Plasma stratification in radio-frequency discharges in argon gas

Abstract

We conducted experimental studies and computer simulations of standing striations in capacitive coupled plasma in Argon gas. Standing striations were observed at frequencies 3.6, 8.4 and 19.0 MHz, in a pressure range 0.05-10 Torr, tube radius R=1.1 cm, for a certain range of discharge currents (plasma densities). Numerical simulations revealed similar nature of standing striations in CCP and moving striations in DC discharges under similar discharge conditions. Comparison of computer simulations with experimental observations helped clarify the nature of these striations. The non-linear dependence of the ionization rate on electron density is shown to be the main underlying mechanism of the stratification phenomena. DOI:XXXPlasma stratification often occurs in laboratory devices and has been observed in space plasmas [1]. The most studied are striations in direct current (DC) glow discharges [2]. Moving striations are usually observed in noble gases, while standing striations are typical for molecular gases. The nature of moving striations (also called ionization waves) in noble gases is relatively well understood [3,4]. Plasma stratification in molecular gases remains poorly understood.Standing striations have been also observed in radiofrequency (RF) discharges [5]. In Capacitively Coupled Plasma (CCP) created between two wires wrapped around a long dielectric tube, standing striations were observed in Argon over a certain range of frequencies and gas pressures [6,7]. Their wavelength is proportional to the tube radius and decreases with increasing gas pressure. The wavelength varies discontinuously with changing the interelectrode distance in such a way that an integer number of standing waves always forms between the electrodes. The wavelength weakly depends on driving frequency in the range from 3 MHz to 60 MHz. 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]. By changing the driving frequency and the distance L between the electrodes, the researchers were able to change the number of striations. 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. The nature of the striations remained unclear...

Argon self‐organized striation plasmas under kPa pressure level

“…The development of instability in the radial direction causes the discharge contracting into a thin cord (constriction) to eliminate the stabilizing factor and leads to a nonlinear dependence of the ionization rate on plasma density in the longitudinal direction (striation). [7,16,28]…”

“…The nonlinear dependence of the ionization rate on the plasma density is the common underlying mechanism in DC striations of noble gases. [6,7] Though AC RF striations have been found to be generated due to the ionresonance between the driving RF and the eigenfrequency of the ion-ion plasma. [3,8,9] The generation and development of self-organized striation plasmas (SSPs) were found to be sensitive to gas pressure.…”

Electron kinetics in standing and moving striations in argon gas

“…Moving and standing striations were observed in both direct current (DC) and radio-frequency (RF) discharges. 7,8 There are several type of striations which can be described using either the hydrodynamic or kinetic approach. 9 At high currents, the electrons are thermalized due to electron-electron Coulomb collisions.…”

“…Then, one can use the hydrodynamic approach for striations modeling. 8 At high gas pressures, the hydrodynamic approach is justified by frequent electron-neutral collisions. At intermediate pressures, the electron kinetics is non-local and stratification can be described only using the kinetic approach.…”

Experimental and numerical study of the plasma in coaxial capacitive coupled radio frequency discharge

“…29,30 The non-linear dependence of the ionization rate on the electron density is shown to be the main underlying mechanism of the striation phenomena. 34 1. Effect of gas pressure and RF power In this section, the effect of the gas pressure and RF power on discharge parameters are presented, such as the voltage and current amplitude (V MAX , I MAX ), the phase between voltage and current (f), the discharge impedance (Z), and the self-bias voltage (V bias ).…”