Plasma sources using long linear microwave field applicators: main features, classification and modelling

Abstract

Microwave plasma sources using elongated field applicators that extend over distances large in comparison with the free-space wavelength are in demand for many applications, mainly for the processing of large surfaces of materials that can be moved transversely with respect to the applicator. Various long applicators have been presented in the literature but no unified description is available to assist the potential user in selecting the one best suited for his or her particular needs. This paper is intended to meet this need. All applicators of this kind provide an electromagnetic wave from which originates the electric field sustaining the discharge. However, they differ in some essential aspects which reflect on their performance and on the modelling of the resulting plasma. We propose to classify these applicators first into two main categories according to the way the power flows to the plasma: (i) transmission-line applicators; and (ii) antenna applicators. Then, in each category, we need to distinguish two possible wave characters: travelling and standing. This leaves us with four systems to examine. For each of these systems, we briefly approach discharge modelling, mainly to examine the longitudinal distribution of plasma properties and establish guidelines for the methodical design and operation of these plasma sources. Finally, we evaluate the advantages and disadvantages of the four systems.