TIME-RESOLVED DETERMINATION OF THE ELECTRON-ENERGY DISTRIBUTION FUNCTION IN A DC PULSED PLASMA

We present an experimental device and a numerical procedure which allo ws the time-resolved sampling and analysis of electrical I-V probe cha racteristics in a low frequency (less than or equal to 1 kHz) DC pulse d plasma used for iron nitriding. Plasma parameters such as electron d ensity and energy and plasma potential are deduced from the experiment al determination of the electron energy distribution function f(E). Th e use of the second derivative technique allows us to follow the time variation of the electron population in the early (t less than or equa l to 2 ms) afterglow. The non-Maxwellian form of f(E) in the afterglow is explained in terms of kinetic processes leading to vibrationally e xcited states of molecular nitrogen which are involved in the plasma r eactivity. Particular study of the real duration of the afterglow show s that the charged species remain in the gas phase up to 14 ms after t he discharge is cut off. These results, combined with the study of f(E ) in the afterglow, lead us to establish time ranges for the discharge and post-discharge durations which appear favourable for the nitridin g process.