Work Function

This is the energy required to remove an electron from the highest filled level in the Fermi distribution of a solid to a point a long way away, at absolute zero.

where

I = thermionic current [A]

T = temperature [K]

φ = work function [eV]

A = constant with a theoretical value of 120 Acm^-2K^-2

k = Boltzmann’s constant = 1.38062x10^-23 JK^-1

In ion engines, the work function of the ionizer must be greater than the ionization potential of the neutral atoms in the propellant gas.