Capacitive discharge mode transition in moderate and atmospheric pressure

Moon

et al. 2018

Advances in Physics: X

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Weakly ionized plasmas at or near 1 atm pressure, or atmospheric-pressure plasmas, have received increasing attention due to their scientific significance and potential for use in a variety of applications, particularly for medicine, agriculture, and food. However, there is a large imbalance between scientific research on plasma physics and applications, which is partly due to the considerable differences in the characteristics of these plasmas compared with those of low-pressure plasmas. This discrepancy is particularly related to the difficulty in performing plasma diagnostics for highly collisional plasmas. Information on electrons (such as the electron density and temperature) is essential since electrons play a dominant role in the generation of active species related to the physical and chemical processes inside the plasma. So far, limited diagnostics have been available for electrons such as Thomson scattering and optical emission diagnostics based on equilibrium models. Here, we review the available diagnostic methods along with their merits and limitations for characterizing electrons in weakly ionized collisional plasmas. Particular attention is paid to continuum radiation-based spectroscopy, which facilitates multidimensional imaging of electron density and temperature. The future impact of these plasmas on relevant fields (i.e. laboratory and industrial plasmas and their applications) is also addressed.

Section: Radio-frequency Capacitive Dischargementioning

confidence: 99%

Electron characterization in weakly ionized collisional plasmas: from principles to techniques

Moon

et al. 2018

Advances in Physics: X

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“…At the voltage above the rightmost point, filaments were generated randomly inside the glow plasma. Although filaments were produced, a transition to the γ-mode (which induces plasma constriction and other significant changes in the plasma characteristics 15 , 16 ) did not occur because the dielectric barrier prevented the current density from exceeding a local maximum; thus, the applied voltage was sufficient to sustain discharge with coexistence of a uniform glow discharge and filaments. The positive slope of the I RMS - V RMS curves (or positive differential conductivities) indicates that the plasmas were in the abnormal glow discharge mode.…”

Section: Resultsmentioning

confidence: 99%

Electron Information in Single- and Dual-Frequency Capacitive Discharges at Atmospheric Pressure

Moon

et al. 2018

Sci Rep

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Determining the electron properties of weakly ionized gases, particularly in a high electron-neutral collisional condition, is a nontrivial task; thus, the mechanisms underlying the electron characteristics and electron heating structure in radio-frequency (rf) collisional discharges remain unclear. Here, we report the electrical characteristics and electron information in single-frequency (4.52 MHz and 13.56 MHz) and dual-frequency (a combination of 4.52 MHz and 13.56 MHz) capacitive discharges within the abnormal α-mode regime at atmospheric pressure. A continuum radiation-based electron diagnostic method is employed to estimate the electron density (ne) and temperature (Te). Our experimental observations reveal that time-averaged ne (7.7–14 × 1011 cm−3) and Te (1.75–2.5 eV) can be independently controlled in dual-frequency discharge, whereas such control is nontrivial in single-frequency discharge, which shows a linear increase in ne and little to no change in Te with increases in the rf input power. Furthermore, the two-dimensional spatiotemporal evolution of neutral bremsstrahlung and associated electron heating structures is demonstrated. These results reveal that a symmetric structure in electron heating becomes asymmetric (via a local suppression of electron temperature) as two-frequency power is simultaneously introduced.

“…The slopes of the I rms - V rms curves slightly increase with the decreasing pressure. According to the one-dimensional (1-D), simple resistor-capacitor series circuit model, which is an acceptable model for atmospheric-pressure capacitive discharges 6 , the slope of the I rms - V rms curve is roughly , where ε 0 is the permittivity in a vacuum, S is the cross sectional area of the plasma, and d is the sheath thickness. Therefore, an increase in the slope as the pressure decreases indicates an increase in the sheath thickness.…”

Section: Resultsmentioning

confidence: 99%

“…However, in contrast to low-pressure plasmas (e.g., under 1 mTorr), plasma properties in the high-pressure regime, including atmospheric pressure, are not well understood; yet, the electron diagnostic for such plasmas still remains a significant challenge. Due to the different plasma characteristics at different pressures in the subatmospheric-to-atmospheric pressure range, characterizing the plasma over such a pressure range is a prerequisite for understanding the underlying principles of plasmas and for industrial plasma applications 6 . One example is the dielectric barrier discharge (DBD).…”

Section: Introductionmentioning

confidence: 99%

Electron heating in rf capacitive discharges at atmospheric-to-subatmospheric pressures

Kim

2018

Sci Rep

Electron heating is a fundamental and multidisciplinary phenomenon in partially ionized gases, from the planet’s ionosphere to laboratory-scale plasmas. Plasmas produced at ambient or reduced pressures have recently shown potential for scientific and industrial applications. However, electron heating, which is strongly coupled to the physicochemical properties of these plasmas, has been poorly understood. We experimentally found the rapid structural transition from non-local to local electron heating in collisional radio-frequency discharges at atmospheric-to-subatmospheric pressures. As the gas pressure decreased from 760 to 200 Torr, the time-averaged electron density increased from 1.3 × 1012 to 1.3 × 1013 cm−3, and the electron temperature decreased from 2.5 to 1.1 eV at the maximum allowable discharge current in the abnormal α-mode in the plasma bulk. The spatiotemporal evolution of the electron temperature clearly shows that the electron temperature increases uniformly throughout the bulk plasma region during sheath expansion and collapse at 760 Torr, but the electron heating weakens with sheath collapse as the gas pressure decreases.