Analysis of Discharge Parameters and Spectroscopic Diagnostic of DBDs

Transcription

1 Analysis of Discharge Parameters and Spectroscopic Diagnostic of DBDs Pooja Gulati Plasma Device Technology, Microwave Tubes Division CSIR-Central Electronics Engineering Research Institute (CSIR-CEERI) Pilani Rajasthan Joint ICTP-IAEA Workshop on Fusion Plasma Modelling using Atomic and Molecular Data, Trieste - Italy

2 CSIR-Central Electronics Engineering Research Institute (CSIR-CEERI), Pilani Foundation was laid on Around 450 Employees Leading Research Institute in India in the field of Electronics Devices

3 Major Research Areas at CSIR-CEERI Pilani 1.Microwave Tubes Gyrotron Klystron Magnetron TWT Plasma Devices Technology 2.Electronic Systems Agri-Electronics Embedded System Digital System Power Electronics 3.Semiconductor Hybrid Microcircuits IC Design MEMS and Microsensors anotechnology & Devices Photonics & optoelectronics Semiconductor Material & Tech.

4 Activities in Plasma Devices Group High Power Plasma Switches: Thyratrons & Pseudospark VUV/UV Excimer Sources based on DBD: Biomedical Applications Surface Treatment Water Purification (jointly with EERI) Plasma Cathode Electron Gun: Electron and Ion Source Plasma Assisted Microwave Sources: Plasma TWT, Pasotron Penning Discharge Devices Ion Sources and VUV Spectroscopy

5 Organization of Presentation Motivation and objectives of the work. Introduction What is Dielectric Barrier Discharges (DBDs)? Advantages and open areas of research and applications of DBDs. Experiments Experimental Setup and testing Results and discussions. Conclusion

6 Motivation In recent time it has been observed that Dielectric Barrier Discharge (DBD) based micro-discharges and micro-arraydischarge plasmas can produce ultraviolet radiation in germicidal wavelength range UV-C( nm) and VUV( nm), UV-B( nm), UV-A ( nm) that can effectively treat impure water, and also can be used for medical and other industrial applications. Our group is working in this area and I am motivated in the spectroscopic studies related to this technology which can in future transit me for large scale plasma related spectroscopic analysis.

7 Objectives To investigate and analyze the characteristics of discharge patterns occurring in the volume discharge (VD) configuration of DBDs filled with inert gases. The traditional metallic diagnostic technique is not useful in the very small geometry of the proposed DBD configuration. Hence to derive the internal electrical and plasma parameters with the help of electrical analysis and spectroscopic diagnostics is the key component of the objectives.

8 Introduction What is Dielectric Barrier Discharge(DBDs)? Dielectric barrier discharges (DBDs), also known as silent discharges or barrier discharges, are generated in discharge configurations with at least one dielectric barrier between the electrodes.

9 Possible DBD Geometries (i). Volume discharge, (ii). Surface discharge (iii). Coplanar discharge Typical dielectric barrier discharge configurations

10 Fabricated Geometries

11 Geometrical Design and Parameters C d1 C g C d2 Dielectric Thickness= 1 mm Electrode diameter =36 mm Electrode Thickness = 1mm Gas gap = 2 mm Dielectric Material Used= Quartz Pressure of Gas=100mbar Gas Used = Helium Dielectric barrier capacitance C d1 = C d2 = pf

12 Experimental Setup Base pressure=1x10-4 mbar Gas filling assembly is used to fill gas at different pressures. Working Pressure~100 mbar Sinusoidal voltage supply up to 2kV peak with frequencies from 30 to 90 khz has been used. Applied voltage and the total current are measured using high voltage probe and Rogowski-type current transformer. Schematic View of experimental setup Oscilloscope and visible spectrometer are interfaced with computer.

13 Testing & Characterization of He DBD Va It Experimental Setup for the DBD Source At breakdown voltage, the discharge begins with some filaments distributed on the dielectric wall. Increasing the applied voltage little bit, number of filaments increases and for further increase in voltage, the discharge finally get diffused. Va(V) Time(u sec) 0.00 It(mA)

14 Using Kirchoff s theorem for the model, we obtain the following equations V = V V (1) I = I I I = I I a d + Total current through DBD and displacement current through gap dvd dvg I dbd = Cd (4) I dg = C g (5) dt dt Diff. (1) with respect to time and replacing (4) and (4) in (1), dva 1 = dt C g ( I dbd I dis I ) + C Rearranging (6), we will get I dis Equivalent electrical circuit of DBDs C g dva ( t ) = (1 + ) I dbd C g C dt d 1 V = + V g dbd d d I dbd dt Vm0 Cd 1 = g Va I dbd dt Vm0 Cd m 1 T / 2 dbd 0 = I dt 2C d 0 tc dbd + The values of dielectric and gas gap voltages are, Where V m0 is memory voltages, ( In case of sinusoidal excitation) V (6) (7) (8) (9) sc (2) dbd dis + dg (3) In case of sinusoidal excitation (10) U Pal et al, J. Phys. D: Appl. Phys. vol. 42, (8pp), U Pal et al, J. Phys.: Confe. Ser.208, , 2010.

15 Results and discussions Volatage (V) Va Vd Vg Vm Input Parameters: Gap capacitance C g (11.30 pf) Dielectric barrier capacitance C d (20.48 pf) Curren nt (ma) It Idbd Idis. Equations are used from references: U Pal et al, J. Phys. D: Appl. Phys. vol. 42, (8pp), U Pal et al, J. Phys.: Confe. Ser.208, , Power (W) Psup. Pdis. Experimental waveforms of dynamic processes occurring in gap (gas: Helium at f = 34.5 khz) for the Parallel plate DBD Geometry. at 100 mbar Time (µs)

16 Spectroscopic Results 4.5x x10 3 HeI Å 3.5x10 3 Intensity(a.u) 3.0x x x x10 3 HeI Å HeI Å HeI Å HeII Å HeI Å HeI Å HeI Å 1.0x x10 2 He I Å (2 3 S-3 3 P) He I Å (2 1 P-4 1 D) W avelength (Å) eutral Helium Lines: He I Å (2 1 S-3 1 P) He I Å (2 3 P-3 3 D) He I Å (2 1 P-3 1 D) He I Å (2 3 P-3 3 S) He I Å (2 1 P-3 1 S)

17 Collisional-Radiative (CR) Model Intensities of the He I lines are calculated using collisionalradiative (CR) model based ADAS code [H. P. Summers, ADAS users manual, JET IR 06 (Abingdon: JET Joint undertaking) (1994)]. With an assumption that the average electron density and temperature in an emission length x, the photon intensity I (λ ul ) of a spectral line can be written from the CR-model as, ~ ~ I ~ ( λ ) = PEC ( x ) + PEC ul recombining e i + excitation From CR-model the ground state populations of atoms and ions is given by, d g d i = = α CR Under steady-state approximation, dt dt i e S CR g e ~ e ~ ( g x ) S α CR CR = i g

18 Under equilibrium condition α S = 0 and So, the condition Purely Ionizing Condition CR i e CR g e i g S = α CR CR S CR For ionizing plasma is >> 1 In true sense the ionizing plasma condition holds well when i / g << S CR / α CR [Fujimoto T. and Sawada K., IFS-DATA-39 (1997)]. α CR S CR and for recombining plasma is << 1 HeI α CR

19 Intensity line ratios Under ionizing condition the term PEC be negligibly small and the line intensity from level u to level l is expressed as, I ~ ( λ ul ) = PEC excitation ~ ~ recombining I( λ ul e ~ ( g x) ) e ~ ( i x) is taken to for a transition The significance of the line ratio technique is that the experimentally observable intensity ratio of two lines (which is not directly dependent on, and ) can be easily i g e obtained from the code as the ratio of corresponding photon emission rate coefficients is given by, I I 1 2 = PEC PEC 1 2 ( ( e e, T e, T e ) )

20 Temperature & Density Calculation Temp. Sensitive Intensity Ratios: / , 5049/ Density Sensitive Intensity Ratios: / , / Calculated Values: Electron Temp. = (6.5±0.5) ev, Density = (3.5±1.5) х10 11 cm -3 Ref. Summers H P 1994 ADAS Users Manual JET IR 06 (Abingdon: JET Joint Undertaking). R. Prakash, et al J. Appl. Phys. vol. 97, no.4, p , 2005.

21 Simulated results of the electron Density using OOPIC-Pro 6.00x nsec 105ns 95ns e (cm -3 ) 4.50x x x Diameter of the electrode (mm) The statistical mechanics have shown that the many small perturbation (errors) that affect a physical system almost always force the measurement to follow the Gaussian distribution. It is usually referred to as simply the normal distribution. Based on this, the average distribution of electron plasma density is derived for entire system geometry using OOPIC-Pro simulation code and if we take line average of the saturated density it would give nearly similar results to the spectroscopic diagnostic measurements, which are in agreement to each other.

22 Conclusion The homogeneous type of discharge has been observed at 100mbar operating pressure for a fixed frequency 34.5 khz in parallel plate DBD cell filled with helium gas. The dynamic evolution of the process in the gap provides the useful information about the electrical characterization of the DBD source. The electron plasma temperatures and electron plasma density obtained for present configuration at 100mbar gas pressure are typically(6.5±0.5)evand(3.5±1.5)х10 11 cm -3 respectively. The existence of such density and temperature in this source is useful for existence of higher metastable states which needs to be further investigated.

23 Acknowledgement Dr. Ram Prakash, CSIR-CEERI, Pilani Mr. U. Pal, CSIR-CEERI, Pilani and all other group members

24 Thank You