PLASMA DYNAMICS AND CHARGING CHARACTERISTIC OF A SINGLE NOZZLE ION HEAD

1. PLASMA DYNAMICS AND CHARGING CHARACTERISTIC OF A SINGLE NOZZLE ION HEAD Jun-Chieh Wanga) Napoleon Leonib), Henryk Bireckib), Omer Gilab), and Mark J. Kushnera) a)University of Michigan, Ann Arbor, MI 48109 USA mjkush@umich.edu, junchwan@umich.edu b)Hewlett Packard Research Labs, Palo Alto, CA 94304 USA napoleon.j.leoni@hp.com, henryk.birecki@hp.com, omer_gila@hp.com NIP-28 September 9-13 2012 * Work supported by Hewlett Packard Research Labs

2. University of Michigan Institute for Plasma Science & Engr. AGENDA • Introduction to micro-Dielectric Barrier Discharges (mDBD) • Current extraction • Description of model • mDBD Scaling • mDBD sustained in N2 (1 atm) • Electron current extraction vs rf frequencies • Charge per pulse and pulse number vs frequencies • Electron current extraction vs dielectric constant • Concluding Remarks NIP28_2012

3. University of Michigan Institute for Plasma Science & Engr. DIELECTRIC BARRIER DISCHARGES • The plasma in DBDs is sustained between parallel electrodes of which one (or both) is covered by a dielectric. • When the plasma is initiated, the underlying dielectric is electrically charged, removing voltage from the gap. • The plasma is terminated when the gap voltage falls below the self-sustaining value and so preventing arcing. • On the following half cycle, a more intense electron avalanche occurs due to the higher voltage across the gap from previously charged dielectric. • http://www.calvin.edu/~mwalhout/discharge.htm NIP28_2012

4. University of Michigan Institute for Plasma Science & Engr. MICRO-DBDs • Microplasmas (10s to 100s m) are interesting for planar current sources due to the ability of fabricating large arrays. • Non-arcing, micro-DBDs (mDBDs) using rf voltages are attractive for these arrays due to inexpensive mass production or area-selective modification. • “Microplasma stamps” based on mDBDs are applied to area-selective surface modification at atmospheric pressure. Ref: J. Phys. D: Appl. Phys. 41, 194012 (2008) NIP28_2012

5. University of Michigan Institute for Plasma Science & Engr. SCALING OF MICRO-DBDs • At atmospheric pressure, plasma formation and decay times can be a few ns whereas the rf period is 10s to 100s ns – the mDBD may need to be re-ignited with each cycle. • Electron extraction from the mDBD may require a third electrode and so the structure, electron emitting properties and dynamics are important to the operation. • We have computationally investigated the extraction of electron current from mDBDs: • Frequency • rf voltage • Material properties NIP28_2012

6. Gas Phase Plasma Plasma Hydrodynamics Poisson’s Equation Liquid Phase Plasma Bulk Electron Energy Transport Kinetic “Beam” Electron Transport Surface Chemistry and Charging Neutral Transport Navier-Stokes Neutral and Plasma Chemistry Ion Monte Carlo Simulation University of Michigan Institute for Plasma Science & Engr. MODEL: nonPDPSIM • 2-unstructured mesh with spatial dynamic range of 104. • Fully implicit plasma transport. • Time slicing algorithms between plasma and fluid timescales. Radiation Transport Circuit Model NIP28_2012

7. University of Michigan Institute for Plasma Science & Engr. MODELING PLATFORM: nonPDPSIM • Poisson’s equation: • Transport of charged and neutral species: • Surface Charge: • Electron Temperature (transport coefficient obtained from Boltzmann’s equation) • Radiation transport and photoionization: NIP28_2012

8. University of Michigan Institute for Plasma Science & Engr. SECONDARY ELECTRONS FROM SURFACES • An electron Monte Carlo Simulation is used to follow trajectories of secondary electrons from surfaces. • Structured EMCS mesh overlayed onto unstructured fluid mesh – E-fields interpolated onto mesh. • Pseudo-particles are launched from sites on surfaces for emission produced by • Ion bombardment • Photon fluxes • Electric field enhanced thermionic emission. NIP28_2012

9. University of Michigan Institute for Plasma Science & Engr. mDBD GEOMETRY • The mDBD is a “sandwich” cylindrical geometry. • rf electrode buried in printed-circuit-board • Grounded electrode separated from rf by dielectric sheet – 35 m hole. • Biased extraction electrode across a gap of 500 m • rf: 1.4 kV at 2.5-25 MHz. • Extraction: 2 kV, ballasted (Rb= 100 kΩ) to prevent arcing or run away current NIP28_2012

10. MINMAX University of Michigan Institute for Plasma Science & Engr. 20 MHz N2 (1 atm): SURFACE CHARGE, [e]  [e] (2x1015 cm-3, 5 dec)  [e] (2x1015 cm-3, 5 dec) •  (-2x1016~2x1016 cm-3) • Vrf= 1.4 kV, 20 MHz, 2 kV bias and ballast resistor 100 kΩ • Vrf=+1.4 kV: Electrons starts to be extracted out of cavity. • Vrf= 0: Top electrode extracts electrons. Avalanches in depleted cavity. • Vrf=-1.4 kV: Positively charged dielectric reduces voltage drop. • Vrf= 0: E-plume extinguishes. e-flux neutralizes positively charged dielectric. NIP28_2012 Animation Slide-GIF

11. MINMAX University of Michigan Institute for Plasma Science & Engr. 20 MHz N2 (1 atm): E/N, Te, Se+ Ssec • [Se+ Ssec](3x1024 cm-3 s-1,3dec)  Te (0.1~15 eV) • E/N (40~4000Td) • Larger E/N occurs at Vrf zero-crossing due to previously charging of dielectric. Te and Se follow E/N. • Ionizations due to secondary “beam” electrons at surface seeded by positive ions and UV photons from long lived N2 excited states are commensurate with bulk ionization. • Conductive plasma shields out electric field and lowers E/N which reduces ionization in the cavity. NIP28_2012 Animation Slide-GIF

12. University of Michigan Institute for Plasma Science & Engr. ELECTRON CURRENT EXTRACTION vs FREQUENCY 20MHz 10MHz • Peak currents at top electrode tend to increase with increasing frequency. • Pre-pulse and single main pulse at higher frequencies, multiple pulses at lower frequencies. • Longer periods allow periodic bursts of ionization in the mDBD cavity, launching new pulses. rf Voltage Top Electrode Current 5MHz 2.5MHz • Vrf = 1.4 kV, 20-2.5 MHz,2 kV bias, 100 kΩ ballast resistor NIP28_2012

13. University of Michigan Institute for Plasma Science & Engr. ELECTRON CURRENT EXTRACTION vs FREQUENCY • 2.5 MHz, Experiment • 2.5 MHz, Simulation • Simulation shows good agreement with experiments. • The multiple pulsing may transition between 2 and 3 peaks (or more) as discharge conditions evolve in the gap. NIP28_2012

14. MINMAX University of Michigan Institute for Plasma Science & Engr. 2.5 MHz N2 (1 atm): SURFACE CHARGE, [e]  [e] (5x1015 cm-3, 8 dec)  [e] (5x1015 cm-3, 8 dec) •  (-3x1016~3x1016 cm-3) • Vrf= 1.4 kV, 2.5 MHz, 2 kV bias and ballast resistor 100 kΩ • Vrf=+1.4 kV: Dielectric charged negatively. Electron plume starts to be extracted. • Vrf=0: Avalanche has occurred. 1st E-plume has been extracted. E/N increases. • Vrf=-1.4 kV: 2 pulses extracted due to the periodic burst of ionization. • Vrf= 0: E-flux neutralizes positively charged dielectric. NIP28_2012 Animation Slide-GIF

15. University of Michigan Institute for Plasma Science & Engr. CHARGE COLLECTION vs. TIME • Charge collection as a function of time increases at higher frequency due to higher repetition rate. • Charge collection is limited by short rf period and capacitance of underlying dielectric. Charge/pulse is not a strong function of frequencies. • Vrf = 1.4 kV, 25-2.5 MHz,2 kV bias, 100 kΩ ballast resistor NIP28_2012

16. University of Michigan Institute for Plasma Science & Engr. CHARGE COLLECTION vs. TIME • Charge collection increases with rf voltage due to the higher ionization rate in the mDBD cavity. • The larger plasma density in the cavity is extracted as a more dense plume. • Vrf = 1.4-5.6 kV, 25 MHz,2 kV bias, 100 kΩ ballast resistor NIP28_2012

17. University of Michigan Institute for Plasma Science & Engr. CURRENT EXTRACTION vs DIELECTRIC CONSTANT 25MHz 20MHz εr=10 20 • Charging of the insulator in part controls current collection on the top electrode. • Larger dielectric constant of the insulator allows larger current collection. • This effect is not linear with . 10MHz 15MHz • Vrf = 1.4 kV, 25-10 MHz, 2 kV bias, 100 kΩ ballast resistor NIP28_2012

18. University of Michigan Institute for Plasma Science & Engr. CHARGE COLLECTION vs. TIME • Since the intervening dielectric acts as a capacitor, its charging characteristics scale with εr. The surface charge required to charge the dielectric to the voltage increase linearly with εr • Charge collection increases with increasing capacitance of dielectric. • Vrf = 1.4, 25 MHz,2 kV bias, 100 kΩ ballast resistor, εr=2-20 NIP28_2012

19. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • First principles computer modeling has been applied to investigation of single nozzle ion head in electrographic printing. • Peak currents collected at top electrode (2.5-25MHz) tend to increase with frequency. • More current pulses per rf cycle at lower frequency due to periodic burst of ionization in the mDBD cavity. • Charge collection as a function of time increases with rf frequencies, voltages and capacitance of dielectric. • The current increases with increasing dielectric constant of insulator. NIP28_2012