Pulsed microplasmas generated in truncated paraboloidal microcavities: Simulations of particle densities and energy flow

Microplasmas generated within cavities having the form of a truncated paraboloid, introduced by Kim et al (2009 Appl. Phys. Lett. 94 011503), have been simulated numerically with a two-dimensional, fluid computational model. Microcavities with parabolic sidewalls, fabricated in nanoporous alumina (Al ₂O ₃) and having upper (primary emitter) and lower apertures of 150m and 75m in diameter, respectively, are driven by a bipolar voltage waveform at a frequency of 200kHz. For a Ne pressure of 500Torr and 2s, 290V pulses constituting each half-cycle of the driving voltage waveform, calculations predict that 10nJ of energy is delivered to each parabolic cavity, of which 26-30% is consumed by the electrons. Once the cathode fall is formed, approximately 65% and 8% of the input energy is devoted to driving the atomic ion and dimer ion currents, respectively, and the peak electron density of 6×10 ¹²cm ³ is attained 90ns following the onset of the first half-cycle (positive) voltage pulse. Specific power loading of the microplasma reaches 150kWcm ³ and the loss of power to the wall of the microcavity drops by as much as 24% when the excitation voltage is increased from 280 to 310V. The diminished influence of diffusion with increasing pressure is responsible for wall losses at 600Torr accounting for <20% of the total electron energy.