A critique of models of electronegative plasmas

This paper seeks to bring together into a common format work on electronegative plasmas, carried out over several years by different authors, as well as indicating how the differences have arisen. In the case of oxygen at higher pressures the dominant loss mechanism is detachment rather than recombination and the difference arose because the importance of associative detachment had not been recognized in some work. More generally, the concept of ambipolar diffusion does not carry over to electronegative plasmas. In particular the expressions given by Thompson (Thompson J B 1950 Proc. Phys. Sec. 73 818-21), quoted by Massey (Massey H S W 1974 Negative lans (Cambridge: Cambridge University Press)) and again by Lieberman and Lichtenberg (Lieberman M A and Lichtenberg A J 1994 Principles of Plasma Discharges and Materials Processing (New York: Wiley-Interscience)), are wrong because they are based on the mathematical inconsistency of taking the negative ion and electron densities to be proportional, while simultaneously obeying Boltzmann relations with different temperatures. In general, electronegative plasmas are structured and there is agreement on this point; furthermore, the ratio of attachment rate to ionization rate is an important parameter in determining the structure. It is shown that a relationship exists between the central negative ion/electron density ratio and the generation and loss processes, namely ionization, attachment, detachment and recombination, over a wide range of pressures. The means of determining the plasma balance equation and so relating the ionization rate to the plasma dimension, the pressure and other discharge parameters are indicated.