1. OPTIMIZATION OF O 2 ( 1  ) YIELDS IN PULSED RF FLOWING PLASMAS FOR CHEMICAL OXYGEN IODINE LASERS* Natalia Y. 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 June 2006 * Work supported by Air Force Office of Scientific Research and NSF. ICOPS2006_Natalie_01

2. Iowa State University Optical and Discharge Physics AGENDA  Introduction to eCOIL  Description of the model  Spiker Sustainer excitation vs CW for improving yield  Optimization of O 2 ( 1  ) yields in Spiker Sustainer excitation:  Power  Carrier frequency  Spiker frequency  Duty cycle  Higher pressure operation  Concluding remarks ICOPS2006_Natalie_02

3. Iowa State University Optical and Discharge Physics ELECTRICALLY EXCITED 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 eCOIL 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.  In this talk, S-S techniques will be computationally investigated. ICOPS2006_Natalie_03

4. Iowa State University Optical and Discharge Physics TYPICAL EXPERIMENTAL CONDITIONS  Laser oscillation has been achieved using He/O 2 flowing plasmas to produce O 2 ( 1  ) using capacitively coupled rf discharges.  I 2 injection and supersonic expansion (required to lower T g for inversion) occurs downstream of the plasma zone. ICOPS2006_Natalie_04  Ref: CU Aerospace

5. Iowa State University Optical and Discharge Physics O 2 ( 1 ∆) KINETICS IN He/O 2 DISCHARGES  Main channels of O 2 ( 1 Δ) production:  Direct electron impact [0.9 eV].  Excitation of O 2 ( 1 Σ) with rapid quenching to O 2 ( 1 Δ).  Self sustaining is T e =2-3 eV. Optimum condition for O 2 ( 1 Δ) production is T e =1-1.2 eV.  Significant power can be channeled into excitation of O 2 ( 1 Δ). ICOPS2006_Natalie_05

6. 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 Δ).  Example: He/O 2 =1/1, 5 Torr, Global kinetics model ICOPS2006_Natalie_06

7. Iowa State University Optical and Discharge Physics  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 ICOPS2006_Natalie_07

8. Iowa State University Optical and Discharge Physics  Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady algorithms.  Individual fluid species diffuse in the bulk fluid. DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT ICOPS2006_Natalie_08

9. Iowa State University Optical and Discharge Physics 2D-GEOMETRY FOR CAPACITIVE EXCITATION  Cylindrical flow tube 6 cm diameter  Capacitive excitation using ring electrodes.  Base case: He/O 2 = 70/30, 3 Torr, 6 slm.  Yield: Flow ICOPS2006_Natalie_09

10. Iowa State University Optical and Discharge Physics NON-SELF SUSTAINED DISCHARGES: SPIKER SUSTAINER  27 MHz, He/O 2 = 70/30, 3 Torr T e (eV) MINMAX t = 2 - 15 µs ANIMATION SLIDE 0 - 2.5 eV ICOPS2006_Natalie_10  Spiker sustainer consists of modulated rf excitation.  T e decreases during low power sustainer as there is excess ionization.  During startup transient, as electron density and conductivity increase with successive pulses, T e decreases.

11. Iowa State University Optical and Discharge Physics CW vs SPIKER SUSTAINER EXCITATION  T e in bulk plasma is reduced from 2.7 to 2.0 eV with factor of two larger n e ; Dissociation is lower, O 2 ( 1  ) larger.  V SS /V CW =2.5, 20% duty cycle, 13.56 MHz/1 MHz  3 Torr, He/O 2 =0.7/0.3, 6 slm MINMAX  CW  Spiker-Sustainer ICOPS2006_Natalie_11 Flow

12. Iowa State University Optical and Discharge Physics  Increasing carrier frequency improves efficiency of O 2 ( 1  ).  Higher ionization efficiency at high frequency enables lower T e.  CW: Lowering T e towards T e-opt is generally a benefit  SS: Decreasing T e below T e-opt lowers total excitation efficiency.  He/O 2 =70/30, 3 Torr  V SS /V CW =2.5, 20% dc, 1 MHz-SS CW vs SS: CARRIER FREQUENCY ICOPS2006_Natalie_15

13. Iowa State University Optical and Discharge Physics  Pulse power format is critical in determining efficiency for a given power deposition.  Larger V SS /V CW shifts power into ionization, allowing lower T e during sustainer.  Too large V SS /V CW produces too much ionization, lowering T e below T e-opt.  He/O 2 =70/30, 3 Torr, 40 W  20% dc, 27 MHz/1 MHz-SS SS FORMAT: V SS /V CW ICOPS2006_Natalie_16

14. Iowa State University Optical and Discharge Physics  Ideal spiker is a delta-function producing instant ionization at high efficiency.  With fixed V SS /V CW, lower power in spiker may reduce efficiency.  Increasing sustainer pulse length provides better utilization of low T e.  Too long a sustainer allows T e to increase towards self sustaining value.  He/O 2 =70/30, 3 Torr, 40 W, 20% dc SS FORMAT: SPIKER AND SUSTAINER PULSE LENGTH ICOPS2006_Natalie_17

15. Iowa State University Optical and Discharge Physics  Yield for SS is larger than CW; both increasing with power.  CW: Decrease in T e from above T e-opt to near T e-opt improves efficiency.  SS: Decrease in T e from near T e-opt to below T e-opt decreases efficiency.  CW and SS converge at high power.  He/O 2 =70/30, 3 Torr  V SS /V CW =2.5, 20% dc, 13.56 MHz/1 MHz CW vs SS: POWER DEPOSITION ICOPS2006_Natalie_14

16. Iowa State University Optical and Discharge Physics OPERATING AT HIGHER PRESSURES: GLOBAL MODEL  Many system issues motivate operating eCOILs at higher pressures.  If quenching is not important, [O 2 ( 1  )]  pressure for constant eV/molecule.  Significantly sub-linear scaling results in decrease in yield with increasing pressure.  O 3 is a major quencher.  Gas heating at high pressure reduces O 3 production and increases O 3 destruction.  O 3 kinetics and T g control are very important. ICOPS2006_Natalie_18

17. Iowa State University Optical and Discharge Physics OPERATING AT HIGHER PRESSURES: FULL 2D HYDRO  Large yields can be obtained at the edge of the plasma zone.  Up to 20-30 Torr, O 3 formation and quenching decrease yield.  >30-40 Torr, gas heating and constriction produce locally high yield that is rapidly quenched.  Reduction in yield is progressively determined by:  O 3 quenching  Gas heating  Discharge stability  He/O 2 =70/30, 25 MHz ICOPS2006_Natalie_19

18. DISCHARGE STABILITY WITH PRESSURE Iowa State University Optical and Discharge Physics  Operating at higher pressures often encounter discharge stability issues.  Constriction of discharge occurs due to smaller mean- free-paths.  Asymmetry in plasma begins to occur due to downstream rarefaction being greater.  He/O 2 =70/30, 25 MHz FLOW [e] 10 10 cm -3 T e (eV) 3 Torr, 40 W 50 Torr, 670 W MAX 0 3 Torr, 40 W 50 Torr, 670 W ICOPS2006_Natalie_21 ANIMATION SLIDE

19. Iowa State University Optical and Discharge Physics  Spiker-sustainer strategies can be effective in lowering T e into more optimum regime for exciting O 2 ( 1  ).  Higher carrier frequencies (either CW or SS) produce larger n e and lower T e and so are beneficial.  Advantage of SS is marginal at higher powers due to T e being naturally lower.  High pressure operation can produce larger densities of O 2 ( 1  ) at high yields with careful management of  Ozone density  Gas temperature  Stability CONCLUDING REMARKS ICOPS2006_Natalie_22