Plasma sampling - a versatile tool in plasma chemistry
In order to determine the nature of the charged constituents of plasmas, various sampling techniques have been applied succesfully in different laboratories. It is the aim of this paper to show the limitatiorof these techniques due to sources of error, caused by effects occurring in the plasma sheath in front of a sampling orifice and effects within it, such as reactive collisions or collisional breakup, mass dependent transmission of the ions or the change of the energy distributions of the various types of ions in collisions during the extraction process. INTRODUCTION Plasma sampling had its early beginnings in experiments concerned with the nature of anode and cathode rays (Refs. 1-4). Although these experiments date back to the end of the last century, it was not until the fifties that the sampling technique was recognized as a powerful tool in plasma diagnostics (5—8) .During extensive investigations of combustion processes in 1953 Calcote and King measured the dependence of the ion concentration on the distance from the luminous flame zone in propane air mixtures (5). In 1957 Pahl and Weimer started systematic studies on the dependence of the wall current in a positive column on the discharge parameters and analyzed the positive ion current penetrating through a tiny orifice in the wall of the discharge vessel (Fig.1) with a magnetic mass spectrometer. Fig. 1. Apparatus of Pahl and Weimer (Ref.8) used for the investigation of the wall current in a positive column burning in a glass tube. + Present address: SRI International, Molecular Physics Laboratory, Menlo Park, Ca. 94025, USA.