1. OPTIMIZING PULSE WAVEFORMS IN PLASMA JETS FOR REACTIVE OXYGEN SPECIES (ROS) PRODUCTION* Seth A. Norberg a), Natalia Yu. Babaeva b) and Mark J. Kushner b) 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 65 th 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. 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 University of Michigan Institute for Plasma Science & Engr. GEC2012

3. 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 O 2 or H 2 O 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/O 2 plasma jets flowing into room air. GEC2012 University of Michigan Institute for Plasma Science & Engr.

4. ATMOSPHERIC PRESSURE PLASMA JETS (APPJ) GEC2012 Figures from X. Lu, M. Laroussi, and V. Puech, Plasma Sources Sci. Technol. 21 (2012)  Coaxial He/O 2 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. University of Michigan Institute for Plasma Science & Engr.

5. FORMATION OF EXCITED STATES IN APPJ GEC2012  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 N 2 creates a ring of N 2 (C 3 π).  Ref: G. V. Naidis, J. Phys. D: Appl. Phys. 44 (2011). University of Michigan Institute for Plasma Science & Engr.

6. 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 University of Michigan Institute for Plasma Science & Engr.

7.  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. MODELING PLATFORM: nonPDPSIM GEC2012 University of Michigan Institute for Plasma Science & Engr.

8.  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 nonPDPSIM: NEUTRAL FLUID TRANSPORT GEC2012 University of Michigan Institute for Plasma Science & Engr.

9. PLASMA JET: GEOMETRY AND CONDITIONS  Quartz tube with inner pin electrode and grounded rink electrode.  Cylindrically symmetric  He/O 2 flowed through tube.  Air flowed outside tube as shroud.  -30 kV, 1 atm  He/O 2 = 99.5/0.5, 20 slm  Surrounding humid air N 2 /O 2 /H 2 O = 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 University of Michigan Institute for Plasma Science & Engr.

10. 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/O 2 and air.  He/O 2 = 99.8/0.2, 20 slm  Air = 0.5 slm GEC2012 Animation Slide MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr.

11. Animation Slide PLASMA JET  One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O 2 = 99.8/0.2, no ground electrode.  Plasma bullet moves as an ionization wave propagating the channel made by He/O 2.  T e has peak value near 8 eV in tube, but is 2-3 eV during propagation of bullet.  [e] and ionization rate S e (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. GEC2012 MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr.

12. ELECTRON DENSITY GEC2012 Animation Slide MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr.  One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O 2 = 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* + N 2  He + N 2 + + e) at periphery aids plasma formation, but air diffusion and increase in required E/N dominates.

13. PLASMA BULLET SHAPE GEC2012 University of Michigan Institute for Plasma Science & Engr. A few slides on “waveform” Figure from X. Lu, M. Laroussi, and V. Puech, Plasma Sources Sci. Technol. 21 (2012)  One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O 2 = 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 (10 7 cm/s).

14. ROS/RNS PRODUCED IN PLASMA  RONS produced by plasma jet plasma include NO, OH, O, O 3 and O 2 (a). (Densities shown are from 1 pulse.)  O 2 (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/O 2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. GEC2012 Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale

15. ROS PRODUCED IN PLASMA  ROS densities increase along the jet with increase of diffusion of air into the jet.  O 2 (a) and O 3 are longed lived (for these conditions), and will accumulate pulse- to-pulse, subject to advective flow clearing out excited states.  1 atm, He/O 2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. GEC2012 University of Michigan Institute for Plasma Science & Engr.

16. RNS DENSITIES  RNS are created through the interaction of the He/O 2 jet with air.  N 2 * [N 2 (A) and N 2 (C)] have peak densities of 10 14 cm -3 (from 1 pulse).  Due to high thresholds of these electron impact processes, densities are center high where T e is maximum in spite of higher density of N 2 near periphery.  1 atm, He/O 2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode. GEC2012 Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale

17. RNS PRODUCED IN PLASMA GEC2012 University of Michigan Institute for Plasma Science & Engr.  Annular to center peaked RNS densities from exit of tube to end of plume.  1 atm, He/O 2 = 99.8/0.2, -30 kV, 20 slm, no ground electrode.

18. PLANER GEOMETRY: T e SEQUENCE  1 atm, He/O 2 = 99.8/0.2, 35 kV, 20 l/min  Surrounding humid air N 2 /O 2 /H 2 O = 79.5/20/0.5  Pulse rise time 25 ns  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. University of Michigan Institute for Plasma Science & Engr. GEC2012 Cathode

19. EFFECT OF PULSE RISE TIME  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 S e and electron temperature T e  1 atm, He/O 2 = 99.8/0.2, 35 kV, 20 l/min, surrounding humid air N 2 /O 2 /H 2 O = 79.5/20/0.5 University of Michigan Institute for Plasma Science & Engr. GEC2012 Cathode  Rise time 25 ns  Rise time 75 ns  Rise time 5 ns  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

20.  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. CONCLUDING REMARKS GEC2012 University of Michigan Institute for Plasma Science & Engr.

21. Back Up Slides

22. DEPENDENCE ON VOLTAGE WAVEFORM GEC2012 University of Michigan Institute for Plasma Science & Engr. In each plot, electron temperature is used to represent the plasma bullet. 1 atm, He/O 2 = 99.8/0.2, 20 slm 1.25 ns rise to -30 kV pulse with no ground electrode 2.25 ns rise to -10 kV pulse with ground electrode 3.25 ns rise to -30 kV pulse with ground electrode 4.50 ns rise to -30 kV pulse with ground electrode. Animation Slide MIN MAX Log scale 1. 2. 3.4.