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

Journal Article · 31 January 2018 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.5016072 · OSTI ID:1515027

Kawamura, E. ^[1]; Lieberman, M. A. ^[1]; Lichtenberg, A. J. ^[1]

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1. Univ. of California, Berkeley, CA (United States)

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%H₂O 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 n₀ in the plasma bulk was stationary. In this work, we first conduct 1D PIC simulations of a 1 mm gap He/2%H₂O APP, driven by a sinusoidal current at a low frequency of f = 50 kHz such that ω = 2πf is well below the ionization frequency ν_iz. In this case, n₀ varies with time, and we observe a time-varying instability which quasistatically depends on n₀(t). At each phase of the rf cycle, the discharge resembles a dc discharge at the same n₀. At higher frequencies (200 kHz–1 MHz), ω approaches ν_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 μs 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 ν_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 n₀, T_e, ν_iz, as well as terms for the nonlinear saturation and noise floor of the striation amplitude. Furthermore, the time-varying global model predictions are compared to the PIC simulations, showing reasonable agreement.

• Research Organization: Univ. of Michigan, Ann Arbor, MI (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0001939
• OSTI ID: 1515027
• Alternate ID(s): OSTI ID: 1418891
• Journal Information: Physics of Plasmas, Vol. 25, Issue 1; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English