New ion grid, BN feedthrough, postmortem on old one

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New ion grid, BN feedthrough, postmortem on old one

Postby Doug Coulter » Thu Feb 02, 2017 4:43 pm

Got the new BN based feed through in, with a new ion grid, now waiting for the base pressure to resume, but the trick of bringing up the tank with argon, working fast, baking hard for half an hour, and running a discharge before pumping it hard again - it's down to e-6 mbar in about 2 hours - only about factor 100 to go to get to my "real" base pressure. Useable now. Dayum, it's nice to have the right chit and the experience to use it well. I'll admit to a little luck, and that's nice too.
May as well start with the good news.

This was taken with a pi camera and enhanced with the Gimp software - it's really pretty dim with about 25 watts in, which is actually a good sign - not much going to waste. Heck, it was probably making neutrons, but I didn't fire up the whole mess. Golly, I didn't even wind up having to reach in the front door for this.
Here's what it looks like in the bake-out heater light (500w in a shiny box is LOUD):
Weird color from overloaded cam dynamic range?

And from outside with one of those big ballast resistors I test with on there as a kludge:
Testing 1,2,3.

So, it's baking out with full pumping speed as I write this, till the sun goes away, then I'll put the turbo on standby and reset the trip points for the backup pump and see how she is tomorrow. So far (knock on wood) this is the smoothest and easiest change I've ever made that involved coming up to STP and changing a flange and other major pieces. Feeling lucky, punk?

Now about that old one and why I even bothered with this. As they say, a picture is worth some words, so here goes.
The old one

There was nothing wrong with the tantalum loop grid, really. The function here is just to make ions, and if anything, sloppy focus etc is probably better for that.
You might see where due to marginal previous performance I'd turned down the center Al stalk a little where it goes through the grounded stuff. It was marginal at around 10kv. And duh, I forgot and we tried 20 on it the other day, and it just plain failed. What's of more interest to me here is the side we can now see - this was the "back" side. Look at all that metalization on the glass! Well then. Probably some sputtering from elsewhere, but also the old bugaboo - hot ions + glass = silicon.

To help with some free info - in the pic is also the grid wires I now use - TIG rods, special order tiny, and that grey block at the top is the buffing compound I used on the new one (6061-T6 Al) - the stuff seems to be color coded to some standard. It's the aggressive grade, at any rate.

I got away with the gorrilla tape and hose clamp on the 3/4" thickwall tubing but it's a definite fail on 1" and larger...hence the nice new PVC clamp.
For completeness, here's an old one for 1" pipe that failed - the new one is overkill (but will hold up the insulating/shielding pipe nicely) and the old one is, well, cracked - too wimpy.

I may add more, I got lots of pix of this process. But this gives the gist.
Not quite up to snuff.
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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