1. 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 October 2005 * Work supported by Air Force Office of Scientific Research and NSF

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

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

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

5. University of Illinois Optical and Discharge Physics SPIKER SUSTAINER TO LOWER T e  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 T e (E/N).  T e is closer to optimum for exciting O 2 ( 1  ).  Example: He/O 2 =1/1, 5 Torr, Global kinetics model GEC_2005_05

6. Iowa State University Optical and Discharge Physics 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: DESCRIPTION OF the MODEL: CHARGED PARTICLES, SOURCES GEC_2005_06

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

8. Iowa State University Optical and Discharge Physics GEOMETRY FOR CAPACITIVE EXCITATION Cylindrical flow tube 6 cm diameter Capacitive excitation using ring electrodes. He/O 2 = 70/30, 3 Torr, 6 slm. Yield: GEC_2005_08 Flow

9. Iowa State University Optical and Discharge Physics TYPICAL PLASMA PROPERTIES (13 MHz, CW) O 2 ( 1 Σ) and O densities are maximum near peak power deposition. O 2 ( 1 ∆) increases downstream while O 2 ( 1 Σ) is quenched to O 2 ( 1 ∆). 3 Torr, He/O 2 =0.7/0.3, 6 slm Power, [e], O, O 2 ( 1 Σ) and O 2 ( 1 ∆) GEC_2005_09 MINMAX O 2 ( 1 ∆) yield on Axis

10. Iowa State University Optical and Discharge Physics  Spiker-sustainer (S-S) consists of pulsed modulated rf excitation.  High power pulses produce excess ionization and allow discharge to operate nearer to optimum T e for O 2 ( 1 ∆) production.. SPIKER-SUSTAINER: VOLTAGE WAVEFORM 27 MHz, 120 W, 1 MHz Carrier, 20% duty cycle GEC_2005_10

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

12. Iowa State University Optical and Discharge Physics S-S vs CW : PLASMA PROPERTIES GEC_2005_12 O 2 ( 1 Σ ) is quickly collisionally quenched to O 2 ( 1 ∆) after the plasma zone. O 2 ( 1 ∆) is quenched slowly. O atom production nearly equals O 2 ( 1 ∆). 13 MHz, 40 W, 3 Torr, He/O 2 =0.7/0.3, 6 slm Spiker-Sustainer CW

13. Iowa State University Optical and Discharge Physics GEC_2005_13 Dissociation fraction decreases when using S-S. Lower T e enabled by S-S reduces rate of dissociation while increasing rate of excitation of O 2 ( 1  ). S-S vs CW: O 2 ( 1  ) PRODUCTION AND O 2 DISSOCIATION Spiker-Sustainer CW 13 MHz, 120 W, 3 Torr, He/O 2 =0.7/0.3, 6 slm

14. Iowa State University Optical and Discharge Physics S-S vs CW: ELECTRON TEMPERATURE GEC_2005_14 Increasing power and increasing intra-pulse conductivity enables lowering of T e. The effect is more pronounced with S-S. 13 MHz, 3 Torr, He/O 2 =0.7/0.3, 6 slm

15. Iowa State University Optical and Discharge Physics S-S vs CW: O 2 ( 1 ∆) YIELD AND PRODUCTION EFFICIENCY GEC_2005_15 S-S raises yields of O 2 ( 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/O 2 =0.7/0.3, 6 slm Efficiency

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

17. Iowa State University Optical and Discharge Physics 13 MHz vs 27 MHz : O 2 ( 1 Δ) YIELD  The efficiency of S-S increases with rf frequency by producing a higher [e] and lower T e.  Reduction in T e shifts operating point closer to optimum value, increasing yield by 10% to 20%. GEC_2005_17 3 Torr, He/O 2 =0.7/0.3, 6 slm Spiker-Sustainer CW

18. Iowa State University Optical and Discharge Physics GOING TO HIGHER RF FREQUENCIES? Increasing frequency above 27 MHz further decreases T e but improvements, if any, are small. At sufficiently high frequencies, T e may decrease below that for optimum O 2 ( 1  ) production (e.g., 40 MHz, T e = 0.5 eV) GEC_2005_18 Optimum T e 3 Torr, He/O 2 =0.7/0.3, 6 slm 27 MHz vs 40 MHz T e vs frequency

19. Iowa State University Optical and Discharge Physics CONCLUDING REMARKS  S-S method can raise yields of O 2 ( 1  ) compared to CW excitation by lowering pulse average T e.  The efficiency of S-S methods generally increase with increasing rf frequency by producing  Higher electron density,  Lower T e  Going to very high frequencies may reduce T e below the optimum value for O 2 ( 1  ) production. GEC_2005_19