Session DF2: Moderate Pressure Capacitively Coupled Plasmas

Hybrid Plasma Simulation of Capacitively-Coupled O2 Plasma at Moderate Pressure

Plasma modeling is performed for oxygen RF capacitively coupled discharges at moderate pressures of a few Torrs. In this model, all neutral and charged species are primarily treated as fluids.  The secondary electrons are treated kinetically using a Monte Carlo model,. Our fluid model includes continuity equations for charged and neutral species, drift-diffusion approximation for electron flux, the momentum conservation equation for ions, and the Poisson equation for electric potential. The secondary electrons emitted from the surface are treated kinetically to model the acceleration these electrons experience when going through the sheath. This kinetic secondary electron model is coupled with the fluid bulk plasma model so that the ionization caused by these secondary electrons is captured in the physics of simulations. The electrode potential, electrode current, and the phase difference between potential and current are compared with the experiments conducted in the modified Gaseous Electronics Chamber (mGEC) chamber at the University of Texas Dallas. The modeling results along with diagnostics can provide understanding of plasma behavior at moderate pressure. The modeling results of complex chemistry in oxygen plasma is significant where electronegativity of the plasma due to O^- ions plays an important role.  Variations in plasma characteristics with gas pressure and power is investigated.      

Characterization of Capacitively-Coupled Argon Plasma at Moderate Pressure using Fluid-MCS Hybrid Model

Radio-frequency (RF) capacitively-couple plasma (CCP) in pure Argon at the moderate pressure regime (a few Torr) is characterized using a one-dimensional fluid-MCS hybrid plasma model. In this regime, the collisional mean free path is generally smaller than the domain size, hence a fluid model is appropriate to the characterize the bulk plasma. The model includes continuity equations for charged and neutral species, drift-diffusion approximation for electron flux, the momentum conservation equation for ions, and the Poisson equation for electric potential. Secondary electrons emitted from the electrode surfaces accelerate across the sheath, gaining significant amount of energy. A Monte Carlo model for secondary electrons is used to accurately compute production rates of species, which are coupled to the fluid plasma model. As these electrons reach an energy level below a threshold, they are merged to the bulk electrons. At low pressures, these high energy beam electrons can exit the domain at the opposite electrode without any significant heavy particle collisions. At high pressures, secondary electrons undergo collisions, losing their energy to the plasma and equilibrating with bulk electrons. At moderate pressures, the behavior is more complex and needs to be modeled accurately.    In this study, the role of the secondary electrons on the plasma dynamics at the moderate pressure is demonstrated. The spatio-temporal characteristics of the plasma are illustrated for different secondary electron emission coefficients. The voltage-current characteristics of the plasma model are corroborated using electrical measurements in a symmetric CCP.   

Tuning of Radio-Frequency (RF) Plasmas by a Perpendicular Magnetic Field

Recently, there has been a surge of interest in exploring the tunability of plasma for high-frequency applications. This interest stems from the wide range of tunability of plasma permittivity and electrical conductivity offered by plasma and the unique characteristic of being the only natural material capable of providing negative permittivity. Traditionally, plasma has been tuned by adjusting parameters such as pressure, input power, and excitation frequency. However, an alternative and promising approach involves manipulating plasma properties using an external magnetic field. Currently, there is a lack of readily available information regarding the electrical properties of a capacitively coupled plasma sustained by radiofrequency (RF) power under the influence of a perpendicular magnetic field. Past research has proposed manipulating plasma properties, such as electron number density, to facilitate communication during spacecraft reentry blackout. However, these models relied on bulky electromagnets, which can be impractical in specific scenarios.

This study investigates RF plasma under the influence of a magnetic field generated by a permanent magnet positioned at a specific location along the electrode gap. The pressure within the system is varied within the range of tens to hundreds of millitorr, and the impact of plasma excitation frequency on properties such as plasma conductivity, permittivity, reflection, and transmission is examined—the probing frequencies used in the study span from 100s of MHz. The primary objective of this research is to minimize plasma loss by reducing plasma conductivity while maintaining the necessary negative permittivity. To pursue this objective, the results are compared across three scenarios: no plasma, plasma operating without a magnetic field, and plasma subjected to a perpendicular magnetic field.   

Nanosecond Pulsed Plasma Discharge in High-speed Flow

Nanosecond Pulsed High-Frequency Discharge (NPHFD) plasma has presented advantages in the ignition for challenging applications, such as a scram-jet. It forms three inter-coupling regimes in flowing mixtures: the fully-coupled regime with short inter-pulse time (IPT) and high ignition probability (P_I), the partially coupled regime with intermediate IPT and low P_I, and the decoupled regime where IPT is long, and P_I becomes a function of the pulse number (N) and single pulse ignition probability. In this study, such regimes are investigated experimentally in a high-velocity fuel-air flow (10 – 100 m/s). The NPHFD plasma with approximately 5 – 6 ns full width at half maximum, minimum IPT of 5 ms, maximum discharge voltage of 20 kV, and maximum average power output of 500 W. The deposition energy, N and IPT are varied for parametric study over their effects on the ignition characteristics. Periodic elongations of the plasma arc are observed. The elongation has been shown to follow the residual hot ignition kernel formed from previous discharge pulses. The size and period of the elongation are a function of the inter-electrode gap distance, flow velocity, and gas composition. The deposition energy also changes with such elongation. These significantly influence the inter-coupling regimes, P_I, and ignition kernel characteristics. This study focuses on the effects of such elongation and the mechanism behind such a phenomenon.   

Effect of light ellipticity on laser induced plasma

A mathematical model is formulated, that describes the laser beam propagation in non-linear media for the case of an arbitrary beam polarization and a spatially inhomogeneous refractive index.  We consider the general wave equation derived directly from Maxwell equations and look for the solution in a form of the wave with a variable complex amplitude and cylindrical symmetry to study the plasma generation, defocusing and Kerr effects. The initial plasma distribution was created using the theory of multiphoton and tunneling ionization assuming an initial Gaussian and azimuthal Gauss-Bessel beam at the focus. The ionization yield of photoelectrons, the femtosecond filament behavior, plasma density and electron temperature depend on the polarization state of the incident photons.The analysis of the limiting solutions in the paraxial approximation for the case of azimuthal, linear and radial polarizations is performed. The dependence of the defocusing and Kerr effects on the light ellipticity is studied.   

Dynamics of charged liquid surface using a shallow water approach

As is well known, a charged plane surface of a liquid conductor becomes instable with respect to spatially periodic perturbations when the electric field is higher than the critical value (the Tonks-Frenkel instability) [1]. As a result of the instability development, multiple cone-like structures form on the surface of the conducting liquid. The physical processes that govern the formation of these structures and the saturation of the instability are not completely understood now. To describe the dynamics of charged liquid metal in 3D, shallow water equations for the height of the liquid metal and its velocity were formulated [2]. The effects of the surface tension of the liquid and "negative pressure" due to the electric field were included in the model. In this work we extend the model [2] by calculating self-consistently the electric field on the surface of the liquid metal and analyze the importance of the field electron emission and metal heating in the process of the growth of several cone-like structures on the surface of the metal. The problem is of interest for studying explosive cathode emission, electrospray and physical processes in electron and ion sources with liquid electrodes.   

References:

[1]  L.D. Landau and E.M. Lifshitz, Electrodynamics of continuous media, Volume 8, Pergamon Press, 460 p. (1984).

[2]  <span id="cke_bm_129C" style="display:none"> Zehua Liu,Kentaro Hara, and Mikhail N. Shneider, Dynamics of electrified liquid metal surface using shallow water model, Phys. Fluids 35, 042101 (2023).   

Decay of Mechanically Driven Axial Counter-Current in a High-Speed Rotating Cylinder Using DSMC Simulations

The decay of mechanically driven axial counter-current along the axial direction in a high speed rotating cylinder is studied for wall pressure P_w in the range 20 to 100 m-bar using two dimensional Direct Simulation Monte Carlo (DSMC) simulations. The shape & magnitude of the radial-profile of the axial mass flux is investigated quantitatively at various axial locations and the axial-decay is characterized by a universal exponential function with varying exponent & pre-exponential factor based on the wall pressure and hence the hold up. The analysis shows that as the wall pressure is increased from 20 to 100 m-bar, the shift in the inversion point (corresponds to zero axial mass flux) along the axial length is significant ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)). The analysis further indicates that the decay of axial counter-current influences both the flow profile efficiency (E_F) and the circulation efficiency (E_C) to a great extent, and plays an important role in deciding the separation performance of the gas centrifuge machine. The DSMC simulation results are compared with the analytical results for the decay length based on Dirac equation of high speed approximation ( Z_D = (1/ 2η) (1/(4.82 A⁶)) ((P_wall M_W )/(R_g T )) (V_θ R²_wall ) [ 1 + ((( γ-1) M_W V_θ²)/(4 γ R_g T ))² ] ^1/2 , and found good agreement (error within 15%). Here, Z_D is the decay length, η is the gas viscosity, A is the stratification parameter A= (M_W V_θ²/(2 R_g T))^1/2, P_wall is the wall pressure, M_W is the molecular weight, R_g is the universal gas constant, T is the uniform gas temperature, V_θ is the peripheral velocity, R_wall is the radius of the cylinder, γ is the specific heat ratio (C_P/C_V), and the parameter B = (((γ-1) M_W V_θ²)/(4 γ R_g T)) represents the ratio of adiabatic force to angular momentum force.

  

Computational analysis of the influence of Ar/H2 and arc current on the in-flight particle in plasma spraying

Plasma spraying is extensively used for the deposition of any material, including metal and ceramics, owing to its high thermal energy. Various process parameters are required from the plasma torch to the substrate for the deposition of coatings. The correlation between process parameters and properties of in-flight particles is not well understood due to the complexity and number of process parameters. Specifically, arc current and gas mixtures significantly impact both heat and momentum transfer of the plasma jet to the particles. In the present study, computational simulation was used to predict the behavior of Al2O3 and MgAl2O4 particles under process conditions such as arc current and gas mixtures. The simulation results of the plasma jet using thermal plasma modeling were coupled with discrete phase modeling to predict in-flight particle behavior. The temperature and velocity of the plasma jet and particles were calculated for various arc currents and Ar/H2 flow rates. Also, the temperature and velocity of the in-flight particles were compared to experimental data. The relationship between the plasma jet and in-flight particles was investigated using a simulation model combined with the thermophysical properties of gas mixtures.