 so there's no separate screw needed. The jury is out on this one as my main HV FT failed during initial tests. It stuck out further than the original, exposing itself to the larger tank right at the end. That definitely didn't work out well - and it got very hot. I since spaced it back 3/4" and now it acts more normally, and produces neutrons - lots for the lower voltage my HV FT will now take - time for new glass *again*, rats. The focus on the poisser and rays is noticeably finer, we''ll see how it works out.
To hold the upper endplate on, I brazed a bit of material onto the tungsten rods where they came through the holes in the .020" thick Ti washer. Can't fold those over to spot weld, they just break.
BAG-24 brazing compound (silver, nickel, copper, no CD) wets tungsten and a little blob at the end of the rods keeps the endplate on. It's also solderable for other uses in the tank. Nothing practical wets the titanium (all of which is grade 2 pure).

The last one (on the right) is a substitute for my two loop little grid used as an ion source. It's all pure Ta, except for the Ti screw to bolt it to that FT. I am getting "interesting" and "non-intuitive" results with this one, more later - I have other repair issues before I can really test things.

My feed throughs have been 1" OD pyrex tubing as the outer layer, with 3/4" quartz as a slip in addition, with a 3/8" copper rod in the middle, with a thinwall quartz tube passed over that too. This generally works well to hold off 50kv at least, but sometimes fails due to the parts being reduced to silicon metal by hot D ions. I've added an in-tank shield of 1" ID copper pipe, which helps a lot, flared out near the grid end. Due to paschen's law, this doesn't get a glow discharge between the back of the grid and the grounded (normally) copper pipe. This weekend, I added a smaller 1/2" feedthrough to connect to that pipe and play games with - find out which polarity current is being drawn, and maybe act as a faraday probe or antenna for RF input to kind of selectively move the electrons vs the ions (60::1 speed difference for the same field, so games might be possible to get the charge separation as desired at the right times and things like that). However, arcing through all three layers of "glass" makes it not useful till I replace them with fresh non conductive stuff.

I am considering using aluminum for the feedthrough end endplate. It seems in other tests that anodized aluminum is a pretty decent insulator and doesn't attract ions. Obviously, in a cylinder grid, a lot of ions hit the center of that endplate, and heat it up - Al would normally melt, but in this case, the feedthrough is a good heatsink, and if anodized, maybe they won't be attracted to it so much - this is the case in my other feedthrough that has an Al center conductor. Might save a lot of wasted power and heat, or melt - but you never know if you don't try.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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Location: Floyd county, VA, USA

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