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D. Vacher , P. André, G. Faure LAEPT, Clermont University, Clermont-Ferrand, France M. Dudeck

Definition of a new level one test case measurements of equilibrium radiation from an inductively coupled plasma in the near-UV to near-IR spectral region for a Titan-type N 2 -CH 4 mixture. D. Vacher , P. André, G. Faure LAEPT, Clermont University, Clermont-Ferrand, France M. Dudeck

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D. Vacher , P. André, G. Faure LAEPT, Clermont University, Clermont-Ferrand, France M. Dudeck

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  1. Definition of a new level one test case measurements of equilibrium radiation from an inductively coupled plasma in the near-UV to near-IR spectral region for a Titan-type N2-CH4 mixture D. Vacher, P. André, G. Faure LAEPT, Clermont University, Clermont-Ferrand, France M. Dudeck Institut Jean Le Rond d’Alembert, University of Paris 6, France M. Lino da Silva Centro de Física dos Plasmas, Instituto Superior Técnico, Lisboa, Portugal

  2. Contents • 1. Description of the experimental set-up • 2. Discussion on the problems encountered concerning the behaviour of plasma • 3. Spectroscopic acquisitions in the [300-850]nm region • 4. Intensity radial evolution of the C2 swan and CN violet systems • 5. Estimations of temperature(s) • 6. Observation of carbon formation inside the torch • 7. Conclusion

  3. HF generator 64 MHz DEFI Systèmes CCD Plasma gas N2 – CH4 (98%-2%) ST 138 controller Experimental set-up Main characteristics Inductively coupled plasma type : ICP-T64 Frequency : 64 MHz Tuning : Automatic adaptation Inductor : Seven-turn air-cooled coil Operating pressure : Atmospheric pressure Torch : 28 mm internal diameter quartz tube Optical set-up Spatial resolution : 0.5 mm Quartz optical fibre Spectrometer :Chromex 500 IS, 500 mm focal length, Czerny-turner mounting Entrance slit : e = 100 m Gratings : 600/1200/1800 grooves.mm-1 Detector : CCD EEV 1152  1242 pixels

  4. U = 4.57 kV Ia = 460 mA Ig = -042 mA U = 4.09 kV Ia = 724 mA Ig = -017 mA Electrical characteristics of the ICP torch applied current Applied power (kW) Voltage (kV) applied voltage • Encounteredproblem : • (the ignition of plasma is realized with pure N2) • Loss of power injected when CH4 is added • Instability of the plasma • - Random extinction of the plasma Current (mA) P  1kW applied power grid current Time (min) Injection of 2% of CH4 Resolution of the problem : To start the plasma directly with the N2-CH4 mixture CH4 flow is cut : (a filter is placed in front of the video camera) P = 2.1 kW P = 2.96 kW

  5. Why the injection of 2% of CH4 into a pure nitrogen plasma cuts it ? • Is it an electronic problem with the automatic adaptation of impedance ? • Can the physical parameters explain this problem ? • Electrical conductivity • Viscosity • Thermal conductivity

  6. Electrical conductivity Electrical conductivity (S/m) Electrical neutrality : N2 e- N+ Titan  e- C+ for T<6000K  e- N+ for T>6000K Temperature (K) The electrical conductivity may be a influent parameter in order to explain the encountered problem with the plasma of Titan atmosphere (extinction of the plasma)

  7. Viscosity Viscosity (Pa.s) Temperature (K) The viscosity is not a influent parameter in order to explain the encountered problem with the plasma of Titan atmosphere (extinction of the plasma)

  8. Thermal conductivity Thermal conductivity (W/m.K) Possible effect ? Temperature (K)

  9. Comparison of spectra between pure N2 and N2-CH4 plasma

  10. CN C2 Molecular structure Wavelength (nm) Spectrum of a N2(98%)-CH4(2%) plasma : last series of measurements Intensity (a.u.)

  11. The optical acquisitions are made between the 4th and the 5th coil Radial evolution of CN for the N2-CH4 plasma 5 4 3 2 1 0 3 2 1 0 x (nm) x (nm) Radial evolution of C2 for the N2-CH4 plasma x (nm)

  12. Radial profiles for C2 and CN after abel inversion C2 CN Intensity (a.u.) Radial position (nm) Radial position (nm) - No problem appear with the application of the Abel inversion with the CN specie - Difficulty to apply the Abel inversion with the C2 specie. The value on the plasma axis is strongly dependant from the shape of the fit near the 0.

  13. Measured spectrum Simulated spectrum TROT=TVIB=4500K Apparatus function: 0.14 nm Simulated spectrum TROT=TVIB=5500K Simulated spectrum TROT=4500K ; TVIB=5000K First results in the estimation of temperature from C2 spectra (G. Faure - LAEPT) Spectrum issued from the plasma axis Normalized intensity Wavelength (nm)

  14. Measured spectrum Simulated spectrum TROT=TVIB=4500K Simulated spectrum TROT=TVIB=5500K Simulated spectrum TROT=TVIB=6500K First results in the estimation of temperature from CN spectra (G. Faure - LAEPT) Apparatus function: 0.14 nm Normalized intensity Wavelength (nm) • - Big disagreement Theory – experiment (self-absorption not taken into account for instant) • Strange behavior of the first band head (0-0) • Broadening phenomena of the spectral lines (not the case for the C2 lines) • Spectral resolution must be increased

  15. TR = 3200 K TV = 3700 K Best agreement obtained for : First results in the estimation of temperature from the C2 spectra (M. Lino Da Silva-IST)

  16. First results in the estimation of temperature from the CN molecule (M. Lino Da Silva-IST) Method for spectral calculations with self-absorption 1) Application of the Abel inversion 2) Estimation of Tv and Tr, followed by the calculation of emission  and absorption α coefficients utilizing the line-by-line code SPARTAN 3) Calculation of the slab emitted intensity using the relationship: I=/ α×(1-exp(α×l) 4) Convolution with a Gaussian apparatus function simulating the slit

  17. TR = 3200 K TV = 3700 K Best agreement obtained for : First results in the estimation of temperature from the CN molecule (M. Lino Da Silva) Evaluation of the thermal disequilibrium  : 1.1 <  < 1.2

  18. Swirl injection of plasma gas Characteristic of the plasma of Titan Observations about the formation of "carbon " inside the ICP torch injector Top of the quartz tube Carbon hair ! Base of the quartz tube

  19. Analysis of the deposited dust on the quartz tube • Images issued from SEM (Scanning Electron Microscopy) with field effect [CASIMIR] • The X-ray analysis didn’t work because of the lack of sample

  20. Analysis of the deposited dust on the quartz tube Similar images have been found in the thesis titled "Carbon nanoparticles synthesis by gas phase non-equilibrium plasma " [M. Moreno, 2006]  Type of the carbon compounds nano-structured : "ruffled paper " particles with a diameter around 35 nm can be observed

  21. No solid carbon appears in the plasma formed with the Mars atmosphere (presence of O2) whereas it is present in the one formed with the Titan atmosphere (2700 K) Plasma composition Mars atmosphere Titan atmosphere Molar fraction C CN Temperature (K) Temperature (K) (2700 K)

  22. Continuation of this study • To understand why the plasma is instable when 2% of CH4 is added to a pure N2 plasma • To acquire the spectral domain [200 – 300]nm in order to verify if carbon lines are present • To determine axial temperature all along the inductor and above • To realize kinetic calculation to confirm that chemical equilibrium is reached inside the inductor • (as it has be done for the CO2-N2 plasma) • To add argon (1%) in order to estimate the atomic excitation temperature • - An additional diagnostic will be soon added to the experimental set-up : LIF, Laser Interferometry

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