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EXCITATION OF O 2 ( 1 Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL IODINE LASERS

EXCITATION OF O 2 ( 1 Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL IODINE LASERS Natalia Babaeva, Ramesh Arakoni and Mark J. Kushner Iowa State University Ames, IA 50011, USA natalie5@iastate.edu arakoni@iastate.edu mjk@iastate.edu http://uigelz.ece.iastate.edu

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EXCITATION OF O 2 ( 1 Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL IODINE LASERS

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  1. EXCITATION OF O2(1Δ) IN PULSED RADIO FREQUENCY FLOWING PLASMAS FOR CHEMICAL IODINE LASERS Natalia Babaeva, Ramesh Arakoni and Mark J. Kushner Iowa State University Ames, IA 50011, USA natalie5@iastate.edu arakoni@iastate.edu mjk@iastate.edu http://uigelz.ece.iastate.edu October 2005 * Work supported by Air Force Office of Scientific Research and NSF

  2. AGENDA • Introduction to eCOILS • Description of the model • O2(1Δ) yield for CW and Spiker-Sustainer Excitation • Optimization with Frequency • Summary Iowa State University Optical and Discharge Physics GEC_2005_02

  3. OXYGEN-IODINE LASERS • In chemical oxygen-iodine lasers (COILs), oscillation at 1.315 µm (2P1/22P3/2) in atomic iodine is produced by collisional excitation transfer of O2(1D) to I2 and I. • Plasma production of O2(1D) in electrical COILs (eCOILs) eliminates liquid phase generators. • Self sustaining Te in eCOILs plasmas (He/O2, a few to 10s Torr) is 2-3 eV. Excitation of O2(1D) optimizes at Te = 1-1.5 eV. • One method to increase system efficiency is lowering Te using spiker-sustainer (S-S) techniques. Iowa State University Optical and Discharge Physics GEC_2005_03

  4. O2(1∆) KINETICS IN NON-EQUILIBRIUM He/O2 DISCHARGES • Production of O2(1∆) is by: • Direct electron impact [0.98 eV] • Excitation of O2(1Σ) [1.6 eV] with rapid quenching to O2(1∆). • Self sustaining is Te = 2-3 eV. Optimum conditions are Te = 1-1.2 eV. • Addition of He typically increases yield by reducing E/N. Iowa State University Optical and Discharge Physics GEC_2005_04

  5. SPIKER SUSTAINER TO LOWER Te • Spiker-sustainer (S-S) provides in-situ “external ionization.” • Short high power (spiker) pulse is followed by plateau of lower power (sustainer). • Excess ionization in “afterglow” enables operation below self-sustaining Te (E/N). • Te is closer to optimum for exciting O2(1D). • Example: He/O2=1/1, 5 Torr, Global kinetics model University of Illinois Optical and Discharge Physics GEC_2005_05

  6. DESCRIPTION OF the MODEL: CHARGED PARTICLES, SOURCES • A computational investigation of eCOILs has been performed with a 2-d plasma hydrodynamics model (nonPDPSIM) to investigate spiker-sustainer methods. • Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique. • Electron energy equation: Iowa State University Optical and Discharge Physics GEC_2005_06

  7. DESCRIPTION OF the MODEL: NEUTRAL PARTICLE TRANSPORT • Fluid averaged mass density, momentum and thermal energy density are obtained using unsteady, compressible algorithms. • Individual species are addressed with superimposed diffusive transport. Iowa State University Optical and Discharge Physics GEC_2005_07

  8. Flow Flow GEOMETRY FOR CAPACITIVE EXCITATION • Cylindrical flow tube 6 cm diameter • Capacitive excitation using ring electrodes. • He/O2 = 70/30, 3 Torr, 6 slm . • Yield: Iowa State University Optical and Discharge Physics GEC_2005_08

  9. MIN MAX TYPICAL PLASMA PROPERTIES (13 MHz, CW) • O2(1∆) yield on Axis • Power, [e], O, O2(1Σ) and O2(1∆) • O2(1Σ) and O densities are maximum near peak power deposition. • O2(1∆) increases downstream while O2(1Σ) is quenched to O2(1∆). Iowa State University Optical and Discharge Physics • 3 Torr, He/O2=0.7/0.3, 6 slm GEC_2005_09

  10. SPIKER-SUSTAINER: VOLTAGE WAVEFORM . • Spiker-sustainer (S-S) consists of pulsed modulated rf excitation. • High power pulses produce excess ionization and allow discharge to operate nearer to optimum Te for O2(1∆) production. • 27 MHz, 120 W, 1 MHz Carrier, 20% duty cycle Iowa State University Optical and Discharge Physics GEC_2005_10

  11. MIN MAX SINGLE SPIKER: Te and ELECTRON DENSITY Te (eV) [e] • Short high power pulse (spiker) is applied , followed by a longer period of lower power. • Te is low after spiker enabling more efficient production of O2 (1Δ). • Excess ionization created by the spiker decays within 10 – 15 µs. • 13 MHz, 40 W Single Spiker • t = 0.5 – 20 s ANIMATION SLIDE 0 - 3.1 eV Iowa State University Optical and Discharge Physics 0 - 2 x 1010 cm-3 GEC_2005_11

  12. S-S vs CW : PLASMA PROPERTIES • CW • Spiker-Sustainer • O2(1Σ ) is quickly collisionally quenched to O2(1∆) after the plasma zone. • O2(1∆) is quenched slowly. • O atom production nearly equals O2(1∆). Iowa State University Optical and Discharge Physics • 13 MHz, 40 W, 3 Torr, He/O2=0.7/0.3, 6 slm GEC_2005_12

  13. S-S vs CW: O2(1) PRODUCTION AND O2 DISSOCIATION • CW • Spiker-Sustainer • Dissociation fraction decreases when using S-S. • Lower Te enabled by S-S reduces rate of dissociation while increasing rate of excitation of O2(1). Iowa State University Optical and Discharge Physics • 13 MHz, 120 W, 3 Torr, He/O2=0.7/0.3, 6 slm GEC_2005_13

  14. S-S vs CW: ELECTRON TEMPERATURE • Increasing power and increasing intra-pulse conductivity enables lowering of Te. • The effect is more pronounced with S-S. • 13 MHz, 3 Torr, He/O2=0.7/0.3, 6 slm Iowa State University Optical and Discharge Physics GEC_2005_14

  15. S-S vs CW: O2(1∆) YIELD AND PRODUCTION EFFICIENCY • Efficiency • S-S raises yields of O2(1∆) by 10-15% at lower powers. • Efficiency decreases with power due to dissociation. • Low power produces the highest efficiency with S-S but requires longer residence times to achieve high yield. • 13 MHz, 3 Torr, He/O2=0.7/0.3, 6 slm Iowa State University Optical and Discharge Physics GEC_2005_15

  16. MIN MAX S-S: ENGINEERING Te FOR YIELD 13 MHz 27 MHz Te (eV) • Intra-pulse Te decreases with increasing rf frequency. • As electron density and conductivity increases with successive pulses, Te decreases. • Average Te with 27 MHz is ≈1 eV, optimum for O2(1∆) production ANIMATION SLIDE • t = 2 - 15 µs 0 - 4.1 eV 0 - 2.5 eV Iowa State University Optical and Discharge Physics GEC_2005_16

  17. 13 MHz vs 27 MHz : O2(1Δ) YIELD • CW • Spiker-Sustainer • The efficiency of S-S increases with rf frequency by producing a higher [e] and lower Te. • Reduction in Te shifts operating point closer to optimum value, increasing yield by 10% to 20%. • 3 Torr, He/O2=0.7/0.3, 6 slm Iowa State University Optical and Discharge Physics GEC_2005_17

  18. GOING TO HIGHER RF FREQUENCIES? Optimum Te • 27 MHz vs 40 MHz • Te vs frequency • Increasing frequency above 27 MHz further decreases Te but improvements, if any, are small. • At sufficiently high frequencies, Te may decrease below that for optimum O2(1D) production (e.g., 40 MHz, Te = 0.5 eV) • 3 Torr, He/O2=0.7/0.3, 6 slm Iowa State University Optical and Discharge Physics GEC_2005_18

  19. CONCLUDING REMARKS • S-S method can raise yields of O2(1D) compared to CW excitation by lowering pulse average Te. • The efficiency of S-S methods generally increase with increasing rf frequency by producing • Higher electron density, • Lower Te • Going to very high frequencies may reduce Te below the optimum value for O2(1D) production. Iowa State University Optical and Discharge Physics GEC_2005_19

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