Diagnostics of high pressure microdischarge plasmas

High pressure (100s of Torr) non-equilibrium microdischarge plasmas have numerous applications including excimer radiation sources, sensors, plasma display panels, ozonizers, and biomedical processes. Plasma diagnostics are indispensable for better understanding of microdischarge physics and optimizing device performance.;Slot-type DC microdischarges were studied using various diagnostics: Laser Thomson Scattering (LTS), Rotational Raman Scattering (RRS), Diode Laser Absorption Spectroscopy (DLAS) and Optical Emission Spectroscopy (OES). With these techniques important plasma parameters---electron density ( ne), electron temperature (Te), gas temperature (Tg) and argon (1s5) metastable density (Nm)---were experimentally measured.;LTS was employed in a novel confocal configuration to perform direct simultaneous measurement of ne and T e for the first time in DC argon microdischarges. RRS provided absolute calibration of the Thomson signal. It was also employed to carry out the first direct spatially resolved 3D Tg measurements in nitrogen DC microdischarges. The results were in reasonable agreement with the predictions of a mathematical model. Spatially resolved measurements of ne and Tg in DC argon microdischarges were also performed using OES with trace gases (N2 and H2O). Spatially resolved Tg across the gap was obtained from analysis of the rotational structure of the first positive band of N2. Tg peaked on the cathode side and slowly decreased towards the anode. Electron densities were extracted from Stark broadening of the Hbeta line. The spatial profile of ne had a maximum in the cathode sheath edge, followed by a minimum in the bulk plasma, and another local maximum towards the anode. This profile was due to a highly contracted positive column. The results obtained by direct (LTS) and indirect (OES) techniques were compared and discussed. It was also revealed that argon metastables play an important part in microdischarge operation.;The first spatially resolved measurements of Nm in Ar microdischarges were carried out using DLAS. Nm peaked near the cathode and decayed rapidly in the bulk plasma. N m increased with increasing pressure, but decreased with increasing current. Neutral gas heating was found to be responsible for this behavior.;This revealed importance of the gas heating in microdischarges was a motivation to develop a new semi-analytical model of the cathode layer that accounts for this effect. The I-V scaling laws obtained by the classical theory of the cathode layer disagreed with experimental results and were shown not to be applicable to microdisharges due to the influence of gas heating. The new model captured the features of experimental I-V curves. The model was used to obtain a rough criterion of the glow-to-arc transition, providing a quantitative measure of discharge stability.