Properties of bipolar DC-pulsed microplasmas at intermediate pressures

Abstract

Summary form only given. Microplasmas generated in spatially confined cavities have applications ranging from electrical switching and radical production to lighting. In these applications, there is often a tradeoff between obtaining a short response time of the plasma and maximizing plasma density, both of which optimize with higher pressure; and obtaining a uniform plasma, which optimizes with lower pressure. These scalings motivate operation at intermediate pressures, tens of Torr to 100 Torr, which by pd scaling corresponds to sizes of the micro-cavity of hundreds of microns. In many cases, the inner surfaces of the microplasma cavities are largely dielectric due to ease of fabrication or to maximize lifetime. These conditions then motivate use of some form of bipolar excitation.In this paper, we discuss results from a computational investigation of scaling of microplasmas excited by pulsed dc-bipolar waveforms with the goal of maximizing the time averaged electron density. The computational platform is the Hybrid Plasma Equipment Model, a 2-dimensional hydrodynamics model in which radiation transport, and electron and ion distributions are addressed using Monte Carlo techniques. We investigated plasmas of 10s-100s Torr excited by short DC bipolar pulses (a few ns) with pulse repetition periods ranging from tens to hundreds of ns using mixtures of rare gases. Cavity sizes are a few hundred microns. Quasi-steady state, time averaged electron densities in excess of 1015 cm-3 in Penning mixtures are predicted. Although ionization by bulk electrons is the major source, the uniformity of the plasma is sensitive to ionization due to sheath accelerated secondary electrons. The behavior of the plasma was asymmetric with respect to the polarity of the voltage pulses, with more ionization occurring on the anodic portion of the cycle, in large part due to the electrically floating dielectric boundaries.