These tubes did not have particularly long lifetimes - usually only a few years of shelf life. Light helium atoms would eventually diffuse through the relatively porous seals changing the gas mix until the tube no longer lases. After a week or so enough helium would often infuse into the tube to allow it to operate again. Once the tube is too far gone, though, this will not work and opening the tube for reprocessing is the only option. The actual plasma tube was a thick capillary tube with a small center bore.
Newer tubes are all hard sealed.
This tube can be used for reconnection to a reprocessing system as well. Given the availability of old HeNe tubes one wonders if it is possible to use other gases in the tube. Indeed there is research to show that a pulsed laser using pure neon gas at about 0.3 torr and a capacitor of about 10nF at 15-20kV will produce several yellow and orange lines. These transitions appear in capillary tubes varying from 20 to 40cm in length. This is certainly worth a try and I intend to experiment with this when a tube is connected to the high-vacuum system for reprocessing.