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Electron Heating in Weakly Ionized Collisional Plasmas
(from left: Professor Wonho Choe and Research Professor Sanghoo Park) A KAIST research team successfully identified the underlying principles behind electron heating, which is one of the most important phenomena in plasmas. As the electric heating determines wide range of physical and chemical properties of plasmas, this outcome will allow relevant industries to extend and effectively customize a range of plasma characteristics for their specific needs. Plasma, frequently called the fourth state of matter, can be mostly formed by artificially energizing gases in standard temperature (25°C) and pressure (1 atm) range. Among the many types of plasma, atmospheric-pressure plasmas have been gaining a great deal of attention due to their unique features and applicability in various scientific and industrial fields. Because plasma characteristics strongly depends on gas pressure in the sub-atmospheric to atmospheric pressure range, characterizing the plasma at different pressures is a prerequisite for understanding the fundamental principles of plasmas and for their industrial applications. In that sense, information on the spatio-temporal evolution in the electron density and temperature is very important because various physical and chemical reactions within a plasma arise from electrons. Hence, electron heating has been an interesting topic in the field of plasma. Because collisions between free electrons and neutral gases are frequent under atmospheric-pressure conditions, there are physical limits to measuring the electron density and temperature in plasmas using conventional diagnostic tools, thus the principles behind free electron heating could not be experimentally revealed. Moreover, lacking information on a key parameter of electron heating and its controlling methods is troublesome and limit improving the reactivity and applicability of such plasmas. To address these issues, Professor Wonho Choe and his team from the Department of Nuclear and Quantum Engineering employed neutral bremsstrahlung-based electron diagnostics in order to accurately examine the electron density and temperature in target plasmas. In addition, a novel imaging diagnostics for two dimensional distribution of electron information was developed. Using the diagnostic technique they developed, the team measured the nanosecond-resolved electron temperature in weakly ionized collisional plasmas, and they succeeded in revealing the spatiotemporal distribution and the fundamental principle involved in the electron heating process. The team successfully revealed the fundamental principle of the electron heating process under atmospheric to sub-atmospheric pressure (0.25-1atm) conditions through conducting the experiment on the spatiotemporal evolution of electron temperature. Their findings of the underlying research data on free electrons in weakly ionized collisional plasmas will contribute to enhancing the field of plasma science and their commercial applications. Professor Choe said, “The results of this study provide a clear picture of electron heating in weakly ionized plasmas under conditions where collisions between free electrons and neutral particles are frequent. We hope this study will be informative and helpful in utilizing and commercializing atmospheric-pressure plasma sources in the near future.” Articles related to this research, led by Research Professor Sanghoo Park, were published in Scientific Reports on May 14 and July 5. Figure 1. Nanosecond-resolved visualization of the electron heating structure. Spatiotemporal evolution of 514.5-nm continuum radiation,Te, Ar I emission Figure 2. Nanosecond-resolved visualization of electron heating. Spatiotemporal evolution of neutral bremsstrahlung at 514.5 nm
KAIST Finds the Principle of Electric Wind in Plasma
(From left: Professor Wonho Choe and PhD Sanghoo Park) A KAIST team identified the basic principle of electric wind in plasma. This finding will contribute to developing technology in various applications of plasma, including fluid control technology. Professor Wonho Choe from the Department of Physics and his team identified the main principle of neutral gas flow in plasma, known as ‘electric wind’, in collaboration with Professor Se Youn Moon’s team at Chonbuk National University. Electric wind in plasma is a well-known consequence of interactions arising from collisions between charged particles (electrons or ions) and neutral particles. It refers to the flow of neutral gas that occurs when charged particles accelerate and collide with a neutral gas. This is a way to create air movement without mechanical movement, such as fan wings, and it is gaining interest as a next-generation technology to replace existing fans. However, there was no experimental evidence of the cause. To identify the cause, the team used atmospheric pressure plasma. As a result, the team succeeded in identifying streamer propagation and space charge drift from electrohydrodynamic (EHD) force in a qualitative manner. According to the team, streamer propagation has very little effect on electric wind, but space charge drift that follows streamer propagation and collapse was the main cause of electric wind. The team also identified that electrons, instead of negatively charged ions, were key components of electric wind generation in certain plasmas. Furthermore, electric wind with the highest speed of 4 m/s was created in a helium jet plasma, which is one fourth the speed of a typhoon. These results indicate that the study could provide basic principles to effectively control the speed of electric wind. Professor Choe said, “These findings set a significant foundation to understand the interactions between electrons or ions and neutral particles that occur in weakly ionized plasmas, such as atmospheric pressure plasmas. This can play an important role in expanding the field of fluid-control applications using plasmas which becomes economically and commercially interest.” This research, led by PhD Sanghoo Park, was published online in Nature Communications on January 25. Figure 1. Plasma jet image Figure 2. The differences in electric wind speeds and voltage pulse
Plasma, an Excellent Sterilizer to Remove Harmful Bacteria
(PhD candidate Joo Young Park, Professor Wonho Choe and PhD researcher Sanghoo Park) KAIST researchers are using plasma to remove bacteria that are stuck to surfaces of plastic bottles and food. This novel technology will contribute to disinfection in medical settings as well as food and agricultural industries. Professor Wonho Choe and his team from the Department of Physics developed a technology that removes biofilm, which is comprised of microorganisms, by using plasma as a non-thermal sterilization method. Plasma contains multiple bactericidal agents, including reactive species. In particular, the chemicals formed in aqueous solution during plasma exposure have the potential for high antibacterial activity against various bacterial infections. The team treated water with plasma to see how effectively bactericidal agents in the plasma water can remove biofilm comprised of harmful microorganism such as Escherichia coli, Salmonella, and Listeria. The team identified that reactive species, including hydroxyl radical, hydrogen peroxide, ozone, nitrite, and superoxide produced during plasma treatment, showed considerable ability to remove the biofilm. Hydrogen peroxide showed the strongest effect removing the biofilm; however, the hydroxyl radical also played a significant role in removing biofilm. Despite having a concentration 100 to 10,000 times lower than other reactive species, the hydroxyl radical showed a high biofilm removal efficacy owing to its strong oxidative power. These findings reveal that plasma can be used as a no-residual and safe sterilization process alternative to conventional methods. With these outcomes, the team is planning to develop and commercialize a technology that can produce hydroxyl radicals with plasma. Professor Choe has registered a patent for flexible packaging materials that facilitate plasma and completed the technology transfer to the startup company, named ‘Plasmapp’, which focuses on commercializing bactericidal technology. “This research outcome will be the foundation for understanding plasma control technology and physicochemical interactions between plasma and microorganisms. It will also become an accelerator for utilizing plasma technology in the medical, food, and agricultural fields,” said Professor Choe. This research, led by PhD candidate Joo Young Park and PhD researcher Sanghoo Park in collaboration with Professor Cheorun Jo’s team from Seoul National University, was published in ACS Applied Materials and Interfaces on December 20, 2017. Figure 1. Flexible packaging materials that facilitate plasma Figure 2. Schematic diagram of biofilm treatment with plasma Figure 3. Concept of plasma application and evaluation result of reactive species' efficacy Figure 4. STERPACK, the product launched by Plasmapp
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