Atomic and Molecular Collisions in the Plasma Boundary

Abstract

The objective of this paper is to provide an introduction to those aspects of atomic collision physics which underly the unavoidably generalised base of cross section data and scaling relationships which is currently employed in plasma modelling. Both experimental and theoretical methods are outlined and, where practicable, general trends in collisional behaviour are illustrated by examples of measured data. Atomic and molecular processes are considered on the basis of their particular relevance to the plasma edge region so that the discussion emphasises the properties of collisions in the regimes of low plasma temperature and low charge state of impurity ions. Nevertheless the basic concepts apply with equal validity throughout the plasma. Particular attention is devoted to recycling of hydrogen atoms and molecules because of its powerful influence upon plasma properties adjacent to boundary surfaces. References are selected with the objective of providing easy access to detailed reviews on topics which perforce cannot be included in this brief account. The method of presentation is firstly to discuss the general roles of atomic and molecular collisions in the plasma edge, then to identify the types of collision involved and subsequently to describe the methods adopted to calculate or measure the relevant cross sections. Cross section data are introduced in increasing order of the complexity of their atomic interactions. Finally the influence of the plasma environment upon atomic collision rates is discussed.