Characterization of hollow cathode and parallel plate microplasmas: scaling and breakdown

We present an experimental comparison of parallel plate and hollow cathode microplasma structures, operated at high pressure with narrow anode–cathode gaps. A moveable anode arrangement with ±12μm spatial resolution was used to vary the electrode separation to cover the pd range between 0.01 and 4 Torr cm for both structures. The hollow cathode discharge was operated with pressure–diameter (pD) product values between 1.8 and 7.2 Torr cm and an aspect ratio of ∼5. Analysis of V –I characteristics shows j/p2 and j 2p scaling for low and high current modes, respectively. Electron temperature estimates between 2 and 3 eV were obtained by solving a numerical collisional–radiative model based on transitions from 4p argon levels. The hollow cathode sheath thickness for both ionizing and non-ionizing sheath conditions was determined analytically and the diffusion equation was solved numerically to estimate the plasma density profile for the ionizing sheath case. (Some figures in this article are in colour only in the electronic version)