Experimental investigation of nanosecond-pulsed dielectric barrier discharge in atmospheric pressure air and its application for direct liquefaction of natural gas Chong Liu Advisor: Dr. Danil Dobrynin Uniformity of high-pressure discharges, especially those ignited in air, has been a topic of interest for long time. Conventionally, as the applied electric field (voltage) increases, the breakdown mechanism changes from uniform Townsend discharge to non-uniform streamer discharge. The focus of this thesis is based on the hypothesis that with application of significant over-voltages, i.e., fast rising pulsed electric fields that allow production of electron density suitable for avalanche-streamer transition significantly before the discharge gap is bridged, may result in development of spatially uniform plasma. This study is devoted to testing this hypothesis and characterization of atmospheric air conventional DBD and DBD ignited under over-voltage conditions. The goals of this thesis are to understand the physics and chemistry of nanosecond pulsed DBD in atmospheric pressure gases, and especially atmospheric air, using experimental techniques, to qualitatively and quantitatively characterize the uniform operating regime of atmospheric pressure DBD, and to evaluate its potential applications. In this thesis, fast imaging of the discharge development on nanosecond time scales in atmospheric air was performed, and transition of DBD from streamer to uniform "overvoltage" mode was shown. A quantitative method was developed for analysis of the discharge uniformity. A nanosecond-pulsed dielectric barrier discharge ignited in atmospheric air was studied by optical emission spectroscopy to investigate the time and space-resolved development of the reduced electric field. The discharge temperature and chemistry were studied as well. The major results obtained in this work can be summarized as follows: 0́Ø It is shown that the discharge operates in two distinctively different modes which appear as "uniform" and "non-uniform" regimes. Qualitative uniformity analysis of the discharge images is performed using chi-square test. 0́Ø It is shown that measured maximum local electric field in the discharge is in a good agreement with these modes. We hypothesize that these results can be qualitatively explained by the absence of individual streamers in the uniform mode due to their overlapping and corresponding decrease of the maximum local electric field to the value of average electric field if the discharge. Due to a strong coupling between discharge physics, and reduced electric field in particular, and plasma chemistry (which in turn determines applications of plasmas), possibility of controlling discharge basic parameters together with its uniformity by simply changing applied voltage or distance between electrodes offers unique and exciting opportunities in a wide range of applications, from treatment of biological tissues to energy applications. The possibility of its application on direct liquefaction of natural gas is investigated as a potential application based on the findings.