Dynamic measurements of the ion saturation current in the plasma plume by a 	 
double-electrostatic probe system were carried out. Regular signals obtained by the electros-	 
tatic probe show good agreement with the stable plasma flow state. Dependence of the flow 	 
steadiness on the plasma generation parameters was discussed. As a fast response method, the 	 
double-electrostatic probe system is feasible to characterize the fluctuations in the plasma jet

吴承康

潘文霞

郭志颖

黄河激

Institute Of Mechanics,Chinese Academy of Sciences

Monitoring of fluctuations in a direct current argon plasma jet at reduced pressure by double-electrostatic probe  H. Huang, W.X. Pan, Z.Y. Guo, C.K. Wu  Institute of Mechanics, Chinese Academy of Sciences, 100190, Beijing, China  Abstract: Dynamic measurements of the ion saturation current in the plasma plume by a double-electrostatic probe system were carried out. Regular signals obtained by the electros-tatic probe show good agreement with the stable plasma flow state. Dependence of the flow steadiness on the plasma generation parameters was discussed. As a fast response method, the double-electrostatic probe system is feasible to characterize the fluctuations in the plasma jet.  Keywords: double-electrostatic probe, plasma stability, reduced pressure.  1. Introduction The use of thermal plasmas in industry has been conti-nuously expanding in the past several decades. The spe-cial characteristics of thermal plasma have made it a widely accepted processing medium with one of the highest energy densities, which accordingly offers high processing rates, high fluxes of reactants and the potential for compact installations [1]. Direct current (dc) plasma is one of the most accessible thermal plasma sources, owing to its flexibility and cost-efficiency. It has been widely applied in a broad variety of industries [2], among which surface modification and coating is by far the most im-portant application. In these applications, large and ho-mogeneous treatment area is usually preferred. However, the high energy density of thermal plasma leads to strong gradients and to non-uniformities of the process. Also, instabilities are encountered reducing the process yield, and the large number of independent process parameters can make process controlling difficult. Particularly, for the case of dc plasma, because of the intrinsic discharge be-havior, the plasma continuously fluctuates during the process. Such non-stationary features of the dc plasma handicap its application to specific tasks, where precise controllability is essential. To improve the controllability of the process requires a fundamental understanding and control of the plasma instabilities and the conditions in the plasma boundaries. Improved controls were achieved through various means such as controlling the arc-electrodes attachment and modifying the plas-ma/environment boundary conditions. To achieve a large plasma volume with moderate temperature and velocity gradient, attempts such as operating the process in a re-duced pressure chamber were developed. In this research, a dc plasma torch with inter-electrode inserts and an ab-ruptly-expanded anode was developed. With this torch, highly stable plasma with relatively large volume was obtained at reduced pressure of 0.1 – 10 kPa. In this re-search, a double electrostatic probe system was used to dynamically measure the ion saturation current (ISC) in the plasma plume. The obtained signal is closely related to the electrical conductivity and was used to characterize the dynamic energy fluctuations of the plasma jet.  2. Experimental details  Gas inGas inPlasma jetWater-coold Inter-electrode insert Arc rootRA B CDPower Unit Fig.1 Schematic diagram of the experimental system. Fig. 1 is the schematic diagram of the experimental system. Arc goes from the tip of the cathode, through the long channel of the inter-electrode insert and reaches the upper surface of an abruptly expanded anode. The size of the channel varies from 7-12 mm while that of the anode ranges from 50-80 mm in diameter. The inter-electrode insert has a floating potential without grounding. All the electrodes are water-cooled. A rectified power supply unit was used to generate plasma. A double-electrostatic probe system was adopted to monitor the ion saturation current in the plasma plume near the torch exit. Four positions were monitored in the present study. Positions A, B and C are along the torch axis, where A is at the torch exit, B is 15 mm and C is 30 mm off the torch exit. Position D is 20 mm away from position A radially. The double-electrostatic probe system consists of a 15 V bias voltage supply, a 500 Ω resistance and two molybdenum electrodes 0.3 mm in diameter. An oscilloscope with na-no-second time resolution was used to monitor the dy-namic voltage change on the ends of the resistance. Pre-vious research showed that under the 15 V bias voltages, the obtained signal represents the ion saturation current in the plasma plume under typical studied conditions. Another oscilloscope was used to monitor the arc voltage and current spectra simultaneously. Argon was used as the plasma gas and the gas flow rates varied from 8.4 to 21 slm. The chamber pressure was controlled to between 500 – 10 kPa and the arc current between 80 – 100 A.  3. Results and Discussion 0 5 10 15 20 25202224262830Ion saturation current (mA)Time (s)Arc current: 100A 90A 80A(a) 0 5 10 15 20020406080100Arc voltage (V)Time (ms)Vmean=70 V       ΔV=11.2 V(b) Fig. 2 (a) Dependence of ion saturation current at posi-tion A on the arc current; (b) Corresponding arc voltage spectrum. Chamber pressure is 0.5 kPa and the total gas flow rate of 16.8 slm.  Fig. 2 (a) shows the dependence of the measured ion saturation current at position A on the arc current. Time dependent ion saturation current signals were recorded by the oscilloscope with a resolution of 1×10-5 s. Previous research showed that the rectified PSU used in the present study has a 300 Hz intrinsic fluctuation [3]. Such fluctua-tion was also detected by the probe system, which indi-cates that the fluctuation in the arc was transmitted to the downstream plasma jet. Besides the 300 Hz intrinsic fluctuation, the ion saturation current signal is rather smooth. This means that there are no significant fluctua-tions caused by other factors in the plasma jet, while the chamber pressure being 0.5 kPa and the total argon gas flow rate being 16.8 slm. The average value of the ion saturation current does not change much with the in-creasing of the arc current. However, the fluctuation be-comes smaller at elevated arc current. This indicates that the plasma jet is more stable at higher arc current of 100 A than those under lower arc current of 80 and 90 A. In pre-vious research, similar dependence was observed when dc plasma was generated under atmospheric pressure using the same type of plasma torch [4]. Higher arc current was found to promote the laminar flow regime in the plasma jet.  -0.010 -0.005 0.000 0.005 0.0100102030 Time (s)0.5 kPa0102030Ion saturation current (mA)5 kPa0102030 at position A at position B at position C10 kPa(a)0 5 10 15 20 25 300102030 0.5 kPa 5 kPa 10 kPaProbability (%)Ion saturation current (mA)(b) Fig.3 Effect of chamber pressure on the measured axial ion saturation current signals. (a): ion saturation current on position A, B and C for different chamber pressures; (b) Distribution of ion saturation current for position A at different chamber pressures.  Although the 300 Hz intrinsic fluctuation is much pro-nounced in the arc voltage spectrum (Fig. 2 (b)), the in-tensity of it is depressed at the torch exit. At a chamber pressure of 0.5 kPa, the detected ion saturation current is quite smooth, as shown in Fig. 2 (a). The largest fluctua-tion is less than 2%, which is much smaller than the cor-responding arc voltage fluctuation of 16% as shown in Fig. 2 (b).   0 20 40 60 808090100110Intensity (a.u.)Time (ms)(f) Fig.4 Time dependence stability of the plasma jet gen-erated at 80 A, 10 kPa. (a) – (e): high speed video cam-era photos of the plasma jets with time intervals of 2 ms and exposure time of 1 μs; (f): Variation of plasma jet intensities with time.  With the increasing of the chamber pressure, the dy-namic stability of the plasma jet becomes worse. Fig. 3 compares the ion saturation current signals at different chamber pressures of 0.5, 5 and 10 kPa. The arc current is kept at 80 A and the total gas flow rate is 16.8 slm. From Fig. 3 (a), it can be seen clearly that the detected ion sa-turation current signal becomes more disordered at higher chamber pressures. Larger perturbations with higher fre-quency superimpose on the 300 Hz intrinsic fluctuations when the chamber pressure is increased to 10 kPa. At the same chamber pressure, from position A to C, the ion sa-turation current becomes slightly smaller with pronounced fluctuations. Fig. 3 (b) shows ion saturation current dis-tributions for position A at different chamber pressures. It is seen that with the increasing of chamber pressure, wid-er distribution of ion saturation current is observable, to-gether with a lower average ion saturation current level. This result shows that at elevated pressure, the electric conductivity of the plasma decreases due to more inten-sive collisions between electrons and ions occur.  Although the detected dynamic ion saturation current signals shows unsteadiness of the plasma jet, the flow field of the plasma is still quite stable when the chamber pressure is up to 10 kPa. Fig. 4 (a) shows the photo of the obtained plasma jet taken by a high speed video camera. The camera speed was set to be 2500 fps and the exposure time to be 1×10-6 s. Previous research shows that when the plasma jet is in a turbulent state, twisted plume can be clearly seen when the exposure time is as short as 1×10-5 s [5]. However, in the present study, even when the expo-sure time is 10 times shorter, no twist and distortion was observed, showing that the plasma jet is still in laminar flow state. However, there are observable changes in brightness and shape of the plasma jet if a series of im-ages were compared. Fig. 4 (b) to (e) shows a conse-quence series of photos with intervals of 5 frames (2 ms in time). A periodical change of the width and brightness of the plasma jet can be seen. By analyzing 200 images taken in 80 ms, the periodicity of the plasma jet intensity change is confirmed to be 300 Hz, which corresponds to the arc voltage fluctuations. -0.010 -0.005 0.000 0.005 0.0101020Time (ms)8.4slm1020Ion saturation current (mA)12.6slm102016.8slm102021slm Fig. 5 Dependence of ion saturation current on gas flow rate at position A with chamber pressure of 10 kPa and arc current of 80 A.  Fig. 5 shows the effect of gas flow rate on the ion satu-ration current signals. In Fig. 5, the chamber pressure is 10 kPa, and the total gas flow rate of argon changes from 8.4 to 21 slm. It can be seen that when the gas flow rate is small in the case of 8.4 and 12.6 slm, the superimposition of higher frequency perturbations on the intrinsic 300 Hz fluctuation is clear. However, when the total gas flow rate is 21 slm, the ion saturation current signal is totally dis-ordered, and no obvious periodicity can be seen. When the gas flow rate is 16.8 slm, a transition of the two modes is observed. 0 5 10 15 20 250102030Ion saturation current (mA)Time (ms) position A position D Fig. 6 Radial distributions of ion saturation current in the plasma jet.  The ion saturation current decreases rapidly in radial direction. Fig. 6 shows that when the probes moves from position A to position D, the detected ion saturation cur-rent decreases from 24 mA to 4 mA. The fluctuation also becomes more remarkable at position D. Compared with Fig. 4, it can be deduced that the large fluctuation on the edge is due to the displacement of the plasma jet. As Fig. 4 shows, the width of the plasma jet changes a lot with time, so when the probes are placed on the edge, the fluc-tuation of ion saturation current is larger than 100%, while in the center it is less than 2%. Above results show that ion saturation current signal is very sensitive to plasma condition changes and is feasible to be used to characterize the steadiness of the plasma jet. It should be noted that, in most cases of thermal plasma processing, the fluctuation of electric conductivity is not the most crucial parameter that affect the process quality, especially when the plasma is only used as a heat source, where temperature fluctuations dominates. However, in processes where active reactants are important such as in thermal plasma chemical vapor depositions, the change of ion saturation current may play an important role in im-proving the film/coating quality.  4. Conclusion Using a double-electrostatic probe system, time and space dependence of the steadiness of an argon plasma was characterized. Dynamic measurement of the ion sa-turation current is a fast response method to reveal the energy fluctuations in a plasma jet generated at reduced pressure of 0.5 – 10 kPa. With the increasing of chamber pressure and total gas flow rate, the dynamic stability of the plasma becomes worse with pronounced higher fre-quency perturbations in ion saturation current. Such fluc-tuation is mainly caused by the displacement of the plas-ma jet.  5. Acknowledgement This work is supported by the National Natural Science Foundation of China under Grant Nos. 50836007 and 10621202.  References [1] Heberlein J 2002 Pure and Applied Chemistry. 74 327 [2] Fauchais P, Montavon G, Vardelle M and Cedelle J 2006 Surface & Coatings Technology. 201 1908 [3] Pan W X, Meng X and Wu C K 2006 Plasma Science & Technology. 8 416 [4] Pan W X, Meng X, Chen X and Wu C K 2006 Plasma Chemistry and Plasma Processing. 26 335 [5] Huang H, Pan W, Guo Z and Wu C 2008 Ieee Transactions on Plasma Science. 36 1052   

Fluctuations in a direct current argon plasma jet at reduced pressure

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Fluctuations in a direct current argon plasma jet at reduced pressure

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Institute Of Mechanics,Chinese Academy of Sciences