Applications of non-thermal plasmas range from material processing to pollution control. In an effort to increase the density of excited states or reactants, or to process non-gaseous media, the scaling of non-thermal discharges to high pressures, liquids, and multiphase systems is of great interest. Supercritical fluids (SCFs) are a special class of high-density fluids that have been investigated in their own rite due to their unique cleaning and solvating properties. Having densities and transport properties intermediate between the gas and liquid states, SCFs have inherent spatial inhomogeneities and are often referred to as "cluster" fluids. This project investigates the nearly unexplored but practically important area of SCF plasmas and discharges. Plasmas sustained in SCFs are expected to have unique chemical reactivity resulting from both the inherent cluster-fluid properties of SCFs; and radical and ion production through electron impact processes in plasma. Even the most fundamental properties of SCF plasmas are largely uncharacterized; so our ability to leverage their unique properties is severely hindered. The project goal is to investigate and characterize discharge initiation and propagation in SCF plasmas. Experimental investigations on electric discharge initiation in supercritical CO2 are being performed over a wide range of supercritical pressures and temperatures. The streamer breakdown mechanism is being investigated including critical electric field, propagation velocity, diameter, branching, and spark transition. Emphasis is placed on the role of spatial inhomogeneities resulting from the inherent nanoscale clustering on plasma properties. Diagnostics include spatially and temporally resolved ICCD imaging, optical spectroscopy, and Schlieren techniques. A comprehensive database on discharge and streamer parameters in SCFs including breakdown voltages, streamer velocities, and spatial characteristics for SCF plasmas is beginning to be assembled. The broader impact involves advances in science, technology, and education. This study is providing fundamental new insights on streamer propagation in inhomogeneous media incorporating clusters of various size and structure. As such, the work is providing a baseline for others to exploit this new class of SCF plasma, which will compliment research on discharges in high-pressure gases and liquids. The research is also advancing knowledge in related areas of dusty plasmas and microplasmas. From a practical perspective, this research opens the area of non-thermal supercritical plasmas for technological benefit. Exploiting the properties of SCF plasmas will likely extend and improve the technologies that now utilize conventional SCFs. Independently, both non-equilibrium plasmas and SCFs have proven to be exceptional processes for oxidizing, etching parts, pollution removal, material synthesis, sensors, bacteria inactivation and polymerization. The combination of both processes holds the promise of significant synergistic enhancements. Research and education are integrated via mechanisms that will serve several important constituencies, including: graduate and undergraduate students and fellow researchers through opening up a new area of plasma science and engineering. Undergraduate students are involved in many aspects of this project through academic credit for undergraduate research.
