Characterisation of microplasmas: parallel plate and hollow cathode configurations

Abstract

This thesis describes the characterisation of microplasma configurations which includehollow cathode (MHC) and parallel plate (PP) structures. A custom built microplasma source was used to investigate its DC microdischarge characteristics at high pressure (p > 10 Torr). High resolution (± 12 µm) control of the gap geometry allowed an investigation of pressure-geometric scaling by varying the discharge gap down to< 10 µm, as well as pressure, allowing for the first time a detailed comparison.The MHC configurations described in this thesis vary in external and internal gap geometry and anode dimensions. Voltage-ampere (V-I) measurements obtained for PP sources and the external anode MHC displayed distinctive low (Mode 1), and high (Mode 2), current modes separated by an oscillation regime. These V - I characteristics were compared to traditional scaling relationships to distinguish mode behaviour. Analysis showed j/p² scaling holds for the Mode 1 gap plasma and j²p scaling for the Mode 2 plasma within the hollow cavity. Electron temperatures were estimated to be between 2 eV and 3 eV using spectroscopy and a collisional radiative model.The hollow cathode sheath models of both ionising and non-ionising sheath conditions were examined analytically. Sheath thicknesses yielded from the models indicated the only realistic assumption is that that these MHC discharges operate with an ionising sheath. The diffusion equation was solved numerically to show a domed plasma density profile (nplpₑₐk > 10¹³ cm⁻³).When the anode was positioned within the hollow cathode, a stable plasma was generated and sustained inside the cathode. V-I characteristics from this configuration showed a new high current mode, Mode 3. Analysis of these characteristics showed that the Mode 2 plasma varies with length depending on current. The placing of the anode further into the cathode has been shown to extend the plasma along its axis, and hence offers a way of creating plasma surface interactions in long tubes. When the Mode 2 plasma approaches the end of the cathode, the plasma expands outside,radially across the cylinder wall, leading to Mode 3 operation.

Date of Award	Jan 2011
Original language	English
Supervisor	Paul Maguire (Supervisor) & Jim McLaughlin (Supervisor)