I was always under the impression that HPS lamps were filled with a small amount of Neon gas to aid in the starting of the lamp.
However, I just recently read that during the 90s HPS lamps were now filled with Xenon gas and was wondering if anyone could provide more information about this?

Can't say I've ever heard of neon in an HPS lamp, but as always I have to admit I may well be wrong. All the examples I looked at on Lamptech show Na/Hg/Xe though.

Standard and high-performance HPS lamps are indeed filled with a xenon start gas, but a certain type of retrofit lamps designed to self-ignite at low voltages are filled with neon with a percent or so of argon (here's an example).

Thanks for the explanation, Max.