Ionization instability induced striations in low frequency and pulsed He/H2O atmospheric pressure plasmas

Abstract

In previous work [Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016)] and [Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], 1D kinetic particle-in-cell (PIC) simulations of narrow gap (1 to 4 mm), high frequency (27 MHz) or dc-driven, He/2%H 2 O atmospheric pressure plasmas (APPs) showed an ionization instability resulting in standing striations (spatial oscillations) in the bulk plasma. We developed a steady-state striation theory which showed that the striations are due to non-local electron kinetics. In both the high frequency and dc-driven cases, the equilibrium electron density n 0 in the plasma bulk was stationary. In this work, we first conduct 1D PIC simulations of a 1 mm gap He/2%H 2 O APP, driven by a sinusoidal current at a low frequency of f ¼ 50 kHz such that x ¼ 2pf is well below the ionization frequency iz. In this case, n 0 varies with time, and we observe a time-varying instability which quasistatically depends on n 0 (t). At each phase of the rf cycle, the discharge resembles a dc discharge at the same n 0. At higher frequencies (200 kHz-1 MHz), x approaches iz , and quasistatic equilibrium at each phase breaks down. The discharge is also driven with a 200 kHz, 50% duty cycle square wave pulse with a short rise and fall time of 0.1 ls in an attempt to directly measure the striation growth rate s during the on-cycle before it saturated. However, the spike in current during the rise time leads to a spike in electron temperature T e and hence iz and s at the beginning of the rise which saturated during the beginning of the on-cycle. To predict the instability growth rate and saturation during and after the current spike, we extend our striation theory to include time-varying n 0 , T e , iz , as well as terms for the nonlinear saturation and noise floor of the striation amplitude. The timevarying global model predictions are compared to the PIC simulations, showing reasonable agreement.

Numerical simulation of the effect of water admixtures on the evolution of a helium/dry air discharge

“…As the level of water increases in the mixture (100-2000 ppm), the most dominant ion becomes H11O5 + due to the fast hydration of the lightest water clusters. Similar results were also observed in the global model [38,27]. The effect of water admixtures on the concentration of the negative charge species is presented in Figure 9.…”

“…[6]. In such discharges the presence of water impurities is unavoidable and has been shown to highly affect the plasma chemistry and dynamic evolution [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In practice, water impurities are due to the operation of the plasma in the ambient humid air and also due to the plasma interaction with biotic surfaces in wet and moist environments.…”

Plasma stratification in radio-frequency discharges in argon gas

“…However, the number of striations per unit length remained constant at different interelectrode distances. For driving frequencies of 70, 29,6, 4.2, and 3.7 MHz, they observed 8,9,10,11,and 12 striations at L=30 cm. Light emission oscillated at the second harmonic of the applied voltage.…”

“…Although discussions of surface-wave and kinetic effects can be found in the literature, no satisfactory explanations of these striations have been proposed so far. Standing striations have been also observed in Inductively Coupled Plasma (ICP) for certain geometries and discharge conditions [8,9].Striations appeared also in Particle-in-Cell (PIC) simulations of plasmas in different gases and gas mixtures [10,11,12]. Plasma stratification, as an example of selforganization at the kinetic level, remains a great challenge for the plasma science and physics of gas discharges [13].Previously, standing striations in argon CCP have been observed in a range of frequencies 3-80 MHz and gas pressure 2 Torr, in a tube with radius R=1.25 cm [7].…”

Effects of excitation voltage pulse shape on the characteristics of atmospheric-pressure nanosecond discharges

“…We also investigated the behaviour of the electron velocity distribution function and the electron energy probability function, and concluded that these deviate significantly from the Maxwell-Boltzmann form, especially in the cathode region of the discharge. These observations demonstrated the advantages (and uniqueness) of particle simulations [23][24][25][26][27][28][29][30] of similar physical systems. We found that the VUV resonance radiation of He atoms is heavily trapped within the high-pressure gaseous environment and photons are absorbed / re-emitted typically several hundred times before leaving the plasma.…”