SPECTRAL INVESTIGATION OF A COMPLEX SPACE CHARGE STRUCTURE IN PLASMA S. GURLUI 1, D. G. DIMITRIU 1, C. IONITA 2, R. W. SCHRITTWIESER 2 1 Faculty of Physics, Al. I. Cuza University, 11 Carol I Blvd., RO-700506 Iaºi, Romania, E-mail: sgurlui@uaic.ro, mailto:dimitriu@uaic.ro 2 Institute for Ion Physics and Applied Physics, University of Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria, E-mail: Codrina.Ionita@uibk.ac.at, Roman.Schrittwieser@uibk.ac.at Received September 25, 2008 Complex space charge structures bordered by electrical double layers were spectrally investigated in argon plasma in the domain 400 1000 nm, identifying the lines corresponding to the transitions from different excited states of argon. The electron excitation temperature in the argon atoms was estimated from the spectral lines intensity ratio. Key words: complex space charge structure, double layer, excitation, argon spectrum, electron temperature. 1. INTRODUCTION Complex space charge structures bordered by an electric double layer frequently appear in plasma in front of an electrode positively biased with respect to the plasma potential when the voltage surpasses a certain critical value [1 5]. They appear in form of single or multiple quasi-spherical luminous plasma bodies attached to the electrode surface and were called anode glow, fireballs or balls of fire. Experimental investigations shown that electron-neutral excitation and ionization reactions play the most important role in the phenomenology [1, 6]. These elementary processes assure the necessary electric charge equilibrium for their existence by compensating the losses of charged particles by recombination and diffusion. Previous researches reported a strong influence of the external optical radiation on the appearance and dynamics of complex space charge structures in plasma [7 9]. An important role is played by metastable and resonance levels of the neutrals. Thus, such complex structures are very suitable for fine optogalvanic studies. Paper presented at the 9th International Balkan Workshop on Applied Physics, July 7 9, 2008, Constanþa, Romania. Rom. Journ. Phys., Vol. 54, Nos. 7 8, P. 705 710, Bucharest, 2009
706 S. Gurlui et al. 2 Here we report on a spectral investigation of a complex space charge structure in argon plasma, in the domain 400 1000 nm. The lines corresponding to the transitions from different excited states of the argon were carefully identified. From the spatial distribution of the spectra in front of the exciting electrode we obtained information about the electron-neutral impact excitations produced by the electrons accelerated in the voltage drop across the double layer at the border of the structure. By using the ratio of two spectral lines intensities, we estimated the electron excitation temperature in the argon atoms inside the structure. 2. EXPERIMENTAL RESULTS AND DISCUSSION The experiments were performed in the double plasma (DP) machine of University of Innsbruck, schematically shown in Fig. 1. The DP machine consists of a large cylindrical non-magnetic stainless steel vessel (90 cm long and 45 cm in diameter), evacuated by a pre-vacuum pump and diffusion pump up to a base pressure of about 10 6 mbar. A metallic grid (marked G in Fig. 1) divides the vessel into two chambers: the source chamber and the target chamber (see Fig. 1). By filling the DP machine with argon to a pressure in the range of 10 4 10 2 mbar, in each of the chambers plasma is created by an electrical discharge (about 100 V discharge voltage) between a hot filament (marked F in Fig. 1) as cathode and the grounded tube as anode. Because of the small dimensions, the filaments collect a negligible part of the ions, whereas most of them are diffusing towards the centre of the vessel. The positive space charge attracts many electrons into this region. In this way, a plasma with a higher Fig. 1 Schematic of the University of Innsbruck DP machine (F filament, G grid, E electrode, PS power supplies).
3 A complex space charge structure in plasma 707 degree of ionization appears. To reduce plasma losses and to improve the homogeneity, rows of permanent magnets (B 1 T on the surface) of alternate polarity are mounted on the walls. In our experiments we used only the target chamber of the DP machine. The plasma created in the target chamber was pulled away from equilibrium by gradually increasing the voltage applied to a tantalum disk electrode (marked E in Fig. 1) with 1 cm in diameter. The background argon pressure was p = 5 10 3 mbar and the plasma density n pl 10 7 10 8 cm 3 (estimated with a plane Langmuir probe). In Fig. 2 the complex space charge structures obtained in front of the electrode E is shown, the voltage between the electrode and ground being V E = 120 V. The diameter of the structure was about 28 mm. This structure was spectrally investigated by a HR4000 Ocean Optics high-resolution spectrometer. The spatial resolution was about 0,8 mm and the spectra were recorded every 2 mm in the axial direction (perpendicular to the electrode surface), starting near the electrode surface until a position outside of the fireball. Fig. 3 shows the optical experimental setup. Fig. 4 shows the axial distribution of the recorded spectra. From this spectral distribution, we extracted in Fig. 5 two spectra near the fireball border, inside and outside the structure, respectively. The excited states of argon, observed inside the fireball (Fig. 5a) are due to electron-neutral impact excitations [1, 6, 8], the electrons being accelerated in the voltage drop across the double layer at the border of the structure. Fig. 6 shows the axial profile of two spectral lines, 1 = 763,51 nm and 2 = 842,46 nm, respectively. The intensities of the spectral lines have maximum values at a distance from the electrode of about 14 mm. The axial profile of the ratio of the two spectral lines intensities is shown in Fig. 7. For the positions Fig. 2 Photo of the complex space charge structures obtained in front of the positively biased electrode immersed into plasma.
Fig. 3 Optical experimental design. Fig. 4 Axial profile of the spectra. a) b) Fig. 5 Optical spectra at two different distances from electrode, inside and outside the fireball, respectively.
5 A complex space charge structure in plasma 709 Fig. 6 Axial profiles of two spectral lines ( 1 = 763.51 nm and 2 = 842.46 nm). Fig. 7 Axial profiles of the intensities ratio of the two spectral lines from Fig. 6. 14 mm < d < 20 mm this ratio is almost constant and the spectral line intensities have maximum values, so we can consider that the conditions for local thermal equilibrium are fulfilled. Thus in this region we can estimate the electron excitation temperature in the argon atoms, using the ratio of the two spectral line intensities [10]: I I A g 1 1 1 2 2 A2g2 1 e E2 E1 kt where I 1,2 are the spectral line intensities, A 1,2 are the Einstein coefficients, E 1,2 are the energy levels, k is the Boltzmann constant and T is the local electron temperature, respectively. By using the values of the parameters from the NIST database [11], we obtained an average local electron excitation temperature in the argon atoms of about T 1,87 ev. The two maxima of Fig. 7 suggest that the energy distribution function of the electrons at the border of the structure is different from that of the electrons inside the structure, which is in good agreement with the existing phenomenological model [1, 12, 13]. (1)
710 S. Gurlui et al. 6 3. CONCLUSION A complex space charge structure confined by an electric double layer was spectrally investigated in low-temperature argon plasma. The axial profile of the recorded spectra emphasized the important role of the electron-neutral impact excitations in the appearance of such structures in plasma. The electron excitation temperature in the argon atoms inside the structure was estimated from the spectral lines intensities ratio. The axial profile of the intensities ratio of two spectral lines revealed the existence of two maxima, corresponding to the double layer at the border of the structure and to the sheath between the structure and the electrode, respectively. Acknowledgments. The work was supported by the National University Research Council Romanian Ministry of Education, Research and Youth, under grant no. 56/2007, cod ID_409 and by an Excellentia fellowship of the University of Innsbruck. REFERENCES 1. M. Sanduloviciu, E. Lozneanu, Plasma Phys. Control. Fusion 28, 585 595 (1986). 2. B. Song, N. D Angelo, R. L. Merlino, J. Phys. D: Appl. Phys. 24, 1789 1795 (1991). 3. S. Chiriac, M. Aflori, D. G. Dimitriu, J. Optoelectron. Adv. Mater. 8, 135 138 (2006). 4. C. Ionita, D. G. Dimitriu, R. W. Schrittwieser, J. Optoelectron. Adv. Mater. 9, 2954 2959 (2007). 5. R. L. Stenzel, C. Ionita, R. Schrittwieser, Plasma Sources Sci. Techn. 17 (2008), 035006. 6. C. Ionita, D. G. Dimitriu, R. W. Schrittwieser, Int. J. Mass Spectrom. 233, 343 354 (2004). 7. M. Strat, G. Strat, Phys. Plasmas 8, 5296 5302 (2001). 8. M. Strat, G. Strat, S. Gurlui, Phys. Plasmas 10, 3592 3600 (2003). 9. M. Strat, S. Gurlui, G. Strat, D. G. Dimitriu, Rom. J. Phys. 49, 81 88 (2004). 10. A. A. Garamoon, A. Samir, F. F. Elakshar, A Nosair, E. F. Kotp, IEEE Trans. Plasma Sci. 35, 1 6 (2007). 11. http://physics.nist.gov/physrefdata/asd/lines_form.html 12. D. G. Dimitriu, M. Aflori, L. M. Ivan, C. Ionita, R. W. Schrittwieser, Plasma Phys. Control. Fusion 49, 237 248 (2007). 13. S. Gurlui, M. Sanduloviciu, M. Strat, G. Strat, C. Mihesan, M. Ziskind, C. Focsa, J. Optoelectron. Adv. Mater. 8, 148 151 (2006).