Microwave N –ArPlasma Torch - ULisboa · A surface wave induced microwave plasma torch is created...
1
Microwave N 2 – Ar Plasma Torch J. Henriques *, E. Tatarova, F.M. Dias and C.M. Ferreira Instituto de Plasmas e Física Nuclear, Instituto Superior Técnico, 1049-001 Lisboa, Portugal *e-mail: [email protected] Nonthermal atmospheric plasmas have attracted much attention in recent years due to many industrial applications such as the creation of nanostructures, decontamination processes, hydrogen production, etc. For such purposes, waveguide-based, atmospheric plasma torches driven by surface waves are an attractive alternative to conventional plasma torches since they are compact, electrodeless, economical, and simple to operate [1]. In this study, a complex investigation of a surface wave driven N 2 -Ar microwave plasma torch operating at atmospheric pressure is presented. The main plasma and wave characteristics are obtained in the framework of a self-consistent 2D model that describes the space structure of this plasma source. Introduction A surface wave induced microwave plasma torch is created using a conventional discharge set-up (see photo below). The field applicator is a wave launcher of the surfaguide type [5]. Microwave power is provided by a ω/2π=2.45 GHz Sairem GMP 20 KED generator, at levels varying between 200 W and 2000 W. A cylindrical, fused quartz discharge tube (with internal and external radii R 1 = 7.5 mm and R 2 = 9.0 mm, respectively) is filled by the working gas mixture of N 2 – Ar at atmospheric pressure. The plasma column is generated within this tube. An overdense discharge plasma is sustained by the electric field of a surface wave guided along the interface between the plasma and the surrounding dielectrics. Due to axial transport, the plasma expands and a post-discharge plasma is formed beyond the discharge zone. Gas flows are controlled via a MKS 247 Readout coupled to three MKS flow meters, the total flow varying between 500 sccm and 10 slm. A spectroscopic imaging system able to couple the plasma-emitted radiation into a SPEX 1250M spectrometer equipped with a nitrogen cooled CCD camera was used to measure 2D(r,z) profiles of emission intensities and line profiles. Abel inversion has been applied to derive the radial profiles from the side-on measurements. Experimental Setup This study was funded by FCT/FEDER in the framework of the project “Microwave Discharges: from Fundamentals to Nanostructures” (PTDC/FIS/108411/2008) Governo da República Portuguesa UNIÃO EUROPEIA - FEDER Programa Operacional Ciência e Inovação 2010 MINISTÉRIO DA CIÊNCIA TECNOLOGIA E ENSINO SUPERIOR Acknowledgments 2D - Imaging optical system Slit CCD Spectrometer SPEX 1250M Imaging Optical Fiber Lens Mass Flow Controllers Ar N 2 H 2 Surfaguide Inverse-Telecentric Lens Iris Plasma Conclusions An experimental and theoretical study of the spatial structure of a microwave (2.45 GHz) plasma torch driven by surface waves in N 2 -Ar mixture has been performed. Spatially resolved emission spectroscopy techniques have been used. The experimental results have been analyzed in the terms of a 2D theoretical model based on a self- consistent treatment of particles kinetics, gas dynamics and wave electrodynamics. The model further determines the two-dimensional discharge structure, i.e., the radial, and axial variations of the main plasma quantities. Strong correlation is shown to exist between the density distributions of plasma electrons and electronically excited states of molecules and atoms. High level of population in metastable states of nitrogen molecules is predicted. High degree of nitrogen dissociation up to 5 % can be achieved. Along the main part of the plasma column there is only a small variation of the gas temperature, which remains close to 5,000 K at the plasma axis. This is followed by a sharp drop to about 400 K in the post-discharge zone (z > 6 cm). The results provide evidence for large radial gradients of the gas temperature (∼ 4,000 K difference between the axial and the wall temperatures). 2D spatial profiles of electron and positive ion densities demonstrate a good agreement with theoretical model. [1] M. Moisan and Z. Zakrzewski, J. Phys. D: Appl. Phys. 24 (1991) 1025. [2] J. Henriques, E. Tatarova, V. Guerra, C.M. Ferreira, J. Appl. Phys. 91 (2000) 5622. [3] M. Pinheiro, B. F. Gordiets and C. M. Ferreira, Plasma Sources Sci. Technol. 8 (1999) 31. [4] C. M. Ferreira, E. Tatarova, J. Henriques and F.M. Dias, J. Phys. D: Appl. Phys. 42 194016 (2009) [5] M. Moisan, M. Chaker, Z. Zakrzewski, and J. Parasczak, J. Phys. E: Sci. Instrum. (1987) 20, 1356. [6] LIFBASE, Database and Spectral Simulation for Diatomic Molecules (v 1.6) – J. Luque e D.R. Crosley, SRI International Report MP-99-009 (1999). References 2D – Experimental Distributions of H - Atom Temperature 4860 4861 4862 0,0 0,2 0,4 0,6 0,8 1,0 r/R = 0.3 z = 4.6 mm T g =5026 K n e =6.2x10 12 cm -3 Mesured Fitted Emissivity (a.u.) λ (Å) 4860 4861 4862 0,0 0,2 0,4 0,6 0,8 1,0 r/R = 0.0 z = 4.6 mm T g =7914 K n e =9.2x10 12 cm -3 Mesured Fitted Emissivity (a.u.) λ (Å) N 2 (20%) - Ar(80%) N 2 (10%) - Ar(90%) Model - 2D Distributions: N 2 (A), N( 4 S) and VDF N 2 (20%) - Ar(80%) 2D – Distributions: N 2 + (B-X), N 2 + Rotational Temperature and N 2 + Density N 2 + (B-X) spectrum, z = 4.5 mm N 2 (10%) - Ar(90%) N 2 (20%) - Ar(80%) Theoretical 2D - Distributions of the Electron Density N 2 (20%) - Ar(80%) Theoretical Theoretical Experimental Model - 2D Distributions: E total , θ and T g Ar(80%) – N 2 (20%) The system of equations considered to describe the plasma source includes [1,2,3,4]: Maxwell’s equations; The dispersion equation for the azimuthally symmetric TM surface mode; The rate balance equations for vibrationally excited states of electronic ground state molecules N 2 (X,v) ; The rate balance equations for the triplet N 2 (A 3 Σ u + ,B 3 Π g ,C 3 Π u ) and singlet N 2 (a’ 1 Σ u - ,a 1 Π g ,w 1 Δ u ,a” 1 Σ u + ) electronic excited states of the nitrogen molecule and the Ar(3p 5 4s) and Ar(3p 5 4p) excited states of Argon atoms; The rate balance equations for N + , N 2 + , N 3 + , N 4 + , Ar + and Ar 2 + ions and electrons; The rate balance equation for the ground state N( 4 S) and the metastable states N( 2 P, 2 D) of nitrogen atoms; The gas thermal balance equation; The equation of mass conservation for the fluid as a whole [3]. Theoretical Model (Experimental Conditions: Q T = 500 sccm, P = 600 W, p = 1 atm)
Transcript of Microwave N –ArPlasma Torch - ULisboa · A surface wave induced microwave plasma torch is created...
Microwave N2 – Ar Plasma Torch
J. Henriques*, E. Tatarova, F.M. Dias and C.M. Ferreira
Instituto de Plasmas e Física Nuclear, Instituto Superior Técnico, 1049-001 Lisboa, Portugal*e-mail: [email protected]
Nonthermal atmospheric plasmas have attracted much attention in recent years due to many industrial applications such as the creation of nanostructures, decontamination processes, hydrogen production, etc. For such purposes, waveguide-based, atmospheric plasma torches driven by surface waves are an attractive alternative to conventional plasma torches since they are compact, electrodeless, economical, and simple to operate [1]. In this study, a complex investigation of a surface wave driven N2-Ar microwave plasma torch operating at atmospheric pressure is presented. The main plasma and wave characteristics are obtained in the framework of a self-consistent 2D model that describes the space structure of this plasma source.
Introduction
A surface wave induced microwave plasma torch is created using a conventional discharge set-up (see photo below). The field applicator is a wave launcher of the surfaguide type [5]. Microwave power is provided by a ω/2π=2.45 GHz Sairem GMP 20 KED generator, at levels varying between 200 W and 2000 W. A cylindrical, fused quartz discharge tube (with internal and external radii R1 = 7.5 mm and R2 = 9.0 mm, respectively) is filled by the working gas mixture of N2 – Ar at atmospheric pressure. The plasma column is generated within this tube. An overdense discharge plasma is sustained by the electric field of a surface wave guided along the interface between the plasma and the surrounding dielectrics. Due to axial transport, the plasma expands and a post-discharge plasma is formed beyond the discharge zone. Gas flows are controlled via a MKS 247 Readout coupled to three MKS flow meters, the total flow varying between 500 sccm and 10 slm. A spectroscopic imaging system able to couple the plasma-emitted radiation into a SPEX 1250M spectrometer equipped with a nitrogen cooled CCD camera was used to measure 2D(r,z) profiles of emission intensities and line profiles. Abel inversion has been applied to derive the radial profiles from the side-on measurements.
Experimental Setup
This study was funded by FCT/FEDER in the framework of the project “Microwave Discharges: from Fundamentals to Nanostructures” (PTDC/FIS/108411/2008)
Governo da RepúblicaPortuguesa
UNIÃO EUROPEIA -FEDER
Programa Operacional Ciência e Inovação 2010
MINISTÉRIO DA CIÊNCIA TECNOLOGIA E ENSINO SUPERIOR
Acknowledgments
2D - Imaging optical system
Slit
CCD
Spectrometer SPEX 1250M
Imaging Optical Fiber
Lens
Mass FlowControllers
Ar N2 H2
Surfaguide
Inverse-Telecentric Lens
Iris
Plasma
Conclusions
An experimental and theoretical study of the spatial structure of a microwave (2.45 GHz) plasma torch driven by surface waves in N2-Ar mixture has been performed. Spatially resolved emission spectroscopy techniques have been used. The experimental results have been analyzed in the terms of a 2D theoretical model based on a self-consistent treatment of particles kinetics, gas dynamics and wave electrodynamics. The model further determines the two-dimensional discharge structure, i.e., the radial, and axial variations of the main plasma quantities. Strong correlation is shown to exist between the density distributions of plasma electrons and electronically excited states of molecules and atoms. High level of population in metastable states of nitrogen molecules is predicted. High degree of nitrogen dissociation up to 5 % can be achieved. Along the main part of the plasma column there is only a small variation of the gas temperature, which remains close to 5,000 K at the plasma axis. This is followed by a sharp drop to about 400 K in the post-discharge zone (z > 6 cm). The results provide evidence for large radial gradients of the gas temperature (∼ 4,000 K difference between the axial and the wall temperatures). 2D spatial profiles of electron and positive ion densities demonstrate a good agreement with theoretical model.
[1] M. Moisan and Z. Zakrzewski, J. Phys. D: Appl. Phys. 24 (1991) 1025.[2] J. Henriques, E. Tatarova, V. Guerra, C.M. Ferreira, J. Appl. Phys. 91 (2000) 5622.[3] M. Pinheiro, B. F. Gordiets and C. M. Ferreira, Plasma Sources Sci. Technol. 8 (1999) 31.[4] C. M. Ferreira, E. Tatarova, J. Henriques and F.M. Dias, J. Phys. D: Appl. Phys. 42 194016 (2009)[5] M. Moisan, M. Chaker, Z. Zakrzewski, and J. Parasczak, J. Phys. E: Sci. Instrum. (1987) 20, 1356.[6] LIFBASE, Database and Spectral Simulation for Diatomic Molecules (v 1.6) – J. Luque e D.R. Crosley, SRI International Report MP-99-009 (1999).
References
2D – Experimental Distributions of H - Atom Temperature
4860 4861 48620,0
0,2
0,4
0,6
0,8
1,0r/R = 0.3z = 4.6 mmT
g=5026 K
ne=6.2x1012 cm-3
Mesured
Fitted
Em
issi
vity
(a.
u.)
λ (Å)4860 4861 4862
0,0
0,2
0,4
0,6
0,8
1,0r/R = 0.0z = 4.6 mmT
g=7914 K
ne=9.2x1012 cm-3
Mesured
Fitted
Em
issi
vity
(a.
u.)
λ (Å)
N2(20%) - Ar(80%)N2(10%) - Ar(90%)
Model - 2D Distributions: N2(A), N(4S) and VDF
N2(20%) - Ar(80%)
2D – Distributions: N2+(B-X), N2
+ Rotational Temperature and N2+ Density
N2+(B-X) spectrum, z = 4.5 mm
N2(10%) - Ar(90%) N2(20%) - Ar(80%)
Theoretical
2D - Distributions of the Electron Density
N2(20%) - Ar(80%)
TheoreticalTheoreticalExperimental
Model - 2D Distributions: Etotal, θ and Tg
Ar(80%) – N2(20%)
The system of equations considered to describe the plasma source includes [1,2,3,4]:
� Maxwell’s equations;� The dispersion equation for the azimuthally symmetric TM surface mode;� The rate balance equations for vibrationally excited states of electronic ground state molecules N2(X,v) ;� The rate balance equations for the triplet N2(A
3Σu+,B3Πg,C
3Πu) and singlet N2(a’1Σu
-,a1Πg,w1∆u,a”
1Σu+) electronic
excited states of the nitrogen molecule and the Ar(3p54s) and Ar(3p54p) excited states of Argon atoms;� The rate balance equations for N+, N2
+, N3+, N4
+, Ar+ and Ar2+ ions and electrons;
� The rate balance equation for the ground state N(4S) and the metastable states N(2P, 2D) of nitrogen atoms; � The gas thermal balance equation;� The equation of mass conservation for the fluid as a whole [3].
Theoretical Model
(Experimental Conditions: QT = 500 sccm, P = 600 W, p = 1 atm)
