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PPS/ICOPS 2001 CONFERENCE Las Vegas, Nevada June 17-22, 2001 RUBIDIUM FILTERED THOMSON SCATTERING MEASUREMENT IN AN ATMOSPHRIC PRESSURE ARGON ARC S.H. Zaidi, Z. Tang, R.B. Miles Department of Mechanical and Aerospace Engineering Princeton University, Princeton NJ 08540 Abstract High temperature, atmospheric pressure plasmas represent a significant challenge for diagnostics. The temperature is too high for physical probes, the plasmas are filamentary with dimensions too small to be resolved by microwave techniques, and the plasma luminosity and Rayleigh scattering background limit optical diagnostics. We report here the measurement of electron temperature and number density in an atmospheric arc discharge by Thomson scattering collected through an optically thick Rb filter. A narrow linewidth, frequency-tunable pulsed Ti:Sapphire laser was employed. The Thomson scattering is frequency broadened by the thermal motion and the ion acoustic coherent motion of the electrons. The linewidth of the Thomson scattering is much greater than the absorption linewidth of the Rb, so it passes through the Rb filter and into a spectrometer. The detector is timegated synchronized with the laser to suppress the plasma luminosity. The frequency spectrum of the Thomson scattered light is fitted to a theoretical model in order to determine the electron temperature and number density. I.INTRODUCTION Experimental diagnostic techniques play a significant role for the understanding of plasma properties and the validation of theoretical predictions. These diagnostics are in high demand in plasma aerodynamics where both intrusive and non-intrusive techniques are used to conduct plasma studies. The use of Langmuir probes is the most popular method for the measurement of electron temperature and electron density in low temperature and weakly ionized plasmas [1]. In spite of their usefulness, the intrusive nature and their restricted application to the low temperature plasmas make these probes unsuitable for high temperature plasma studies. Among the non-intrusive techniques, Thomson scattering has emerged as an important diagnostic tool which can be used to make unambiguous measurements of electron temperature and electron number density in plasmas [2]. Experiments in plasmas with electron densities in the range of 1017 to 1021 cm-3 and electron temperatures in the range of 1eV to 5eV are now being performed [3]. Recent attempts have been made to lower limit for Thomson scattering measurements to below 1012 cm-3 . This density regime is commonly found in glow discharges which are used in industrial application including etching and depositions. In spite of several advantages, Thomson scattering technique does have few limitations which mainly come from the plasma luminosity and the Rayleigh light which is scattered by the system as a back ground light. The Thomson signal has to compete with plasma emission and the elastic background radiation, both of which can mask the Thomson signal. To overcome the problem of interference from the undesired radiation, atomic dispersive resonance filters can be employed. In the present work a Rubidium filter along with a narrow width, frequency-tunable, pulsed Ti:Sapphire laser was used. The laser was tuned to the 780 nm absorption line of Rb vapor. The Rb filter was made optically thick at the laser wavelength, so that the light that is Rayleigh scattered from the neutral species in the plasma and background scattering are filtered out. As the Thomson signal is frequency broadened by the thermal motion and the ion acoustic coherent motion of the electrons, its linewidth becomes much greater than the absorption linewidth of the rubidium. This way the Thomson signal passes through the Rb filter whereas the undesired Rayleigh and background radiation is blocked. Further details of the Thomson technique and the related apparatus are presented in the following sections. II.THOMSON SCATTERING When a laser beam is passed through a plasma, the Thomson component of the scattered light arises form free electrons. The bound electrons scatter the Rayleigh component whereas the stray light comes from elastic scattering from various surfaces in the system. Thomson and Rayleigh components are Doppler broadened but have very different spectral widths because of the much lower mass of electrons compared with that of atoms. The line shape of the Thomson scattering profile is a complicated function of the parameter α which, in turn, is a function of the scattering angle and the plasma characteristics (electron number density and electron temperature). The parameter α is defined as: α = 1/kλD ≈ λ0 /(4πλD sin (θ/2)) (1) where λD is the Debye’s length in the plasma and k is the magnitude of the difference between the scattered wave vector KS and the incident wave vector KO . Since the Thomson scattering is elastic, KS = KO . If α <<1, the scattering comes from the uncorrelated electron motion and leads to incoherent Thomson scattering whereas for α>1 collective electron motion plays a dominant role and leads to coherent Thomson scattering. The factor governing the spectral profile of the Thomson scattering is referred to as the form factor and is denoted by S(K,ω). The simplified expression for the form factor can be obtained by using the Salpeter’s approximations [4] which assume that for most of the practical cases, the electron mass is much less than that of the ion and the electron temperature and the ion temperature are comparable. The experimental detection of the line shapes provides information on plasma temperature and electron number and ion number densities. It would be worth mentioning that the frequency spectrum of the scattered light is strongly dependent on the scattering angle and the motion of the charged particles in the plasma. As compared to large scattering angles, spectral features at small scattering angles are compressed. In this regime stray light is a major interference that must be rejected and attenuated for accurate measurement of the scattering profile. This rejection can be achieved by employing atomic vapor filters as is explained n the next section. III. ATOMIC VAPOR FILTERS Atomic vapor filters have been widely used to suppress Rayleigh signals and the back ground noise which arises from the elastic scattering from medium particles and surfaces. The initial applications involve the successful measurement of Raman intensities of gases only a few GHz away from the Rayleigh line [5]. Recently atomic filters have also been used in Thomson scattering measurements for the same reason. Bakker et. al. [6] used a sodium notch filter to suppress the stray light intensity in performing 90° Thomson scattering experiments in a low density plasma. In the current work an optically thick Rb filter has been employed in the system. Figure 1: A schematic of the experimental arrangement. IV. EXPERIMENTAL RESULTS Figure 1 describes a schematic diagram of the experimental setup. The arc source plasma was an atmospheric argon MAXI-ARC lamp form NIST. In this water-cooled lamp, the arc constricting section is 6.3 mm long with a 4.0 mm diameter dis k. The design of the lamp ensures stable burning of the argon flow. Pure atmospheric argon was supplied to the arc chamber. From the emission spectrum of the lamp it was found that the emission in the infra-red was significantly lower than in the visible region. Hence, it was advantageous to use the infra-red radiation source for Thomson scattering to reduce noise from the plasma emission background. In contrast to Snyder et. al. [7], Bentley [8] and Bakker et. al [6] who performed 90° Thomson experiments, a backward layout has been adopted in this work. With the restricted optical access of the structure of the argon arc lamp, only the forward or backward scattering experiments were possible. The advantages of backward layout lie in a relatively less stray light radiation and the appropriate α parameter for plasma diagnostics. As mentioned earlier, a narrow linewidth pulsed injection seeded Ti: Sapphire laser was used as the light source. The output of the laser was about 50-60 mj/pulse with a line-width of and about 100 MHz which made it an ideal choice for Thomson scattering. However the optical purity of this laser system was only about 99%. Besides the injection seed component, laser emission also consists of a broad ASE component. The elastic scattering from this ASE was found much stronger than the weak Thomson scattering. An ASE reduction filtered was constructed to overcome this problem. This filter consists of two spatial filters and 20-fold prisms which are used as the dispersive elements. Figure 2 shows both the ASE and the filtered ASE emission spectrum when the laser was tuned to the Rb D2 line. The full width at half maximum of the ASE component is about at 2.0 nm whereas the corresponding value for the filtered ASE is about 0.7 nm which is spectrally narrow enough and can be suppressed by an optically thick Rb filter. signal was reduced dramatically when the laser arc was extinguished. Once the Thomson signal was observed, a scan of its spectrum was obtained. The measurement results have been shown in figure 3 which also included the plasma emission spectrum without the laser and the laser emission spectrum without the plasma. The electron number density and the electron temperature was extracted from the Thomson scattering by matching the experimental line profile with the Figure 2: The ASE and the filtered ASE spectrum of the Ti:Sapphire laser. It is worth mentioning that the Thomson scattering is linearly polarized whereas the plasma radiation is unpolarized. Therefore a Glan-Taylor polarizer was employed to pass the light which was polarized parallel to the electric field of the incident laser. The backward scattered light from the argon arc was spatially filtered and was passed through the Rb filter which absorbed the elastic and Rayleigh scattered light. The Thomson component of the scattered light was focused into a monochromator for spectral analysis where a photomultiplier tube (PMT) detected the required signal. The PMT was time gated synchronously with the laser to suppress the plasma luminosity. The signal was BOXCAR averaged, A/D converted, and was recorded by a computer. Before making any Thomson measurements, the detection system was aligned by capturing the Raman features of the carbon oxide gas. Once the satisfactory Raman signals were obtained, the system was ready to measure the Thomson signal. The monochromator was set to 783.7 nm and the laser was tuned to 780.02 nm. Firstly, when the lamp was off, the laser radiation was blocked to check the zero level. Then the arc was ignited and the emission signal strength was monitored. The laser light was again blocked to check the zero drift, and the time gate of the BOXCAR was checked to make sure that the gate captured all the scattered light from the laser radiation. When the laser was focused into the arc, a big signal due to Thomson scattering was observed. The Figure 3: The plot of the measured Thomson scattering spectrum along with the fitted model. theoretical models. The α parameter was 1.17 and the measurement results gave an electron temperature of 0.82 ± 0.06 eV, and an electron number density of 1.61 × 1016 ± 0.05 × 1016 electrons /cm3 . The errors were estimated by the asymptotic standard errors from the fitting. It must be noted that all the Thomson scattering electron features observed so far were asymmetrical. The blue-wing hump was more than 10% to 30% larger than the red-wing hump. This asymmetry feature has also been observed by Snyder and Bentley [7,8]. An example of such a Thomson scattering line profile is shown in figure 4. Although the integrated scattered light intensity is stronger in the blue wing than I the red wing, they have comparable spectral line-shapes and the same value of electron number density and electron temperature can be extracted from both sides. I n the graph, the two sides are separately fitted. The arc lamp was operated at an operating condition different from that used in figure 3. In this case the electron number density was found 5.48 × 1015 cm-3 whereas the electron temperature was about 2.42 eV. [6]: Bakker L.P., Kroesen G.M.W., Thoms on scattering using an atomic notch filter, Review of scientific instruments, Vol. 71, No 5, May 2000, pp 2007-2014. [7] S.C. Snyder, L.D. Reynolds, J.R. Fincke, G.D. Lassahn, J.D. Grandy, T.E. Repetti, Electron temperature and electron number density profiles in an atmospheric press argon plasma jet. Phys. Rev. E., 50:519, 1994. [8] R.E. Bentley, A departure from local thermodynamic equilibrium within a freely burning arc and asymmetrical Thomson electron features, J. Phys. D: Appl. Phys., 30:2880, 1997. Figure 4: The plot of the measured Thomson signal along with the fitted model. V. SUMMARY An optically thick Rb filter has been successfully used to suppress the elastic and Rayleigh scattering background while measuring a Thomson signal from an argon arc plasma which was operating at an atmospheric pressure. The spectral purity of a narrow linewidth Ti:Sapphire laser was achieved by using an ASE filter. The information on the electron temperature and the electron number density was extracted from the Thomson signal by fitting a model curve to the data VI. REFERENCES [1] D. Batani, S. Alba, P. Lombardi, A. Glassi, Use of Langmuir probes in a weakly ionized, steady state plasma with strong magnetic field, Rev. Sci. Instrum. 68 (11), Nov. 1997. [2] J. Sheffield, Plasma scattering of electromagnetic radiation, Academic Press, New York, 1975. [3] C.J. Barth, M.N.A. Beurskens, C.C Chu, A.J.H. Donne, N.J.L. Cardozo, J. Herranz, H.J.V.D. Meiden, F.J. Pijper, A high resolution multiposition Thomson scattering system for the Rijnhuizen Tokamak Project, Rev. Sci. Instrum. 68 (9), Sep. 1997. [4] E.E. Salpetter, Electron Density Fluctuations in a plasma, Physical Review, Vol. 120, No. 5, Dec. 1960, pp 1528-1535. [5] Tang Z., Zaidi S.H., Miles R.B., Density gradient rubidium dispersive absorption filter for low wave number Raman and Thomson scattering, AIAA 20000644, 38th Aero. Scien. Meeting and Exhib., 10-13 January, 2000, Reno, NV.