In a video on Youtube of a HG-free HPS lamp that exhibit the inital run-up of this type of lamp. The white light of the xenon was of course much brighter then in normal HPS lamp because of it high pressure (300 torr vs 50 torr of regular HPS), but the sodium began to vaporize soon after the lamp ignited (Without the 25-45 secs wait of the regular HPS). What cause this phenomenon in HPS lamps without mercury?

The higher xenon pressure mean higher arc voltage, so higher power dissipation in this state, so the arctube heat up and sodium evaporate quicker...