Wen-Cong Chen

Gas temperature, electron density and electron temperature measurement in a microwave excited microplasma

Gas temperature, electron density and electron temperature of a microwave excited microplasma are measured by optical emission spectroscopy. This microplasma is generated in the small gap of a microstrip split-ring resonator in argon at near atmospheric pressure. When less than 100 ppm of water is present in the plasma, the gas temperature can be obtained from the rotational temperature of the hydroxyl molecule (A 2Σ+, v = 0) and the electron density can be measured by the Stark broadening of the hydrogen Balmer β line. According to a collisional–radiative model, the electron temperature can be estimated from the measured excitation temperature of argon 4p and 5p levels. It is found that the values of these parameters (gas temperature, electron density and temperature) increase when the gap width of the resonator is reduced. However, when the microwave power increases, these parameters, especially the electron density, do not vary significantly. Discussions on this phenomenon, being very different from that in the low-pressure bounded discharges, are provided.

Measurement of the temporal evolution of electron density in a nanosecond pulsed argon microplasma: using both Stark broadening and an OES line-ratio method

The temporal evolution of electron density in a nanosecond pulsed argon microplasma is measured using a combination of Stark broadening and the optical emission line-ratio method. In the initial discharge period (0?100?ns), the electron density can reach as high as ?1018?cm?3. It decreases to ?1017?1016?cm?3 in the early afterglow period (100?ns?1??s after the ignition) and ?1016?1013?cm?3 in the late afterglow period (1?20??s). It is demonstrated that the optical emission spectroscopy (OES) line-ratio method can obtain the electron density in the range 1013?1016?cm?3, while in the range 1016?1018?cm?3, the Stark broadening technique with argon 2p?1s lines (in Paschens notation) is a better choice. These results are in good agreement with those from the Stark broadening technique with hydrogen Balmer lines. Finally, a possible mechanism for such a density evolution is briefly discussed.

Determining the electron temperature and the electron density by a simple collisional?radiative model of argon and xenon in low-pressure discharges

A simple collisional–radiative model for argon and xenon is used, in conjunction with optical emission spectroscopy (line-ratio technique), to determine the electron temperature and electron density in low-pressure discharges containing argon and xenon. Satisfactory agreement is obtained between this method and the Langmuir probe for an inductively coupled plasma containing neon, argon and xenon. This method is applied for a capacitive discharge containing fluorocarbon, argon and xenon. The electron temperatures and electron densities obtained under various discharge conditions are compared with those reported in the literature by other techniques.

Electron density and ion energy dependence on driving frequency in capacitively coupled argon plasmas

The dependence of the electron density and the mean ion energy relative to the ground electrode on the driving frequency (13.56?156?MHz) is investigated in capacitively coupled argon plasmas. It is found that at constant rf power, the electron density increases with the driving frequency then starts to level off after reaching some transition frequency. When the driving frequency exceeds this transition frequency, almost all rf power is spent on electron heating and the electron density remains unchanged even when the frequency continues to increase. The measured data is compared with the results from an inhomogeneous plasma model. The variations of the measured electron density and mean ion energy are found to be consistent with the scaling between the electron heating and the ion acceleration power and their dependence on driving frequency, as obtained earlier by Godyak.

Reconstruction of ion energy distribution function in a capacitive rf discharge

A simple one-dimensional ion dynamic model with charge exchange collisions as the predominant ion-neutral reactions in the sheath is used to obtain the ion energy distribution function (IEDF) in a single-frequency collisional capacitive argon discharge. The shape of IEDF strongly depends on the electron density in this model. IEDFs predicted by this model can be in good agreement with those measured by adjusting the electron density at the ion sheath boundary. The electron densities obtained in this way are in good agreement with those from optical emission spectroscopy measurement, which also indicates the validity of the model.

Chromatic-free spatially resolved optical emission spectroscopy diagnostics for microplasma

A chromatic-free spatially resolved diagnostic system for microplasma measurement is proposed and demonstrated, which consists of an optical chromatic-free microscope mirror system, an electron multiplying charge coupled device (EMCCD), and bandpass filters. The diagnostic system free of chromatic aberrations with a spatial resolution of about 6 microm is achieved. The factors that limit the resolution of this diagnostic system have been analyzed, which are optical diffraction, the pixel size of the EMCCD, and the thickness of the microplasma. In this paper, the optimal condition for achieving a maximum resolution power has been analyzed. With this diagnostic system, we revealed the spatial nonuniformity of a microwave atmospheric-pressure argon microplasma. Furthermore, the spatial distribution of the time-averaged effective electron temperature has been estimated from the intensity distributions of 750.4 and 415.8 nm emissions.

An analytical model for time-averaged ion energy distributions in collisional rf sheaths

An analytical model is proposed for time-averaged ion energy distributions (IEDs) in collisional rf sheaths (?i?<?sm, where ?i is the ion mean free path and sm is the sheath thickness), in which charge transfer is the dominant ion-neutral collision mechanism. Our model is different from the model in Israel et al 2006 J. Appl. Phys. 99 093303 in two aspects. Firstly, to calculate the ion flux, we consider ions created in both the space charge region and the quasi-neutral region instead of only the quasi-neutral region. Secondly, to calculate the ion energy, we use an rf-modulated transit time instead of only the average transit time. Consequently, our model is valid over a wider pressure range (from a weakly collisional sheath to a strongly collisional sheath) compared with the model in Israel et al 2006 J. Appl. Phys. 99 093303. Our model shows that, in a collisional rf sheath (argon, 5?Pa, 27.12?MHz and 100?W), 65% of the ion flux in the IED comes from secondary ions created in the space charge region. Results of our model are compared with those obtained by measurement, direct integration method and particle-in-cell/Monte Carlo collision simulation.

Spatial evolution of the electron energy distribution function in a low-pressure capacitively coupled plasma containing argon and krypton

The spatial evolution of the electron energy distribution function (EEDF) in the axial direction of a capacitively coupled plasma with two parallel plate electrodes is investigated using an optical emission line-ratio method for Ar/Kr discharges. When the rf power is increased from 25 to 400?W at a pressure of 400?mTorr, we observe a transition from convex EEDFs to concave ones and a sharp increase in electron density, due to an ??? mode transition, which is believed to be caused by the high-energy electrons originating in the high-voltage sheath. We also investigate the spatial evolution of the EEDF when the pressure is increased from 45 to 500?mTorr at a power of 100?W. The EEDF is uniform at pressures below 180?mTorr and becomes non-uniform at higher pressures, owing to the decrease in the energy relaxation length of the high-energy electrons.

The Evolution of the Optical Emission Pattern From a Pulsed Microwave-Excited Microstrip Split-Ring Resonator Microplasma

The evolution of the emission pattern of a pulsed microwave-excited microstrip split-ring resonator microplasma at different pressures in argon is presented. When the pressure is lower than ~200 torr, the plasma fills in the gap right after the power is ON. Then, the filaments start to form on a time scale of microseconds to tens of microseconds. When the pressure is higher than ~300 torr, the initial discharge region becomes much smaller with a longer filament development time. No filament is observed in helium and neon up to one atmospheric pressure. It is suggested that the diffusion and localized heating of the electrons determine the evolution of the plasma and the formation of the filaments.

Effect of the reactor surface roughness on benzene oxidation in dielectric barrier discharges

Effects of the surface roughness of dielectric barrier discharge reactors on benzene decomposition are analyzed and compared. It is found that increasing the roughness of the reactor surface improves benzene consumption at low specific energy density (SED) and promotes CO formation and carbon balance at high SED. In addition, it is also found that the release of NO and NO2 is restrained in the reactor with a rough surface at high SED. It is suggested that these results are attributed to the surface reactions on the rough surface.