Effect of gas cleanup in low-pressure xenon discharge

Here is a special T4 low-pressure (1.33 mbar) xenon discharge lamp shown right after its first startup (top), and about an hour later (bottom), operated on an HF 13 W fluorescent lamp driver. The difference in light color arises from the presence of gaseous impurities (mainly CO2) in the lamp fill at the time of its first starting. Over time, energetic electrons in the discharge breakup the molecular impurities, whose components (atoms, smaller molecules, radicals) react with various lamp parts, such as the glass wall, the lead wires and the oxide-coated tungsten electrodes. The nonthermal plasma driven between the electrodes is a very energetic environment which results in the gradual disappearance of impurities, leading to the light emission settling on the purplish color of the pure low-pressure xenon discharge. This clean-up procedure is crucially important in plasma studies involving closed vessels to ensure that measured characteristics are not affected by pollutants.

Of historical interest, the whitish light color emitted by carbon dioxide led to the development and introduction of Moore's tube in the early 20th century, a lamp technology which predates the fluorescent tube. Because of the clean up mechanism described above, the CO2 fill had to be replenished regularly via an automated system to ensure a reasonably long service life. Note that in the present case the pressure is much lower than in Moore's tube and the accelerated dissociation of carbon dioxide results in a significant oxygen partial pressure in the discharge, which gives a noticeable greenish tone to the emitted light. Also, at such low pressure (1.33 mbar) and low current intensity (~0.17 A) the xenon discharge is not white.