The collisionless two‐fluid model of a plasma is applied to the drift of ions and electrons downstream from an electron cyclotron resonance source. A plasma potential drop of 20 V is found for the system parameters typical of plasma‐assisted epitaxy of the group III nitrides. This model provides a simple way to design downstream magnet coils to tune the ion acceleration, a key parameter in growth and etching processes. Compared to previous analyses, the present work makes clear the conditions under which an ion accelerating potential actually appears, in terms of the expansion that the plasma undergoes upon exiting from the source region. Published measurements of the potential profile downstream from a N2plasma source can be fit assuming an electron temperaturekTe=3.2 eV, somewhat greater than half the excitation energy of the long‐livedA 3&Sgr;+umetastable state. The analysis is consistent with the plane probe current–voltage characteristics obtained from the growth stage in a plasma‐assisted molecular‐beam epitaxy process. For a 10 W plasma used in the growth of GaN, the value of ion saturation current implies a plasma density at the source of 1010cm−3. Considering the suppression of electron current by the magnetic field, the measured ratio of ion to electron saturation current implies an electron temperature of 3.8 eV. ©1996 American Institute of Physics.