Spectacular failure

A while ago I found that the burner of one of my spare sodium lamps had leaked, thus rendering it useless. Since the lamp in question, a Philips SON-H 220W, is of the retrofit kind, I was curious to see how the very low pressure neon discharge in the outer bulb would look like when energized with a standard 50 Hz neon-sign transformer. The sequence presented above is the result.

Initially, and as expected, the discharge is formed in the outer bulb volume, outside the burner (left picture). While the bright negative glow remains confined around the metal frame, the very low fill pressure results in quite a long electron mean free path which causes the ionization of the neon atmosphere all the way to the glass wall (the fact that we have a neon-argon Penning mixture helps also). This results in a unusually voluminous diffuse discharge normally associated with high-frequency excitations.

Interestingly, because of the nature of the discharge the power dissipated in the lamp was quite significant and led to a strong heating of the leaked burner. As a result, the mercury and sodium vapor pressures increased to the point that a discharge ignited inside the burner, diverting part of the the lamp's input power. The result is the dual discharge shown in the right picture. What is extremely unusual there is that we have two discharges running in parallel, something that normally never happens. The reason why this works here is because those two discharges do not burn simultaneously: when the plasma in the arc tube is conducting, the outside glow is off, and when the drive current has reverted polarity and the burner plasma is off, the outside glow strikes and runs until the discharge in the burner re-ignites. We see both plasmas here because the image was capture in a long exposure (this is in fact a combination of several long exposures as this is an HDR composite).

A very peculiar phenomenon occurred for a short moment before the transition from the single glow (left) to the dual discharge (right) mode. As the lamp's metal frame was heavily sputtered by energetic neon ions from the glow discharge, the burner was being covered by a growing metallic layer. At one point, this conducting layer caused a repetitively transient short-circuit which resulted in the generation of very high voltage spikes from the neon-sign transformer (several tens of kV by my estimate). This led to the production of extremely energetic electrons that resulted in the local bluish luminescence of the glass jacket (check the bulb neck's right side in the left image), the production of ozone and NOx outside the lamp (the smell was unmistakable) and possibly the emission of X-rays.

This lamp failure was really exceptional on many levels and resulted in several rare or unusual phenomena in a very short amount of time! the picture below shows the state of the lamp in the early stage of the experiment (left) and at the end (right), showing the effect of sputtering.