This ampoule was especially made for experiments with high voltage.
This tube does not contain any other “starter” gases, just pure Selenium. It needs a certain temperature to “ingnite”. Put the ampoule under boiling water to increase the pressure inside (Selenium even needs a bit more heat than others). Once illuminated with suitable electrodes, the ampoule keeps the ideal temperature for continuous illumination. It will work with a tesla tube only if heated enough before and during operation.
On the main picture you see the illuminated ampoule. Connected to a 5000V/35kHz transformer via two metal loops. The transformers can be found here.
The tube contains sublimed selenium which evaporates during operation and keeps the necessary pressure to illuminate.