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OPOLE Opole University

Institute of Physics,Plasma Spectroscopy Group

Symmetry

the plasma

produced in

a wall-stabilized d.c. arc

Wall-stabilized arc (Maecker)

Wall-stabilized arc (Shumaker)

Main advantages of the wall-stabilized arc

•very stable (as well spatially as temporally)

•long time of stable work (hours)

•cylindrical symmetry of the plasma

•uniformity of the plasma along the arc axis(neglecting infinitesimally small area near electrodes)

•the plasma is at least close to theLocal Thermal Equilibrium

Usually the discharge is conducted in an inert gas atmosphere with small

admixtures of the element under study.

•Argon

•Helium

temperature: 8 000 – 15 000 (K)

pressure: 1 atmosphere

electron densities: 1015 – 1017 (cm–3 )

Typical parameters of plasma produced in a wall-stabilized arc

Wall-stabilized arc (this work)

Gas inlet-outlet

A rH e

s ide w indow

Gas flow 10

Experiment parametersEstimated Ar

concentration

(per vol.)

Discharge

current

30 10.0 % 45

60 25.7% 4.5%

Optical set-up :A – top view,B – side view;

1a – wall-stabilized arc, 1b – tungsten strip lamp (standard source),

2 – flat mirror,3 – spherical mirror,4 – filter, 5 – spectrograph, 6 – CCD camera, 7 – PC computer, 8 – flat mirror.

Detector tracks 13

Spectra registered in 6545–6685Å range 14

6540 6560 6580 6600 6620 6640 6660 6680

Wavelength (Å)

C athode

A node

Spectra registered in 6945–7095Å range 15

6960 6980 7000 7020 7040 7060 7080

Wavelength (Å)

C athode

A node

What can cause the differences in line intensities?

Changes of plasma parameters (enhancement of the excitation)

Changes in chemical plasma composition(partial pressure or concentration of the species)

Methods

ne = f (FWHM(H))

T = f (ne,Ar I, Ar II)

natoms Ar,H = fBoltzmann(T, )

nions Ar,H = fSaha(T, ne,natoms)

natoms He = patm– k·T · ni

aHe = nHe/nHe( HeI)

nions He = fSaha(T, ne,natoms, aHe)

Method (B){system of LTE equation}

natoms He,Ar,H = fBoltzmann(T, )

nions He,Ar,H = fSaha(T, ne,natoms)

T = p/(k· n)

ne = z· niz

Method (A){partial LTE}

Axial distribution of the temperature at different discharge currents (values on the arc axis). 18

Method (A)

-4 -3 -2 -1 0 1 2 3 410000

i = 30 A i = 45 A i = 60 A

Distance from the arc center (cm)

Method (B)

-4 -3 -2 -1 0 1 2 3 410000

i = 30 A i = 45 A i = 60 A

Axial distribution of the electron density at different discharge currents

(values on the arc axis). 19

Method (A)

-4 -3 -2 -1 0 1 2 3 4

5.0x1015

1.0x1016

1.5x1016

i = 30 A i = 45 A i = 60 A

n e (c

Method (B)

-4 -3 -2 -1 0 1 2 3 4

5.0x1015

1.0x1016

1.5x1016

i = 30 A i = 45 A i = 60 A

n e (c

Axial distribution of the temperature at different plasma compositions (values on the arc axis). 20

Method (A)

-4 -3 -2 -1 0 1 2 3 410000

large amount of Ar small amount of Ar

Method (B)

-4 -3 -2 -1 0 1 2 3 410000

Axial distribution of the electron density at different plasma compositions

(values on the arc axis). 21

Method (A)

-4 -3 -2 -1 0 1 2 3 4

5.0x1015

1.0x1016

1.5x1016

n e (c

Method (B)

-4 -3 -2 -1 0 1 2 3 4

5.0x1015

1.0x1016

1.5x1016

n e (c

Spatial distribution of plasma parameters

(method A, i = 60 A)

cathode

5.0x1015

1.0x1016

1.5x1016

n e (c

cathode anode

Spatial distribution of Argon mass fraction (method A , i = 60 A)

cathode anode 23

Spatial distribution of Hydrogen mass fraction (method A , i = 60 A)

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

cathode anode 24

Spatial distribution of Helium mass fraction (method A , i = 60 A)

cathode

End-on spectra – how to interpret it?

Cathode

6560 6580 6600 6620 6640 6660 6680

W avelength (Å )

Demixing effect

Murphy has shown that in a mixture of two homonuclear gases that do not react with each other the treatment of diffusion can be greatly simplified if local chemical equilibrium is assumed.

In this case, instead of considering the diffusion of individual species separately, one can consider the diffusion of gases.

Here a gas, for example nitrogen, is defined to consist of all the species that can be derived from that gas, for example N2, N2

+, N, N+, N++, and the electrons derived from the ionization of nitrogen molecules and atoms.

A. B. MurphyPhys. Rev. E 55

7473 (1997)

Temperature dependence of the mole fractions of the species present in a mixture of argon and helium if no demixing occurs.

Demixing effect

• mole fraction (or partial pressure) gradient,

Demixing can be caused by:

• frictional forces,

• thermal diffusion,

• external forces (e.g. electric field).

A. B. Murphy, Phys. Rev. Lett. 73,

1797 (1994)

Combined diffusion coefficients for different mixtures of argon and nitrogen.

(a) Mole fraction diffusion coefficient; (b) temperature diffusion coefficient; (c) thermal diffusion coefficient.

Radial distributions of Argon mass fraction (7 different gas mixtures). 31

0.00 0.05 0.10 0.15

0.00 0.05 0.10 0.150.69

0.00 0.05 0.10 0.150.52

0.00 0.05 0.10 0.150.32

0.00 0.05 0.10 0.150.24

0.00 0.05 0.10 0.15

r (cm) r (cm)

r (cm)

Ar mass fraction in gas mixture

r (cm)

Radial distributions of Argon mass fraction (2 different gas mixtures).

0.00 0.05 0.10 0.15 0.200.60

Ar mass fraction (0.89 in gas mixture)

Ar mass fraction in gas mixture

0.40 Ar mass fraction

(0.14 in gas mixture)

ass fraction in arc plasma {in cold gas 0.14}

Radial distributions of temperature (7 different gas mixtures). 33

0.00 0.05 0.10 0.15 0.20

Ar mass fractionin gas mixture

0.89 0.78

0.700.540.370.280.14

r (cm)

Effective temperaturesDdetermined based on intensities of •Ar I 6965.43Å •Ar I 7030.25 Å (E1.5 eV)

-4 -3 -2 -1 0 1 2 3 40

between stabilizing pla tes within stabilizing plates

CATHODE

0.00 0.05 0.10 0.15 0.20 0.25 0.300

r (cm)

CATHODE

0.00 0.05 0.10 0.15 0.20 0.25 0.30-0.2

r (cm)

Ar I 7030.25 Å 0.0

mission coefficient (a.u.)

The End

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