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Rough and dirty vacuum testing: my path towards the 1 Pascal range


Cremona, 2018-ongoing...
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Building simple feedthroughs


Here are some attempts to create some vacuum feedthroughs. In this page you will find some attempts (with various degree of success) for electrical feedthroughs and one gas hose feedthrough, for feeding some argon or other gas into the chamber.

The goals here are:
  1. Develop a simple technique, with no glass-metal or exotic junctions;
  2. Whitstand medium vacuum (I will test up to 50 Pascal for now, as I'm testing them in the red reservoir and it takes a bit of time to pump it down - final goal will be around or below 1 Pa);
  3. For electrical feedthrough, provide multiple connections (like 4 or 6 at least), so that I can plug different things at once.




The idea is really simple and it works pretty well (see drawing above). But it takes some considerations: first, use epoxy resin, not polyester resin. Second, degrease the surfaces. Basically, that's it. If you want additional details on my fails & successes, keep reading.
Here is the index:

Electrical feedthrough - tests 0 and 1 - big failures

Ok for the first tests, the idea was using some resin to glue the wires inside a KF-25 ISO connector. To prevent air leaking through the wires rubber sheath, I cut the wires and tin soldered them to some nickel wires. The resin should glue to the nickel wires and prevent air from leaking in. Then put some heat-shrinking sheath around to prevent contacts during the building phase, and soldered also the other side of the wires. To keep the wires separated, I used a perforated board, where I removed the circular copper base plates. The pictures are quite self-explainatory.



So, at this point I put this into the KF-25 flange, clogged the outer side with electrical PVC black tape and prepared the polyester resin. "Resin is viscous enough and it will get hard quite fast, so it won't flow through the small holes between the tape." No, it was a failure, all the resin flowed through the hole down on the ground.

Lesson learned: you need to clog one side very well (this is even more true for epoxy resin, as it polymerizes much more slowly).

Ok, so let's do it again, from scratch, and this time let's use some hot glue to block the inner side before pouring the resin. Not sure why I didn't block the outer side, as this would be much neater from the vacuum perspective...
Anyway, it kind of worked (although some resin somehow found its way through the hot glue (did it corrode it? did it melt it getting a bit warm?). Anyway, looked good, see images below.



So, the inner side was hot glued, the outer side was a nice block of resin blocking the wires in place (and nichel wires as well). Did it work? No. Some how it wasn't holding the vacuum. Actually, I started pumping down and the pressure went down straight to 600 Pa, then 550 Pa, then it stalled and it started rising again.
What?!
Maybe hot glue is degassing or small bubbles are exploding in the vacuum? Maybe some air trapped inside the wires is leaking all of a sudden? I tried to place the connector alone inside the vacuum chamber, closing all apertures and pumped down, but no, it was not degassing.
So, to make it short, probably everything was leaking there. One last test, let's connect it, let's create some vacuum, and while the pump is sucking, let's put extra hot glue all around the exit.

I actually ended up putting much more than in the picture. Is it working? Yeeeees!! It worked well enough to go down to 250 Pa so that I could test some plasmas, but then all of a sudden it stopped working. Also, something nasty happened inside, because the connectors probably shorted while making some discharges. So, now this setup leaks like hell (I can't get below 1 mbar) and wires are shorted.
FAILURE! Let's move to a new setup.

Lessons learned:
  1. Polyester resin gets hard fast and is very rigid. NOT SUITABLE.
  2. Beware hot glue, it's not that good for vacuum (as you may have guessed since the beginning hahah).

Electrical feedthrough - tests 2 and 3 - success

Ok so, let's do it again, this time with epoxy resin. Below a bad picture of the resin I used, the brand is Mold Art or something like that. It's a bi-component, mixing ratios are 100-60 in mass (10 g of component A, 6 g of component B). Apparently the component A is the base one, while there are a number of variants for component B (called C, D, etc), which give different properties to the resin.



After mixing the resin, I decided to degas it by putting it in the vacuum. Mainly, I was curious to see what goes on, but for sure this should help the performance. So, after putting it in the vacuum, I went down to 10 mbar or maybe below. At the beginning some bubbles on the surface will pop very gently. After a while, bubbles will start to create all inside the volume, even if they were too small to be seen or even if apparently there was none. The bubbles will all come up and start popping, creating a foam. After a while the foam will be disappeared, and you will be good to go. The image below is taken at the end, when most of the foam was gone and the resin was degased.



The first test (test #2) was to see if this epoxy would hold well the vacuum. So I took 4 straight nichel wires, put them inside the KF-25 flange, keeping them in place on the outer end, by using again hot glue. I know I should use something else, but I only had this at hand at the moment. Moreover, the test is mainly to see if this holds the pressure. So, first of all, surface preparation: I rubbed with acetone the nickel wires and the inner wall of the chamber. Then I poured epoxy, this time on the inner side of the KF-16 connector.

Results: some epoxy managed to pass through the hot glue plug, but only a few drops, although it stayed liquid for a number of hours. Watch out: even when it seems that it got plastic, don't displace the guy, but keep it well vertical in position. It will still flow as a liquid.

Ok, when dry, after about 1 day I plugged it to the chamber and tried the vacuum. It was holding it really well. So, I soldered some wires to the inside of the nickel feed-through wires, and put a lot of heat-shrinking tubing around it, to prevent sparks (see pictures below). Then poured additional resin. Some air is probably trapped inside the heat shrinking tube, but this should either evacuate easily, or anyway shouldn't give too much issue. The important thing is that the interior is well isolated from the outer side by the nickel wires embedded in epoxy.



Finally, while building the previous one, I also tried out a slightly different version, by including a teflon spacer that should help to keep everything in place. So, I machined a teflon piece (see image below), and welded nichel wires through it, to the connectors.



Then, clogged the outer side with hot glue to prevent epoxy from flowing out from small holes, and finally poured the resin.

Gas feedthrough

A gas feedthrough can be used to fill the chamber with some gas and try out discharges in a different atmosphere than pure air, or can be used to feed some devices. For electric propulsion for example, you need to feed some xenon/argon/krypton whatever to the device and propel it outside.

So, I basically need a hose going into the chamber. I opted for a KF-16 hose connector that I had (see picture below). The small crystal tubing in the picture fits almost exactly the hole, passing through it. The crystal tube goes inside the chamber for about 15 cm.



So, since the hole is small anyway, I guess hot glue can be just fine. I connected the KF-16 flange to the vacuum system, started the vacuum and put inside the crystal tube. After clogging the tube, air would leak in from the gap between the tubing and the KF-16 connector. So, I warmed up a bit the connector with a torch, and started putting hot fglue on the sides, which would get fantastically sucked in. Don't heat too much or the glue gets too liquid and flows 100% inside. Then, to add some structural strength, I covered everything with a larger hose (gardening-like), and poured abundant hot glue on both sides.

So far, it seems holding the vacuum pretty well. I may update the page in case I have some problems in the future.

Cheers,
Stefano

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