Resonance hairpin probe for electronegative plasma diagnostics

The practical applications of resonance hairpin probe for characterizing electronegative plasma is investigated. In particular the hairpin probe was operated in a time-resolved mode for measuring the electron density evolution during pulsed laser photodetachment of negative ions in an oxygen inductively coupled plasma (ICP) discharge. From the temporal evolution of the electron density, the negative ion temperature is determined from the characteristic diffusion time of negative ions in the photodetached channel. This is compared with an independent method based on the temporal evolution of the plasma potential during photodetachment measured by a floating emissive probe. Temporal evolution of negative oxygen ions in a pulsed dc magnetron discharge is also obtained using pulsed photodetachment and a time-resolved hairpin probe. The results are found to be in reasonably good agreement with those measured from Langmuir probe assisted photodetachment. Two important applications addressed in this thesis are the use of steady state and pulse biased hairpin probes. For overcoming the limitations arising because of the finite sheath width around the resonator pins, the hairpin was systematically biased at strong negative dc potentials (eφ >> kTe) with respect to the plasma. A plot of resonance frequency verses probe bias is shown to provide a unique value for the electron density. Comparison of the sheath corrected electron density based on the step front sheath model and that obtained with a planar Child-Langmuir sheath approximation, were found to be in good agreement at higher densities (above 1x10 16 m-3), however small deviations were observed at lower densities (below 3x1015 m-3) due to deviation from a planar sheath assumption. Finally, qualitative measurement of negative ion parameters is performed by pulse biasing the hairpin probe. Comparison of the temporal evolution of the electron density before and post application of a negative pulse bias allows one to obtain both the negative ion density, negative ion temperature, as well as the positive ion density. The relative measurements of the negative ion parameters using this technique are found to be in reasonably good agreement with the hairpin probe assisted laser photodetachment however the absolute measurements are underestimated by a factor of 10. The discrepancy in the measured absolute negative ion parameters and the limitations behind this technique are discussed