Effect of Anode-Cathode Gap on Microhollow Cathode Discharges

A high pressure hollow cathode (HC) plasma source was designed and fabricated for thin film deposition of bio-compatible coatings on coronary artery stent interiors. We investigate the electrical {\&} optical characteristics of a 10 mm long solid-wall hollow cylinder ($\sim $2~mm ID), as a stent substitute, with precision control of the anode -- cathode (cylinder) gap dimension (to 12 micron). We present VI, Paschen curves and other derived scaling characteristics while OES was employed to determine the effective electron temperature via a collisional-radiative model [1]. An analytical radial sheath model is used to estimate the sheath width and electron density within the cylinder. We observe in the VI characteristics, a number of modes not associated with expansion over the cathode exterior but indicating current-dependent discharge progression along the interior of the cathode cylinder. We also observe deviation from standard pd and j/p$^{2}$ scaling.\\[4pt] [1] Mariotti D et. al., \textit{J. Appl. Phys} \textbf{101} (2007) 013307