Role of confinement in the development of a helium impacting plasma jet at atmospheric pressure

Transcription

1 Role of confinement in the development of a helium impacting plasma jet at atmospheric pressure T. Gaudy 1,2, J. Iacono 1, A. Toutant 1, P. Descamps 2, P Leempoel 2, F. Massines 1 1 : Laboratoire PROcédés, Matériaux et Energie Solaire (PROMES) CNRS, Tecnosud, Rambla de la thermodynamique, PERPIGNAN, France 2 : DOW CORNING EUROPE, SA Parc Industriel - Zone C, Rue Jules Bordet 7180 Seneffe, Belgium Abstract: The aim of this work is to study low temperature point to plane discharges development in an atmospheric pressure helium plasma jet impacting a surface and confined in a 16 mm tube. Two discharge regime directly connected to the gas flow dynamic have been pointed out. In most of plasma jets, the flow dynamic influence the discharge power, the length of the jet, or interaction with air. But in the PlasmaStream configuration, not only power and length are affected, but the flow dynamic lead to the existence of two different discharge modes, influencing quality of deposited films. These two modes have been defined, and studied by short exposure time pictures, current and voltage measurement, and flow dynamic modeling with FLUENT. It has been confirmed that the two discharge modes are linked to two jets flow modes. One is laminar and allows air entrance along the tube wall. The other avoids air entrance in the tube and can be turbulent. 1 : INTRODUCTION: Today, numerous plasma jets are studied as plasma source for surface treatment (1). Most of them flow in the air before impacting the surface. The peculiarity of the studied configuration is that two impacting jets are confined in a single tube of 16 mm diameter. This leads to some interactions between the two jets, and between each jet and the confinement tube. It also prevents the discharge to interact with surrounding air before reaching the surface. The electrodes configuration is point to plane with a thick (12 mm) dielectric on the plane. Two very different visual aspects of the discharge have been observed during deposition experiments, which will be called mode 1 and mode 2 in this paper. This study is dedicated to the understanding of these two modes and the parameters controlling the transition from one mode to the other. At the first stage, visual observation is considered to be enough to study the influence of the experimental parameters on the discharge mode switching. Then short exposure time pictures have been taken to characterize each discharge mode and understand the differences between them. Simultaneously, current and voltage measurements allow highlighting the specificities of the two discharge modes. Finally, flow dynamic modeling with FLUENT has been carried out to try to confirm the link between the discharge appearance and the gas flow modes. 2 : EXPERIMENTAL SETUP: All experiments are carried out with PlasmaStream. Figure 1 - Configuration of the plasma head The PlasmaStream belongs to the family of atmospheric pressure plasma jets, excited by a low frequency high voltage source in the range of 20 khz. The particularities of this set-up are: - The electrodes are needles, while in traditional jets the electrodes are concentric and set in a parallel or perpendicular field arrangement. Needles possesses the unique benefit of (i) creating a gas breakdown using a lower voltage source because of the enhanced

2 Gap (mm) Gap (mm) Transition from mode 1 to mode 2 (75 to 55mm) 3 2,5 2 1,5 1 0, mm 55 mm possible to change the regime by adjusting the flow rate. The space between the two modes is hysteresis like when we switch from mode 1 to mode 2. He flow (L/min) Figure 4 Gap associated to the transition between mode 1 and mode 2 as a function of the gas flow for two tube lengths electric field at the sharp extremity of the needles (ii) giving the possibility of limiting the discharge extinction to the surface because of the drastic field decrease with the distance to the point. - 2 separate plasma jets of 1.6 mm diameter open to a confined space having a diameter 10 times larger. - The jets do not flow out in the ambient air but are confined in a dielectric plasma tube, whose length can vary between 35 and 90 mm. - The plasma tube is located above the substrate that placed on a grounded metallic electrode covered by a dielectric. The space between the bottom of the confinement tube and the substrate (the gap) is adjustable from 0 to several centimeters. In standard conditions, the tube is 75 mm in length with and a gap between 0.5 mm to 3 mm from the grounded electrode. The helium flow is controlled by a mass flow ranging from 0 to 15 slpm. 3 : RESULTS: 3 a) Experimental observations : As shown figure 2 and 3, two strongly different discharge modes are observed, named mode 1 and mode 2 in this paper. In the mode 1, the discharge fills all the tube in an almost homogenous manner. In the mode 2, discharge is localized in two jets, with few micro discharges around. The color is also different: white for mode 1 and pink for mode 2. The most influencing parameters of the discharge mode are the gap, the total flow rate and the tube characteristics. Figure 4 shows the influence of the total flow rate in the confinement tube and the gap. Mode 2 occurs for larger gap. Whatever the flow rate is, the mode can be changed by adjusting the gap, and whatever the gap is, it is almost always 2,4 2,2 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 Figure 2 - Standard pictures of: a) Mode 1 b) Mode 2 Transition from 1 to 2 appears for higher values of gap than transition from 2 to 1. Some seconds are necessary to stabilize mode 2. This observation tends to show that mode 1 is more stable. Speed (m/s) Flow (L/min) Figure 3 - gap associated to the transition from one mode to the other as a function of the gas flow. Red curve: when the gap is increased, black curve when the gap is decreased Some experiments have been done with different length and diameter of confinement tube. With a shorter confinement tube (55 or 35 mm), mode 1 is more difficult to be obtained because gap has to be smaller, whereas with a thinner tube (8 mm diameter) it's more easy. But in all of the cases, there are always two discharge and flow modes that can be obtained by changing gap or flow. 3 b) Short exposure time pictures To try to understand the behavior of these two modes and the origin of the transition, 500 ns exposure time pictures synchronized with the

3 voltage have been taken over one period for the two discharge modes. Pictures are the results of 70 accumulations. The absolute value of the light intensity is changed from one picture to another using the camera software auto-contrast function. So it is not possible to compare light intensity just comparing the pictures. But comparison of the spatial distribution of the energy injected in the two discharge modes is possible. In mode 1 the discharge close to the point is intense, then there is a rather dark area followed by a light uniformly distributed along the tube radius and again an intense discharge between the tube bottom and the plane surface which extend far away from the tube. In mode 2, two filaments are visible from the point to the surface. They follow the gas jets and the light between the tube bottom and the surface is limited. In both the cases, there is always light at the sharp extremity of the needles (top of the confinement tube). An other point that differentiate the two regimes is the breakdown mechanism when the electrode points are positive. In mode 2 and in that mode only, plasma bullets propagate towards the surface (Figure 7). This phenomenon appears before the main positive breakdown. Plasma bullet initiates the main discharge which begins when the "plasma bullet"(2) reaches the surface. It's very specific to the mode 2 as we never observe this phenomenon in mode 1 Figure 5 500ns exposure time picture of the "plasma bullet" independent of the flow rate or the gap values. So with these short exposure time pictures we have seen that the discharge is somewhat different when the flow mode changes. Energy localistion is always more homogeneous in mode 1, and some discharge mecanisms exist only in mode 2. Simultaneously with these experiments voltage and current have been acquired of the discharge. 3 c) Current and voltage analysis : Figure 8 presents the 2 modes current for the same voltage. Current is the superposition of capacitive and discharge current. The dielectric surface charged by the discharge and thus the capacity in series with the discharge, depends on the discharge mode and development. Therefore it is not possible to remove the capacitive component from the current measurement. The voltage shape is not fully sinusoidal as an oscillation is superrimposed to the negative part.the consequence is that the capacitive current becomes positive during the negative half cycle of the voltage. The discharge current being in phase with the voltage it decreases the total current in the zone where the capacitive current polarity is opposite to that of the voltage. So according to figure 8, during a cycle of the excitation, there is one positive and two negative breakdowns. Short exposure time pictures confirm this interpretation. The positive breakdown starts 5µs before origin on the figure 6. The two other breakdowns visible between 15 and 50 µs are negative breakdowns. Mode 1 (black) and mode 2 (red) currents are very similar in their general shape. The number of breakdown and order of magnitude of the total current are the same. A more careful comparison with the pictures allows identifying differences between the 2 modes electrical characteristic. "Plasma bullet" is associated to the small shift at 0 µs. The current amplitude in mode 1 is higher than in mode 2. This can be due to the capacitive part of the current that is more important in mode 1, charged dielectric surface being bigger in this mode. Overall, the voltage current characteristics of mode 1 and mode 2 are similar. But it is possible to identify which mode is active with the little current peak created by the "plasma bullet" propagation before the main positive breakdown. This peak is always visible in mode 2 and never appears in mode 1. Parameters influencing the transition from mode 1 to mode 2 depend strongly on the gas flow. To verify this hypothesis, that the modification of the gas flow is at the origin of the differences between the two discharge modes, flow dynamic modeling with FLUENT software has been done.

4 3 d) FLUENT modeling : The experimental configuration requires a 3D modeling. However, as the discharge regime is controlled by the total flow rate, it is assumed that 2D ax symmetric simulation can give helpful results to understand the reason of the discharge transition. Therefore, we chose to simulate a "virtual" configuration, purely ax symmetric, with only one helium injection at the center of a smaller diameter confinement tube (8 mm diameter). Dimensions have been studied to keep the flow dynamic as close as possible from the original geometry. To validate the modeled configuration, a real plasma head with the new geometry has been manufactured. Reynolds number in the injection channels is between 2000 and 3500, so the transition between laminar and turbulent is possible. The main results given by FLUENT modeling is that in laminar conditions, air enters inside the confinement tube (figure 7). The breakdown voltage of air is 30 times higher than that of He. So air introduction along the tube inner wall confined the discharge in a smaller volume. The other consequence is a higher ionization level related to air N2 Penning ionization by He metastable. This is confirmed by the N2+ emission increase compare to N2 (figure 9) (3). That can explain the color change in the discharge with the diffusion of nitrogen in the plasma zone at the bottom of the confinement tube, were the model's streamlines show that outside gas enter, just over the helium outflow. Air entry Figure 7 - FLUENT in laminar conditions with air entry at the bottom of the confinement tube Figure 8 - FLUENT in turbulent conditions without air entry Figure 6 Current and voltage oscillograms for mode 1 and 2 Figure 9 - OES in mode 1 and 2 showing evolution of the ration N2 / N2+

5 4 : CONCLUSION: In the PlasmaStream configuration, the flow mode (laminar or turbulent) has a very strong effect on the discharge mechanisms, in particular on the spatial behavior of the discharge. Turbulent mode being more homogeneous, almost like a glow discharge in some conditions, helps to control the energy in the confinement tube, mixing the gas and so avoiding coexistence of low energy and very high energy zones. It also prevent from nitrogen diffusion in the plasma zone, so we can more easily control the energy of the plasma in this mode. References: 1. Gas flow dependence of ground state atomic oxygen in plasma needle. Yukinori Sakiyama, Nikolas Knake, Daniel Schröder, Jörg Winter , 2010, APPLIED PHYSICS LETTERS Finite element analysis of ring-shaped emission profile in plasma bullet. Yukinori Sakiyama, David B. Graves, Julien Jarrige, and Mounir Laroussi , 2010, APPLIED PHYSICS LETTERS Neutral gas flow and ring-shaped emission profile in nonthermal RF-excited plasma needle. Graves, Yukinori Sakiyama and David B , 2009, Plasma Sources Sci. Technol., Vol. 18.