How hot does the plasma inside a high-pressure sodium-vapour lamp become?

The plasma within the inner alumina envelope of a high-pressure sodium-vapour lamp reaches temperatures of 1,200 °C (2,200 °F).1

This extreme environment is made possible by the use of translucent alumina, an optical ceramic that can withstand the intense heat and corrosive effects of the sodium and mercury mixture while remaining transparent enough to emit light.213