Jai Hyuk Choi

Development of a dielectric-barrier discharge enhanced microplasma jet

A low-power ultrahigh-frequency-driven inductively coupled microplasma (ICMP) source equipped with dielectric-barrier discharge (DBD) was developed to realize a low-temperature and high-density plasma in fine quartz capillaries with inner diameters of less than 1.0 mm. A stable plasma was generated and its sustainability was independent of the gas flow rate. This plasma jet had a longer plume than that of a thermoelectron-enhanced microplasma jet, and time-resolved characterization suggested interactions between ICMP and DBD jets. By optical emission spectroscopy characterization, the gas temperature and electron density inside a capillary were estimated to be 400–1000 K and 1013–1014 cm−3, respectively.

Gas-temperature-dependent generation of cryoplasma jet under atmospheric pressure

Plasma with a gas temperature below room temperature is not yet fully understood although it is expected to be an attractive tool for applications to material processing. In the present work, gas-temperature-dependent generation of a cryoplasma jet was studied. So far, we have generated a helium cryoplasma jet (296–5K) under atmospheric pressure. At gas temperatures below 20K, the helium excimer, He2, was observed clearly from by optical emission spectroscopy.

Gas-Temperature-Dependent Characteristics of Cryo-Dielectric Barrier Discharge Plasma under Atmospheric Pressure

In the present work, cryo-dielectric barrier discharge (DBD) plasma was generated continuously below room temperature down to liquid nitrogen temperature (296 to 78 K) under atmospheric pressure using parallel indium–tin-oxide (ITO)-coated electrodes. Gas-temperature-dependent optical emission spectroscopy (OES) measurements and discharge pattern observation of cryo-DBD plasma were performed. In the case of helium gas, the discharge mode of cryo-DBD plasma changed from filamentary mode to glow mode as the gas temperature decreased. When introducing a small amount of nitrogen in helium gas, the filamentary discharge mode persisted upon decreasing the gas temperature, although the discharge pattern changed from concentric rings (296 K) to a hexagon-like pattern (78 K).

Temperature-dependent transition of discharge pattern of He/air cryoplasma

Dielectric barrier discharges were generated under atmospheric pressure at temperatures ranging from room temperature down to 88K. The gas temperature of the plasma generated by the discharges was controlled by liquid nitrogen, and a mixture of helium and air was used as the discharge gas. We found that microdischarges exhibited temperature-dependent specific discharge patterns as the temperature decreased. This transition of discharge patterns was closely related to the change in the gap voltage at breakdown. A possible scenario that may explain the pattern of the transition of the microdischarges is discussed.

Electron density and temperature of gas-temperature-dependent cryoplasma jet

A microsize cryoplasma jet was developed and analyzed at plasma gas temperatures ranging from room temperature down to 5 K. Experimental results obtained from optical emission spectroscopy and current–voltage measurements indicate that the average electron density and electron temperature of the cryoplasma jet depend on the gas temperature. In particular, the electron temperature in the cryoplasma starts to decrease rapidly near 60 K from about 13 eV at 60 K to 2 eV at 5 K, while the electron density increases from about 109 to approximately 1012 cm−3 from room temperature to 5 K. This phenomenon induces an increase in the Coulomb interaction between electrons, which can be explained by the virial equation of state.

Analysis of time-resolved optical emission of He cryoplasma at atmospheric pressure

Cryogenic plasma, so-called cryoplasma, was generated under helium at atmospheric pressure at temperatures ranging between room temperature and 79 K. Time-resolved emission spectra of the main species of the He cryoplasmas were measured in order to analyse the optical characteristics that can define the discharge mode. The experimental results clearly show that a decrease in temperature changes the temporal variation of radiation of the measured species and induces a transition of the discharge mode, from pseudoglow to glow, and then to Townsend discharge. This result reconfirms the scenario of temperature-dependent mode transition of cryoplasma.

Optical and electrical analysis of a temperature-dependent mode transition of a helium cryoplasma

A novel plasma source that can be generated at a cryogenic temperature, the so-called cryoplasma, was established under atmospheric pressure. Our cryoplasma system can easily control the processing temperature from room temperature down to 78 K using liquid nitrogen. We employed a dielectric barrier discharge reactor, which was connected to an ac power supply, and we flowed helium as the discharging gas. We found that plasma phenomena such as the discharge pattern and color showed temperature-dependent behavior as the gas temperature decreased. Furthermore, this result was closely related to changes in not only the emission spectra measured by optical emission spectroscopy but also the waveforms of the discharge currents and voltages. We concluded that the change in gas temperature induced a transition in the discharge modes.

Experimental and numerical investigation of time evolution of discharge current and optical emission in helium–nitrogen cryoplasmas

Cryoplasmas represent a class of non-equilibrium plasmas whose gas temperature can be controlled below room temperature. However, so far, the influence of the plasma gas temperature on the plasma chemical reactions has not yet been examined in detail. Here we investigated the time-dependent reaction dynamics related to optical emission in helium?nitrogen cryoplasmas. We acquired voltage and discharge current waveforms, optical emission spectra, and observed a temporal change of the emission intensity in helium?nitrogen cryoplasmas at two experimental conditions (temperatures of temperature detector: 5?K and plasma gas temperature: 28?K (condition A), 40?K and 54?K (condition B)). Two time-dependent phenomena were observed: the first was a longer duration of the discharge current compared to that of helium emission at both conditions A and B, and the second was nitrogen ion emission delayed by about 8??s with respect to the emissions of atomic helium and helium dimers at 40?K. The experimental observations could be reproduced qualitatively by a global reaction model, which took into account the effect of the plasma gas temperature on the reaction rate constants and the diffusion coefficients. The simulations suggested that the reactions related to metastable helium atom were the key reactions, and that the long lifetimes of metastable helium atoms at cryogenic gas temperatures are crucial for the appearance of the time-dependent phenomena. These results imply that the plasma gas temperature is one of the key parameters in non-equilibrium plasma chemistry.

Temperature-dependent transition of discharge pattern during helium cryo plasma

Summary form only given. New cryogenic plasma source, so-called cryo plasma was established under atmospheric pressure. Cryo dielectric barrier discharge (DBD) system could easily control processing temperature from room temperature to 78 K by liquid nitrogen. Reactor chamber for this experiment consisted of two chambers, the inner and outer chamber. We employed a DBD reactor into the inner chamber and connected an AC power supply operating at a frequency of 20 kHz. Helium as discharge gas was introduced into the inner chamber and the operating pressure was kept at atmospheric pressure, with a steady state condition. And the outer chamber played a role to control and sustain gas temperature by flowing liquid nitrogen as a refrigerant. After experiments, we found out the plasma properties, such as discharge patterns and color, showed the temperature- dependent behavior as gas temperature went down. And this result was closely related with the change of emission spectra measured by optical emission spectroscopy. Conclusively, the change of gas temperature induced a transition of discharge mode. The detail will be discussed at the conference.