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Abstract
In this thesis, we use diode laser absorption spectroscopy and optical emission spectroscopy to characterise nanosecond repetitively pulsed discharges (NRPDs) generated between two parallel-plate electrodes in quiescent, sub-atmospheric argon and argon-nitrogen mixtures.
We have developed a fast, absorption-based diagnostic technique to make time-resolved measurements of the translational temperature and number density of metastable argon atoms generated in sub-atmospheric NRPDs. Measurements were made both during and after the discharge pulse with a time-resolution of \SI{2}{ns}. As far as we know, these measurements are the first of their kind with this time-resolution and capability of being able to make time-resolved temperature and number density measurements both during and after the discharge pulse. We report measurements with maximum temperature and number density uncertainties of $\pm$\SI{45}{K} (peak temperature of \SI{1890}{K}) and $\pm$\SI{8E13}{m^{-3}} (peak number density of \SI{1.4E16}{m^{-3}}), respectively. For the pulse energies investigated, we show that it is physically impossible to elevate the temperature of the bulk gas in the discharge volume to the measured metastable Ar temperatures. Our absorbance measurements suggest that only a small fraction ($\sim10^{-7}$) of the bulk gas is kinetically excited, with translational temperatures significantly higher than that of the bulk gas.
We used ro-vibrational spectra of N$_2(C)$ (second positive system) molecules obtained via emission spectroscopy to make time-resolved rotational and vibrational temperature measurements as a function of the nitrogen mole fraction and pulse energy in N$_2$-Ar NRPDs. We report rotational temperature measurements, which were found to be close to the initial ambient gas temperature for the mole fractions and pulse energies investigated. We demonstrate that the rotational temperature measurements are a good approximation of the translational temperature of the ground state nitrogen molecules. Our measurements show that the bulk gas does not heat up significantly during the discharge pulse, as predicted with the energy balance calculations. In addition, we show that the vibrational temperature is an order of magnitude greater than the rotational temperature of N$_2$(C) molecules, thus demonstrating the highly non-equilibrium nature of NRPDs. We also show that the spectral irradiance of the nitrogen second positive system increases with argon content, however, the mole fraction dependence on the vibrational temperature is unclear due to the uncertainty in the vibrational temperatures. We report translational temperature measurements of metastable Ar made in N$_2$-Ar NRPDs, which show that there is kinetic energy transfer between the metastable argon atoms and nitrogen molecules. The magnitude of the additional vibrational excitation due to collisions with metastable argon atoms is presently unknown.
We performed a parametric study on argon NRPDs to investigate the effect of pressure, pulse repetition frequency (PRF) and pulse energy on the temperature and number density of metastable argon atoms. Using known kinetic process rates, we show that the most likely cause for the reduced lifetime of the metastable atoms is due to collisional quenching with water vapour molecules. We show that the peak temperature decreases linearly at higher PRFs and the $1/e$ decay time constant of the number density remains unchanged above a PRF of \SI{10}{kHz}. We propose an empirical relationship to describe the pulse energy dependence on the peak translational temperature and the $1/e$ decay time constant of the metastable Ar number density. The number density measurements across the electrode gap were shown to be consistent with the intensity distribution across a striated positive column. We also show that the peak temperature measured at the cathode is higher than those measured at the anode and the centre of the electrode gap, consistent with known discharge physics.