Experimental Studies On Characteristics Of Pulse-modulated Radio-frequency Atmospheric Pressure Discharges

Radio frequency atmospheric pressure glow discharges (RF APGDs) are widely used in surface modification and biomedical due to the advantage of no need of vacuum system, low breakdown voltage and easy to produce large-area uniform plasma. However, in practical applications, the gas temperature rise in continuous wave (CW) RF APGDs is higher, which limits the applications of RF APGDs. In order to reduce the rise of gas temperature in CW RF APGDs, several research groups attempted pulse-modulated (PM) RF APGDs. In these works, the influences of PM parameters on breakdown characteristics, transient evolution, discharge mode and pattern were researched, but there are still a series of problems to be further explored and discussed. Therefore, the following works are done in this thesis:I.Calculation of PM RF APGD power1. The discrete ? probe was calibrated in situ, and PM RF APGD power and the phase were obtained by applying Discrete Fourier Transform (DFT) to the measured RF voltage and current signals. From the calculation results, the obtained conclusions are as follows:(1) in the pulse rising (falling) edges, both the pulse discharge power oscillates; the oscillation disappears on the plateau of the pulse. (2) in the final stage of the pulse falling edge, the phase is lower than -90°. The analytic expressions were derived using the integral method, and the power and the phase were calculted simultaneously using DFT. The calculation results show that the oscillation is originated from the non-zero oscillating net power of the capacitor(s) of a RC load. The oscillation amplitude increases with the increase of the ample window length or rising (falling) edge slope.2. For given PM RF numerical signals, by analyzing DFT-power of a pure capacitive load, it was found that the RF power and the phase at special time instants are equl to the real absorbed power (0 W) and phase(-90°). These special time instants are defined as the time instants of the zero capacitor power (denoted as TPINCZERO hereafter). For different RC load, two empirical formulae for the TPINCZERO and the phase extremum were derived by analyzing the relationship between TPINCZERO and the phase extremum, thus the active power of a real PM RF APGD can be determined, the procedures are as follows:(1) applying DFT to the measured RF voltage and current signals to obtain the power and the phase; (2) finding the time instants of the minimal (maximal) phase in the pulse rising (falling) edge, and selecting the empirical formula to determine the TPINCZERO; (3) getting the powers and the phases at the instants of TPINCZERO, that is the active power and the phase of a real PM RF APGD.3. Numerical simulations were made to study the error of the DFT-calculated active power. It was found that the error of the DFT-calculated active power is almost independent of the load type, the ratio of conventional stationary impedance moduli of parallel RC and series RC or the impedance phase angle, but increases with the increase of the slope of PM RF voltage amplitude vs. time. Compared with the PMT signal in the experiments, a calculation approach to determining the DFT-calculated active power is reliable.?.Studies on breakdown characteristics in PM RF APGDs1. A new approach to determine is proposed the gas breakdown by the steep increase of the plasma-absorbed RF power (denoted as Ppa hereafter) and obtain breakdown voltage (denoted as Vgb hereafter) in a PM RF APGD. The effects of the averaged Papa and the pulse off-time (denoted as Toff hereafter) on the Vgb were studied. The results show that the Vgb increases with increasing Papa at low pulse frequency, and decreases with the increasing Papa at high pulse frequency. For the same Papa, the Vgb at the higher duty ratio is smaller than that at the lower duty ratio. The obtained Vgb vs. Toff is the presence of three characteristic regions:(A) in the region of Toff>10 ms, the Vgb hardly changes, only decreases slightly. (B) In the region of 0.05 ms<Toff<10 ms, the Vgb decreases remarkably. (C) In the region of Toff<0.05 ms, the changing rate of Vgb with Toff slows down once again.2. The effect of the Toff on the mean value of breakdown delay time (denoted as Td hereafter) was studied. The results show that:The dependence of obtained Td on the Toff can be roughly divided into three characteristic regions:(I) in the large Toff domain of region, Td hardly changes. (?) In the moderate Toff domain of region, Td decreases remarkably with the decrease of Toff. (?) In the small Toff domain of region, Td decreases slowly with the decrease of Toff. Based on the changing characteristics of the Vgb vs. Toff, the charged particles during the afterglow of argon pulsed RF APGD is found to undergo three different diffusion loss mechanisms. The contribution of nitrogen atom in the recombination at the alumina surfaces to Td was experimentally validated by introducing a trace amount of nitrogen gas into the discharge box together with the high purity argon at different Toff.3. In a large range of Toff, The luminescence properties of the initial breakdown stage in PM RF APGD are as following:(1) in the region Toff>50 ms, the breakdown occurs randomly at local radial locations. (2) In the region 50 ms>Toff>10 ms, the number and the randomness of breakdown locations increase with the decreasing Toff. (3) In the region 10 ms>Toff>0.05 ms, the breakdown is uniform glow. (4) In the region 0.05 ms>Toff>0.04 ms, the glow and the pattern coexist in the initial breakdown stage. (5) In the region 0.04 ms>Toff, the breakdown is pattern. After the breakdown, the variation of discharge shapes and size in different characteristic regions with time are as following:when Toff is in characteristic regions (1) or (2), discharge after gas breakdown has jumping-expand along the radial direction, then evolves gradually into uniform glow plasmas. The required time of developing into uniform glow plasmas is closely related to the electron diffusion during the pulse off. When Toff is in characteristic region (3), the electron diffusion not only meet the requirements of uniform breakdown, but also avoid the delivery, accumulation of spatial non-uniformity, so both breakdown and subsequent discharge are uniform glow. When Toff is in characteristic regions (4) or (5), discharge after gas breakdown is glow-pattern or glow.4. At a fixed pulse duty ratio, first breakdown discharge was studied. The results show that first breakdown voltage increase with the increase of pulse frequency, which is explained by the relationship between breakdown delay time and voltages.III.Studies on mode transition in PM RF APGDsThe a-y mode transition is first confirmed with discharge current-voltage characteristic and glow image in PM RF APGD. Compared with the voltage at the ?-? mode transition in CW RF APGD, the corresponding voltage in PM RF APGD is larger, and decreases with the increasing pulse duty ratio. The features of discharge current-pulse duty ratio curves in PM RF APGD were discussed and explained, combined with the gas temperature of PM RF APGD and the threshold voltage at the a-y mode transition.The time evolution of discharge images taken with a high speed camera was studied, the results show that the discharge evolves from a to y mode in the pulse rising edges when the pulse RF voltage (power) is enough high.IV.Studies on discharge voltage and current waveforms in PM RF APGDsIn order to elucidate the complex evolution of pulse voltage\current waveforms in PM RF APGD, we firstly investigated positive\negative feedback effects in CW RF APGD. The results show that the series capacitor of impedance matching network (denoted as Cs hereafter), corresponding to the boundary of positive\negative feedback region (denoted as Cs max hereafter), decreases with increasing discharge power, the simplified circuit model of PM RF APGD is used to explain positive\negative feedback region, thus reveals the influence of positive\negative feedback on the complex time evolution behavior of pulse voltage\current waveforms and discharge power. In the negative feedback region, wide spikes as well as undershoots occur in voltage\current waveforms and the plasma absorbed RF power during the pulse rising edge, but disappear in the positive feedback region. The voltage\current waveforms are found to be associated with the rising edge length of PM RF signal and the discharge power. The time evolution behavior of pulse discharge waveform is complicated when the rising edge length of PM RF signal is 5 ns, and relatively simple when the rising edge length of PM RF signals is increased to 15?s. A PM RF APGD can transit from the positive to negative feedback region when the Cs is slightly smaller than the Csmax.Undershoot is found to be associated with the negative feedback effect, the discharge power, the rising edge length and the pulse off-time. In case of a fast rising edge time with a moderate pulse frequency, a high RF power discharge is operated in the negative feedback region at the same time, undershoots are so sufficiently larger that PM RF APGD is extinguished and re-ignited during the initial pulse phase. In addition, it was found that there are obvious distortions for RF voltage\current waveforms, but the distortions of the current waveforms is larger than that of the voltage waveforms. DFT analysis shows that the RF waveform contains odd number harmonic components.