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An atmospheric pressure plasma jet visually appears as a continuous plasma, but is actually comprised of a series of fast moving ionization waves, commonly called plasma bullets. Plasma bullets are the visible front of ionization streamers. Studying their spatiotemporal formation and propagation can help understand the plasma dynamics. In this work, the formation and lifetime of a high voltage pulsed dc plasma jet and the subsequent bullets were observed using time-resolved synchronized measurements with an intensified charge-coupled device (ICCD) camera and an ICCD spectrometer. The operating voltage, helium flow rate, and pulse width were varied to observe changes in bullet behavior. The measured velocities of the plasma bullets largely depended on voltage but showed that flow rate can have an effect. The formation of OH and excited N 2 (2+) was measured at the nanosecond timescale. It was seen that the formation of these species begins at the visible bullet, but the peak emission and decay occur ∼50-100 ns after bullet passage, indicating a finite time of reaction, excitation, and emission. Observing the temporal formation of plasma bullets and reactive species provides insight into some of the reactions occurring in the discharge and how to tailor the plasma operating conditions.

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As human innovation continuously expands the knowledge base for life beyond Earth, the need for self-sufficient spacecraft is essential. With that, space crop production facilities are ever expanding for research and development. A current area of key interest is seed sanitation before transport from ground to the International Space Station (ISS). Sanitation practices are performed to mitigate any potential biohazard and to ensure the viability of the seed. Conventional methods involve fumigation of seeds or chemical processes but are not effective with all seed types. Therefore, plasma technology was implemented in this research to explore low-temperature plasmas as an alternative means for seed sanitation without the need for chemicals. This project investigated the viability of plasma as a means for sanitation by incorporating three different plasma types within the study. For the treatment of Cherry Belle radish seeds, the optimal system was a radio frequency (RF) sub-atmospheric plasma chamber. Treatments of 100 W for 10 minutes or longer with the Diener system consistently reduced microbial loads by 90% or more. While 20-minute treatments caused reductions in germination rate, a treatment of 15 minutes with the Diener system at 100 W consistently resulted in germination rates above 80% after 1 month of seed storage. For the 5 and 10 minute treatments at a pressure of 187 mTorr and power of 100 W, growth was also accelerated. Additionally, plasma provided 90% reduction of Escherichia coli and Bacillus pumilis and a 99% reduction of Fusarium ozysporum on inoculated seeds. Overall, the plasma systems show promising potential but require further exploration.

An in-depth study of plasma activated water (PAW) generation was conducted to link changes in power supply, electrode material, input gas, and treatment time to the resulting reaction chemistry. These...

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