I'm not going to embarrass myself by guessing and getting it wrong, I'll do it by admitting I haven't a clue instead

Yep, same here...does look intriguing.

Ah, interesting the sodium discharge is contained in the middle narrow section. That’s suggest the sodium was placed there. I did something similar in a very small scale once electrodeless with some metallic sodium salvages from an old lamp. Not sure if you are using metallic sodium here or a salt. I can’t see any evidence in the tube. I would also guess you would have externally heated the middle section to produce the sodium vapour .

Sodium was placed in the tube indeed, but not as a metal, and not specifically in the middle section of the discharge tube. Also, that part was not heated by any external means, only the electrical discharge is the source of heat but that is not the determining factor here. This lamp does not work like any other ones, that's why I managed to get a strong sodium emission at a relatively low operating temperature - note that this discharge tube operates in free air without any means of heat conservation.

Plot thickens What temperature is the tube running at it not preheated? Is it heated by the discharge or not really warm at all? Are you HF coupling the tube so no internal electrodes? Just wondering if there is enough kinetic energy in the constricted area to knock out the sodium emission form a relatively cold discharge. It appears to be producing strong sodium lines anyway. Could you get way with doing this with table salt (NaCl)

Ha ha, you're getting there but you're missing something crucial. In all fairness I should say that it's not something obvious. To answer your questions I did not use table salt here and the discharge is capacitively coupled indeed. The tube is warm but certainly not as hot as classic low-pressure sodium discharge burners. You can touch it just after power is switched off, it won't burn your fingers.

That is really impressive! I have no clue electrically speaking what might work and what won't. So I may say something really dumb by guessing does it run on 2-phase live?

@AgentHalogen_87 Don’t worry it’s not dumb, glad it’s not just me trying to figure it out. I think the idea of 2-phase may come from the floating high frequency nature of the supply. Officially, this does exist but haven’t seen it used anywhere.

@Max I’m interested in the sodium compound you are using. I’m guessing it’s something that’s quite easy to disassociate the sodium ions from, but what would you have to hand? Possibly sodium carbonate or sodium hydroxide. It is likely to require quite a strong field, I am tempted to suspect using you’re RF, like it’s in a microwave oven. However, you have already mentioned capacitive coupled so lower frequency supply. I get the feeling I’m still missing something crucial.

@Max, with you it's never something obvious.!
That's what makes it so fascinating.!

I'm not here to share banalities, there are other lighting sites for that

AgentHalogen_87, Tuopeek - Good guess about the floating HF HV power supply (~30 kHz to be exact), this is what I use indeed. So, it is correct to say that the lamp runs on two-phase live, but this is not crucial to the way this discharge operates, it could also run properly on a ground/neutral-and-phase circuit.

Tuopeek - I don't exactly know the nature of the sodium compound used here either, it could be oxides, carbonates, etc. All I can say is that this was introduced as a by-product of something else. As for the release mechanism of sodium, something important is missing here indeed.

Hmm, if you haven't added a compound then it is possible for the sodium to originate from the gas walls of the tube. This is noticeable when working with glass but the temperature need to be at the melting point I think. I could be released from an electrode coating but this is electrodeless and this would also give an indication to the location of the emitter. As it looks balanced here I still think it could be from the glass walls... Is that possible at low temperatures!

Given the arc tube is kinked, is it some kind of chemical compound with sodium and argon, that requires the narrower diameter tube to force the sodium to excite?