A few additional notes:
(1) There are traces of other gases in the mix as well. Water vapor and Argon for example. Each addresses some secondary need of the laser,
(2) Over time some of the metal electrodes erode and that metal combines with the various gases to create new ingredients in the gas mixture. (Especially the cathode.)
(3) One of the problems is CO2 disassociating into CO instead (affected by the catalyst in the helical tube).
(4) Another aging problem is erosion of the ZeSn window and internal mirror. Both contribute to the loss of beam quality as the tube ages. (loss of focus or creation of satellite beam spots).
(5) On my K40 (and the few others I’ve seen), the manufacturer chose to wire the WP and (TH and TL) reversed such that the WP was actually being driven from the controller board. I’ve analyzed the internal circuitry of several other laser power supplies, and the WP line driven this way means the the laser never is used in pre-ionization mode. When WP is off, the output current is off. If properly used, WP being on, enables pre-ionization and the tube is running at a current just under that necessary to start lasing. Then the combination of (TH and TL) driven by the controller board will shift the current from pre-ionization to full on (where full on is determined by the potentiometer setting). This is more important for fine engraving than for cutting.
(6) With the ballast resistor used on many of these machines, if the tube is idling at preionization current levels, then the node from the ballast resistor to the tube return, can sit at several hundred volts, which could be dangerous. (But I don’t have equipment to verify how high of voltage.)