Study of the floating potential in a glow discharge plasma using Langmuir Probe

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1 Journal Homepage: Study of the floating potential in a glow discharge plasma using Langmuir Probe Sudeshna Lahiri, Physics, Dinabandhu Mahavidyalaya, WB, India Rena Majumder 1, Physics, Bhairab Ganguly College, India Dola Roy Chowdhury, Physics, Techno India, Kolkata, India Ranjan Kr. Saha, Physics, Mahadevananda Mahavidyalaya, WB, India M.S. Janaki, Physics, Saha Institute of Nuclear Physics, India A.N.S. Iyengar, Physics, Saha Institute of Nuclear Physics, India Article Record: Received Sept , Revised paper received Nov , Final Acceptance Dec Available Online December Abstract In this experiment, we have measured plasma floating potential using a Langmuir probe. Here we have used DC glow discharge argon plasma. The whole experiment was carried out in different conditions such as with and without presence of external inhomogeneous magnetic field, with different gas filling pressure, etc. Experimental results show that there is a variation in DC floating potential with the variation in gas filling pressure (0.055mbar to 0.210mbar) and with external magnetic field (390 Gauss to 10 Gauss). When the Langmuir probe is not biased, a potential is created in the probe due to fluctuations in the plasma. We have analysed these fluctuations by nonlinear time series analysis where we have plotted raw signals, phase diagram etc. From power spectrum plots at pressure 0.11 mbar, a series of dominant frequencies has been observed and those peaks are shifted towards lower frequency side and corresponding power also decreases as the magnetic field is decreased. Hurst exponent shows an increasing trend at pressure 0.11 mbar and this indicates presence of long range correlation in system dynamics. Keywords: Plasma, floating potential, Langmuir probe, Hurst exponent. 1. Introduction In a glow discharge plasma, a large voltage ( 100 V) accelerates free electrons to gain sufficient speed to cause ionization on collision with neutral gas particles. The gas in the tube is usually at low pressure (~ mbar), so collisions are not so frequent that the electrons fail to reach the speed required for ionization. Langmuir probe is one of the most important plasma diagnostic tools used in plasma laboratories. One of its use is to measure plasma floating potential. If a probe or wire is placed in a plasma, it will collect more electrons than ions (as electrons move faster than ions) and ultimately get negatively charged. Due to extra collection of negative charges, electron current will decrease and ion current will increase. Thus a balance condition will arise where the resulting potential is called floating potential. At this time the probe will not draw any current. Floating potential measurement is useful to understand relative trends in the acceleration process of electrons and ions in the plasma. Due to nonlinear behaviour of the DC glow discharge plasma different 1 Corresponding Author s renabkp021@gmail.com 1

2 types of fluctuations arise in it. Introduction of external dipolar magnetic field also gives rise to interesting nonlinear behaviour in the plasma. The nonlinear time series analysis of fluctuations in floating potential help us to realize instabilities in the linear plasma used in space for delivering thrust to satellite. 2. Review of Literature In material processing, gas lasers (Chiad, 2009) and thin film deposition (Scanlan, 1991) DC glow discharges are widely used. There are few basic discharge parameters like floating potential, electron temperature etc. which are necessary to be measured that parameters experimentally. It is also necessary to understand their dependence on pressure, external magnetic field etc. Using mass spectroscopy, optical spectroscopy or microwave spectroscopy (Gordillio Vazquez, 2007), discharge parameters can be measured. In this experiment we have used single Langmuir probe to measure floating potential. Though this method cannot sample entire velocity distribution of electrons it is a simple and powerful technique as it collects very small electron current, while keeping the remaining plasma condition undisturbed. In low density, the plasma floating potential depends on the shape of the Langmuir probe (Chen, 2001). Floating potential shows an abrupt decrease at a certain value of negative concentration (Shindo, 1991) with relatively low negative ion temperature and this is applicable to digital etching technique. In low pressure plasma (Chen, 1965), electron and ion currents change linearly with ionization density. But in high pressure, collision dominated plasmas, the electron current depends on electron density and its temperature whereas the ion-current depends on ion density and probe-sheath geometry (Clements, 1974). If an external magnetic field is applied, then the relationship between the plasma and the floating potential becomes more complicated, since in that case, the velocity distribution of ions become non- Maxwellian. Even today there is ample scope for research on the effect of magnetic field on plasma parameters (Samantaray, 2017). Instead of Langmuir probe, other novel probes (e.g., Adamek, 2004) can be used to measure plasma potential directly in presence of strong magnetic field. In this case, floating potential of the probe becomes equal to the plasma potential. In the presence of magnetic field, the accuracy of measurement of the floating potential depends on orientation of the probe and ratio of the probe radius to the Larmor radius of ion or electron (Hofer, 2003). Nonlinear time series analysis of experimental data obtained from glow discharge plasmas has been carried out by many of the scientists (Nurujjaman, 2006, 2007, 2008, 2009, 2010). Some of the important works are as follows: long range correlation in chaotic oscillations (Lahiri, 2012), Gottwald Melbourne test for chaos (Roy Chowdhury, 2012), series resonance effect in capacitively coupled radio frequency glow discharge plasma by homogeneous discharge model (Bora, 2014), order to chaos transition (Mitra, 2014), multi-scale dynamics in externally excited glow discharge plasma (Deka, 2015). Investigation of coherent modes and their role in intermittent oscillations using empirical mode decomposition has also been studied (Shaw, 2016). In the present work we have considered the DC floating potential and its fluctuation obtained from a glow discharge plasma in presence and absence of external inhomogeneous magnetic field. We have also studied its variation with different gas filling pressures and external magnetic field conditions. Analysis has been done using the MATLAB software. 3. Experiment In Plasma Physics Division of Saha Institute of Nuclear Physics (SINP), there is a glow discharge plasma device (Nurujjaman, 2009; Lahiri, 2012). It is basically a cylindrical cathode made of stainless steel (diameter 10 cm and length 20 cm), through which there is a wire shaped anode of 1.6 mm thickness. The whole setup is mounted inside a vacuum chamber, which is evacuated by an oil rotary pump to attain a base pressure of 0.03 mbar and then filled with argon gas (Ar) to obtain the working 2

3 pressure. A photograph of the experimental device is shown in Figure 1a., and a schematic diagram of experimental set up is shown in Figure 1b. Figure1a. Experimental Setup Figure1b. Schematic diagram of the experimental setup Bar Magnet In this study we have measured DC floating potential by using a cylindrical Langmuir probe made by tungsten wire of diameter 0.5 mm and length 20 mm which is soldered to a Teflon coated stainless steel, with the variation of discharge voltage at a fixed gas pressure. By changing the gas pressure from mbar to mbar the whole experiment has been repeated. A photograph of the DC glow discharge plasma is shown in Figure 1c. Figure1c. Glow discharge plasma in the absence of external magnetic field 3

4 Floating potential fluctuation has been measured at a fixed gas pressure of 0.11 mbar. During this measurement a bar magnet has been placed just outside the cathode surface. The distance of the bar magnet has been increased from the cathode surface by shifting it outwards up to 6 cm from the vessel containing plasma. We have also measured the magnetic field intensity by using a flux meter which shows a variation from 390 to 10 Gauss. At each position of the bar magnet the fluctuating signal has been stored by a Tektronix Oscilloscope which is then used for computer analysis. When the bar magnet is placed very close to the cathode surface (~1 cm) plasma glow has been observed as shown in Figure 1d. Figure 1d. Glow discharge plasma in the presence of external magnetic field. 4. Results and Discussion The floating potential (V f ) in an unmagnetized, quiescent, Maxwellian plasma is related to the plasma potential (V p ) by the relation V f = V p + ( kt e e )ln (0.6 2πm e M i )..(1) (Merlino, 2007) 4

5 Where T e is electron temperature, m e is electron mass, M i is ion mass and k is Boltzmann constant. We have measured DC floating potential at a particular gas filling pressure for different discharge voltages. Now the pressure is varied from mbar to 0.21 mbar and same experiment is repeated at each value of the pressure. Figure 2a shows the variation of floating potential at pressure P= mbar which indicates nonlinear increase in floating Figure 2(a.). Variation of floating voltage with discharge voltage at pressure 0.055mbar. potential with the increase of discharge voltage. Figure 2b (P=0.081mbar) shows the increase of V f with discharge voltage but in this case the nature of increase is different. Fig. 2c shows that V f decreases slowly with the increase of discharge voltage and becomes approximately constant when discharge voltage is ~320 V and above. Fig.2d shows that when the pressure is 0.14 mbar and above V f decreases with the increase of discharge voltage at a faster rate but if the discharge voltage is ~ 295V and above, V f attains a nearly steady value. We have also investigated the variation of V f with the variation of pressure from mbar to 0.21 mbar at constant discharge voltage (320V). Figure 3 shows that V f decreases with the increase of pressure, at very low pressures the rate of decrease is more rapid than that at high pressures. 5

6 Figure 2(b). Variation of floating voltage with discharge voltage at pressure 0.081mbar. Figure 2(c). Variation of floating voltage with discharge voltage at pressure 0.1and 0.12 mbar Figure 2(d). Variation of floating voltage with discharge voltage at pressure 0.14,0.16, 0.19, 0.21 mbar Figure 3. Variation of floating voltage with pressure at constant discharge voltage At very low pressure the density of gaseous neutral particles is low inside the electrodes, with the increase of discharge voltage, the electrons get accelerated and those energetic electrons make more ionization collisions with low density neutral gaseous particles. Consequently, more electron current flows which increase the floating potential. If the applied pressure is high i.e. 0.1mbar and above, the density becomes relatively high which increases the probability of collisions and due to that the electrons cannot gain sufficient energy from the field to make ionization collisions. Thus the drift velocity of the electrons gets reduced and they become more sluggish. Due to increase of non-ionization 6

7 collision probability between electrons and neutral particles, less number of electrons can reach the probe and less bias voltage is required to balance the electron flux by the ion flux. Presence of external magnetic field in the plasma creates electron cyclotron fluctuations apart from that of electron plasma fluctuations and ion acoustic fluctuations present in un-magnetized plasma. The behaviour of generated electron cyclotron waves is different along and perpendicular to the direction of the magnetic field. In the presence of external magnetic field, the relationship between floating potential and plasma potential becomes more complicated and in that case equation (1) is not strictly valid. In our work we have investigated interesting nonlinear behaviour of plasma by introducing an external magnetic dipolar field produced by a bar magnet placed outside the plasma chamber. A fireball like structure is observed near the cathode surface (Figure 1d) of the chamber which creates the fluctuation in the floating potential. If a dipolar magnetic field is applied from outside the plasma chamber, electrons emitted from the surface of the cathode by secondary emission process move along the magnetic field lines and are reflected by sheaths present close to the cathode surface. Due to collisions between those reflected electrons and the neutral gas particles, a dense plasma (Shaw, 2017) is formed near the cathode surface. When the secondary emitted electrons from the cathode surface move the through cathode sheath across the magnetic field, they get energized and ionize the gas. Due to increased ionization, glow region near the cathode is formed. Raw signals of fluctuation have been shown in Figure 4 [(a) (f)] where magnitude of the magnetic field decreases from 390 Gauss to 10 Gauss. Phase diagram of the filtered signals has been plotted in Figure 5 [(a) (f)]. These figures indicate that the signals are more or less quasi periodic. Fast Fourier Transform of the signal was done by MATLAB software. Figure 4. [ (a)-(f)]: Raw signals 7

8 Figure 5. [ (a)-(f)]: Phase space plots Power spectrum plots show variation of power with respect to frequency in KHz in Figure 6 [(a) (f)]. A number of prominent frequencies (31 KHz, 62KHz, 93 KHz, 124KHz etc.) are observed in the spectrum when the bar magnet is at distance 1cm from the vessel. These are the fundamental, second, third and fourth harmonics respectively. The powers at these frequencies show decreasing tendency and are shifted towards the lower frequency side of the spectrum as the magnetic field changes from 390 Gauss to 10 Gauss. This frequency shifting is a very important feature in nonlinear signal analysis. Figure 6. [(a)-(f)]: Variation of power with frequency Logarithm of frequency vs. logarithm of power are shown in Fig.7 [(a) (f)]. 8

9 Figure 7. [(a)-(f): log(power) vs log(frequency) Hurst exponent (Roy Chowdhury, 2012) has been calculated from the slope (β) of these plots by using the relation β = 2H +1. Hurst exponent vs. magnetic field intensity is shown in Figure 8. It is clear that Hurst exponent increases from 0.4 to 0.65 as the magnetic field decreases from 390 Gauss to 10 Gauss and this indicates the system dynamics tends to long range correlation. Figure 8. Variation Hurst exponent with magnetic field 5. Conclusion DC floating potential in DC glow discharge plasma using Argon gas has been measured at different pressures (0.055 mbar 0.21 mbar) by varying discharge voltage. In the low pressure region (0.055 mbar and mbar) DC floating potential increases with the increase of discharge voltage while it decreases with increase of discharge voltage if the applied pressure is 0.10 mbar and above. The first observation may be due to production of more ions whereas the second observation is due to nonionization collision between electrons and neutrals. It is also observed that if an external inhomogeneous magnetic field is applied using a bar magnet from outside the chamber, DC floating potential remains constant with the variation of discharge voltage. The presence of bar magnet just outside the cathode 9

10 forms a localized glow region near the cathode surface. The increase of magnetic field intensity increases the intensity of the glow region due to increase of electron density in that region. Fluctuation of floating potential shows quasi periodicity. It is also clear from power spectrum plots. Power spectrum plots indicate presence of dominant frequency and its harmonics. It is apparent that the decreasing power of harmonics is due to the decreasing magnetic field. Shifting of dominant frequencies to the lower frequency region with the decrease of magnetic field is a typical nature of nonlinear signal. Hurst exponent has been estimated from the slope of log power vs. log frequency plots. Hurst exponent increases with decrease of applied external magnetic field. The increasing tendency of the value of Hurst exponent indicates long range behaviour of the system dynamics. 6. Acknowledgement The authors would like to acknowledge the Director, Saha Institute of Nuclear Physics for his continuous encouragement to carry out the experiments and analyse the data. The authors also happy to acknowledge the student Mr. Pankaj Kr. Shaw and technical staff Mr. Asok Ram for their constant help to complete this work. We also like to thank Prof Sandip Sarkar, Saha Institute of Nuclear Physics for introducing us with the MATLAB software. References Adamek J., Stockel J., Stockel J., Hron M., Ryszawy J., Tichy M., Schrittwieser R., Ionita C., Balan P., Martines E. & Oost G.V. (2004). A novel approach to direct measurement of the plasma potential., Czechoslovak Journal of Physics, 54 (Suppl.C) Bora B. & Soto L. (2014). Influence of electrode configurations in dual capacitively coupled radio frequency glow discharge plasma., Journal of Physics: Conference series, vol. 591, Conference 1. Chen F. F. (1965). Electric probe Plasma Diagnostic Techniques. Edited by Huddleston & Leonard, Academic Press, New York Chen F.F. & Amush D. (2001). The floating potential of cylindrical Langmuir probes, Physics of Plasmas,8(11): Chiad B. T., Al-Zubaydi T. L., Khalaf M. K. & Khudiar A. I. (2009). Construction & characterization of a low pressure plasma reactor using glow discharge, Journal of Optoelectronics & Biomedical Materials, 1 (3): Clements R. M. & Smy P. R. (1974). The floating potential of a Langmuir probe in a high-pressure plasma, Journal of Physics D., Applied. Physics. 7, Deka U., Rao A. & Nurujjman M. (2015). Multistage dynamics in externally excited glow discharge plasma, Physica Scripta, 90(12), Gordillo-Vazquez F.J., Camero M. & Aleixandre C. (2007). Spectroscopic measurements of the electron temperature in low pressure radio frequency Ar/H 2 /C 2 H 2 and Ar/H 2 /CH 4 plasmas used for the synthesis of nanocarbon structures, plasma sources, Science. Technology. 15,

11 Hofer R.R. & Gallimore A.D. (2003). Recent results from internal and very-near-field plasma diagnostics of a very high specific impulse Hall Thruster., 28 th International Electric Propulsion Conference., 037. Lahiri S., Roy Chowdhury D. & Iyengar A.N.S. (2012). Long range correlation in the oscillations of a dc glow discharge plasma, Physics of Plasmas, 19, Merlino R.L. (2007). Understanding Langmuir probe current Voltage characteristics, American. Journal of Physics. 75 (12), Mitra V., Sarma A., Janaki M.S., Iyenger A.N.S., Sarma B., Marwan N., Kurths J., Shaw P.K., Saha D., Ghosh S. (2014). Order to Chaos transition studies in a DC glow discharge plasma by using recurrence quantification analysis., Chaos, Solitons & Fractals, 69, Nurujjman M.D. & Iyengar A.N.S. (2006). Chaotic to ordered state transition of cathode sheath instabilities in DC glow discharge plasmas, Pramana Journal of Physics, 67(2), Nurujjman M., Narayanan R. & Iyengar A.N.S. (2007). Parametric investigation of nonlinear fluctuation in Dc glow discharge plasma, Chaos: An interdisciplinary non linear science, 17(4), Nurujjman M., Iyengar A.N.S., & Paramananda P. (2008). Noise- invoked resonances near a homo clinic bifurcation in the glow discharge plasma, Physical Review E, 78(2), Nurujjman M., Narayanan R. & Iyengar A.N.S. (2009). Continuous wavelet transform based on time scale and multi fractal analysis of nonlinear oscillations in hollow cathode glow discharge plasma, Physics of Plasmas, 16, Nurujjman M. & Iyengar A.N.S. (2010). Dynamics of an excitable glow discharge Plasma under external forcing, Physical Review E, 82(5), Roy Chowdhury D. & Iyengar A.N.S., Lahiri S. (2012). Gottwald Melbourne (0-1) test for chaos in a plasma, Nonlinear Processes Geophysics, 19, Samantaray S., Paikarray R. & Ghosh J. (2017). On the effect of magnetic field upon plasma parameters, International Journal of Science and Research, 6(1). Scanlan J.V. (1991). Langmuir probe measurement in MHz Discharge Ph.D thesis, School of Physical Sciences, Dublin City University, Shaw P.K., Samanta S., Saha D., Ghosh S. Janaki M.S. & Iyengar A.N.S. (2017). A Localised Cathode Glow in the presence of a Bar magnet and its associated Nonlinear dynamics, Physics of Plasmas, 24, Shaw P.K., Ghosh S., Saha D., Janaki M.S. & Iyengar A.N.S. (2016). Investigation of coherent modes and their role in intermittent oscillations using empirical mode decomposition., Physics of Plasmas, 23(11), Shindo H & Horiike Y. (1991). Floating potential in negative-ion containing plasma, Japanese Journal of Applied Physics, 30, Part-1. No.-1,