OPOLEOpole University Institute of Physics, Plasma Spectroscopy Group I am from.. 1

Symmetry of the plasma produced in a wall-stabilized d.c. arc 2

Wall-stabilized arc (Maecker) 3

Wall-stabilized arc (Shumaker) 4

Main advantages of the wall-stabilized arc 5 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 the Local Thermal Equilibrium

Usually the discharge is conducted in an inert gas atmosphere with small admixtures of the element under study. 6 Argon Helium

temperature: – (K) pressure: 1 atmosphere electron densities: – (cm –3 ) Typical parameters of plasma produced in a wall-stabilized arc 7

Wall-stabilized arc (this work) 8

Gas inlet-outlet 9

Gas flow 10

Experiment parameters 11

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. 12

Detector tracks 13

Spectra registered in 6545–6685Å range 14

Spectra registered in 6945–7095Å range 15

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) 16

Methods n e = f (FWHM(H  )) T = f (n e,  Ar I,  Ar II ) n atoms Ar,H = f Boltzmann (T,  ) n ions Ar,H = f Saha (T, n e,n atoms ) n atoms He = p atm – k·T ·  n i a He = n He /n He (  HeI ) n ions He = f Saha (T, n e,n atoms, a He ) Method (B) {system of LTE equation} n atoms He,Ar,H = f Boltzmann (T,  ) n ions He,Ar,H = f Saha (T, n e,n atoms ) T = p / (k·  n) n e =  z· n i z Method (A) {partial LTE} 17

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

Axial distribution of the electron density at different discharge currents (values on the arc axis). 19 Method (A) Method (B)

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

Axial distribution of the electron density at different plasma compositions (values on the arc axis). 21 Method (A) Method (B)

Spatial distribution of plasma parameters (method A, i = 60 A) 22

Spatial distribution of Argon mass fraction (method A, i = 60 A) 23

Spatial distribution of Hydrogen mass fraction (method A, i = 60 A) 24

Spatial distribution of Helium mass fraction (method A, i = 60 A) 25

End-on spectra – how to interpret it? 26

Demixing effect 27 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 N 2, N 2 +, N, N +, N ++, and the electrons derived from the ionization of nitrogen molecules and atoms.

A. B. Murphy Phys. Rev. E (1997) 28 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). 29

A. B. Murphy, Phys. Rev. Lett. 73, 1797 (1994) 30 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

Radial distributions of Argon mass fraction (2 different gas mixtures). 32

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

Effective temperatuers Effective temperatures Ddetermined based on intensities of Ar I Å Ar I Å (  E  1.5 eV) 34

The End 35