Utilizing Raman Scattering to Explore the Non-equilibrium Thermodynamics of Plasma.

We describe a nonintrusive method to measure the vibrational distribution and concentration of molecules in non-equilibrium plasma, such as O₂, CH₄, H₂, CO, and CO₂. This method utilizes Raman scattering to determine the chemical makeup and temperature of a sample of plasma. A 2-cavity laser pulse stretching technique is used in this method to increase the amount of time it takes the laser pulse to pass through the sample. The 2-cavity laser pulse stretching technique uses two beam splitters to split the laser pulse. One part of the laser pulse is then sent through a cavity created by a set of mirrors while the other part continues to the second beam splitter and second cavity. This technique triples the length of the pulse from approximately 20 ns to approximately 60 ns. The method originally used a Herriott cell to increase the number of passes the laser makes through the sample which would amplify the gathered signal. The cell that was created did not have a great enough vertical resolution for the tests so only one extra pass will be used. The method was used to collect Rayleigh scattering data from ambient air and will be used to collect Raman scattering data from ambient air and a methane flame in the near future.