This is a flame diode.  A hot filament is used to rectify a high voltage in a similar manner to a vacuum tube. Except it didn’t work for me.
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It was based on this excellent video and article where two triodes are made and form a nice oscillator.

The photos above show the setup with a 6 kV 30 mA neon sign transformer (NST). I am using a MAPP gas burner with a tungsten filament attached to it. The anode is a titanium mesh cylinder from a pool chlorinator. The only time I seemed to get proper rectification was when sparks formed, rather than as a diffuse process as I had hoped. The DC meter shows 7 mA of rectification but it was rather hit and miss and most of the time there was no useful rectification. I am sure the flame position is critical and that the cathode should be fairly cold. It is glowing red hot here.

The tungsten filament oxidises after a while too. I had hopes of making a big propane powered triode to run a VTTC. That would have been impressive.

Probably worth revisiting sometime though, to make a triode with proper grid control. However, I have only ever made only standard valve circuit before so lots of variables even with the flame diode working OK.

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