Jean-Pierre Boeuf

Pattern formation and propagation during microwave breakdown

During microwave breakdown at atmospheric pressure, a sharp plasma front forms and propagates toward the microwave source at high velocities. Experiments show that the plasma front may exhibit a complex dynamical structure or pattern composed of plasma filaments aligned with the wave electric field and apparently moving toward the source. In this paper, we present a model of the pattern formation and propagation under conditions close to recent experiments. Maxwells equations are solved together with plasma fluid equations in two dimensions to describe the space and time evolution of the wave field and plasma density. The simulation results are in excellent agreement with the experimental observations. The model provides a physical interpretation of the pattern formation and dynamics in terms of ionization-diffusion and absorption-reflection mechanisms. The simulations allow a good qualitative and quantitative understanding of different features such as plasma front velocity, spacing between filaments, maximum plasma density in the filaments, and influence of the discharge parameters on the development of well-defined filamentary plasma arrays or more diffuse plasma fronts.

Rotating structures in low temperature magnetized plasmas—insight from particle simulations

The EXB configuration of various low temperature plasma devices is often responsible for the formation of rotating structures and instabilities leading to anomalous electron transport across the magnetic field. In these devices, electrons are strongly magnetized while ions are weakly or not magnetized and this leads to specific physical phenomena that are not present in fusion plasmas where both electrons and ions are strongly magnetized. In this paper we describe basic phenomena involving rotating plasma structures in simple configurations of low temperature EXB plasma devices on the basis of PIC-MCC (Particle-In-Cell Monte Carlo Collisions) simulations. We focus on three examples: rotating electron vortices and rotating spokes in cylindrical magnetrons, and azimuthal electron-cyclotron drift instability in Hall thrusters. The simulations are not intended to give definite answers to the many physics issues related to low temperature EXB plasma devices but are used to illustrate and discuss some of the basic questions that need further studies.

Ionization?diffusion plasma front propagation in a microwave field

A 1D model of the expansion of a collisional plasma under the combined effect of diffusion and ionization is presented. It is shown that a simple quasi-neutral model of the plasma using an effective diffusion coefficient can accurately describe the plasma front propagation. The effective diffusion coefficient describes the transition from free electron diffusion in the plasma front to ambipolar diffusion in the bulk. Comparisons with exact solutions from a drift-diffusion–Poisson model show excellent agreement not only in the simple case of a constant ionization frequency, but also when the plasma front propagation is due to microwave breakdown. In the latter case the plasma model is solved together with Maxwells equations and the ionization frequency in the front is modulated in time due to the formation of standing waves in the plasma front region, leading to the formation of plasma patterns. The effect of electron–ion recombination on the observed plasma pattern is discussed.

Three dimensional simulations of pattern formation during high-pressure, freely localized microwave breakdown in air

Recent experiments have demonstrated that a freely localized 100u2009GHz microwave discharge can propagate towards the microwave source with high speed, forming a complex pattern of self-organized filaments. We present three-dimensional simulations of the formation and propagation of such patterns that reveal more information on their nature and interaction with the electromagnetic waves. The developed three-dimensional Maxwell-plasma solver permits the study of different forms of incident field polarization. Results for linear and circular polarization of the wave are presented and comparisons with recent experiments show a good overall agreement. The three dimensional simulations provide a quantitative analysis of the parameters controlling the time and length scales of the strongly non-linear plasma dynamics and could be useful for potential microwave plasma applications such as aerodynamic flow and combustion control.

Multi-scale gas discharge simulations using asynchronous adaptive mesh refinement

Abstract The breakdown of a gas gap at high products of pd (pressure × distance) is a multi-scale phenomenon in both time and space. This is especially true when the plasma is interacting with a gas flow, a problem of considerable recent interest in the context of aerodynamic applications of surface discharges. This paper presents a contribution to the numerical modeling of such discharges. We describe here a new approach for adaptive meshing which is suitable for use with the explicit asynchronous integration scheme described in our previous publication. Rather than relying on a family of nested grids as is commonly done, this technique is based on a single unstructured mesh with possible non-conforming cells at the interface between coarse and fine areas. Substantial computational time saving has been achieved for a surface dielectric barrier discharge configuration of the kind proposed as plasma actuators for flow control.

Modeling of an advanced concept of a double stage Hall effect thruster

We present a study of the principle and operation of a two-stage Hall effect thruster, the SPT-MAG, using a two-dimensional quasineutral hybrid model coupled with a Monte Carlo simulation of electron transport. The purpose of the two-stage design is the separation of ion production and acceleration in two separate chambers, the ionization stage and the acceleration stage, with separate control of acceleration voltage and total ionization. The originality of the SPT-MAG lies in the magnetic field configuration in the ionization chamber. Electrons are confined by this magnetic field while ions are supposed to be trapped in the electric potential well supposedly resulting from the magnetic configuration, and guided toward the acceleration stage. The acceleration stage is similar to the channel of a conventional Hall effect thruster. The purpose of this paper is to clarify the physics of the SPT-MAG and to understand the formation of the positive ion trap in the ionization chamber. Using a hybrid model and a ...

Modeling of double stage Hall effect thruster

Hall effect thrusters (HETs) are ion sources used for satellite station keeping and orbit raising. In Single Stage HETs, the same electric field is responsible for electron heating and ion acceleration. We present a new HET concept where ionization and acceleration are separated in two different stages. This double stage HET allows for a more versatile operation and a separate control of thrust and specific impulse.

ADI-FDTD modeling of microwave plasma discharges in air towards fully three-dimensional simulations

Abstract Plasma formation and propagation during microwave breakdown has been extensively studied during the last decades. Numerical modeling of the strong coupling between the high frequency electromagnetic waves and the plasma is still a challenging topic due to the different time and space scales involved. In this article, an Alternative Direction Implicit (ADI) formulation of the Finite Difference Time Domain method for solving Maxwell’s equations coupled with a simplified plasma model via the electric current is being proposed, leading to a significant reduction of the computational cost as the CFL criterion for stability of the FDTD method is being removed. An energy estimate has been used to prove the unconditional stability of the ADI-FDTD leapfrog scheme as well as its coupled formulation. The computational efficiency and accuracy of this approach has been studied in a simplified case. The proposed method is applied and validated in two dimensional microwave breakdown in air while its computational efficiency allows for fully three dimensional simulations, an important step for understanding the complex nature and evolution of a microwave plasma discharge and its possible applicability as an aerodynamic flow control method.

Finite Volume Time Domain modelling of microwave breakdown and plasma formation in a metallic aperture

Abstract The modelling of plasma formation during microwave breakdown is a difficult task because of the strong non-linear coupling between Maxwellʼs equations and plasma equations, and of the large plasma density gradients that form during breakdown. An original Finite Volume Time Domain (FVTD) method has been developed to solve Maxwellʼs equations coupled with a simplified fluid plasma model and is described in this paper. This method is illustrated with the study of the shielding of a metallic aperture by the plasma generated by an incident high power electromagnetic wave. Typical results obtained with the FVTD method for this shielding problem are shown.

Experimental protocol and critical assessment of the Pockels method for the measurement of surface charging in a dielectric barrier discharge

The behaviour of dielectric barrier discharges (DBDs) is controlled by the successive charging and discharging of the dielectric surface. Measuring the space and time evolution of the charges on the dielectric layers is of great interest to optimize these discharges and better understand the transition from homogeneous to filamentary regimes and the formation of patterns. Such measurements can be performed using a diagnostic technique based on the Pockels effect, where an electro-optic crystal is used as the dielectric layer of a DBD and as a probe of the electric field across the layer. The surface charge density is related to this field and can, in some limited conditions, be simply deduced from it. The principle of this technique is relatively simple but the necessary care that must be taken in its application to DBDs has not been described in detail in the literature. The aim of this paper is to provide a critical assessment of the method and to illustrate its application in the case of a low pressure dielectric barrier glow discharge.