Literature Survey

Recently, atmospheric-pressure plasma (APP) has attracted considerable attention, mainly because, unlike low-pressure plasmas, APP does not require the use of vacuum equipment and it is increasingly used in modern science and technology applications. In the following we focus on introducing the AP DBD literature surveys which are restricted along this line. A frame work of the literature survey is revealed in Figure 1. 3.

1.2.1 Atmospheric Pressure Dielectric-barrier Discharges

Dielectric-barrier discharges, or simply barrier discharges, have been known for more than a century. First experimental investigations were reported by Siemens in 1800s [Kogelschatz, 2003]. DBD used in the industry usually operate at 1 atm, therefore we focus the plasma in the atmospheric operating pressure. Depending on discharge conditions and geometry, the DBD may appear in two discharge forms:

homogeneous and filamentary.

The filamentary discharge consists of a large number of narrow (about 100μm) filaments stochastically distributed over the electrode area. The discharge current has

the form of multiple peaks (microdischarges), about 10 ns in duration. There is a large number of works where filamentary discharge (a most common form of the DBD) has been simulated, for example [Papageorghiou et al., 2009]. Most of the works are devoted to the study of a single microdischarge on the basis of a fluid model.

Under certain conditions (for example, frequency and applied voltage…), DBD is homogeneous along the plane of the electrodes. The homogeneous discharge allows one to treat surfaces more uniformly. The homogeneous barrier discharge was obtained in helium [Massines et al., 1998; Mangolini et al., 2002], neon [Trunec et al., 2001], nitrogen [Tepper et al., 2000; Sègur et al., 2000], air [Tepper et al., 2000] and other gases. The homogeneous discharge is realized in two forms: Townsend and glow discharge.

The glow form (atmospheric pressure glow discharge) is characterized by higher discharge current (hundreds of milliamperes) and abrupt alteration of the electric field in the discharge gap. On the contrary, the discharge current in the Townsend discharge is much smaller (units of milliamperes) and the electric field is practically homogeneous along the discharge axis. This Townsend of the homogeneous discharge was observed in most working gases [Mangolini et al., 2002; Tepper et al., 2000;

Mangolini et al., 2002]. In the present research, we take nitrogen as the working gas, because the homogeneous barrier discharge in nitrogen appears mostly as a Townsend discharge.

1.2.2 Simulations and Experiments on DBD with Pure Nitrogen

A Townsend-like barrier discharge in nitrogen at 7 kHz frequency and sinusoidal voltage is studied experimentally and theoretically by [Golubovskii et al., 2006]. The experimental results are compared with the calculations of the existence range of barrier discharges in different forms, which were performed on the basis of a fluid

model. The influences of the barrier material and thickness are discussed while the permittivity of dielectric is 2.2-8.0 and thickness of dielectric from 1.0 to 2.0 mm. It is shown that the lower the permittivity of barriers is, the wider is the range of parameters where the discharge is homogeneous. However, it is considered in this thesis that the effects of permittivity in the more high frequencies higher than 60 kHz with realistic distorted sinusoidal AC power source.

The properties of a barrier discharge in nitrogen near the transition from the Townsend mode to the filamentary mode are studied on the basis of a two dimensional fluid model by [Maiorov et al., 2007]. It is shown that the widening of the stability region for the discharge with small gap distance is proved. Therefore, in the thesis considered the effects of gap distance in the more high frequency 60 kHz with realistic distorted sinusoidal AC power source.

An one-dimension fluid model for pure nitrogen atmospheric pressure plasma was studied [Choi et al., 2006]. The influences of different driving frequencies and voltage amplitudes are discussed while the amplitude of sinusoidal applied voltage is 6-10 kV and frequency changes from 10 to 20 kHz. The increase of the amplitude and frequency of external voltage lead to the increase of plasma density. In addition, the dielectric constants of the barrier materials have also shown a strong influence on the discharge structure. The simulation of pure nitrogen barrier discharge and the nitrogen with different content of oxygen are presented.

1.2.3 Simulations and Experiments on DBD with Nitrogen Mixed Oxygen

The nitrogen with small admixtures of oxygen barrier discharge at frequency of 6.95 kHz was studied [Brandenburg et al., 2005]. The electric characteristics with different external admixture of O2 and the spectroscopic diagnostics are measured

from experiment, and the discharge current and displacement current are compared with numerical and experiment results. The transition to the filamentary mode and the discharge mechanism are discussed, this transition starts from less of oxygen (about 450 ppm) to the micro-discharges, which are generated at higher admixtures of O2

(about 1000 ppm).

The techniques of spatially resolved cross-correlation spectroscopy (CCS) and current pulse oscillography were used to carry out systematic investigations of the barrier discharge (BD) by [Kozlov et al., 2005]. Under the experimental conditions being considered (symmetrical electrode configuration ‘glass–glass’, gap width of about 1 mm, feeding voltage frequency of 6.5 kHz), in the binary N2/O2 mixtures at atmospheric pressure. Special attention was devoted to the investigation of the transition between the filamentary and diffuse modes of the BD, this transition being caused by the variation of oxygen content within the range 500–1000 ppm. The spatio-temporal distributions of the filamentary BD radiation intensities were recorded for the spectral bands of the 0–0 transitions of the second positive (λ = 337 nm) and first negative system of molecular nitrogen (λ = 391 nm). In the case of the diffuse mode, the spectral bands λ = 337 nm, λ = 260 nm (0–3 transition of the γ -system of NO) and λ = 557 nm (radiation of ON2 excimer) were used in the paper.

1.2.4 Numerical Approaches for Plasma Simulation

Generally speaking, there are four different types of approaches to plasma simulation, which can be applied in different plasma conditions. They include: (1) direct Boltzmann equation solver, (2) Particle-in-Cell with Monte-Carlo collision method (PIC-MCC), (3) fluid modeling, and (4) hybrid fluid-PIC method. We are only focus in fluid modeling as following in turn.

Fluid model of plasma is based on partial differential equations which describe

the macroscopic quantities such as density, flux, average velocity, pressure, temperature or heat flux. The governing equations can be derived from the Boltzmann equation by taking velocity moments of the Boltzmann equation with some assumptions [Meyyappan, 1994; Gogolides et al., 1992; Pitaevskii et al., 1981]

including the continuity equation, momentum equation and energy equation. The fluid descriptions break down for highly rarefied plasma, or intense nonlocal effect induced by strongly electric filed. Related publications of fluid modeling could be found in numerous articles, e.g. [Ventzek et al., 1993; Lymberopoulos et al., 1995; Bukowski et al., 1996], and are not reported here.

There are generally two types of fluid modeling techniques: (1) local field approximation (LFA) and (2) local-mean-energy approximation (LMEA). The former assumes the input electric power into the plasma is fully balanced by the power dissipated by ionization, while the latter solves the electron energy density equation directly. Although the transport and rate coefficients of electrons are obtained from the solution of stationary spatially homogeneous Boltzmann equation, the consequences are quite different. It has been shown that the use of LMEA is generally much better than the LFA because of the former considers non-local effect of electron energy distributions [Grubert et al., 2009].