The hot plate is made of a metal that has a large work function and can withstand high temperatures, e.g. tungsten or rhenium.
A high value of the hot plate work function and a low ionization potential of the metal makes for a low potential barrier for an electron in the alkaline metal to overcome, thus making the ionization process more efficient.
Sometimes barium is used instead of an alkaline metal due to its excellent spectroscopic properties.
The low temperature makes it possible to create a plasma column that is several ion gyro radii across.
Since the alkaline metals are solids at room temperature they will stick to the walls of the machine on impact, and therefore the neutral pressure can be kept so low that for all practical purposes the plasma is fully ionised.