Session SR3: Sheaths, Patterns, and Waves

Live

Striations, as plasma self-organization, emerge from an ionization instability in DC discharges. Similar patterns have been reported in RF discharges, but the physical origin remained unknown. We propose a mechanism from a fluid model in which transport coefficients have been computed from a 0-D kinetic model [1]. In the quasineutral regime, the electron flux $\Gamma_e$ and the energy flux $H_e$ are expressed as a function of the plasma density gradient $\nabla n_e$ and electronic temperature gradient $\nabla T_e$ and transport coefficients $D_a, \mu_e, \chi_e \ \rm{ and }\ \kappa_e$ (e.g. for energy $H_e=\chi_e \nabla n_e+ \kappa_e \nabla T_e$). When the electron distribution is non-Maxwellian, off-diagonal terms $\chi_e$ and $\mu_e$ may be non-zero and unstable regimes may develop. Using the BOLSIG+ kinetic model at low Ar pressure, we showed that off-diagonal terms may be sufficiently negative to overcome diffusive effects, leading to an instability. This model reproduces all experimental features observed in an annular RF plasma: (1) axisymmetry is broken above a critical pressure, (2) azimutal modulations of the plasma iincreasing with pressure, (3) axisymmetry is recovered at higher pressure. [1] D\'esangles et al., \textit{Phys. Rev. Lett.} \textbf{123}, 265001 (2019)   

Live

A sheath forms over surfaces in contact with plasmas to balance the currents of electrons and ions incident from the plasma. In the vicinity of an electrically non-uniform material (e.g. spatially varying conductivity), two-dimensional (2D) kinetic effects may occur in the sheath. A 2D direct kinetic (DK) simulation for charged particle transport is capable of resolving spatial differences that arise in the plasma sheath as a result of these electrically disparate, adjacent materials. An Eulerian method is used to model the evolution of ion and electron transport in a collisionless sheath, describing the behavior of a plasma in two-dimensional (2D2V) discretized phase space. The DK technique is coupled with a solution of Poisson's equation for the electric potential. The model is verified with analytical theory for a quasi-one-dimensional case. A conservative boundary condition for particle injection is implemented at the sheath edge. Spatial differences that arise within the sheath as a result of the electrically disparate, adjacent materials are discussed.   

Live

It has been predicted that the ion flow in an ion presheath can excite ion-acoustic waves and that subsequent wave-particle interactions significantly enhance the effective Coulomb collision rate within the presheath. Increased collisionality could explain why ion and electron velocity distribution functions are measured with varying degrees of thermalization at different locations in the presheath. Recent LIF measurements have directly confirmed that the waves are present throughout the presheath, even in regions where linear theory predicts stability. This suggests that the waves are being reflected from the ion sheath, but wave reflection could not be directly diagnosed by the experiment. Here, we use PIC simulations to corroborate the existence of the waves by comparing the power spectrum of density fluctuations to the linear ion-acoustic dispersion relation. The simulations are used to quantify both wave reflection and wave-particle scattering rates that have not been measured experimentally. Wave-particle scattering rates were quantified by calculating time correlations between the distribution fluctuations and the electric-field fluctuations \textless f E\textgreater , while the reflected power was calculated from power spectra.   

Live

Understanding plasma self-organization in the anode region of DC discharges is far from complete. In this paper, we analyze the formation of electron distribution function, ion generation and transport, and the electric potential distribution near the anode for a wide range of discharge conditions (gas pressures, plasma densities, and the size of the anode with respect to device dimensions (such as the tube radius R and length L for classical discharge geometry)). Four cases can be distinguished in plasmas of nobble gases depending on the relative values of the spatial discharge dimensions, the electron mean free path, the electron energy relaxation length, and the distance over which the electrons gain kinetic energy equal to the first excitation potential of atoms. The developed kinetic theory predicts the length the anode region, the spatial distribution of the electric field, and the anode potential with respect to plasma in these four cases. Simple analytical solutions obtained for some limited cases are compared with results of computer simulations based on the numerical solution of a kinetic equation for electrons coupled with ion generation and transport, and Poisson equations. New insight is provided to explain previous experimental observations.   

Not Participating

Self-organized plasma attachment patterns occurring on the surface of the cathode in DC microdischarges have been reported by several groups. It is theorized that patterns occur due to mechanisms in the near-cathode space-charge sheath [1]. In this work, the pattern formation and evolution of such patterns are mapped along the IV characteristic of the discharge. The sensitivity of the occurrence of the pattern or change in pattern spatial morphology in response to pressure changes is characterized. Changes in optical emission are quantified at the bifurcations points where the pattern first appears due to a differential change in discharge current or pressure. A fast camera is used to track pattern evolution through time. This ongoing study will compare experimental findings with Benilov's model. This work aims to contribute to the validation of models based on the theory of self-organization in bistable nonlinear dissipative systems [1]. It has been reported that the appearance of such patterns is highly dependent on the presence of background gas contaminants. This effort will also explore the impact of trace gas species on pattern appearance. [1] M. S. Benilov, \textit{Plasma Sources Sci.Technol. }2014.   

On Demand

The ion sound waves can be driven unstable by the subsonic ion flow in a finite length plasma as a result of the coupling (mediated by plasma boundaries) of the ion sound waves propagating in opposite directions. In this work, the nonlinear regime of this instability is studied by particle simulations with kinetic ions and Boltzmann electrons. There are two types of the ion sound instability that are well identified in the linear theory and simulations: aperiodic zone [$Re(\omega )=0$] and oscillatory zone [$Re(\omega )\ne 0$]. We study here the nonlinear regime and show that the mode saturation results in coherent nonlinear structures. For oscillatory instability, the instability saturates with the oscillating virtual anode structure resulting in the ion beam acceleration and formation of system-long ion hole (vortex in phase space). For aperiodic instability, the mode saturates with either the virtual anode scenario or a global oscillation, depending on the initial condition.   

Harmonic generation in multipactor-induced plasma ionization breakdown

Multipactor and plasma ionization breakdown near a single dielectric surface frequently occur in high power microwave transmission devices and high field microwave discharges. The harmonics can cause coupling of modes and other adverse signal effects in practical devices. In this work, we report observations of higher harmonics generation of the normal electric field in multipactor-induced plasma ionization breakdown in argon using kinetic particle-in-cell simulations. The observed harmonic frequency is around ten times the fundamental microwave frequency, but is significantly lower than the electron plasma frequency. A theoretical model reveals that two-stream instability is the fundamental mechanism of higher harmonics generation in the collisional regime. Higher harmonics are reduced at higher pressure. Similar higher harmonics are also demonstrated in ionization breakdown processes in helium and xenon. Additionally, a propagating double layer-like structure is observed for helium. The physical mechanism is attributed to the response of light ions to two neighboring reversed normal electric fields and the localized space charge effect.