  We investigate the spatially and temporally resolved electron kinetics in a
homogeneous electric field in argon gas, in the vicinity of an emitting
boundary. This (transient) region, where the electron swarm exhibits
non-equilibrium character with energy gain and loss processes taking place at
separate positions (in space and time), is monitored experimentally in a
scanning drift tube apparatus. Depending on the strength of the reduced
electric field we observe the equilibration of the swarm over different length
scales, beyond which the energy gain and loss mechanism becomes locally
balanced and transport properties become spatially invariant. The evolution of
the electron swarm in the experimental apparatus is also described by Monte
Carlo simulations, of which the results are in good agreement with the
experimental observations, over the domains of the reduced electric field and
the gas pressure covered.
