STRONGLY DAMPED SURFACE-WAVES IN PLASMAS .1. THE WKB APPROXIMATION

The propagation of highly damped surface waves in plasmas, i.e. plasmas with large axial electron density gradients is dealt with. The WKB approximation adopted in previous models of surface wave discharges is not applicable in this situation. A key parameter of discharge behaviour is nu/omega, where nu is the electron-atom collision frequency and omega is the excitation frequency of the wave. Discharges in which nu/omega << 1 are generally referred to as 'weakly collisional'. However, the range of validity of the WKB approximation depends also on the local value of the electron density. Even if nu/omega << 1, this approximation breaks down for sufficiently low electron densities (e.g. near the end of the discharge), while in some cases WKB may adequately model discharges in which nu/omega >> 1. The limits of validity of the WKB approximation are examined, both for published experimental results and parameters relevant to our own experiments on fluorescent lamps. In a second paper we will show solutions for cases where the axial density gradients are high, using the formalism developed previously for electrostatic surface waves.