The ionization fraction of helium is determined by the temperature and density of the plasma. At a particular density

and temperature

, some fraction of helium will be neutral

, some fraction will be singly ionized

, and some fraction will be doubly ionized

. At the same time, the number of electrons per helium nucleus

is determined by charge neutrality. You can set

with

in cgs units (

to see the ionization fraction as temperature

varies from 40,000 K to 200,000 K.