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I made a argon plasma tube with 954 bronze aluminum bronze electrodes. After running for some time the anode (left) face is a silver colour. The cathode is on the right.

Does anyone know what this is? Is a deposit of some kind? Or has some reaction occurred with the aluminum bronze after being bombarded by electrons? Could this perhaps be the aluminum the aluminum bronze from the cathode being sputtered since aluminum has a lower melting temperature than the copper in the bronze?

enter image description here

In another set of electrodes of the same material the anode did not get a silver colour but instead just got discoloured dark burn marks.

enter image description here

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  • $\begingroup$ What does the tube look like? Any idea how hot those get? $\endgroup$
    – Jon Custer
    Sep 22 at 20:00
  • $\begingroup$ @JonCuster 45mm inner diameter tube fuse quartz tube. Electrodes are about 40" apart. The electrodes, in question, of the first photo are 39mm diameter. The second electrodes are 44mm in diameter. They don't seem to get very hot at all. If I take them out of the tube at most they feel a nice pleasant warmth in my hand, if that. Sometimes they aren't warm at all. $\endgroup$
    – DKNguyen
    Sep 22 at 20:28
  • $\begingroup$ The thing that these two pair of electrodes gets different outcomes,- may indicate that they might not be so identical at all or other conditions may not be equal. Like, maybe one pair has more impurities of whatsoever type and/or plasma intensity, exposion differencies ? Also if some catalyst may act, some level of oxidation may happen. Just my thoughts... $\endgroup$
    – Agnius Vasiliauskas
    Sep 22 at 20:33
  • $\begingroup$ @AgniusVasiliauskas The electrodes are turned from the same rod if that makes a difference. The second photo electrodes are slightly domed (or rather ramped to a peak) since the parting tool wasn't aligned properly when they were made and flexed. The first electrode photos are flat as they should have been. They are also slightly smaller in diameter compared to the second photo electrodes (39mm vs 44mm diameter). $\endgroup$
    – DKNguyen
    Sep 22 at 20:35

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Well analysis of second pair of electrodes, namely left one, revealed this picture, after I did these operations (in GIMP): $\text{grayscale} \to \text{histogram equalization} \to \text{exposure modification}$ :

enter image description here

So indeed second pair doesn't match first one effect, due to the mentioned fact that electrodes shape is convex-like. So in this case plasma beam may get some profile which get's focused to the edges of electrodes.

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