Hydrodynamic approximation for ions in high-pressure rf glow discharges

The use of the hydrodynamic approximation to describe the motion of ions in weakly ionized plasmas subjected to a high-frequency external electric field is critically examined. It is shown that if the frequency of this field is comparable with or larger than the ion collision frequency, then the response of the ions to the alternating electric field cannot be described by ion mobility. A possible alternative approach to the problem is presented.