This is for non-relativistic electron emission
and transport in vacuum.

Assumes no initial electron
velocity.

Assumes no ions (ions generally
increase electron current 100% (half impedance), but are a small fraction
of total current)

Effective area can be spreading of the electron source.

According to Bruce Weber (NRL), this equation only works up to about
500kV. Above this, replace the 2.34 with 2.00 in the coefficient
and the 3/2 with 4/3 in the exponent for a better fit.