Very easy, in fact. Anyone can do it in half-hour or less. No
secret gas recipe of "Thuringian glassblowers" was needed ;)

What I did:

Took two 10mm EGL electrodes. Cleaned activation coating with
vinegar. Washed with clean water.

Dried overnight. Next, took about 30cm of 12mm lead tube. Blown
some bubbles. Spliced closed T-arm with couple of bubbles too.
Spliced electrodes. Lightly bombarded couple of times, evacuated in
between to remove water vapours. Cooled and backfilled with about
1.5mm of simple plain air. Hooked 20 ma HF transformer. Adjusted
fill for best effect. Volia! When on, glows pink nitrogen and
periphery of bubbles and T-arm are filled with dim lemon-green
glow. When off, aterglow continues for about 1-2seconds, just like
in "Thuringian videos" - "nachleuchtendem Geissler Rohre" on
Youtube. 1-2 mm of air is sweet point IMHO. Scientific texts I
found on internet suggest that oxygen is nessessary and this glow
most likely is a chemiluminescense of nitrogen oxide when it
combines with oxygen. Not sure if non-activated
electrodes and exactly lead tube are needed. In fact, if you ever
bombarded thick tube with bubbles, you may be already noticed that
yellowish glow.