WDS'14 Proceedings of Contributed Papers Physics, 242 248, 2014. ISBN 978-80-7378-276-4 MATFYZPRESS Electric Probe Diagnostic of Supersonic Thermal Plasma Jet O. Hurba 1,2 and M. Hrabovský 2 1 Charles University Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. 2 Institute of Plasma Physics AS CR, v.v.i., Praha, Czech Republic. Abstract. This paper describes how electric probes can be used as relatively simple and efficient tool for investigation of structure of flow field in thermal plasma jet at the low pressure. A boundary of plasma jet between nozzle of plasma torch and ambient air was investigated by means of array of moving electric probes. Were defined boundaries of the conducting region and their change with pressure change. Introduction Recent years have seen increase rates of technology development at the heart of its containing thermal plasma dc arc jets at low pressures such as plasma spraying, deposition of thin films or plasma synthesis. These technologies are widely used in aeronautics and medical industries among others. [Fauchais et al., 2001] The rapid development of industry gives jolt to more detailed research in this area. Most of the studies were performed with relatively low power of the jets and for pressures near 1 bar [Meulenbroeks et al.,1994, 1995; Mazoufre et al., 2002]. Less attention has been paid to the jets in pressure range between 10 mbar and 0.2 bar. In this pressure range a transfer of flow regime takes place from the subsonic turbulent jet controlled dominantly by an entrainment of cold ambient gas to the supersonic jet. Plasma processes inside the jet are most important. Effect of chamber pressure on the structure of the jet flow and deviation from local thermodynamic equilibrium (LTE) in dc arc plasma jet was studied in [Selezneva et al., 2001] for pressure range of 0.06 to 0.39 bar. A diagnostic extensively used for study plasma jets is optical emission spectroscopy (OES), which is non-intrusive and gives information about the plasma species. However, determination of the temperature, for example, which is obtained using the Boltzmann plot method, relies on the assumption of LTE, which is no longer satisfied at low working pressures. The result of the deviation from LTE is that the heavy particle, electron and excitation temperatures are different. Therefore, different interpretations of OES measurements in plasma jets at reduced pressure have been developed and are mainly used to evaluate the deviation from the LTE as a function of the working pressure [Sember et al., 2002; Selezneva et al., 2002]. Another experimental diagnostic tool, the enthalpy probe system, has been used in compressible supersonic plasma jets but was limited to a pressure range down to 0.2 bar [Jodoin et al., 2002]. In parallel to the experimental research, numerous simulations have been developed, in particular, two temperature models for low pressure plasma jets, but they lack experimental validation [Han et al., 2001]. On the other hand, plasma jets expanding at pressures below1 mbar have been extensively investigated both numerically and experimentally [Burm et al., 2001]. In this paper we investigate low-pressure (0.01 0.1 bar) dc arc plasma jets generated in water/argon-stabilized arc [Březina et al., 2001; Hrabovský et al., 2002]. The experiments descripted in this article were conducted using electric (statical) probe. The application of electric probe in thermal plasmas as classical Langmuir probe is complicated by the interaction of probe with plasma and by variety of mechanisms influencing probe current. Nonetheless, various impotent information can be obtained from electric probe measurement. The probe was applied for measurement of plasma potential. [Hrabovský et al., 2005] This paper presents the description the application of moving probe for study of the structure of thermal plasma jets generated by arc plasma torch at the low pressure. Experimental setup The experiments were carried out with plasma torch WSP H 500. The plasma torch configuration is shown in Figure 1. The cathode part of the torch is arranged similarly like in gas torches. Gas is supplied along the cathode; vortex component of the gas flow assures proper stabilization of arc in the cathode nozzle. Plasma flows through the nozzle into the second part of the torch where the arc 242
column is surrounded by the water vortex. The vortex is formed in three cylindrical segments with a tangential water injection in the same way like in water-stabilized torches. The segments are separated by two exhaust gaps; water is exhausted out of the arc chamber. The interaction of the arc column with the water vortex causes evaporation from the inner surface of the vortex. The steam is mixed with the plasma flowing from the cathode section, the overpressure produced in the arc chamber due to evaporation accelerates the plasma created from the mixture of steam and gas towards the exit nozzle. [Hrabovský et al., 2004; Kovitya et al., 1979] The anode of the torch is realized by a rotating cooper disk that is located out of the arc chamber about 4 mm downstream of the exit nozzle. Both the cathode and anode have internal water cooling. The diameter of the exit nozzle was 5.7 mm. Vacuum chamber and two vacuum pumps allow taking measurements in the different pressure from 1 bar to 3 mbar. The measurements were performed for arc current 200 A, argon flow rate 12.5 slm, steam flow rate in plasma jet was determined by evaporation rate from stabilizing water vortex and was 0.2 g/s. Figure 1. Schematic of the plasma torch. 1 3 2 Figure 2. Probe-moving system (1 holder of the electric probe, 2 cylindrical spiral spring, 3 synchronization module angle position sender). 243
Figure 3. Schematics of the experimental arrangement. In the experiments we used platinum probe 1 mm diameter. The probe was fixed to the probemoving system (Figure 2). In this system cylindrical spiral spring is moving the probe like pendulum in vacuum chamber and gives the possibility to move the electric probe through the thermal plasma jet with a velocity 1.5 m/s. The length of the measuring tip was 3 mm. The probe movement was monitored by means of the angle position resistor sender and recorded on the oscilloscope together with the probe signal. Probes were positively biased by connecting to the anode via resistor R L (Figure 3). Due to the potential drop on the anode attachment the plasma in free jet downstream the attachment has negative potential against the anode that is determined by the value of anode attachment voltage drop (5 15 V depending on the jet conditions). Thus, for probe connected to the anode, electron currents flow to the probe. The probe potential and current are determined by plasma potential and resistance R L. Measurements were performed for various values of resistor R L. When using resistors with high resistance of several MΩ the potential of the probe corresponded to the floating potential and was approximately equal to plasma potential [Hrabovský et al., 2005] Series of experiments was made with different axial distances from plasma torch nozzle exit (x = 30, 40, 60, 80 and 100 mm). Experimental results Probe potential was recorded during passage of the probe across the jet. For each point of the investigated area, we conducted three experiments under identical conditions. The signal of the probe was processed using Matlab code. From probe signals we determined dependence of probe potential on radial position within the jet. This dependence, determined from three successive measurements at the same conditions, is in Figure 4 (There and further on figures the zero on r-axis corresponds to the axis of the torch nozzle center and conducting jet is shifted out from this axis on anode rotation side.). Strongly fluctuating probe signals were obtained when plasma disturbances and inhomogeneity s flowed along the probe (Figure 4 left). The averaged signals are shown in Figure 4 right. It can be seen that although the probe signals are highly fluctuating, the averaged signals are relatively well reproducible and the region of non-zero probe currents can be determined. Effect of pressure and resistance R L on probe currents is illustrated in Figures 5 and 6. Probe currents at axial position x = 40 mm for several values of resistor R L and pressure are shown here in dependence on radial position within the jet. It can be seen that the radial positions corresponding to conducting plasma area are well defined independently on biasing resistance and thus on probe current and potential. The conducting region is much broader for pressure 9 mbar than for pressure 80 mbar due to higher jet expansion at lower pressure. Relatively high electron currents up to 5 ma flow to the probe in wide region with the diameter 80 mm in case of pressure 9 mbar, although initial diameter of plasma jet at the torch nozzle exit is 5.7 mm. This is the consequence of jet expansion at reduced pressures. 244
Figure 4. Probe potential as a function of radial position (original and average signals). Figure 5. Probe currents for 80 mbar with using different resistor R L on the 40 mm from the nozzle of plasma torch. Figure 6. Probe currents for 9 mbar with using different resistor R L on the 40 mm from the nozzle of plasma torch. Summarize information received from the four series of experiments, we can create 3D maps of the saturation current the study area. 245
Figure 7. Triaxial structure of the flow field of plasma jet at the pressure P = 30 mbar. Figure 8. Triaxial structure of the flow field of plasma jet at the pressure P = 40 mbar. Figure 9. Triaxial structure of the flow field of plasma jet at the pressure P = 80 mbar. Figures 7, 8, and 9 show the spatial distribution of the probe currents for lowest resistance 4.7 kω and pressures 30, 40 and 80 mbar. The red color corresponds to the highest probe current (tens of ma) and the dark blue represents zero probe current. It can be seen how the dimensions of conducting region around plasma jet change with pressure changes and with the distance from the torch nozzle 246
exit. The region of existence of plasma with relatively high conductivity around the jet can be up to tens of mm in diameter and extents up to several centimeters from the torch exit. Summary and conclusion Figure 10 gives dependence of diameter of conducting area on pressure, derived from measurements at various axial positions. Figure 11 presents dependence of diameter conductive region of the jet on axial position for the different pressures. It is interesting that with increasing pressure of 9 mbar to 30 mbar the diameter of the conducting region is reduced more strongly than a further increase in pressure to 80 mbar. Following conclusions can be derived from the measurements: with increasing pressure the diameter of conducting zone decreases; in cases where the pressure is less than 30 mbar the diameter of the conducting area increases with distance from the plasma torch from 30 mm to 80 mm; at the pressures P = 30, 40 and 80 mbar the diameter of the conducting zone with distance from the plasma torch increases only up to a maximum, then there is a gradual decrease in the diameter of the conducting region; with increasing pressure from 30 to 80 mbar a maximum possible value of the diameter is closer to the nozzle of the plasma torch. Figure 10. The diameters of conduction area under different pressure on 30, 40, 60 and 80 mm from the plasma torch. Figure 11. Dependence of diameter conductive region of the jet from the axial position 247
In this paper we described electric probe investigation of the free plasma jet in positions downstream of the anode region of DC arc plasma torch. 3D maps of probe currents were created for plasma jet area between 30 and 80 mm from the plasma torch nozzle for 30, 40 and 80 mbar. We have defined the boundaries of the conducting plasma region and we determined how it varies with pressure. The results presented in this paper explain the behavior of the supersonic plasma jet at low pressure and can be used to quantify the deviation from LTE. The mapping of the measured physical properties of the jet can also serve as input for modeling. In the future we are preparing a series of experiments in close proximity to the anode. Results expected in the study of this area will be used for analysis of the anode attachment mechanisms. References Březina, V., M. Hrabovský, M. Konrád, V. Kopecký and V. Sember, New plasma spraying torch with combined gas-liquid stabilization of arc, Proc. of 15th Int. Symp. on Plasma Chemistry (ed. A. Bouchoule et al.), Vol. III, 9 13 July, Orleans, 1021 1026, 2001. Burm K T A L, W. J. Goedheer and D. C. Schram, Plasma expansion in the preshock region, J. Appl. Phys. 90 2162, 2001. Fauchais, P., A. Vardelle and B. Dussoubs, Quo vadis thermal spraying, J. Thermal Spray Technology, 10 1683, 2001. Han P. and X. Chen, Modeling of the supersonic argon plasma jet at low gas pressure environment, Thin Solid Films, 390 181, 2001. Hrabovský, M. and V. Kopecký, Visualization of Structure of boundary layer between thermal plasma jet and ambient air by moving electric probes, IEEE Trans. on Plasma Science, vol. 33, no. 2, 2005. Hrabovský, M., Generation of thermal plasma jets in liquid-stabilized and hybrid gas-liquid plasma torches, Power Beams&Materials Processing, PBAMP (ed. A. K. Das, A. V. Bapat, A. K. Sinha), Allied Publ. Priv. Ltd., 29 39, Mumbai, 2002. Hrabovský, M., V. Kopecký, V. Sember, T. Kavka, O. Chumak, Properties of hybrid water/gas dc arc plasma torch, IEEE Trans. on Plasma Science, TPS0333, 2004. Jodoin B., M. Gindrat, J. L. Dorier, C. Hollenstein, M. Loch and G. Barbezat, Modelling and diagnostics of a supersonic DC plasma jet expanding at low pressure, Proc. ITSC (International Thermal Spray Conference) pp 716 720, 2002. Kovitya, P., J. J. Lowke, A. D. Stokes, Theory of arc clogging in nozzles, Symposium on High Voltage Switching Equipment, Sydney, N.S.W, 1979. Mazoufre, S., V. Lago, M. Lino da Silva, M. Dudeck, Plasma formation during high speeed flights in upper layers of the earth atmosphere, AIAA/AAAF-2002-5272, 11th Int. Conf. Space Plane and Hypersonic Systems and Technologies, Orleans, 2002. Meulenbroeks, R. F. G., M. F. M. Steenbakkers, Z. Quing, M. C. M. van den Sanden, D. C. Schram, Four ways to determine the electron density in low-temperature plasma, Phys. Rev. E 49, 2272, 1994. Meulenbroeks, R. F. G., R. A. H Engeln, C. Box, I. De Bari, M. C. M. van den Sanden, J. A. M. van der Mullen, D. C. Schram, Influence of molecular processes on the hydrogen atomic system in an expanding argonhydrogen plasma, Phys. Plasmas 2, 1002, 1995. Selezneva, E., M. Rajabian, D. Gravelle, M. I. Boulos, Study of the structure and deviation from equilibrium in direct current supersonic plasma jets, J. Phys. D: Appl. Phys. 34, 2862 2874, 2001. Selezneva, S. E., V. Sember, D. V. Gravelle, M. I. Boulos, Spectroscopic validation of the supersonic plasma jet model, J. Phys. D: Appl. Phys. 35, 1338 1349, 2002. Sember, V., D. V. Gravelle and M. I. Boulos, Spectroscopic study of a supersonic plasma jet generated by an ICP torch with a convergent-divergent nozzle, J. Phys. D: Appl. Phys. 35, 1350 1361, 2002. 248