OPTIMIZING PULSE WAVEFORMS IN PLASMA JETS FOR REACTIVE OXYGEN SPECIES (ROS) PRODUCTION*

1. OPTIMIZING PULSE WAVEFORMS IN PLASMA JETS FOR REACTIVE OXYGEN SPECIES (ROS) PRODUCTION* Seth A. Norberga), Natalia Yu. Babaevab)and Mark J. Kushnerb) a)Department of Mechanical Engineering University of Michigan, Ann Arbor, MI 48109, USA norbergs@umich.edu b)Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA nbabaeva@umich.edu, mjkush@umich.edu http://uigelz.eecs.umich.edu 65th Annual Gaseous Electronics Conference Austin, TX, October 22-26, 2012 * Work supported by Department of Energy Office of Fusion Energy Science and National Science Foundation

2. University of Michigan Institute for Plasma Science & Engr. AGENDA • Atmospheric Pressure Plasma Jets (APPJ) • Description of model • Plasma jet model • Propagation of plasma bullet • Radical production at fringes of jets • Planar plasma jet model • Concluding remarks • Special Acknowledgement – • Prof. Annemie Bogaerts • Mr. Peter Simon GEC2012

3. University of Michigan Institute for Plasma Science & Engr. ATMOSPHERIC PRESSURE PLASMA JETS (APPJ) • Plasma jets provide a means to remotely deliver reactive species to surfaces. • In the biomedical field, low-temperature non-equilibrium atmospheric pressure plasma jets are being studied for use in, • Sterilization and decontamination • Destruction of proteins • Bacteria deactivation • Plasma jets typically consist of a rare gas seeded with O2 or H2O flowing into room air. • Plasma produced excited states and ions react with room air diffusing into plasma jet to generate ROS (reactive oxygen species) and RNS (reactive nitrogen species). • In this talk, we present results from computational investigation of He/O2 plasma jets flowing into room air. GEC2012

4. University of Michigan Institute for Plasma Science & Engr. ATMOSPHERIC PRESSURE PLASMA JETS (APPJ) • Coaxial He/O2 plasma jets into room air were addressed. • Needle powered electrode with and without grounded ring electrode. • In these configurations, plasma bullets propagate into a flow field. • Figures from X. Lu, M. Laroussi, and V. Puech, Plasma Sources Sci. Technol. 21 (2012) GEC2012

5. University of Michigan Institute for Plasma Science & Engr. FORMATION OF EXCITED STATES IN APPJ • Prior experimental and modeling results have shown that jet produced excited states undergo reaction with air at boundary of jets. • For example, excitation transfer from He* to N2 creates a ring of N2(C3π). • Ref: G. V. Naidis, J. Phys. D: Appl. Phys. 44 (2011). GEC2012

6. University of Michigan Institute for Plasma Science & Engr. MODELING PLATFORM: nonPDPSIM • Poisson’s equation: • Transport of charged and neutral species: • Charged Species:  = Sharffeter-Gummel • Neutral Species:  = Diffusion • Surface Charge: • Electron Temperature (transport and rate coefficients from 2-term spherical harmonic expansion solution of Boltzmann’s Eq.): GEC2012

7. University of Michigan Institute for Plasma Science & Engr. MODELING PLATFORM: nonPDPSIM • Radiation transport and photoionization: • Poisson’s equation extended into materials. • Solution: 1. Unstructured mesh discretized using finite volumes. 2. Fully implicit transport algorithms with time slicing between modules. GEC2012

8. University of Michigan Institute for Plasma Science & Engr. nonPDPSIM: NEUTRAL FLUID TRANSPORT • Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady, compressible algorithms. • Individual neutral species diffuse within the single fluid, and react with surfaces GEC2012

9. University of Michigan Institute for Plasma Science & Engr. PLASMA JET: GEOMETRY AND CONDITIONS • Quartz tube with inner pin electrode and grounded rink electrode. • Cylindrically symmetric • He/O2 flowed through tube. • Air flowed outside tube as shroud. • -30 kV, 1 atm • He/O2 = 99.5/0.5, 20 slm • Surrounding humid air N2/O2/H2O = 79.5/20/0.5, 0.5 slm • Fluid flow field first established (5.5 ms) then plasma ignited. • Ring electrode is dielectric in analyzed case. GEC2012

10. University of Michigan Institute for Plasma Science & Engr. MINMAX Log scale PLASMA JET: DIFFUSION OF GASES • Flow field is established by initializing “core” of He in room air, and allowing gas to intermix. • Room air is entrained into jet, thereby enabling reaction with plasma excited species. • The mixing layer is due to diffusion at the boundary between the He/O2 and air. • He/O2 = 99.8/0.2, 20 slm • Air = 0.5 slm Animation Slide GEC2012

11. University of Michigan Institute for Plasma Science & Engr. MINMAX Log scale PLASMA JET • One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O2 = 99.8/0.2, no ground electrode. • Plasma bullet moves as an ionization wave propagating the channel made by He/O2. • Te has peak value near 8 eV in tube, but is 2-3 eV during propagation of bullet. • [e] and ionization rate Se (location of optical emission) transition from hollow ring to on axis. • Bullet stops when mole fraction of He is less than 40%. • Plasma has run for 66 ns. Animation Slide GEC2012

12. MINMAX Log scale University of Michigan Institute for Plasma Science & Engr. ELECTRON DENSITY • One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O2 = 99.8/0.2, no ground electrode. Plasma has run for 66 ns. • Electron density transitions from annular in tube and exit to on axis. • As air diffuses into He, the self sustaining E/N increases, progressively limiting net ionization to smaller radii. • Penning ionization (He* + N2 He + N2+ + e) at periphery aids plasma formation, but air diffusion and increase in required E/N dominates. Animation Slide GEC2012

13. University of Michigan Institute for Plasma Science & Engr. PLASMA BULLET SHAPE A few slides on “waveform” • One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O2 = 99.8/0.2, no ground electrode. Flow at 5.5 ms. Plasma has run for 66 ns. • Bullets propagate at speeds similar to conventional ionization waves (107 cm/s). • Figure from X. Lu, M. Laroussi, and V. Puech, Plasma Sources Sci. Technol. 21 (2012) GEC2012

14. MINMAX Log scale University of Michigan Institute for Plasma Science & Engr. ROS/RNS PRODUCED IN PLASMA • RONS produced by plasma jet plasma include NO, OH, O, O3 and O2(a). (Densities shown are from 1 pulse.) • O2(a) and O are formed in tube. • NO and OH are in plume, resulting from diffusion of humid air into jet. • Significant RONS production outside core partly due to photoionization & photodissociation. • 1 atm, He/O2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. Animation Slide GEC2012

15. University of Michigan Institute for Plasma Science & Engr. ROS PRODUCED IN PLASMA • ROS densities increase along the jet with increase of diffusion of air into the jet. • O2(a) and O3 are longed lived (for these conditions), and will accumulate pulse-to-pulse, subject to advective flow clearing out excited states. • 1 atm, He/O2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. GEC2012

16. MINMAX Log scale University of Michigan Institute for Plasma Science & Engr. RNS DENSITIES • RNS are created through the interaction of the He/O2 jet with air. • N2* [N2(A) and N2(C)] have peak densities of 1014 cm-3 (from 1 pulse). • Due to high thresholds of these electron impact processes, densities are center high where Te is maximum in spite of higher density of N2 near periphery. • 1 atm, He/O2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. Animation Slide GEC2012

17. University of Michigan Institute for Plasma Science & Engr. RNS PRODUCED IN PLASMA • Annular to center peaked RNS densities from exit of tube to end of plume. • 1 atm, He/O2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. GEC2012

18. University of Michigan Institute for Plasma Science & Engr. PLANER GEOMETRY: Te SEQUENCE • Fluid module is run first (8 ms) to establish steady-state mixing of Helium and ambient air. • Then, a pulse of different rise time (tens of ns) is applied. Cathode • 1 atm, He/O2 = 99.8/0.2, 35 kV, 20 l/min • Surrounding humid air N2/O2/H2O = 79.5/20/0.5 • Pulse rise time 25 ns GEC2012

19. University of Michigan Institute for Plasma Science & Engr. EFFECT OF PULSE RISE TIME • Rise time 75 ns • Rise time 5 ns • Rise time 25 ns Cathode Cathode • Bullet formation time inside tube 7 ns • Propagation time 13 ns • Bullet formation time inside tube 22 ns • Propagation time 17 ns • Bullet formation time inside tube 47 ns • Propagation time 33 ns • Bullet formation time inside the tube and propagation time increases with the increase of the pulse rise time. • Shorter rise time results in more intensive IW: higher electron impact sources Se and electron temperature Te • 1 atm, He/O2 = 99.8/0.2, 35 kV, 20 l/min, surrounding humid air N2/O2/H2O = 79.5/20/0.5 GEC2012

20. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • Conducted a proof of concept for modeling the plasma bullet and gained information about radical species in the trail of the bullet. • Significant densities of reactive oxygen and nitrogen species are created by the dry chemistry of the atmospheric pressure plasma jet. • Future modeling work includes: • Plasma bullet behavior for different polarities. • Varying discharge geometry to reproduce results. • Different mixtures of feed gas to optimize desired ROS/RNS production. • Impact effects of jet on a surface. GEC2012

21. Back Up Slides

22. MINMAX Log scale University of Michigan Institute for Plasma Science & Engr. DEPENDENCE ON VOLTAGE WAVEFORM 1. 2. 3. 4. • In each plot, electron temperature is used to represent the plasma bullet. • 1 atm, He/O2 = 99.8/0.2, 20 slm • 25 ns rise to -30 kV pulse with no ground electrode • 25 ns rise to -10 kV pulse with ground electrode • 25 ns rise to -30 kV pulse with ground electrode • 50 ns rise to -30 kV pulse with ground electrode. Animation Slide GEC2012