Title: Multi-mode ionization instability induced striations in RF driven He/H₂O atmospheric pressure plasma (APP) discharges

Abstract

Previously [E. Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016) and E. Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], one dimensional (1D) particle-in-cell (PIC) simulations of 1 to 4mm gap, He/2%H₂O atmospheric pressure plasma discharges showed an ionization instability resulting in bulk striations. Assuming that the ionization rate coefficient K_iz is related to the root mean square electric field E by K_iz ∝ E^q, a striation theory showed that q<0 is a necessary condition for the instability. A local calculation yielded q>0, implying that nonlocal electron kinetics are required for the instability. Wider gaps can fit in a wider range of wavelengths, resulting in multi-mode striations. Previously, we assumed a single mean q value for each discharge, and did not calculate q for each mode separately. Here, we develop a wavelength resolved global striation model and apply it to 1D PIC simulations of 4mm gap discharges with input currents J_rf = 0.04 to 0.30A/cm². We first examine a base case at 0.23A/cm² and observe a mixture of unstable modes within a window of wavelengths λ. At shorter λ, the modes are suppressed by diffusion. At longer λ, we observe a transition to locality inmore » which q becomes less negative with increasing λ, approaching its local positive value and stabilizing the modes. The window of unstable modes shifts to shorter λ with increasing J_rf, causing the modes to be suppressed by diffusion at higher J_rf. At lower J_rf, the decrease in bulk plasma density with decreasing J_rf suppresses the striations.« less

Authors:

Kawamura, E.  ^[1];

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• Search DOE PAGES for ORCID "0000-0003-2176-7377"

Lieberman, M. A.  ^[1];

• Search DOE PAGES for author "Lieberman, M. A."
• Search DOE PAGES for ORCID "0000-0002-2841-2870"

Lichtenberg, A. J. ^[1]

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

Publication Date:

Wed Sep 11 00:00:00 EDT 2019

Research Org.:

Univ. of Michigan, Ann Arbor, MI (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Fusion Energy Sciences (FES)

OSTI Identifier:

1610605

Alternate Identifier(s):

OSTI ID: 1561337

Grant/Contract Number:  

SC0001939

Resource Type:

Accepted Manuscript

Journal Name:

Physics of Plasmas

Additional Journal Information:

Journal Volume: 26; Journal Issue: 9; Journal ID: ISSN 1070-664X

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; physics