The development of uniform atmospheric pressure glow discharges: appearance of stable glow discharge in air, argon, oxygen and nitrogen at atmospheric pressure using a 50 Hz source (S Okazaki et al 1993 J. Phys. D: Appl. Phys. 26 889)

The development of atmospheric pressure plasmas (APPs) for chemical synthesis, mat­ erials processing, environmental cleanup and now for biotechnology has been continually challenged by the desire, and the need, to produce uniform glow discharges. A scaling law in low temperature plasmas states that if ionization occurs only from the ground state, the gas density N is constant, and the E/N (electric field/gas density) is constant, the product Nτ is constant, where τ is the characteristic time that the plasma will come into a steady state— or become unstable [1]. APPs, by virtue of their high pressure (large N), have small values of τ. This instability formation time is typically so short at atmospheric pressure that feed­ back control systems cannot respond quickly enough to prevent an instability. The end result is that strategies for obtaining stable and uniform plasmas in APPs have typically relied on passive techniques where active intervention by the user is not required. The most common type of instability in APPs is the formation of an arc in which the discharge collapses into a small hot, intense region of plasma which often damages the electrodes. One of the features of APPs is that they are non­equilibrium, which means that the electron temperature can be controllably higher than the ion or gas temperature. This non­equilibrium nature of the plasma enables selective production excited states, ions and photons. The high temperature of the arc works against this selectivity by producing an equi­ librium set of conditions, not unlike combustion. Although arcs are highly desirable in many applications, for low gas temperature, selective processing, non­thermal plasmas are usually called for. The first step towards producing uniform APPs was the prevention of arcs. Perhaps the first example of a passive technique to stabilize an APP and prevent arcs is the dielectric­barrier­discharge (DBD) [2], invented by Ernst Werner von Siemens in 1857 as a means to produce ozone. The DBD is passively stable by inserting capacitance in series with the current flowing through the plasma—the dielectric barrier. By charging this series capacitance, the current through the plasma is cut off prior to an arc forming. Although the DBD is stable, it is usually not a uniform glow discharge. The plasma in a DBD is usually composed of a forest of small filaments having diameters of hundreds of microns [3]. Although this filamentary discharge serves well in many applications, it is often also desir­ able to have a uniform glow discharge. This means that on a microscopic basis, the plasma is spatially uniform without filaments, gradients or other inhomogeneities. In their article, Appearance of stable glow discharge in air, argon, oxygen and nitrogen at atmospheric pressure using a 50 Hz source, Satiko Okazaki, Mashuhiro Kogama, Makoto Uehara and Yoshihisa Kimura addressed this technological challenge—the desire and need Mark J Kushner