A transient model of a low‐pressure Cs‐Ba diode which operates in a collisionless Knudsen mode is developed and benchmarked with experimental measurements. The model couples the external circuit to the plasma discharge in the diode. The model calculates the electron energy, plasma density, densities of ground state and first four excited states of neutral Cs atoms, sheath potentials, Cs coverage on the electrode surfaces, forward voltage drop, and discharge current as functions of time, as well as the current‐voltage (I‐V) characteristic of the diode. Calculated terminal voltage and current characteristics are in good agreement with experimental values. ©1995 American Institute of Physics.