Kinetic processes controlling N2 vibrational distribution, electron temperature and electron density in nanosecond pulse, nonequilibrium plasma, electric discharges are studied through laser scattering diagnostic techniques. The experiments are conducted in high pulse energy (≥4 mJ/pulse), nanosecond pulse gas discharge plasmas at moderate pressures (75-200 torr) in nitrogen, air, helium, H2-He and O2-He mixtures.
In electric discharges, local energy loading is a function of the electron number density (ne) and electron temperature (Te). Furthermore, electron temperature, and more specifically, electron energy distribution function (EEDF) control the electron energy partition in nonequilibrium plasmas by controlling the rates of critical kinetic processes including ionization, vibrational and electronic excitation, and recombination of molecules, atoms and electrons in the gas discharge. Thus, obtaining time-resolved, quantitative measurements for these values (ne, Te, and EEDF) is critical in understanding the energy requirements for sustaining these discharges, as well as discerning how electron energy is partitioned among different molecular energy modes, and which excited species and radicals are generated in the plasma. Furthermore, in molecular plasmas, significant electron energy is loaded into vibrational modes. Study of temporally resolved vibrational distribution function (VDF) and vibrational temperature (Tv) is important in quantifying vibrational energy loading and relaxation in these plasmas. This affects the rate of temperature rise in nanosecond pulse discharges and the afterglow, as well as rates of vibrationally stimulated chemical reactions, such as NO formation. Applications of these studies include plasma flow control (PFC), plasma assisted combustion (PAC), electrically excited laser development and various plasma bio-medical applications.
Time-resolved N2 vibrational distribution function (VDF) and first-level N2 vibrational temperature have been measured via spontaneous Raman scattering in a nanosecond pulse, nonequilibrium, single-filament gas discharge sustained between two spherical copper electrodes. Gases studied include nitrogen and air (P=100 torr). Highly nonequilibrium N2 VDFs have been observed (vibrational levels up to v=12 significantly populated and detected). Results in nitrogen have been compared with a 0-D, master equation kinetic model.
A Thomson scattering diagnostic, including a solid state Nd:YAG laser as the pump source, a custom-made glass test cell, and custom-built triple-grating spectrometer has been developed. Thomson scattering has very low signal intensity, and is therefore highly susceptible to several types of interference. Rayleigh scattering interference has been filtered from the spectra by using a spectral mask in the spectrometer, while a second slit was used to provide critical stray light rejection. Background interference due to plasma emission has been subtracted.
Time-resolved electron number density, electron temperature and electron energy distribution function (EEDF) have been measured via the Thomson scattering diagnostic. Studies of two highly nonequilibrium plasma environments have been conducted, including a nanosecond pulse, single-filament discharge sustained between two spherical copper electrodes, as well as a nanosecond pulse near surface discharge (which develops initially as a surface ionization wave). Studies in helium, as well as mixtures of H2 in helium and O2 in helium have been conducted in the single-filament discharge, while a study in helium has been conducted in the near surface discharge. Results in helium for the single-filament discharge have been compared with a 2-D, axisymmetric, kinetic model. Electron density measurements in these experiments ranged from 1013 - 1015 cm-3, while electron temperatures were observed to range from 0.1 - 7.0 eV.