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


Cremona, 2018-ongoing...
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First plasmas


Here are some of the very first plasmas produced by the current setup (click to enlarge)!



The setup was supposed to be a couple of L shapes glued to a plexiglass base. The glue supporting the left electrode (cathode) actually melted, and it came down a bit. So, a slightly improved version followed (see picture below):



Air plasmas

I used a faulty feedthrough for this test, so the lowest pressure reachable was around 250 Pa. The gas is simply the residual air in the system. Other gases are coming soon. Anyway, here are some tests at different pressures!!

air-plasmas-pressures

Argon plasma

Using argon to make the plasma is way more fun! Discharges ignite super easily in argon (ionization energy is about 15 eV, slightly higher than for atomic nitrogen, but the latter absorbs additional energy dissociating from N2 to N, plus it has vibrational degrees of freedom etc etc). To feed Ar in the system I used the gas feedthrough (click here): first I evacuated the chamber down to some 500 Pa, then I refilled it using some argon for tig welding applications. Not sure how pure it is. Also, some air was surely contaminating, and some additional air may have leaked in during the duration of the test (especially at lower pressure, where it takes about 20 minutes of pumping in the current setup).

In Ar, even at slightly sub-atmospheric pressure you have super long sparks, super tiny and durable, like bright slightly pink-ish hot wires, and they move in a crazy way. Then as you decrease the pressure, they stabilize and you go in a regime pretty similar to the one encountered for air in the previous pictures. Finally, as the pressure goes down again, you get into yet another regime, where the plasma is not anymore discharge-like, but is everywhere. Probably the presence of the metallic reservoir walls plays a role here. Click on image to enlarge.

argon-plasma-pressures

Note that colors are not super realistic, being taken by a cellphone camera. The argon is slightly whiter and more pink-ish than how it appears in the pics. Also, pressures are a rough estimation, and I may pretty well be off by some 50 Pa in this image, but it should give the idea.

Cheers,
Stefano

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